Module 2: Cells, Microscopes, Biological Molecules, Nucleotides, Enzymes, Membranes, Cell Division.

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  • Created by: Sophhoyle
  • Created on: 28-03-16 16:22

Eukaryotic Cells.

  • Prokaryotic organisms are single celled and small e.g. bacteria. 
  • Where as, eukaryotic cells are larger and more complex e.g. animal and plants.
  • Organelles- A part of a cell, each one has a specific function.


  • Plant cells have a few extra organelles: Chloroplasts 
  • A cell wall with plasmodesmata (channels for exchanging substances.)
  • A vacuole (containing cell sap.)
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  • Plasma (cell surface) membrane- The membrane found on the surface of animal ells and inside the cell wall of plant cells. It is made mainly of lipids and proteins. Its function is to regulate the movement of substances into and out of the cell. It also has receptor molecules, which allow it to repond to chemical messengers.
  • Cell wall-It is a ridgid structure that surrounds plant cells. It is made of the carboydrate, cellulose. Its function is to support plant cells.
  • Nucleus-It is a large organelle surrounded by a nuclear envelope, containing pores. The nucleus contains chromatin, and a structure called the nucleolus. Its function is to control the cells activities. DNA contains instructions to make proteins. The pores allow substances to move between the nucleus and cytoplasm. Also, the nucleolus makes ribosomes.
  • Lysosome-It is a round organelle surrounded by a membrane, with no clear internal structure. Its function is to digest invading cells and break down worn out cell components. It contains digestive enzymes to do this.
  • Ribosome-  It is a small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum (ER). It is made of proteins and RNA. Its function is to be the site where proteins are made.
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Organelles Part 2.

  • Rough endoplasmic reticulum (RER)- It is a system of membranes enclosing a fluid filled space. The surface is covered with ribosomes. Its fuction is to fold and process proteins.
  • Smooth endoplasmic reticulum (SER)- It is similar to the RER, but has no ribosomes. It functions to synthesise and process lipids.
  • Vesicle-It is a small fluid-filled sac in the cytoplasm, with a membrane surrounding it. Its function is to transport substances in and out of the cell, and between organisms.
  • Golgi apparatus-It is a group of fluid-filled, membrane-bound, flattened sacs. Vesicles are often seen at the edges. It functions to process and package new lipids and proteins. It also makes lysosomes.
  • Mitochondria-It is oval-shaped and has a double membrane. The inner membrane is folded to form cristae. Inside the matrix, it contains enzymes involved in respiration. It functions to be the site of aerobic respiration, and produces ATP.
  • Chloroplast-It is a small, flattened structure found in plant cells. It is surrounded by a double membrane, and has thylakoid membranes inside. Its function is the site where photosynthesis takes place.
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Organelles Part 3.

  • Cetriole- It is a small, hollow cylinder, made of microtubules. It is found in animal cells, but only some plant cells. It functions during the seperation of chromosomes in cell division.
  • Cilia-It is small, hair-like structures found on the surface membrane of some animal cells. It functions to allow the movement of substances along the cell surface.
  • Flagellum-It is a longer version of the cilia. They stick out from the cell surface and are surrounded by the plasma membrane. It functions to propel the cell forward, using the contraction of the microtubules.
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Protein Production.

Protein production takes place, involving many organelles, here is the process:

  • Proteins are made at the ribosomes.
  • The ribosomes on the RER make proteins that are excreted or attached to the cell membrane. The free ribosomes in the cytoplasm make proteins that stay there.
  • New proteins produced at the rough ER, are folded and processed in the rough ER.
  • They are then transported from the ER to the golgi apparatus in vesicles.
  • At the golgi apparatus, the proteins are processed further.
  • The proteins then enter more vesicles, to be transported around the cell.

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The organelles in cells are surrounded by a cytoplasm. The cytoplasm has a network of protein threads running through it. These proteins are called the cytoskeleton.

In eukaryotic cells, the protein threads are arranged as microfillaments and microtubules.

  • The microtubules and microfillaments support the cells organelles, keeping them in position.
  • They also help to strengthen the cell and maintain its shape.
  • They are also responsible for the movement of materials.
  • The proteins of the cytoskeleton can cause the cell to move.


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Prokaryotes Vs Eukaryotes.

