AQA Biology AS Unit 1: Cells and Movement In and Out

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3.1 Investigating the structure of Cells


  • Light microscopes use relatively long wavelengths so can only distinguish between two objects at 0.2 micrometres away.
  • Using a beam of electrons instead has shorter wavelengths so can see as close as 0.1 nanometres.


  • Object is the material under the microscope
  • Image is the appearance of the object under the microscope
  • Magnification is found by dividing the size of image by the size of object


  • Kilometres (km): 1000 metres
  • Millimetre (mm): 0.001 metres
  • micrometres:      0.000001 metres
  • nanometres: (nm): 0.000000001 metres
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Investigating the structure of Cells Continued


  • Minimum distance apart two objects can be in order to see them separately
  • Resolution depends on the wavelength or form of radiation used
  • Greater resolution means greater clarity so the image is more precise
  • Increasing magnification increases the size of the image not always the resolution

Cell Fractional

  • This process breaks up cells and different organelles
  • Before the process the tissue is placed in cold, isotonic, buffered solution:
  • Cold: to reduce enzyme activity that might break down organelles
  • Isotonic: prevents organelles bursting or shrinking due to osmosis
  • Buffered: maintain a constant pH
  • There are two stages
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Investigating the structure of Cells Continued


  • Cells are broken up by a homogeniser (blender)
  • The resultant fluid (homogenate) is filtered to remove any complete cells and debris


  • Fragments are separated using a ultracentrifuge
  • Tubes of homogenate are spun at high speeds to create centrifugal force
  • For animal cells:
    • Filtrate tubes spun at slow speed
    • Heaviest organelle (nuclei)  forced to the bottom of tube to form thin sediment
    • Fluid at top (supernatant) is removed leaving sediment of nuclei
    • Supernatant transferred to tube and spun at higher speed
    • The mitochondria are forced to the bottom
    • The process then continues: goes from nuclei, mitochondria, lysosomes and ribosomes
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3.2 The Electron Microscope

  • Light microscopes have poor resolution due to long wavelengths
  • Electron microscope has advantages as:
    • Have very short wavelength so have good resolution
    • Electrons are negatively charged so beam can be focused using electromagnets
  • Due to electrons being absorbed by molecules a vacuum needs to be used
  • Two electron microscopes:
    • Transmission Electron Microscope (TEM)
    • Scanning Electron Microscope (SEM)


  • Electron beam is focused onto the specimen
  • The parts of the specimen that absorb the electrons appear dark
  • An image is formed on a screen: Photomicrograph
  • Specimen must be thin to allow electrons to penetrate
  • The image may contain artefacts that are not part of the specimen
  • Complex staining is used but image still appears black and white
  • Slow, complicated process to form 3D images
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The Electron Microscope Continued


  • Limitations of TEM apply to SEM
  • However specimens don't have to be thin
  • The electrons are penetrated from above passing back and forth across the portion
  • Can build a 3D image due to electrons being scattered by the specimen due to contours
  • It has a lower resolving power than TEM at 20nm
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3.3 Structure of an Epithelial Cell

  • Epithelial cells are eukaryotic cells so have a distinct nucleus and possess membrane bounded organelles
  • Prokaryotic cells e.g. bacteria are different
  • Epithelial cell absorb and secrete

The Nucleus

  • Contains hereditary material and controls cell activity
  • It is around 10-20 micrometres
  • The Nucleus contains:
    • Nuclear envelope: double membrane surrounding the nucleus, outer membrane is continuous with the endoplasmic reticulum of the cell often having ribosomes on the surface. Controls the entry and exit of materials in and out of nucleus
    • Nuclear pores: allows passage of large molecules e.g. RNA out of nucleus
    • Nucleoplasm: granular, jelly like that makes up bulk of nucleus
    • Chromatin: DNA found in nucleoplasm, diffuse form that chromosomes take up when cell not dividing
    • Nucleolus: Spherical body within nucleoplasm, manufactures ribosomal RNA and assembles ribosomes
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Structure of an Epithelial Cell Continued

  • Functions of the nucleus:
  • Acts as a control centre through production of mRNA and protein synthesis
  • Retains genetic material of the cell as DNA/Chromosomes
  • Manufacture ribosomal RNA and ribosomes

