• Created by: sam9991
  • Created on: 25-11-16 12:31

Introduction to Biological Molecules

Covalent Bonding - atoms share a pair of electron in their outer shell. As a result both full outer shell, stable, molecule formed

Ionic Bonding - ions with opposite charge attracts one another. The electrostatic attraction is known as an ionic bond

Hydrogen Bonding - the electrons within a molecule are not evenly distributed but tend to spend more tine at one position. This region is more negatively charged that the rest of the molecule. This molecule is polarised and is a polar molecule. The negative region and positive region attract each other. A weak electrostatic bond is formed between the two. Each individually weak collectively form important forces which alter physical properties E.g Water

  • Monomers + Monomer = Polymer process called polymerisation
  • Condensation rection - reaction that produces water.  Hydrolysis reaction - water used to break bonds
  • Metabolism - all chemical processes that take place in living organisms
  • Mole - unit for measuring the amount of a substance (mol). A mole is the molecular mass in grams 
  • Molar solution (M) - solution that contains one mole of solute in each litre of solution
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Monosaccharides, Disaccharides and Polysaccharides

Monosaccharides -sweet, soluble, general formula (CH2O)n  where n from 3-7 E.g Glucose, Galactose and Fructose. Glucose is a hexose sugar C6H12O6 has two isomers α glucose and β glucose

Disaccharides - monosaccharides + monosaccharides = disaccharides condensation reaction occurs, glycosidic bond forms. Disaccharide + water = 2 monosaccharides, hydrolysis reaction


  • Glucose + Glucose = Maltose
  • Glucose + Galactose = Lactose
  • Glucose + Fructose = Sucrose

Polysaccharides - very large, insoluble (allows them to be suitable for storage) are a polymer formed by combinimg many monosaccharides joined by glycosidic bonds formed by condensation reaction. Hydrolysed into disaccharides or monosaccharides. E.g Starch and Cellulose


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Polysaccharide found in parts of plants in small grains, seeds or storage organs (potatoe tubers). Major energy source and is made up of chains of α glucose monosaccharides linked by glycosidic bonds formed by condensation reaction

Structure is suitable

  • Insoluble and therfore doesn't affect Ψ so water not drawn out by osmosis
  • large and insolube so doesn't diffuse out
  • compact so can store a lot in small space
  • when hydrolysed forms α glucose which is easily transported and used in respiration
  • branched forms have many ends which can be acted on by enzymes so glucose monomers rapidly released

Starch is not found in animals cells instead glycogen is used

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Found in animals and bacteria but never in plant cells. It has a very similar structure to starch but shorter chains and more branched

Nicknames 'animals starch' as in animals it is stored as small granules mainly in the muscles and the liver. The mass of carbohydrated that is stored is small because fat is the main storage in animals

Structure suits it for storage

  • insoluble and doesn't tend to draw water into the cells by osmosis
  • as insoluble does not diffuse out of cells
  • compact so a lot can be stored in a small space
  • highly branched so more ends acted on by enzymes so more rapidly broken down to form glucose monomers which used in respiration
  • being highly branched is important a animals have high metabolic rate and respiratory rate than plants as are more active
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Made of β glucose instead of α glucose. It has straight unbranched chain that run parallel to each other with hydrogen bonds forming cross linkages (individually hydrogen bonds weak overall they are strong)

Major component of plant cell walls and provides rigidity to the plant cell and prevent cell from bursting when osmosis occurs by exerting an inwards pressureto stop further influx of water

Structure of cellulose suited to its function

  • make up of β glucose and form long straight unbranched chains
  • chain run parallel to each other and are crossed linked by hydrogen bonds which add collective strength
  • these are then grouped together to form microfibrils which are group together to form fibres which provide more strength
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Role of Lipids

Source of energy - when oxidised lipids provide more than twice the energy of the same mass of carbs and release valuable water

Waterproofing - lipids are insolube in water. Both plant and insects waxy, lipid cuticles that conserve water, while mamals produce an oily secretion from glads in the skin

Insulation - fats are slow conductors of heat and when stored beneath body surface it helps to retain body heat. They also act as electrical insulators around nerve cells

protection - stored around delicate organs such as the kidney

Fats are solid at room temperature (10-20°C) while oils are liquids

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Have three fatty acids and a glycerol. Each fatty acid forms an ester bond with glycerol in a condensation reaction, Hydrolysis of triglyceride prodces glycerol and three fatty acids

Difference in fats and oild is variation in the fatty acids. There are over 70 different fatty acids but they all have a carboxyl group (-COOH) and a hydrocarbon chain

If there is no carbon-carbon double bond then it is unsaturated. If there is 1 double bond then it is monounsaturated if there is more than 1 carbon-carbon double bond then it is polyunsaturated

Structure related to properties

  • high ratio of energy storing carbon-hydrogen bonds to carbon atoms an therefore excellent source of energy
  • low mass to energy ratio making them good storage molecules as lots of energy can be stored in small volume. (benefits animals as reduces mass they have to carry)
  • large, non-polar and insoluble in water it doesn't affect osmosis
  • high ratio of hydrogen to oxygen atoms when oxidised they produce water which is important for organism in dry deserts
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Similar to Triglyceride but has one fatty acid molecule replaced with a phosphate molecule

  • Phosphate molecule attracts water (Hydrophillic) as are polar
  • Fatty acid molecules repel water (Hydrophobic)

Structure related to propeties

  • polar molecules as has hydrophillic phosphate head and hydrophobic tail. This means inaqueous environment phospholipid molecules form a bilayer within cell surface membrane. This forms a hydrophobic barrier between the inside and outside of the cell
  • Hydrophilic phosphate head helps to hold the surface of the cell membrane together
  • Structure allows glycolipids to form by combining carbs within the cell surface membrane which are important in cell recognition
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Structure of an amino acid and formation of Peptid

Are basic monomer units which combine to make a polyumer called polypeptide which can combine to form proteins. There are about 100 amino acids of which 20 occur naturally in proteins. These 20 occur in all living organism providing evidence of evolution

Every amino acid has four different chemical groups

  • Amino group (-NH3) - basic group which amino part of name is from
  • Carboxyl grouo (-COOH) - an acidic group which acid part of name is from
  • Hydrogen atom (-H)
  • R (side) group - a variety of different chemical groups. Each amin acid has a different R group. This allows the 20 naturally occuring amino acids to be different

Formation of Peptide Bonds

Amino acids combine to form dipeptides by a condensation reaction. The water is made by combining an -OH from the carboxyl group and a -H from the amino group. The two amino acids become linked by a peptide bonf between the carbon of the carboxyl group of one amino acid and the nitrogen of the amino group of the other.  This bond can be broken by hydrolysis to for 2 amino acids

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Primary Structure of protein

Series of condensation reaction and many amino acid monomers can be joined together by polymerisation which results in a polypeptide forming.

The sequence of amino acids in a polypeptide forms primary structure of protein and is determined by DNA. There is a limitless number of possible combination and therefore types of primary protein structure

Primary structure of a protein determines the shape and function a change in a single amino acid can change the shape of the protein and may stop it carrying out its function as shape is specific to its function

A simple protein may consist of a single polypeptide chain but  normally proteins are made up of a number of polypeptide chains

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Secondary Structure of Proteins

Linked amino acids have -NH and -C=O group either sie of peptide bond.

The Hydrogen in the -NH bond has a + charge while the Oxygen in the -C=O bonds has a - charge

These groups form weak bonds called hydrogen bonds which causes the long polypeptide chain to be twisted into a 3D shape known as a α helix

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Tertiary Structure and Quaternary Structure of Pro


α helices of secondary proteins can be twisted and folded even more to give the complex and specific 3D structure of each protein

This structure is maineded by different bonds where they occur depends on primary structure

  • Disulfide bridges - fairly strong not easily broken
  • Ionic bonds - formed between carboxyl and amino groups that are not involved in forming peptide bonds. They are weaker then Disulfide bridges and easily broken by changes in pH
  • Hydrogen bonds - are eaily broken


Large proteins form complex molecules containing lots of individual polypeptide chains that are linked in various ways. There may also be non-protein (prosthetic) groups auch as iron-containing haem group in haemoglobin.

3D strucutre determined by primary structure and is important to its function

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Protein Shape and Function

Proteins have may different roles depending on their molecular shape

  • Fibrous proteins such as collagen have structural function
  • Globular proteins such as enzymes and haemoglobin carry out metabolic function

Fibrous proteins are long and thin and tend to have structural roles, such as bone, hair, cytoskeleton and muscle. They are alway composed of many polypeptide chains. Polypeptides form long chains running parallel to each other, these chains are linked by disulphide bridges making the proteins very stable and strong

Majority of proteins are globular, they have a compact ball-shaped structurem forming a coiled shape. This includes enzymes, membrane proteins, receptors and storage proteins. Hydrophobic groups point to centre of molecule and Hydrophilic groups are exposed outside making globular protein soluble

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Enzyme Structure

They are globular protein and have a specific 3D shape because of their amino acids sequence. A specifc region of the enzyme is functional (active site) which is made up of a small number of amino acids which forms a small depression within a larger enzyme molecule

Substrate - the molecule on which the enzyme acts

Enzyme-Substrate complex - when the substrate fits neatly into the active site

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Induced fit model of enzyme action

Is a scientific model to show how the active site forms as the enzyme and substrate interact. A change in the environment of the enzyme leads to a change in the enzyme that forms the functional active site.

The enzyme is flexible and can mould itself around the substrate, the enzyme has a certain general shape but this alters in the presence of the substrate.

As the enzyme changes its shape the enzyme puts a strain on the substrate which distorts the bonds in the substrate and lowers the activation energy needed to break the bond

Any changes in the enzyme's environment is likely to change its shape. Just by colliding with its substrate is a change so its shape will change (induced fit)

Enzyme-substrate molecule → Enzyme-substrate complex → Enzyme + product molecule

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Lock and key model of enzyme action

Thought that enzymes work in the same way as a key. Each key haas a specific shape that fits and will only work in a single lock. This is similarr to substrate as it will only fit the active site of a particular enzyme.