  • Prokaryotes are extremely small (less than 2 um), where as eukaryotes are larger cells (about 10-100 um.)
  • Prokaryotes have circular DNA, where as eukaryotes have linear DNA.
  • Prokaryotes have no nucleus and have DNA free in the cytoplasm, where as eukaryotes have a nucleus and have DNA inside this.
  • Prokaryotes have a cell wall made of a polysaccharide, but not cellulose or chitin, where as eukaryotes have no cell wall (animals), a cellulose cell wall (plants) and chitin cell wall (fungi).
  • Prokaryotes have few organelles and no membrane-bound organelles, where as eukaryotes have many organelles and other membrane-bound organelles present.
  • Prokaryotes have a flagella made of the protein flagellin in a helix structure, where as eukaryotes have a flagella made of microtubule proteins arranged in a 9+2 formation.
  • Prokaryotes have smaller ribosomes, where as eukaryotes have larger ribosomes.

Bacterial cells are one example of a prokaryote. They are 1/10th the size of a eukaryote cell. And it is often difficult to look at their internal structure under a normal microscope.

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Microscopes Basics.

Magnification- is how much bigger the image is compared to the specimen (sample you're looking at.)

Resolution- is how detailed the image is. Specifically, how well a microscope can distinguish between two points that are close together.

The image size, actual size, or magnification can be calculated using the following formula:


  •  You also need to know the conversions for microscope units:
  • mm x1000= um
  • um x1000= nm
  • To find the smaller value, divide by 1000.
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Light Microscopes And Laser Scanning Confocal Micr

Light Microscopes.

  • Light microscopes use light.
  • They have a lower resolution than electron microscopes, they have a max resolution of 0.2 micrometres (um).
  • They are used to look at whole cells or tissues.
  • There max magnification is usually about x 1500.

Laser Scanning Confocal Microscopes.

  • They use laser beams to scan a specimen, which is tagged with fluorescent dye.
  • The laser causes the dye to give off light. This light is then focused through a pinhole onto a detector. This is hooked on a computer, and can generate a 3D image.
  • The pinhole means that out of focus light is blocked, so the image is clearer.
  • They can be used to look at objects at different depths in thick specimen.


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Electron Microscopes.

Electron Microscopes.

  • These use electrons instead of light to form an image.
  • They have a higher resolution than light microscopes, so give more detailed images.
  • There are two types of electron miscroscopes:

Transmission electron microscopes (TEM) 

  • Use electromagnets to focus a beam of electrons, which is then transmitted through the specimen.
  • Denser parts absorb more electrons.
  • TEMs are good because they provide high resolution images, but can only be used on thin specimen.

Scanning electron microscopes (SEM)

  • Scan a beam of electrons across the specimen, knocking electrons from it, which are gathered in a cathode ray tube to form an image.
  • The images show the surface, but can be 3D, but are lower resolution.
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Comparing Microscopes.

Comparing the three microscopes.

  • Light microscopes have a maximum resolution of 0.2um and a maximum magnification of x 1500.
  • TEM (transmission electron microscopes) have a maximum resolution of 0.0002um and a maximum magnification of x 1,000,000.
  • SEM (scanning electron microscopes) have a maximum resolution of 0.002um and a maximum magnification of x 500,000.


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  • Often in light microscopes and TEM microscopes, the beam of light (electrons) passes through the object being produced. And an image is formed because some parts absorb more light.
  • However, sometimes the object being viewed is completely transparent, this is because the light rays just pass straight through.
  • To avoid this, an object can be stained.

For a light microscope:

  • Use some kind of dye, e.g. methylene blue or eosin.
  • The stain is taken up by some parts, causing a contrast.
  • Different stains are used to make different things show up.
  • More than one stain can be used at once.

For a electron microscope:

  • Objects are dipped in a solution of heavy metal (e.g. lead).
  • The metal ions scatter, again creating contrast.
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Preparing A Microscope Slide.

  • If you want to look at a specimen, you must put it on a slide first.
  • A slide is a ***** of clear glass or plastic.Slides are usually flat, but some have a small dip or well.
  • There are two main ways to prepare a slide:

Dry mount:

  • Your specimen needs to let light through, a thin slice must be taken for a slide.
  • Use tweezers to pick up a specimen and put it in the middle of a clean slide.
  • Pop a cover slip on the top.

Wet mount:

  • Pipette a small drop of water on a slide, then use tweezers to place a specimen on this water.
  • When putting the cover slip on, stand the slip upright, next to the water. Then, carefully tilt and lower so it covers the specimen, try not to get air bubbles.
  • A stain can then be added.
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Water-the basics.

  • Water is a reactant in a lot of important chemical reactions, e.g. hydrolysis.
  • Water is a solvent, which mean some substances dissolve in it.
  • Water transports substances, this is because it is a liquid and a solvent.
  • Water helps with temperature control, because it has a high specific heat capacity and a high latent heat of evaporation.
  • Water is a habitat, because it is a solvent and has good temperature control. It also becomes less dense when it freezes so many organisms can survive and reproduce.