The Mitochondrion

  • Rod shaped made up of following:
  • Double membrane: surrounds organelle, outer one controls entry and exit of material, inner membrane is folded to form cristae
  • Cristae: Provide large surface area for attachment of enzymes for respiration
  • Matrix: semi rigid material containing protein, lipids and DNA allowing mitochondria to produce own proteins, enzymes for respiration found in matrix
  • Responsible for the production of ATP from carbohydrates
  • The higher the metabolic rate of a cell the larger the mitochondria with more cristae
  • Epithelial cells use lots of energy to absorb substances from the intestine by active transport
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Structure of an Epithelial Cell Continued

Endoplasmic Reticulum

  • System of sheet like membranes spreading through the cytoplasm of the cells
  • Membranes enclose fattened sacs (cisternae)
    • Rough endoplasmic reticulum (RER) has ribosomes on its outer surface: provides large surface area for protein and gycoprotein synthesis and a pathway for transport of materials e.g. proteins throughout the cell
    • Smooth endoplasmic reticulum (SER) more tubular in appearance: synthesises, stores and transports lipids and carbohydrates
  • Cells that need to make and store large quantities of carbohydrates, proteins and lipids have many ER


  •  Increase surface area to allow efficient absorption
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Structure of an Epithelial Cell Continued

Golgi Apparatus

  • More compact than SER, it consists of stack of membranes that form cisternae with rounded hollow structure called vesicles
  • Proteins and lipids produced by ER passed through golgi in sequence
  • Golgi modifies the proteins often adding non protein compounds e.g. carbohydrates
  • It 'labels' them so they can be accurately sorted and sent to correct destinations
  • Vesicles transport them to the cell surface so they can fuse with the membrane and be released
  • Functions:
    • Add carbohydrate to proteins to form gycoproteins
    • Produce secretory enzymes
    • Secrete carbohydrates e.g. those used to make cell walls
    • Transport, modify and store lipids
    • Form lysosomes
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Structure of an Epithelial Cell Continued


  • Formed when vesicles contain enzymes e.g. proteases, lipases
  • As many as 50 enzymes can be contained in one lysosome
  • Isolate potentially harmful enzymes from the rest of the cell before releasing them outside/ into phagocytic vesicle
  • Functions:
    • Break down material ingested by phagocytic cells e.g. white blood cells
    • Release enzymes to outside cell to destroy material around cell
    • Digest worn out organelles so useful chemicals can be reused
    • Break down dead cells (autolysis)


  • Two types:
    • 80S: found in eukaryotic cells, 25nm in diameter
    • 70S: found in prokaryotic cells, slightly smaller
  • Have 2 subunits one large, one small which contain RNA and protein so used in protein synthesis
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3.4 Lipids

  • Contain carbon, hydrogen and oxygen
  • Proportion of oxygen to carbon and hydrogen smaller than in carbohydrates
  • Insoluble in water
  • Soluble in organic solvents like alcohols
  • Main groups are triglycerides, phospholipids and waxes


  • Main role in plasma membranes
  • Phospholipids contribute to flexibility of membranes and the transfer of lipid soluble substances across them
  • Energy Source: when oxidised lipids provide more than twice energy as carbohydrate
  • Waterproofing: Mammals have oily secretion from the sebaceous glands in the skin
  • Insulation: fats are slow conductors of heat, when stored can retain body heat
  • Protection: often stored around delicate organs
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Lipids Continued

  • Fats are solid at room temperate
  • Triglycerides have 3 fatty acids combined with glycerol
  • Each fatty acid forms a bond with glycerol in a condensation reaction
  • Over 70 fatty acids which all have -COOH (carboxyl) group with a hydrocarbon chain
  • Saturated: no carbon double bonds due to maximum possible hydrogen
  • Monounsaturated: one carbon double bond
  • Polyunsaturated: more than one carbon double bond


  • Contain 2 fatty acids, glycerol and a phosphate molecule and is made up of two parts:
  • Hydrophilic 'head': interacts with water not with fat
  • Hydrophobic 'tail': otients itself away from water but mixes with fats
  • phospholipids are polar

Test for Lipids - Emulsion Test

  • Add ethanol to sample, shake, add water, a cloudy colour indicates lipid
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3.5 The cell-surface membrane

  • Plasma membranes: membranes around/in the cell that have the same basic structure