Supported by observations that enzymes are specific in the reactions that they catalyse. The shape of the substrate(key) exactly fits the active site of the enzyme (lock). Hence known as the lock and key model

However scientists have observed that other molecules could bind to enzymes at sithes other than the active site. They then alter the activity of the enzymes, which suggests that the enzyme's shape was being altered by the binding molecule. So its structure was not rigid but flexible

Alternative model was proposed called induced fit model

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Competitive inhibitors

Competitive Inhibitors

Have a molecular shape similar to that of the substrate which allows them to occupy the active site of an enzyme, they therefore have to compete with the substrate for the available active sites.

The difference between the concentration of the inhibitor and the substrate which determines the effects that this has on enzyme activity

  • if substrate concentrateion is increased the effect of the inhibitor is reduced

The inhibior is not permanently bound to the active site so when it leavesanother molecule can take its place, this could be a substrate or inhibitor molecule depending on how much present. The substrate molecules will occupy the active site but the greater te concentration of the inhibitor the longer it will take

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Non-competitive inhibitors

They attach themselves to the enzyme at a binding site (allosteric site) which is not active, this alters the shape of the enzyme and active site so substrate can no longer fit it so enzyme cannot function. As not competiting and increase in substrate concentration doesn;t decrease the effect of the inhibitor

Control of metabolic pathways

They are a series if reaction in which each step is catalysed by an enzyme, which happen in the tiny space inside a single cell.

The enzymes that control a pathway are often attached to the membrane of a cell organelle in a very precise sequence. Inside each organelle optimum conditions for the funtioning of the enzyme are provided.  To keep a steady concentration of a particular chemical in a cell, the same chemical often acts as an inhibitor of an enzyme at the start of a reaction.

If the concentration of the end product increases then there will be greater inhibition of enzyme A and less end product will be produced and concentration will return to normal. If the concentration of the end product falls there will be less of it to inhibit enzyme A, more end product profuced and concentration will return to normal.This is known as end-product inhibition and type of non-competitive inhibition

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Nucleotide Structure

Made up of three components:

  • Pentose sugar (have five carbons)
  • Phospohate group
  • Nitrogen-containing organic bases (cytoside, thymine, uracil, adenine and guanine)

They are all joined by a condensation reaction to form a single nucleotide (mononucleotide). Two mononucleotides may be joined by condensation reaction between the deoxyribose sugar of one mononucleotide and the phosphate group of another forming a phosphodiester bond creating a dinucleotide. This contines to form long chains (polynucleotides)

Base Pairing

Bases on the two strands of DNA attach to each other by hydrogen bonds.A joins with T and      G joins with C, as a result of complementary pairing

The concentration of A and T, G and C are always the same in DNA. However, the ratio of A and T to G and C varies from species to species

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RNA and DNA Structure


Ribonucleic acid (RNA) is a polmer made up of nucleotides. It is a single, short, polynucleotide chain which pentose sugar is ribose and organic bases are A,T,C,U

One type of RNA transfers genetic information from DNA to the ribosomes. Ribosomes are made up of proteins and another type of RNA. A third type of RNA is involved in protein synthesis


In DNA pentose sugar is deoxyribose and the organic bases are A,T,C,G and is made up of two strands of nucleotides (polynucleotides). Each strand joined by hydrogen bonds between bases.The phosphate and deoxyribose wind around each other to fom a double helix and form backbone of the DNA molecule

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Stability and function of DNA

Stability of DNA

  • Phosphodiester backbone protects the chemically reactive organis bases inside the helix
  • hydrogen bonds link organic base pairs forming bridges (rungs) between the phosphodiester uprights. 3 hydrogen bonds between C and G, higher proportion of C-G more stable it is
  • there are oter forces between base pairs that hold the molecules together (base stacking)

Function of DNA

There are 3.2 billion base pairs in DNA of mammal cells. There is infinite varitey of sequences and will provide genetic diversity. Function depends on sequence of base pairs

  • Very stable strcture from generation to generation with no changes as rarely mutates
  • 2 seperate strands joined by hydrogen bonds allows them to seperate during replication and protein synthesis
  • large so can carry lots of genetic information. Base pairs allow to replicate and transfer info
  • By having base pairs with helical cylinder of deoxyribos-phosphate backbone the genetic information is protected from being corrupted by outside chemicals and physical forces
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DNA Replication

Cell divison occurs in two stages Nuclear division (nucleus divides by mitosis and meiosis) and Cytokinesis (whole cell divides). Before nucleus divide DNA must replicat to ensure that the daughter cells have identical genetic infomation

Semi-conservative replication

  • requirments: 4 types of nucleotide (A,G,C,T), both strands of DNA to act as template, DNA polymerase and source of chemical energy to drive process


  • DNA helicase breaks hydrogen bonds seperating double helix into two strans and it unwinds
  • Exposed polynucleotide strands acts as a template to which complementary nucleiorides bind to specific base
  • Nucleotides are joined together in a condensation reaction by DNA polymerase to form 'missing' polynucleotide strand on each of the original strands of DNA
  • Each new DNA molecule contains one original DNA strands (half the original DNA has been built into each of the new DNA molecule
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Structure and storage of ATP

Structure of ATP

Is a Phosphorylated molecule made out of adenine (nitrogen-containing base), ribose (pentose sugar that acts as a backbone) and phosphates (chain of three phosphate groups)


Bonds between the phosphate groups are unstable and have a low activation energy so they are easily broken. When they break they release a large amount of energy

ATP + H2O→ ADP + Pi + Energy

Water is used to convert ATP to ADP by hydrolysis and is catalysed by ATP hydrolase (ATPase)

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Synthesis of ATP

Conversion of ATP to ADP is reversible and therefore energy can be used to add an inorganic phosphate to ADP to re-form ATP. This reaction is catalysed by ATP synthase. Water is also removed by a condensation reaction

The synthesis of ATP from ADP involves the addition of phosphate molecules to ADP

This can occur in three ways:

  • In chlorophyll-containing plant cells during photosynthesis (photophosphorylation)
  • In plant and animal cells during respiration (oxidative phosphorylation)
  • In plant and animal cells when phosphate groups are transferred from donor molecule of ADP (substrate-level phosphorylation)
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Roles of ATP

  • Instability of phosphate bonds means not good at long-term energy store (Fats,carbs and glycogen are better) therefore it is an immediate energy source for a cell and is not stored in large quantities as it can be quickly produced within the mitochondria of cell that need it. Cells like muscle fibres and epithelium of small intestines require more energy for movement and active transport so have many mitochondria
  • Each ATP molecule releases less energy than a glucose molecule. So energy released via ATP is smaller and more managable compared to glucose
  • Hydrolysis of ATP to ADP is a single reaction that releases immediate energy, whereas glucose is a long series of reaction and energy release takes longer

Used in processes such as:

  • Metabolic processes (to build up macromolecules from basic units E.g starch from glucose)
  • Movement (provide energy for muscle contraction (for filaments to slide past each other shortening overall length of the muscle fibre))
  • Active transport (to chage shape of carrier proteins in plasma membrane to move against concentration gradient)
  • Secretion (form lysosomes
  • Activation of molecules (Pi released phosphorylate other compounds to make them more reactive lowering activation energy   
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Microscopy and Magnification


  • Microscopes are instruments that produce a magnified image of an object, a convex glass lens acts as magnifying glass, they are more effective if in pairs like a compound microscope.
  • Long wavelengths of light means a light microscope can distinguish between two objects if they are 0.2μm or further. This can be overcome by using beams of electrons rather than beams of light, with shorter wavelengths the beam of electrons in the electron microscope can distinguish between two objects only 0.1nm

Kilometer (km) =103m                   Magnification = (size of image) / (size of real object)

Metre (m) = 1m

Millimetre (mm) = 10-3m

Micrometre (μm) =10-6m

Nanometre (nm) =10-9m

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Cell Fractionation

Process where cells are broken up and the different organelles they contain are seperated. Before it can take place it must be in

  • Cold - to reduce enzyme activity that might break down the organelles
  • Isotonic - to prevent organelles bursting or shrinking due to osmotic gain or loss
  • Buffered - so pH doesn't fluctuate as it could alter structure of the organelles or affect functioning of enzymes

Two stages

Homogenation - cells broken up by homogeniser (blender) to release organelles from cells. Fluid left is homogenate which is filtered to produce filtrate

Ultracentrifugation - filtrate is placed in a centrfuge and spun at slow speed, heaviest organelle sinks to form a pellet and fluid left is supernatant, supernatant is transferred and spun again. This process is repeated

Results:  Nuclei 1st at 1,000 revolutions per min. Then mitochondria at 3,500 revolutions per min. Then Lysosomes at 16,500 revolutions per min

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Transmission Electron Microscope (TEM)

An electron gun produces a beam of electrons that is focused onto the specimen by a condenser electromagnet. In a TEM beam passes through a thin section of specimen parts of it is absorbs electrons and appear dark, other parts allow to pass through so appear bright. Image produced to give a photomicrograph.

Resolution of 0.1nm but hard to achieve as it can be hard to prepare the specimen without limiting the resolution and the higher energy electron beam and may destroy the specimen


  • whole system must be in a vacuum and therefore specimens cannot be alive
  • staining process is required but the images is not in colour
  • specimen must be extremely thin
  • Image may contain artefacts (things that result from the way the specimen is prepared) they may appear on the finished photpmicrograph but are not part of specmimen. Therefore not always easy to be sure what we see on a photomicrograph exists in that form
  • produces a 2D image but you can build up a 3D image by looking ay the series of photomicrographs produces. But this is slow and complicated
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Scanning Electron Microscopes (SEM)

SEM is similar to TEM as it directs a beam of electrons on to the surface of the specimen rather than penetrating it from below. The beam is then passes back and forth across a portion of the specimen in a regular pattern. The electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface

You can build up a 3D image by computer analysis of the pattern of scattered electrons and secondary electrons produced.

Limitation of TEM also apply to SEM except that specimens don't need to be extremely thin as electrons so not penetrate

The basic SEM has a lowerr resolving power than a TEM around 20nm but is still ten times better than a light microscope

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Measuring cells and Calibrating the eyepiece

When using a light microscope you measure the size of the objects using an eyepiece graticule (a glass disc that is placed in the eyepiece of a microscope) a scale is etched on the glass disc which is typically 10mm long and divided into 100 sub divisions. The scale cannot be used to directly measure the size of objects because each lens will magnify so it must be calibrated for a particular objective lens once done it remain in the same place if same lens is used. Good to record the results of the calibration for a object and leave it attached so save recalibrating

Calibrating the eyepiece graticule

  • Need to ise a special microscope slide called a stage micrometer, which is also has a scale etcjed onto it. When the eyepiece graticule scale and stagemicrometer scale are lined-up it is possible to calculate length of the ivisions of eyepiece
  • should be able to see that 1 units on micrometer scale = 4 units on graticule scale, also each unit on the micrometer scale = 10μm each unit on graticule equals 10/4 = 2.5μm
  • To calculate scale by dividing the differences in magnification
  • E.g if an objective lens magnifying X40 gives calibration of 25μm per graticule unit, then an objective lens magnifying X400 (10 times greater) will mean a graticule unit is equivalent to 25μm/10 = 2.5μm
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Eukaryotic Cell Strucutre - The Nucleus

Most prominent feature and contains the organism's hereditary material and controls the cell's activity. Usually spherical and between 10 and 20μm in diameter

  • Nuclear envelope - double membrane surrounding nucleus. Outer membrane is continuous with ER of cell often has ribosomes on surface. Controls the entry and exit of material in and out of nucleus and contain the reatction taking place within it
  • Nuclear pores - allows passafe of large molcues such as mRNA out of nucles, around 3,000 in each nucleus eas 40-100nm in diameter
  • Nucleoplasm - granular, jelly-like material that makes up the bulk of the nucleus
  • Chromosomes - consists of protein-bound linear DNA
  • Nucleolus - small spherical region within nucleoplasm which manufactures ribosomal RNA and assembles the ribosomes. There may be more than one nucleolus in a nucleus

Functions of the nucleus 

  • Act as a control centre through production of mRNA and tRNA and protein synthesis
  • Retain generic material in form of DNA and chromosomes
  • Manufacture ribosomal RNA and ribosomes
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Eukaryotic Cell Strucutre - The Mitochondrion

Rod-shaped and 1-10μm in length and made up of

  • Double membrane - controls the entry and exit of material. The inner two membranes is folded to form extensions (cristae)
  • Cristae -  extensions of the inner membrane which some species extend across the whole width of the mitochondrion. These provide large surface area for attachment enzymes and other proteins involved in respiration
  • Matrix - makes up remainder of the mitochondrion and contains proteins, lipid, ribosomes and DNA that allows it to control the production of some of own proteins. Many enzymes involved in respiration are found in the matrix

Are sites of aerobicstages of respirsation (krebs cycle and oxidative phosphorylation).Are responsible for production of ATP from respiratory substrate (E.g glucose), the number and size of the mitochondria and number of cristae are high in cells that have high levels of metabolic activity and require large supply of ATP.

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Eukaryotic Cell Strucutre - The Chloroplasts

Carry out photosynthesis and vary in shape and size but are typically disc-shaped 2-10μm long and 1μm diameter. Main features:

  • Chloroplast envelope - double plasma membrane surround it, selective about what enters and leaves the chloroplast
  • Grana - stacks of up to 100 disc-shaped structures (thylakoids), they also have photosynthetic pigment (chlorophyll) within them, and have tubular extensions that join thylakoids with adjacent grana. The grana are where the first stage of photosynthesis (light absorption) takes place
  • Stroma - fluid-filled matrix where 2nd stage of photosynthesis takes place, is also starch grains


  • Granal membrane provide large SA for attachment of chlorophyll, electron carriers and enzymes that carry out 1st stage of photosynthesis and attached to membrane
  • Stroma possesses all enzymes needed to make sugar for 2nd stage of photosynthesis
  • Chloroplasts contain DNA and ribosomes so can make proteins needed for photosynthesis quick and easily
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Eukaryotic Cell Strucutre - The Endoplasmic Reticu

3D sheet-like membranes spreading through the cytopplam of the cells, continuous with outer nuclear membrane and encloses a network of tubules and flattened sacs (cisternae) There are two types:

Rough ER (RER) has ribosomes on outer surface of membranes and functions are to:

  • Provide large surface area for the synthesis of proteins and glycoproteins
  • Provide a pathway for transport of material, especially proteins throughout the cell

Smooth ER (SER) lacks ribosomes on its surface and is often more tubular and functions are to:

  • Synthesis, store and transport lipids
  • Synthesis, store and transport carbohydrates
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Eukaryotic Cell Strucutre - The Golgi Apparatus

In almost all eukaryotic cells and is similar to SER in structure excep it is more compact. It consists of a stack of membranes that make up flattened sacs or cisternaewith small rounded hollow structure called vesicles.Proteins and lipids produced by ER passes through the Golgi and is modifided (often by adding non-protein components). Also is labelled allowing them to be sorted and sent to their correct destinations. Once stored the proteins and lipids are transported to Golgi vesicles which are pinched off from the end of the Golgi cisternae. These vesicles may move to the cell surface where they fuse with the membrane and release their contents to the outside


  • Add carbohydrates to proteins to form glycoproteins
  • Produce secretory enzymes such as ones secreted by pancrease
  • Secrete carbohydrates such as those used in making cell walls in plants
  • Transport, modify and store lipids
  • Form lysosomes

Well developed in secretory cells, such as epithelial cells that line the intestines

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Eukaryotic Cell Strucutre - Lysosomes

Formed when vesicles produced by the Golgi apparatus contain enzymes such as proteases. They also contain lysozymes enzymes that hydrolyse the cell wall of certain bacteria, there is a many as 50 enzymes in a single lysosome and is up to 1μm diameter. They isolate enzymes from the rest of the cell before releasing them with to the outside or into a phagocytic vesicle within the cell


  • Hydrolyse material ingested by phagocytic cells such as white blood cells and bateria
  • Release enzymes to the outside of the cell (ecocytosis) in order to destroy material arount the cell
  • Digest worn out organelles so that the useful chemicals they are made of can be re-used
  • Completely break down cells after they have died (autolysis)

Given roles of lysosomes they have a large number secretory cells such as epithelial cells and phagocytic cells

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Eukaryotic Cell Strucutre - Ribosomes

Small cytoplasmic granules found in all cells. They may occur in the cytoplasm or be associated with the RER. There are two types, depending on the cells in which they are found:

  • 80S - found in eukaryotic cells around 25nm in diameter
  • 70S - found in prokaryotic cells, mitochondria and chloroplasts and is slightly smaller

Ribosomes have two subunits - one larger and one small - each which contain ribosomal RNA and proteins, small, occur in small numbers and account for 25% of dry mass of a cell.

They are the site of protein synthesis

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Eukaryotic Cell Strucutre - Cell Wall

Consists of microfibrils of cellulose embedded in a matrix. Cellulose microfibrils have large strength and contribute to overall strength of cell wal. Features 

  • Number of polysaccharides such as cellulose
  • Thin layer called middle lamella which marks boundary between adjacent cell walls and cements adjacent cells together


  • To provide mechanical strength in order to prevent the cell bursting under the pressure created by osmotic entry of water
  • To give mechanical strength to the whole plant
  • To allow water to pass along it and contribute to the movement of water throught the plant

Cell walls of algae made up of either cellulose or glycoproteins or both

Cell walls of fungi do not contain cellulose but have a mixture of nitrigen-containing polysaccharide called chitin, and a polysaccharide called glycan and glycoproteins

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Eukaryotic Cell Strucutre - Vacuoles

Fluid-filled sac bounded by a single membrane, within mature plant cells there is usually one large central vacuole. The single membrane around it is called a tonoplast. A plant vacuole contans a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins


  • Support herbaceous plants and herbaceous parts of woody plants by making cells turgid
  • Sugars and amino acids may act as temporary food store
  • The pigments may colour petals to attract pollinating insects
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Cell Specialisation

Cells of a multicellular oganism perform certain basic functions. However no one cell can provide the best conditions for all functions, therefore the cells are specialised in different ways to perform a particular role. Each cell has evolved more to suit the role it carriers out

Embryos are initially identical, but as it matures each cell takes on its own individual characteristics that suits its function that it will perform when its mature.

All cells in an organism are produced by mitotic divisions from the fertilised egg, because they all have exactly the same gene . They become specialised because every cell contains the genes needed for development. Some of these genes are expressed in one cell at one time, different genes are switched on in each type of specialised cell the rest are switched off.

Not just shape what varies but also each of their organelles E.g muscle or sperm cell will have many mitochondira while bone cells have few. White blood cells have many lysosomes while muscles have few

Cells of multicellular organisms have evolved to become more and more suited to one specialised function. These cells are adapted to their own particular function and perform it more effectively. Resulting in whole organism functions efficiently

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Collection of similar cells that perform a specific function

Epitherlial tissues - found in animals and consist of sheets of cells. They line surfaces of organs and often have protective or secretory function. There are similar types made up of thin, flat cells that line organs where diffusion takes place E.g Alveloi of lungs and ciliated epithelium that lunes a duct like trachea. Cillia used to remove musus over the epithelial surface.