  • Important components of the cell surface membrane:
  • Due to the hydrophilic heads pointing outwards and the hydrophobic tails pointing to the centre forming a bilayer sheet
  • Function:
    • Allow lipid soluble substances to enter and leave the cell
    • Prevent water soluble substances entering and leaving the cell
    • Make the membrane flexible


  • Arranged more randomly than phospholipids, embedded into bilayer:
    • Extrinsic proteins: give mechanical support or in conjunction with glycolipids as cell receptors for molecules such as hormones
    • Intrinsic proteins: span the bilayer acting as carriers to transport water soluble molecules or are enzymes
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The cell-surface membrane Continued

  • Funtion of the proteins in the membrane:
    • Provide structural support
    • Act as carriers transporting water soluble substances across membrane
    • Allow active transport across the membrane by forming ion channels for ions
    • Form recognition sites by identifying cells
    • Help cells adhere together
    • Act as receptors

Fluid-Mosaic Model

  • Fluid: individual phospholipid molecules can move relative to each other, making membrane flexible
  • Mosaic: proteins embedded in phospholipid bilayer vary in shape and size
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3.6 Diffusion

  • Diffusion is passive transport where energy comes from natural inbuilt motion of particles
  • It is the net movement of molecules or ions from a region where they are in high concentration to one with a lower concentration
  • Dynamic Equilibrium: where particles are equally distributed although individual particles continually move

Rate of Diffusion Factors

  • Concentration gradient: the greater the difference the faster diffusion
  • Area: larger surface area gives faster diffusion
  • Thickness of surface: thinner exchange surface gives faster diffusion
  • Diffusion is (surface area X difference in concentration)/ length of diffusion path
  • Diffusion is also effected by nature of plasma membrane and the size of the diffusion molecules
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Diffusion Continued

Facilitated Diffusion

  • Relies on kinetic energy of the diffusing molecules
  • Occurs down a concentration gradient
  • Occurs at specific points on plasma membrane where have special protein molecules
  • The proteins form water filled channels across the membrane ( protein channels)
  • Channels allow water soluble ions and molecules like glucose to pass through much quicker than the plasma membrane
  • The channels are specific so open in the presence of certain molecules
  • Alternate form involves carrier proteins
  • When particular molecule is present it binds to the protein
  • It causes it to change shape so the molecule is released
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3.7 Osmosis

Solutions and Water potential

  • The passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a partially permeable membrane
  • A solute is a substance that dissolves in a solvent
  • Water potential is measured in pressure e.g.kilopascals, it is the pressure created by water molecules
  • Pure water has a water potential of zero, when solute is added it lowers potential
  • The more solute added the more negative the water potential
  • Water moves by osmosis from a less negative water potential to a more negative potential

Explanation of Osmosis

  •  Both solute and water molecules are in random motion due to kinetic energy
  • When water potentials on either side are equal there is no net movement of water
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Osmosis Continued

Osmosis and Animal Cells

  • If a red blood cell is placed in water, water will start to be absorbed as it has lower water potential causing the cell to burst
  • If a red blood cell is placed in a solution that has a lower water potential the water will leave the cell causing it to shrivel
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3.8 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 energy and carrier molecules
  • ATP is needed for active transport: used for direct movement of molecules and by using a concentration set up by direct active transport which is co-transport
  • Carrier protein molecules act as pumps
  • The process is selective

Direct Active Transport

  •  Carrier proteins accept the molecules/ions to be transported
  • Molecule binds to receptor on channels
  • On inside ATP binds to the protein causing it to split into ADP and a phosphate molecule
  • The molecules are released on the other side as the protein has changed shape
  • The phosphate molecule binds to ADP to form ATP again causing the protein to revert to its original shape
  • Different to facilitated diffusion as it occurs against a concentration gradient
  • Sodium-Potassium pump removes sodium ions and takes in potassium ions at the same time
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3.9 Absorption in the Small Intestine

Villi and Microvilli

  • Glucose is absorbed  through the wall of the small intestine
  • Villi cover the small intestine and have thin walls lined with epithelial cells that have a rich blood supply
  • Villi are situated at the interface between the lumen of the intestine and the blood and other tissue inside the body
  • Properties increase efficiency of absorption:
    • Increase surface area for diffusion
    • Thin walled reducing distance
    • Able to move  so maintain concentration gradient
    • Good blood supply
  • Epithelial cells possess microvilli called the brush border
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Absorption in the Small Intestine Continued