Xylem - occuts in plants and is made up of number of similar cell types. Used to transport water and minerals ions throughout the plant and also gives mechanical support

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A combination of tissues that are coordinated to perform a variety of functions, they often have one major function. Made up of tissues such as

  • Muscles to churn and mix stomach contents
  • Epithelium to protect the stomach wall and produce secretions
  • Connective tissue to hold together the other tissues

In plants a leaf is an organ and is made up of tissues such as

  • Palisade mesophyll - made up of leaf palisade cells that carry out photosynthesis
  • Spongy mesophyll - adapted for gaseous diffusion
  • Epidermis - to protect the leaf and allow gaseous diffusion
  • Phloem - to transport organic materials away from the leaf
  • Xylem - to transport water and ions into the leaf

Not easy to determine which structures are organs. Blood capillaries are not organs whereas arteries and veins are both organs. All 3 structures have same major function to transport blood. Capillaries are made up of one tissue (epithelium) whereas arteries and veins are made up of many tissues (epithelial, muscle and others)

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Organ Systems

Organs work together as a single unit know as an organ system. These systems may be grouped together to perform a particular functions more efficiently. Organ systems in humans:

  • Digestive system - digests and processes food. It is made up of organs which include salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver
  • Respiratory system - used for breathing and gas exchange. It is made up of organs which include trachea, bronchi and lungs
  • Circulatory system - pumps and circulates blood. It is made up of organs which include heart, arteries and veins
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Structure of a bacteria cell

Bacteria occur in every habitat they are versatile and adaptable. They are small ranging from 0.1 to 10μm. Their structure is simple

  • Cell wall which is made up of murein (polymer of polysaccharides and peptides)
  • Protect themselves by secreting a capsule of mucilaginous slime around the wall
  • Cell surface membrane which has cytoplam containing 70S ribosomes
  • Ribosomes are smaller that those in Eurkaryotic cells (80S) still synthesise proteins
  • Store food as glycogen granules and oil droplets
  • Have genetic material in the form of circular strand of DNA. They also have smaller circular pieces of DNA called plasmids
  • Can reproduce themselves independently and may give the bacterium resistance to harmful chemicals such as antibiotics
  • Plasmids are used as carriers of genetic information (vectoes) in genetic engineering
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Are acellular, non-living particles that are smaller than bacteria ranging in size from 20-300nm. They contain nucleic acids such as DNA or RNA as genetic material but can only multiply inside living host cells.  The nucleic acidds is enclosed within a protein called the capsid. Some viruses like HIV are surrounded by a lipid envelope which has attachment proteins (if no lipid envelope the capid will have attachment proteins) which are essential to allow the virus to identify and attach to a host cell.

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Compare ear prokaryotic and eukaryotic cells

Prokaryotic cells

Eukaryotic cells

No true nucleus only an area where DNA is found

Distinct nucleus, with a nuclear envelope

(Pro) DNA is not associated with proteins

DNA is associated with proteins called histones

Some DNA may be in the form of circular strands called plasmids

There are no plasmids and DNA is linear

No membrane-bounded organelles

Membrane-bounded organelles such as mitochondria are present

No chloroplasts, only bacterial chlorophyll associated with cell surface membrane in some bacteria

Chloroplasts present in plants and algae

Ribosomes are smaller (70S)

Ribosomes are larger (80S)

Cell wall made of murein (peptidoglycan)

Where present cell wall is made mostly of cellulose (or chitin in fungi)

May have outer mucilaginous layer called a capsule

No capsule

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Division of a cell that results in each of the daughter cells having an exact copy of the DNA of the parent cell. Except in the rare event of a mutation, the genetic make-up of the two daughter nuclei is identical to parent nuclei

Interphase - period during which the cell is not dividing and includes the replication of DNA. Two copies of DNA after replication remain joined at the centromere. Mitosis is a continuous process but is divided into four stages Prophase, Metaphse, Anaphase and Telophase (Cytokinesis)

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Mitosis - Prophase

Chromosomes first become visible initially as long thin threads which later are shortened and thickened.

Animal cells contain two cylindrical organelles called centrioles which move to opposite ends (poles) of the cell. From each of centrioles spindle fibre develop which span the cell from pole to pole these are called spindle apparatus.

Plant cells don't have centrioles but do develop a spindle apparatus, centrioles are therefore not essential to spindle fibre formation.

The nucleolus disappears and the nuclear envelope breaks down, leaving the chromosomes free in the cytoplasm of the cell. These chromosomes are drawn towards the equator of the cell by spindle fibres attached to the centromere

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Mitosis - Metaphase and Anaphase


Chromosomes are seen to be made up of two chromatids each is an identical copy of DNA from the parent cell. The chromatids are joined by the centromere.

Some microtubules from the poles are attached and the chromosomes are pulled along the spindle apparatus and arrange themselves across the equator of the cell


Centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosomes part.

The chromatids move rapidly to opposite poles of the cell and now are chromosomes.

The energy for the process is provided by the mitochondria which gather around the spindle fibres. If cells are treated with chemicals that destroy the spindle, the chromosomes remain at the equator, unable to reach the poles

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Mitosis - Telophase and cytokinesis

Chromosomes reach their poles and become longer and thinner, they disappearr leaving chromatin. Spindle fibres disintergrate and nuclear envelope and nucleolus re-form. Finally the cytoplasm divides in a process called cytokinesis

Summary of mitosis

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Cell Divison in prokaryotic cells and Replication

Cell division in prokaryotic cells

Takes place by binary fission 

  • Circular DNA molecule replicate and both copies attach to cell membrane
  • The plamids also replicate
  • Cell membrane begins to grow between two DMA molecules and begins to pinch inwards diving the cytoplams into two
  • Cell wall forms between the two molecules of DNA dividing the original cell into two identical daughter cells each with a single copy of DNA and variable number of copies of plasmids

Replication of viruses

Cannot undergo cell division, instread they replicate by attaching to their host cells with attachment proteins. Tey inject their nucleic acids into host cell. The genetic information from the viruses nucleic acid provide instruction for host cell's metabolic process to start producing virus components, nucleic acid, enzymes and structural proteins which assemble into new viruses

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Importance of mitosis

It produces daughter cells that are gentically identical to parent cells. Why make copies

  • Growth - two haploids (sper and ovum) fuse to form a diploid cell that has all genetic information to form a new organism. It resembles its parents all the cells that grow from this original cell must be genetically identical. Due to mitosis
  • Repair - if cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost
  • Reproduction - single-celled organisms divide by mitosis to give two new organisms. Each new organisms is genetically identical to the parent organism
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Cell cycle

Cells that do not divide continuously ungergo a regular cycle of division seperated by periods of cell growth. There are three stages

  • Interphase - occupies most of the cycle and sometimes known as resting phase because no division takes place
  • Nuclear division - nucleus divides either into two (mitosis) or four (meiosis)
  • Division of the cytoplasm (cytokinesis) - follows nuclear division and is the process by which the cytoplasm divides to produce two new cells (mitosis) or four new cells (meiosis)

Length of a complete cell cycle varies greatly amongst organisms. Typically a mammal cell takes about 24 hours to complete a cell cycle which is about 90% is interphase

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Cancer and control of mitosis

Cancer is a group of disease (around 200 in total) caused by a growth disorder of cells. It is a result of damage to the genes that regulate mitosis and the cell cycle which leads to uncontrolled growth and divisiom of cells. As a result a group of abnormal cells (tumour) develop and constantly expand in size. Tumours develop in any organ but mostly in lungs, prostate glands (males), breast and ovaries (female), large intestine, stomach, oseophagus and pancreas. A tumour becomes cancerous if it changes from denign to malignant

Most cells divide by mitosis to increase the size of tissure during growth or to replace dead and worn out cells (repair). The rate of mitosis can be affected by environment of the cells and by growth factors. It also is controlled by two types of genes. A mutation to one of these gene results in uncontrolled mitosis. The mutant cells are usually structurally or functionally different from normal cells. Most mutate and die

The ones that survuve are capable of dividing to form clones of themselves and forming tumours. Malignant tumours grow rapidly, less compact and more likely to be life-threatening while benign ones grow slowly, more compact and less likely life-threatening, 

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Treatment of cancer

Involves killing, dividing cells by blocking a part of the cell cycle. The cell cycle is disrupted and cell division and stops cancer growth. You can use drugs to treat it (chemotherapy) to disrupt cells cycle by

  • Preventing DNA from replicating
  • Inhibiting the metaphase stage of mitosis by interfering with spindle formation

Problems with drugs is that they disrupt cell cycles of normal cells and they are more effective against rapidly dividing cells.

As cancer cells have fast rate of division they are damaged more then greater degree than normals cells. These cells like hair producing cells which divide rapidly are vulnerable to damage, which explains hair loss seen in patients undergoing cancer treatment

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Cell surface membrane - Phospholipids

Important components of cell-surface membrane

  • Hydrophilic head - points to the outside of the membrane attracted by water on both sides
  • Hydrophobic head - points into the centre of the cell membrane repelled by water on both sides

Lipid-soluble material moves through the membrane through the phospholipid. Function of the phospholipids in the membrane are:

  • Allow lipid-soluble substances to enter and leave the cell
  • Prevent water-soluble substances entering and leaving the cell
  • Make the membrane flexible and self-sealing
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Cell surface membrane - Protein

Proteins are embedded into the phopholipid bilayer in two ways

  • Some occur in surface and never extend across it. They act to give either mechanical support to membrane or with glycolipids as cell receptors for molecules such as hormones
  • Some span from one side to the other. Some are protein channels which from water-filled tubes to allow water-soluble ions to diffuse across the membrane. Others are carrier proteins that bind to ions or molecules (e.g glucose and amino acids) and change shape in order to move them across the membrane

Functions of proteins in the membrane

  • Provide structural support
  • Act as channels transporting water-soluble substances across the membrane
  • Allow active transport across the membrane through carrier proteins
  • Form cell-surface receptors from identifying cells
  • Help cells adhere together
  • Act as receptors E.g Hormones
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Cell surface membrane - Cholesterol

They add strength to the membranes. They are very hydrophobic and play important role in preventing loss of water and dissolved ions from cell. They also pull together fatty acid tails of phospholipid molecules limiting their movement and other molecules without making membrane too rigid

Functions of cholesterol

  • Reduce lateral movement of other molecules including phspholipids
  • Make the membrane less fluid at high temperatures
  • Prevent leakage of water and dissolved ions from the cell
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Cell surface membrane - Glycolipids


Made ups od carbs covalently bonded with lipids. Carb portions extends from bilayer into outside watery environment where acts as cell-surface receptor for specific chemicals

Functions of glycolipids

  • Act as recognition sites
  • Help maintain the stability of the membrane
  • Help cells attach to one another to form tissues
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Cell surface membrane - Glycoproteins


Carn chains attached to many proteins on outer surface membrane and act as receptors more specifically for hormones and neurotransmitters

Function of glycoproteins

  • Act as recognition sites
  • Help cells to attach to one another to form tissues
  • Allow cells to recognise one another E.g lymphocytes can recognise organisms own cells
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Permeability of cell-surface membrane

It controls the movement of substance into and out of the cell. Most molecules do not freely diffuse across it becuase many are

  • Not soluble in lipids therefore cannot pass through the phspholipid layer
  • Too large to pass through the channels in the membrane
  • The same charge as the charge of the protein channels so they are repelled
  • Electrically charfe (polar) and therfore have difficulty passing through non-polar hydrophobic tails in the bilayer
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Fluid-mosaic model of the cell-surface membrane

Way various molecules are combined into structures known as fluid-mosaic model 

  • Fluid - because individual molecules can move to another, which gives the membrane a flexible structure that is constantly changing in shape
  • Mosaic - because proteins are embedded in the bilayer which vary in shape, size and pattern similar to the way of stones or tiles in a mosaic
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The net movement of molecules or ions from a region where they are more highly concentrated to one where their concentration is lower until evenly distibuted

Is a passive transport in that energy comes from natural inbuilt motion of particles than external sources such as ATP

  • All particles are constantly in motion due to the kinetic energy they have
  • Motion is random with no set pattern to way particles moves
  • Paticles are constantly bouncing off one another as well as off other objects

Particles that are concentrated together distribute themselves evenly as a result of diffusion

Most molecules can't easily pass across the membrane few molecules can small, non-polar molecules such as oxygen and carbon dioxide

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Facilitated diffusion

Charged ions and polar molecules don't easily diffuse because of the they are hydrophobic  faty-acid tails. Movement of these molecules are made easier by facilitated diffusion by channels and carrier that go throught the membrane

It is a passive process that relies on kinetic energy NO ATP from respiration, it occurs down a concentration gradient but it occurs on points on the membrane where there are protein channels and carrier proteins

Protein channels

Form water-filled hydrophilic channel across the mebrane allowing water specific water-soluble ions to pass through, they are selective only open to specfic ion if not present will close. Ions binds with channel to change shape then close one side and open the other

Carrier proteins

Molecules that specific to protein is present binds with proteins causes it to change shape in way that molecules is released to inside of the membrane. No external energy needed, moved from region of high concentration to low using kinetic energy

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The passage of water from a region where it has a higher water potential to a region where it has a lower water potential trough a selectively permeable membrane

Cell-surface membranes and other plasma membranes around organelled are selectively permable, they are permeable to water molecules and few other small molecules but not to larger molecules


  • Both solute and water molecules are in random motion due to kinetic energy
  • Selectively permeable membrane only allows water across it not solute
  • Water molecules diffuse from high water potential to low water potential (down water potential gradient)
  • At point where water potential are equal a dynamic equilibrium is established and there is no net movement of water
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Water potential and osmosis and animal cells

Highest value of water potential (pure water) is zero and other values are negative. The more negative the value the lower the water potential

Animal cells such as red blood cells (RBC) contain variety of solutes dissolved in cytoplasm. If RBC placed in pure water it will absorb water because has lower water potential. Cell surface membranes are very thin (7nm) and are flexible and cannot stretch very much.

The cell-surface membrane will therefore break bursting the cell and releasing the contents (haemolysis). To prevent this happening animal cell live in a liquid which has same water potential as the cells. Liquid in blood plasma has same water potential

If RBC placed in a solution with a water potential lower than its own water leaves by osmosis and the cell shrinks and becomes shrivelled

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Active transport

The movement of molecules or ions into or out of a cell from a region of lower concentration to a region of higher concentration using ATP and carrier proteins

ATP is used to

  • Directly move molecules and to individually move molecules using a concentration gradient whic has already been set up by direct active transport known as co-transport

Different from passive forms of transport

  • Metabolic energy in the form of ATP, substances are moved against concentration gradient from low to high, carrier proteins involved to acts a pumps and process is very selective with specific substances being transported
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Active transport

Direct active transport of single molecules or ion

  • carrier proteins span membrane and bind to molecule or ion to be transported across, the moleculs or ion binds to receptor sites on carrier proteins, inside the cell ATP binds to the protein causing it to split into ADP and Pi, molecules or ion released to other side, phosphate molecule released from protein causing protein to revert to original shape ready to repeate recombining to form ATP

Sometimes more than one molecules or ion may be moved in the same direction at the same time by active transport. Ocassionally the molecules or ion is moved into a cell/organelle at same times a different one being removed an example of this is the sodium-potassium pump

In the sodium-potassium pump, sodium ions are actively removed from the cell/organelle while potassium ions are actively taken in from the surrounding. This process is essential to a number of important procesess in the organism including the creation of nerve impulses

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Movement across membranes

The epithelial cells lining the ileum posess microvilli. These finger-like projections on the cell-surface membrane about 0.6μm.

Collectively termed brush border because when viewing them under a light microscope they look like the bristles on a brush.

The microvilli provide more surface area for insertion of carrier proteins through which diffusion, facilitated diffusion and active transport take place

Another mechanism to increase transport across membranes is to increase the number of protein channels and carrier proteins in a given area

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Role of diffusion in absorption

Diffusion is the net movement of molecules or ions from a region of high concentration to low concentration

As carbs and proteins are digested continuously. There is normally a higher concentration of glucose and amino acids within the ileum them in the blood. There is therfore a concentration gradient down which glucose moves by facilitated diffusion from inside the ileum into the blood.

Given that the blood is constantly being circulated by the heart, the glucise absorbed is being removed by the cells as they use it during respiration, which helps to maintain the concentration gradient between the inside of the ileum and the blood. Meaning the rate of movement by facilitated diffusion across epithrlial cell-surface membranes is increased.

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Role of active transport in absorption

Diffusion results in concentration on either side becoming equal. Meaning that not all the available glucose and amino acids are being absorbed by diffusion and may pass out the body. This doesn't happen because glucose and amino acids are also being aborbed by active transport meaning all the glucose and amino acids should be absorbed into the blood.

The mechanism which they are absorbed from the small intestine is an example of co-transport, because either glucose or amino acids are drawn into the cells along with sodium ions that have been actively transported out by the sodium-potassium pump

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Sodium-potassium pump

What happens

  • Sodium ions are actiely transorted out of the epithelial cells, by the soidum-potassium pump into the blood. This takes place in protein-carrier molecules found on the surface membrane
  • This maintains high concentration of sodium ions in the lumen of  the intestine than inside epithelial cells
  • Sodium ions diffuse into the epithelial cells down the concentration gradient through a different type of protein carrier (co-transport protein) into the cell-surface membrane
  • As sodium ions diffuse through second carrier protein they carry either amino acid molecules or glucose molecules into the cells with them
  • Glucose/amino acids pass into the blood plasma by faciliated diffusion using another type of carrier
  • Both sodium ions and glucose/amino acids move into the cell, while sodium ions move down their concentration gradient, the glucose molecule move against their concentration gradient. It is the sodium ion concentration gradient that powers the movement of glucose/amino acids into the cells. This makes it an indirect form of active transport
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Immune system

Ten million of people die from infectious diseases, many survive or appear not to be affected.

Any infection is an interaction between the pathogen and the body's defence mechanism. Sometimes the pathogen overwhelms the defence and the person dies. Sometimes the defence overwhelms the pathogen and the person recovers.

Having overwhelmed the pathogen the body's defence seems to be better prepared for the second infection from the same pathogen and can kill it before it can cause harm. This is know and immunity and the main reson why some people are unaffected by certain pathogens

There is range of intermediates between the stages but they depend on the state of health of the person. A fit, healthy adult rarely die from an infection. Those not healthy, young and elderly are more vulnerable

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Defence Mechanisms

Human body has a range of defences to protect itself from pathogens. Some are general and immediate defences like skin form a barrier to entry of pathogens and phagocytosis. Others are more specific, less rapid but long lasting

Responses involve a type of white blood cell called lymphocyte and take two forms

  • Cell-mediated responses involing T lymphocytes
  • Humoral responses involving B lymphocytes
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Recognising your own cells

To defend the body frominvasion by foreign material, lymphocytes must be able to distinguish the body's own cells and molecules (self) and foreign cells (non-self). If they can't do this they would destroy the organism own tissues.

Each cell self or non-self has specific molecules on its surface that identify it. These molecules can be a variety of types, proteins are most important because have specific tertiary structure/3D structure that distinguishes one cell from another. They allow the immune system to identify: Pathogens (like HIV), non-self material (cells from organism same species), toxins (produced by certain pathogens like bacteria that causes cholers), abnormal body cells (cancer cells).They are all potetially harmful first stage is removing them.

Implication - people who have tissure or organ transplants, sometimes there immune system recognises them as non-self even though from same species it attempts to destroy. To minimise this they try to get a close match from people genetically similar and can use immunosuppressant drugs.