Role of Diffusion in Absorption

  • Carbohydrates are  digested continuously, normally greater concentration of glucose within the small intestine than in the blood
  • The blood is constantly circulated, the glucose is absorbed then removed by cells, used for respiration
  • Villi contain muscles that contract and relax mixing the content of the small intestine allowing glucose to be absorbed helping maintain a concentration gradient

Role of Active Transport in Absorption

  • Not all available glucose is absorbed in diffusion
  • Glucose is absorbed by co-transport as drawn into the cell with sodium ions that have been transported by the sodium-potassium pump
  • Na ions actively transported out of epithelial cell by the sodium-potassium pump to the blood, this is done in one protein carrier molecule
  • Much higher Na ions concentration in lumen so they diffuse into epithelial cells down the concentration gradient through co-transporter protein, the Na ions couple with the glucose which goes into the cell with them
  • The glucose passes into the blood plasma by facilitated diffusion. It is the Na concentration gradient that powers the glucose movement
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3.10 Cholera

  • Caused by Vibrio Cholerae characterised by the flagellum, it is a prokaryotic cell

Prokaryotic cells:

  • No true nucleus, no nucleolus, no chromosomes, no membrane bounded organelles, no chloroplasts, smaller ribosomes (70S), no endoplasmic reticulum, golgi apparatus or lysosomes and has a cell wall made of peptidoglycan

Eukaryotic cells:

  • Distinct nucleus, nucleolus present, chromosomes, membrane bounded organelles, chloroplast present in plants, ribosomes larger (80S), endoplasmic reticulum present and if cell wall is present it is made from cellulose
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Cholera Continued

Structure of bacterial cell

  • Small size from 0.1 to 10 micrometres
  • Have a cell wall made from a mixture of polysaccharides and peptides, capsule, flagella, cell surface membrane, circular DNA and plasmid
    • Cell Wall: physical barrier that protects against mechanical damage
    • Capsule: protects from the cells helping bacteria stick together
    • Cell surface membrane: controls entry and exit of chemicals
    • Flagellum: aids movement
    • Circular DNA: possesses genetic information for replication of cells
    • Plasmid: possesses genes to aid survival in adverse conditions
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Cholera Continued

How Cholera causes disease

  • Main symptoms are diarrhoea and dehydration
  • Transmitted by ingestion of water that is contaminated with faecal matter
  • Causes symptoms in the following ways:
  • Almost all vibrio cholerae killed by acid in stomach, surviving bacteria reach the small intestine using their flagella to propel themselves
  • Produce toxic proteins, one part of the protein binds to carbohydrate receptors. Only epithelial cells of the small intestine have the receptors explaining why cholera toxins effect only this part of the body. Other part enters epithelial cells causing the ion channels to open causing the choride ions to flood the lumen
  • Loss of chloride ions from epithelial cells raises water potential of cells and increases water potential of the lumen causing water to flow from the cells to the lumen
  • Loss of ions in the epithelial cells causes ions to move to surrounding tissues e.g. the blood causing water to move by osmosis from the blood and other tissues to the intestine
  • This loss of water from the blood causes the symptoms
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3.11 Oral Rehydration Therapy

What Causes Diarrhoea?

  • It is an intestinal disorder which causes:
    • Damage to the epithelial cells
    • Loss of microvilli due to toxins
    • Excessive secretion of water due to toxins
  • Dehydration occurs and it can be fatal

What is oral re-hydration therapy?

  • Drinking water is ineffective:
    • It is not being absorbed from the intestine
    • It doesn't replace electrolytes lost
  • There is more than one carrier protein that absorbs sodium in the epithelial cells
  • As Na ions are absorbed water potential of the cells falls causing water to enter cells by osmosis
  • Re-hydration solution needs to contain: water, sodium, glucose, potassium and other electrolytes like chloride and citrate ions
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Oral Rehydration Therapy Continued

  • Water: used to rehydrate tissues
  • Sodium: replaces Na ions lost and make optimum use of alternate carrier proteins
  • Glucose: stimulates uptake of Na ions from intestine and provides energy
  • Potassium: replace lost K ions and cause appetite
  • Other electrolytes: reduce imbalance
  • The ingredients can be mixed into a powder that is administered with little training
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