High probability you have protein with complementary proteins to pathogen that enters the body, if do then will stimulate for it to divide to increase numbers so can effectively destroy it called clonal selection and explains lag time between exposure and bringing under control

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How lymphocytes recognise cells belonging to the b

  • Ten million different lymphocytes each capable of recognising a different chemical shape
  • In the fetus, lymphocytes are constantly colliding with other cells
  • Infection in the fetus is rare because it is protected from the outside by mother and placenta
  • Lymphocytes will collide almost the body's own material (self)
  • Some lymphocytes will have receptors that exactly fit those of body's cells
  • Lymphocytes either die or are suppressed
  • Remaining lymphocytes might fit foreign material (non-self) and respond to foreign material
  • In adults lymphocytes produced in the bone marrow initially only encounter self-antigens
  • Any lymphocytes that show immune response to self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes
  • No clones of these anti-self lymphocytes will be in the blood, leaving only non-self antigens
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Two types of white blood cells: Phagocytes (ingest and destroy the pathogen by phagocytosis) and Lymphocytes (Involved in immune response)


Large particles like types of bacteria can be engulfed by cells in the vesicles fromed from cell-surface membranes. Phagocytes provide important defence against the pathogens that manage to enter the body, some travel in the blood but can move out into tissues

  • Chemical products of pathoogens or dead, damaged and abnormals cells act as attractants causing phagocytes to move towards the pathogen
  • Phagocytes have several receptors on their cell-surface membrane that recognise and attach to chemicals on the surface of the pathogen
  • Phagocytes engulf the pathogen to from a vesicle called phagosome
  • Lysosomes move towards the vesicles and fuse with it
  • Enzymes called lysozymes present within lysomsone which destroy ingested bacteria by hydrolysis
  • Soluble products from breakdown of pathogen are absorbed into the cytoplasm of the phagocyte
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Is any part if an organism or substance that is recognised as non-self (foreign) by the immune system and stimulate an immune response.

They are usually proteins that are part of the cell-surface membranes or cell wall of invading cell such as microorganism, abnormal body cells or cancer cesll

Presence of an antigen triggers production of antibodies

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There are two types of responses non-specific (Phagocytosis which occurs for any infection) and specific (rescts to specific antigents)

Specific response is slower at start but can provide long-term immunity and depends on type of white blood cell (lymphocyte) there are two types B lymphocytes (B cells) and T lymphocytes (T cells)

B lymphocytes (B cells)

  • They mature in the bone marrow and are associated with humoral immunity which involved antibodies what are present in body fluids or humor such as blood plasm

T lymphocytes (T cells)

  • They mature in the thymus glands and associated with cell-mediated immuntiy involving body cells
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Cell-mediated immunity (Cellular response)

Lymphocytes respond to an organism's own cells that have been infected by non-self material from different species and respond to cells from other individuals as are genetically different therefore have different antigens on membrane that their own cells

T Lymphocytes distinguish as invader because: Phagocytes (engluf and hydrolyse pathogen and present pathogens on cell surface), body cells invade (viruses present some antigens on their surface), transplant cells (same species different antigens on surface), caner cells (different from normals body cells and different antigens presented)

Cells that display foreign antibodies called antigen-presenting cells. T lymphocytes (T cells) respond to antigens on body cells are receptors on T cells respons to single antigen

  • Pathogen invades call or taken in by phagocytosis, and phagocytes places antigens from pathogen on its cells membrane
  • Receptors on helper T cells fit onto these anigens and activate T cells to divide by mitosis and form a clone of genetically identical cells
  • Clone T cells then either: develop into memory cells (help with respone in the future to same pathogen), stimutate phagocytes (for phagocytes), stimular B cells (to divide and secrete antibodies), activate cytotoxic T cells
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How cytotoxic T cells kill infected cells

They kill abnormals cells and body cell infected by pathogens by producing a perforin which is a protein that makes in holes in the cell-surface membrane.

The holes mean that the membrane becomes permeable to all substances and the cell dies. This is why the cell-surface membrane is important for survival

T cells are most effective against viruses becuase viruses replicate inside cells. As they use living cells to replicate by killing off some the body cells it stops the viruses from multiplying and infecting more cels

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Humoral immunity

Involves antibodies which are soluble in blood and tissue fluid. There are 10 million B cells each produced specifc antibody in response to a specifc antigen.When antigen on surface of pathogen, foreign cell, toxic or damaged or abnormal cell enters the blood or tissue fluid a B cell with that has the antibody and complementary shape attached to antigen and pulls into the B cell by endocytosis and presents of antigens on its surface (processed). Helper T cells bind to the processed antigens and stimulate B cells to divide by mitosis to form a clone of B cells to produce antibodies specific to antigen (clonal selection) which helps for rapid response. Some pathogens produce toxins and each toxin acts as an antigen, therefore many different B cells are cloned to produce specific antibody (monoclonal antibodies)

Each clone develops into two types of cells

  • Plasma cells - secrete antibodies into blood plasma, only last few days, but make 2000 antibodies every second, lead to destruction of antigen, is primary immune response
  • Memory cells - secondary immune response, live longer, don't produce antibodies but circulate in blood and tissue fluid. When encounter same antigen divide rapidly and develop into plamsa cells (to produce antibodies and destroy pathogens ) and memory cells which circulate again and help with long-term immunity, increase amount of antibodies, release them faster and quicker response
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Role of B cells in immunity

  • The surface antigen of an invading pathogen are taken up by a B cells
  • The B cell processes the antigens and presents them on its surface
  • Helper T cell attach to the processed antigens on the B cells and acitate the B cells
  • B cells now activated divide by mitosis to give a clone of plasm cell
  • Cloned plasma cells produce and secrete the specific antibody that fits the antigen on the pathogen's surface
  • The antibody attaches to antigens on the pathogen and destroys them
  • Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and develop into plasma cells that produce antibodies which is a secondary immune response
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Proteins with specific binding sites synthesised by B cells. Ehen body infected by non-self material a B cell produces a speficic antibody. This specific antibody reacts with an antigen on the surface of the non-self material by binding to them. Each antibody has two identical binding sites which are complementary to specific antigen. The variety is because they are made of proteins.

Antibodies made of four polypeptide chains. One pair are long and called heavy chains while shorter chains called light chains.

Each antibody has specific site that fits onto specific antigen form a antigen-antigen complex, binding site called variable reigion.

Each consists of a sequence of amino acids that from 3D shape that binds directly to specific antigen. Rest of anitbody called constant region which binds to receptors on cells such as B cells

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How the antibody leads to destruction of the anitg

Different antibodies lead to the destruction of an antigen in different ways

When antigen is a bacterial cells antibodies assist in its destruction is two ways

  • Cause agglutination of bacterial cells, forming clumps of bacterial cells making it easier of phagoctes to locate them as they are less spread-out within the body
  • Are markers to stimulate phagocytes to engulf the bacterial cells
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Medication to cells to specific drugs by attaching

Monoclonal antibodies can be used to target specific substances and specific cells. One type of cells thay can target is cancer cells. Monoclonal antibodies can be used to treat cancer in a number of ways by direct monoclonal antibody therapy.

  • Monoclonal anibodies are produced that are specific to antigens on cancer cells
  • These anitbodies are given to a patient and attach themselves to the receptors on the cancer cells
  • They attach to the surface on the cancer cells and block the chemical signals that stimulate their uncontrolled growth

Advantage of direct monoclonal antibody therapy is that since the antibodies are not toxic and are highly specific they lead to fewer side effects

Indirect monoclonal antibody therapy involved attaching a radioactive or cytotoxic drug that kills the cell to the monoclonal antibody. When the antibody attaches to the cancer cell it kills them

Monoclonal antibodies (magic bullets) are used in small doses and target specific sites. Smaller doses as cheaper and reduce side effects

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Medical diagnosis and Pregnancy testing

Medical Diagnosis

  • Monoclonals antibodies are used to diagnose infections and produce results quicker and are important in diagnosing certain cancers. For example men with prostate cancer oftern produce more prostate specific antigen (PSA) leading to high levels in the blood.
  • Monoclonal antibodies that interact with the antigen and you an then obtain a measure of PSA in the blood sample. Higher than normals PSA is not a diagnosis of the disease but give an early warning for possible needs for further teast

Pregnancy Testing

  • Important to know as soon as possible. Pregnancy kits that can be used at home rely on fact that placenta produces a homorne called human chorionic gonadatrophin (hCG) and found it mothers urine
  • Monoclonal antibodies present on the test ***** on pregnancy test are linked to coloured particels. If hCG present in the urine it binds to these antibodies and hCG-antibody-colour complex forms and moves along the ***** until is trapped by different type of antibody creating a coloured line
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Ethical use of monoclonal antibodies

Development of monoclonal antibodies provides society with power and opportunity to treat disease in different ways but it has raised some ethical issues

  • Production of monoclonal antibodies involved mice to produce antibodies and tumour cells. Production of tumour cells involved deliberately introducing cancer to mice. Guidelines have been drawn up to minimise any suffering
  • Monoclonal antibodies been used successfully to treat diseases like cancer and diabetes saving lives. But in treatment of multiple sclerosis there have been some deaths. It is therefore important for patients to have full knowledge of risks and benefits and must give informed constent
  • In March 2006 six healthy volunteer took part in trial of new monoclonal antibody but within minutes they suffered multiple organ failure probably as a result of T cells overproducing chemicals to stimulate immune response or attacking body tissue. They all survived but it raised issues about way drug trials are conducted

Must combine issues raised, scientific knowledge to make a decision about use. Must balance advantage and dangers to make an informed decision at individual, local, national and global level about ethical use of drugs

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Immunity is the ability of an organism to resist infections and takes two forms

Passive Immunity

  • Produced by intoduction of antibodies into individuals from outside source. No direct contract with pathogen or antigen. Antibodies not produced and replaces, no memory cells formed no lasting immuntiy. E.g anti-venom for snake bitres

Active Immunity

  • Produces by stimulating production of antibodies by own immune system. Direct contact with pathogen or antigen, immuntiy takes time and long lasting and has two types:
  • Natural active immunity - results from individual becoming infected with a disease under normal circumstances, body produces own antibodies and continues do do for many years
  • Artificial active immunity - Forms basis of vaccination and involved introducing immune response without them suffering symptoms of the disease.

Material introducted by the vaccination is called a vaccine

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Features of a successful vaccination programme

Vaccination is used as a precautionary measure to prevent individuals contracting a disease. Some programmes of vaccination have success other don't.

Success depends of number of factors:

  • A suitable vaccine must be economically available in sufficient quantities to immunise most of the vulnerable population
  • There must be few side-effects from vaccination. Unpleasant side-effects may discourage individuals in the population from being vaccinated
  • Means of producting, storing and transporting the vaccine must be available. This usually involved technologically advanced equipment, hygienic conditions and refrigerated transport
  • There must be the means of administering the vaccine properly at the appropriate time, which involves training staff with appropriate skills at different centres throughout the population
  • Must be possible to vaccinate the vast majority of the vulnerable population to produce herd immunity
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Herd immunity

When a large proportion of the population has been baccinated to make it difficult for a pathogen to spread within that population. Based on idea that pathogens are passed from induvidual to individual when in close contact. So in majority are immune then less likely come in contact with an infected person

It is important because it is never possible to vaccinate everyone in a large population

Percentage of population to achieve herd immunity is different for each disease. So to achieve herd immunity vaccination best carried out at one time, so for a certain time there are few people in the population with the disease and transmission of pathogen is interrupted

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Why vaccination may not eliminate a disease

  • Fails to induce immunity to certain people E.g people with defective immune systems
  • People may develop the disease immediately after vaccination but before immunity levels are high enough to prevent it
  • Pathogen may mutate so antigens change suddenly therefore vaccines become ineffective because new antigen on pathogen not recognised by immune system and immune system doen't produce the antibodies to destroy the pathogen. (antigenic variability)
  • Many varieties of a particular pathogen and it is impossible to develope a vaccine that is effective against all of them
  • Certain pathogen hide from the body's immune system either by concealing themself inside cells or by living in places out of reach (inside intestines)
  • People may have objections to vaccination for religious ethical or medical reasons
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Structure of human immunodeficiency virus (HIV)

HIV causes the disease aquired immune deficiency syndrome (AIDS)

  • On the outside is a lipid envelipe embedded with attachment proteins.
  • Inside the envelop there is a protein layer called capsid that encolses 2 single strands of RNA and some enzyme. One of the enzymes is reverse transcriptase (catalyses production of DNA from RNA) reverse reaction carried out by transcriptase
  • By having reverse transcriptase and ability to make DNA from RNA means HIV is a virus called retrovirus
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Replication of the HIV

  • Infection of HIV enters the bloodsream and circulate around the body
  • Protein of HIV binds with protein called CD4 which occurs of different cells but HIV frequently attached to helper T cells
  • Protein capsid fuses with cell-surface membrane. The RNA and enzyme of HIV enters the helper T cell
  • HIV reverse transcriptase converts the virus's RNA into DNA
  • Newly made DNA moves into the helper T cell's nucleus and inserts its DNA
  • HIV DNA creates mRNA using cell's enzymes. The mRNA contains instructions for making new viral proteins and the RNA to go into new HIV
  • mRNA passes out of the nucleus through a nuclear pore and uses cell's protein synthesis mechanism to make HIV particles
  • HIV particles brealk away fom helper T cells taking piece of cell-surface membrane which forms lipid envelope

Once infected with HIV person is HIV +. However replication of HIV often goes into dormancy and only leads to AIDS years later

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How HIV causes symptoms of AIDS

HIV attacks helper T cells. HIV causes AIDS by killing or interfering with the normal functioning of helper T cells

An unifected person normally has between 800 and1200 helper T cells in each mm3 of blood.

A person suffereing from AIDS has 200 per mm3 of blood.

Without a sufficient number of helper T cells the immune system cannot stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens

Memory cells may also become infected and destroyed therefore the body is unable to produce an immune response and becomes susceptible to other infections and cancers

Many AIDS sufferers develop infections of lung, intestine, brain, eyes as well as weight loss, diarrhoea and can cause death

HIV doesn't kill directly but it can cause ill health and eventually death

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ELISA test

Enzyme linked immunosorbant assay. It uses antibodies to detect presence of proteins and quantity. Very sensitive and can detect small amount of molecules


  • Apply sample of a surface (slide) which antigens will attach
  • Wash the surfacee several times to remove unattached antigens
  • Add the antibody that is specific to antigen you are trying to detect and leave to bind together
  • Wash the surface to remove excess antibodies
  • Add seconds antibody that will bind to first antibody, which has enzyme attached to it
  • Add colourless substrate of the enzyme which acts on the substrate to change it into a colour product
  • The amount of antigen present is relative to intensity of colour
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Why antibiotics are ineffective against viral dise

Antibiotics prevent bacteria from making normal cell walls

In bacteria water constantly enters by osmosis, this would normally cause the cell to burst but due to cell wall it doesn't. The wall is made of murein (peptidoglyan) which doesn't easily stretch. As water enters the cell it expands and pushes aganist the cells wall. The cell wall resists expansion and further entry of water

Antibiotics like penicillin inhibit certain enzymes required for synthesis and assembly of peptide croos-linkages in cell walls. This weakens the walls making them unable to withstand pressure. As water enters naturally by osmosis the cell bursts and bacterium dies.

Viruses rely on host cell to carry out their metabolic activity and therefore lack their own metabolic pathways and cell structure. As a result antibiotics are ineffective becuase there is nothing for them to disrupt

Viruses also have a protein coat rather than a murein cell wall, so doesn't hae sites where antibodies can work. When viruses are within an organism's own cells antibiotics cannot reach them

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Surface area to Volume ratio

Exchange takes place at surface of an organism, but materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surface of the orrganism must be large compared with its volum

As organisms become larger, their volume increases at a faster rate then their SA. Because of this diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms. Even if the outer surface could supply enough of a substance it would take too long for it to reach the middle of the organism if diffusion was method of transport

Organism have evolved

  • Flattened shape so that no cell is ever far from the surface
  • Specialised exchange surfaces with large areas to increase the SA:Vol
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Features of specialised exchange surfaces

For effective transfer of materials across specialised exchange surfaces by diffusion or active transport, surfaces show following characteristics

  • Large suface area relative to the volume of the organism which increase the rate of exchange
  • Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
  • Selectively permeable to allow selected material to cross
  • Movement of the environmental medium E.g air to maintain diffusion gradient
  • A transport system to ensure the movement of interal medium E.g blood in order to maintain a diffusion gradient

Being thin, specialised exchange surfaces are easily damaged and dehydrateed. They are therefore often located inside an organism.  Where they are located inside the body, the organism need to have a means of moving the external medim over the surface E.g Means of ventilating the lungs in a mammal

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Gas exchange in single-celled organisms

They are small and therefore have a large surface area to volume ratio.

Oxygen is absorbed by diffusion across their body surface, which is covered by a cell-surface membrane.

Carbon dioxide from respiration diffuses out across their body surface

Where a living cell is surrounded by a cell wall this isn't an additional barrier to the diffusion of gases

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Gas exchange in insects

Insects have evolved to conserve water. Increase in SA doesn't help to conserve water because it will evaporate from it. For gas exchange insects have enolved an internal network of tracheae which are supported by ring to stop them collapsing. They are divided into small deadend tubes called tracheoles which exten throughout the body tissue. Oxygen brough directly to respiring tissue and short pathway from tracheole to body cells

How gases move in and out of tracheal system

  • Along diffusion pathways - as oxygen is used up there is low concentration at end og tracheoles and there is a carbon dioxide in the opposite direction as it is produces as oxygen is used up
  • Mass transport - as muscles contrat they squeeze trachea enabling movement of air in and out 
  • Ends of tracheoles are filled with water - muscles respire by anaerobic respirateion which produces lactate which is soluble and lowers water potential. Water moves into the cells from the tracheoles by osmosis. Water in ends of tracheoles decreases in volume as air drawn up 

Gases enter and leave by tiny pores called spiracles on body surface that have valves

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Structure of gills

Located in fish behinf te head made up of gill filaments which are stacked in pilles like book pages. At right angles to filaments are gill lamellae, they increase SA. Water taken in through the mouth and forced over the gills and out through opening on each side of the body. Water flows over the gill lamellae and there is a flow of blood in the opposite direction this is called  (countercurrent flow). This ensures maximum gas exchange achieved

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Countercurrent exchange principle

Blood and water that flow over gill lamellae are in opposite directions. This means that

  • Blood is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. Therefore diffusion of oxygen from water to blood takes plae
  • Blood with little oxygen in it meets water which has most of oxygen removed

As a result diffusion gradient maintained across gill lamellae. 80% of oxygen available in water is absorbed into the blood of the fish. If in same direction gradient only maintained across part of length so only 50% of oxygen absorbed into the blood

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Structure of plant leaf and gas exchange

Gas exchange in pplant similar to insects because:

  • No living cell is far from external air and therefore source of oxygen and carbon dioxide
  • Diffusion takes place in of air which makes it more rapid than if it were in water

Short, fast diffusion pathway. Also air spacess inside a leaf have larger SA compared to volume. There is no specific transport system for gases to move in a through the plant by diffusion. Most gas exchange occurs in the leaves as it is adapted for rapid diffusion

  • Many small pores (stomata) and no cell is far from a stoma and is short diffusion pathway
  • Lots of interconnecting air-spaces throughout mesophyll so gases can readily come in contact with mesophyll cells
  • Large SA of mesophyll cells for rapid diffusion
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Minute pores that occur mainly on the leaves especially the underside

Each stoma (singular) is surrounded by a pair of special cells (guard cells) which can open and close the stomatal pore, to control the rate of gas exchange

This is important because organisms lose water by evaportation. Plants have evolved to balance gas exchange and control of water loss. This is done by closing stomata when water loss is high

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Limiting water loss in insects

Insects have adapted to reduce water loss

  • Small SA:Vol ratio to minimise area which water is lost
  • Waterproof covering over their body to create a rigid outer skeleton of chitin covered with a waterproof cuticle
  • Spiracles which are openings of the tracheae at the body surface and these can be closed to reduce water loss. The need for oxygen occurs when the insect is at rest

These mean that insects cannot use their body surface to diffuse respiratory gases in the way a single0celled organism does. Instead they have an internal network of tubes called tracheae that carry air containing oxygen directly to the tissues

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Limiting water loss in plants

Certain plants have a restricted supply of water and have evolved to limit water loss through transpiration these are called xerophyted. They have modified their leaves

  • Thick cuticles - waxy cuticle on leaves form waterproof barrier. 10% water loss from leaves, thicker the cuticle the less water can escape
  • Rolling up of leaves - protects the lower epidermis from the outside trapps a region of air within the rolled leaf. This region becomes saturated with water vapour and has a high water potential. There is no water potential gradient between the inside and outside of the leaf so no water loss E.g Marram grass
  • Hairy Leaves - thick layer of hairs on leaves this trapps moist air next to the leaf surface, water potential gradient is reduced and less water is lost by evaportation
  • Stomata in pits or grooves - traps moist air next to the leaf and reduces water potential gradient
  • Reduce SA:Vol - smaller it is the slower the rate of diffusion and the rate of water loss can be reduced
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Are site of gas exchange in mammals and are inside the body because air is not dense enough to support and protect them also the body would lose water and dry out

They are supported by the ribcage. The ribs can be moved by the muscles between them, they are ventilated by tidal stream of air ensuring that the air within them is constantly replenished. The main parts of gas exchange system are:

  • Lungs - pair of lobed structures made up of highly branched tubles called bronchioles which end in tiny air sacs called alveoli
  • Trachea - flexible airway that is supported by rings of cartilage to prevent from collapsing when pressure falls. Walls are mafe up of muscle lined with ciliated epithelium and goblet cells
  • Bronchi - division of trachea, one for each lung and produce mucus to trap dirt particles and have cilia that move the mucus and dirt towards the throat
  • Bronchioles - brancing of bronchi, have muscles lined with epithelial cells, muscle allows them to constrict to control flow of air in and out of alveoli
  • Alveoli - air sacs diameter of 100-300μm, there is collagen and elastic fibres between them which allow them to stretch as they fill with air and spring back as expel CO2
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Breathing in is an active process and occurs by:

  • External intercostal muscles contract, while the internal intercostal muscles relax
  • The ribs are pulled upwards and outwards, increasing the volume of the thorax
  • The diaphragm muscle contracts, causing it to flattern, which increases the volumn of the thorax
  • The increased volume of the thorax results in reduction of pressure in the lungs
  • Atmospheric pressure is now greater then pulmonary pressure so air is forced into the lungs
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Breathing out is a passive process and occurs by:

  • The internal intercostal muscles contract, while the external intercostal muscles relax
  • The ribs move downwards and inwards, decreasing the volume of the thorax
  • The diaphragm muscles relax and it pushes up the contents of the abdomen that were compressed during inspiration. The volume of the thorax further decreases
  • The decreased volume of the thorax increases the pressure in the lungs
  • The pulmonary pressure is now greater than the atomspheric pressure so air is forced out of the lungs

During normal breathing the recoil of the elastic tissue in the lungs is the main cause of air being forced out. Only under more strenuous conditions such as exercise does the muscle play a major part

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Role of the alveoli in gas exchange

300 million alveloi in each human lung there total surface is around 70m2 (half area of tennis court). Each alveolus is lined with epithelial cells 0.05μm – 0.3 μm thick. Around wach alveolus is a network of pulmonary capillaries that are 0.04μm – 0.2 μm thick

There is fast diffusion between alveoli and the blood because:

  • RBC are slowed as they pass through the pulmonary capillaries allowing more time for diffusion
  • Distance between the alveolar air and RBC is reduced as RBC are flattened against the capillary wall
  • Walls of alveoli and capillaries are very thin and is therefore short diffusion distance
  • Alveoli and pulmonary capillaries have larfe SA
  • Breathing movements constantly ventilate the lungs and the action of the hear constantly circulates blood around the alveoli. Together these ensure steep concentration gradient of exchange gases
  • Blood flow through the pulmonary capillaries maintains a concentration gradient
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Major parts of the digestive system

  • Oesophagus - carries food from mouth to stomach
  • Stomach - muscular sac, inner layer produces enzymes, it stores and digests food especially protein (glands produces enzyme to digest protein)
  • ileum (small intestines) - long muscular tube, it further digests food by enzymes produced in its walls and by glands. Inner wall folded into villi and microvilli to increase SA helps with absorption of products of digestion into the blood
  • Large intestine - absorbs water, most of water from secretions of digestive glands
  • Rectum - final section, faeces are stored here before being removed via anus by egestion
  • Salivary glands - situated near the mouth, pass their secretions via a duct into the mouth. The secretions contain enzyme amylase which hydrolyses starch into maltose
  • Pancreas - large gland situated below the stomach, produces secretions called pancreatic juice which contain proteases to hydrolyses proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch
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Physical Breakdown

Large food is broken down into smaller pieces (e.g by teeth), provides a large SA for chemical digestion. Also another example is food churned by muscles in the stomach

Chemical Breakdown

Hydrolyses large, insoluble molecules into smaller soluble ones, carried out by enzymes. They split up molecules by adding water to the chemical bonds. Enzymes are specific so more than one enzyme need to hydrolyse large molecules. One to hydrolyse into large molecules into secretions which are hydrolyses into smaller molecules

  • Carbohydrases - hydrolyse carbohydrates to monosaccharides
  • Lipases - hydrolyse lipids (fats and oils) into glycerol and fatty acids
  • Proteases - hydrolyse proteins to amino acids
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Carbohydrate digestion

  • More that one enzymes used to hydrolyse large molecule (one hydrolyses the molecule into smaller sections, another hydrolyses them futher into their monomers)
  • Enzymes produced in different parts of digestion as important that enzymes added to the food in correct sequence
  • First amylase (produced in mouth + pancrease), it hydrolyses alternate glycosidic bonds of the starch molecule to produce a disaccharide (maltose)
  • Maltose is hydrolyse into monosaccharide (alpha glucose) by maltase (produced in lining of ileum)
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Carbohydrate digestion in humans

  • Saliva enters mouth from salivary glands and mixed with food during chewing
  • Saliva contains salivary amylase which starts hydrolysing any start in the food into maltose. It also contain mineral salts to help maintain neutral pH as optimum for the amylase
  • Food swallowed and enters stomach, acidic conditions which denatures amylase and prevents hydrolysis of starch
  • Then passes into small intestine and mixed with pancreatic juice which contains amylase which allows hydrolysis of starch to continue (alkaine salts also produced to maintain neutral pH)
  • Muscles in intestine wall push food along the ileum
  • Epithelial lining of the ileum produced maltase and is a membrane-bound disaccharidase so is not released into the lumen

Other membrane-bound disaccharides:

  • Sucrase - hydrolyses the single glycosidic bond in surose to produce its monosaccharides glucose and fructose
  • Lactase - hydrolyses the single glycosidic bond in lactose to produce its monosaccharides glucose and galactose
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lipid digestion

  • lipids hydrolyses by lipases which are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides
  • monoglyceride - is a glycerol molecule with single fatty acid molecule


  • lipids (fats and oils) split up into tiny droplets called micelles by bile salts (produced in liver) by emulsification which increases surface area fo the lipid speeding up action of lipases
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Protein digestion

  • Proteins are large, complex molecules that are hydrolyses by peptidases (proteases)

There are different types of peptidases

  • Endopeptidases - hydrolyse peptide bonds between amino acids in central region of a protein molecule forming a series of peptide molecules
  • Exopeptidases - hydrolyse peptide bonds on the terminal amino acids of peptide molecule formed by endopeptidases (they release dipeptides and single amino acids)
  • Dipeptidases - hydrolyse the bond between the 2 amino acids of a dipeptide. They are membrane-bound and part of cell-surface membrane of the epithelial cells lining the ileum
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Structure of the ileum

  • Wall of ileum is folded and has finger-like projection about 1mm long called villi which are have thin walls with rich network of capillaries and have large surface area to increase rate of absorption
  • Villi are between the lumen of the intestines and the blood and other tissues and part of an exchange surface adapted to absorb produces of digestion

They increase efficiency of absorption by:

  • increasing the surface area for diffusion
  • very thin walls so reduced diffusion distance
  • contain muscle so can move which helps to maintain concentration gradient (because as they move it mixes contents of ileum)
  • well supplies with blood vessels so that blood can carry away absorbed molecules which also maintain diffusion gradient
  • villi have microvilli which further increase the surface area for absorption
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Absorption of amino acids and monosaccharides

  • Digestion of proteins produces amino acids
  • Digestion of carbs produce monosaccharides such as glucose, fructose and galactose etc.
  • Absorption of these products happens by diffusion and co-transport
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Absorption of triglycerides

  • Monoglycerides and fatty acids combine with bile salts to emulsify the lipid droplets forming micelles
  • Micelles are tiny around 4-7nm in diameter
  • Micelles will eventually come into contact with the epithelial lining and will break down releasing monoglycerides and fatty acids (as non-polar they diffuse across membrane into the epithelial cells)
  • Inside the epithelial cell monoglycerides and fatty acids are transported to the endoplasmic reticulum and are combined to form triglycerides
  • They continue to the Golgi apparatuse and combine with cholesterol and lipoproteins to form chylomicrons 
  • Chylomicrons are able to move out of the cell by exocytosis
  • They then enter the lymphatic capillaries called lacterals which are found at the centre of each villus (or villi)
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These revision notes are really useful... helped me a lot ...

Thanks a bunch!!!!!! ❤❤❤



These are so useful!! Thank you



are these for the new spec?



  1. This is great, as it is all together. Thanks a bunch


OMG so useful I love you so much from Reese



what do you call a crazy person with a communicable disease? A psychopathogen *dabs*




Thank you very much for the notes.

just a question: do these notes apply to all of A-level biology, specifically for the AQA spec

I've just finished my GCSE's and I just want an insight to what I'll be facing the upcoming academic year. 

I will be grateful for an answer... 

Thanks again. 

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