Water Structure.

  • A molecule of water is one atom of oxygen (O), joined to two atoms of hydrogen (H) by shared electrons.
  • Because both hydrogens are negative, and the oxygen is positive, the hydrogens are left with a slight negative charge.
  • This makes a water molecule polar, as it has a slight negative and slight positive charge.
  • Atoms in water molecules are attached by hydrogen bonding.
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Water's Functions.

Hydrogen bonds give water a high specific heat capacity.

  • Specific heat capacity-the energy needed to raise the temperature of one gram of a substance by one degree celsius.
  • The hydrogen molecules between water molecules absorb a lot of energy.
  • So water takes a lot of energy to heat it up.
  • This means that water does not experience rapid temperature changes, which makes it a good habitat- as the temperature is likely to be more stable then on land.

Hydrogen bonds give water a high latent heat of evaporation.

  • It takes a lot of energy (heat) to break the bonds between water molecules.
  • Latent heat of evaporation-a lot of energy is used up when water evaporates.
  • This is beneficial for mammals, as water is great for cooling.

Waters polarity makes it very cohesive.

  • Cohesion-the attraction between molecules of the same type. E.g. water molecules are very cohesive as they are polar, this helps water flow.
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Water's Functions 2.

Water's polarity makes it a good solvent.

  • A lot of important substances in biological reactions are ionic (e.g. salt).
  • This means they are made from one postively charged atom and one negatively charged atom. (e.g. salt is made from a positive sodium ion and a negative chloride ion.)
  • As water is polar, the slightly positive end of a water molecule will be attracted to the negative ion, and the slightly negative end of the water molecule will be attracted to the positive ion.
  • This means ions will get totally surrounded by water molecules, and dissolve.
  • This makes it useful as a solvent.

Water is less dense when solid.

  • At low temperatures, water freezes turning from liquid to solid.
  • Water molecules are held further apart in ice, than liquid because each water molecule forms four hydrogen bonds to other molecules, making a lattice shape.
  • This is beneficial as a habitat, as ice freezes on top and fish can survive under.
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Carbohydrates-the basics.

  • Most carbohydrates are polymers.
  • Polymer-a molecule made up of many similar, smaller molecules (monomers) bonded together.
  • The monomers that make up carbohydrates are called monosaccharides.
  • Glucose is a monosaccharide with six carbon atoms- this means its a hexose monosaccharide.
  • There are two forms of glucose alpha glucose and beta glucose.(
  • Glucose's structure is related to its function as the main energy source in animals and plants. Its structure makes is soluble, so it can be easily transported. Its chemical bonding contain a lot of energy.
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Carbohydrates-the basics.

  • Ribose is a monosaccharide with five carbon atoms, this makes it a pentose monosaccharide.
  • You must know its structure:


  • All carbohydrates are made up of the same three chemical elements- carbon (C), hydrogen (H), and oxygen (O). For every carbon atom, there are usually two hydrogen atoms and one oxygen atom.
  • Monosaccharides are joined by glycosidic bonds.
  • During synthesis, a hydrogen atom on one monosaccharide bonds to an hydroxl group on the other, releasing a molecule of water.
  • The reverse of this reaction is hydrolysis, gaining a water molecule.
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Three Polysaccharides To Know. (Starch)

  • A disaccharide is formed when two monosaccharides join together.
  • A polysaccharide is formed when more than two monosaccharides join together.


  • Cells get energy from glucose. Plants store excess glucose as starch.
  • Starch is a mixture of two polysaccharide of alpha glucose- amylose and amylopectin.
  • Amylose- a long, unbranched chain of a glucose. The angles of a glycosidic bond give it a coiled structure. This makes it compact.
  • Amylopectin- a long, branched chain of a glucose. Its side branches allow enzymes to get at the glycosidic bond quickly.
  • Starch is insoluble in water, so it does not cause water to enter the cell by osmosis.
  • This makes them good for storage.


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Three Polysaccharides To Know. (Glycogen And Cellu


  • Animal cells get energy from glucose also. But animals store excess glucose as glycogen- another polysaccharide of alpha-glucose.
  • Its structure is very similar to amylopectin, except that it has loads more side branches coming off it. Loads of branches mean that the stored glucose is released quickly, which is important for energy release.
  • Its also a very compact molecule, so is good for storage.


  • Cellulose is made of long, unbranched chains of beta-glucose.
  • When beta-glucose molecules bond, they form straight cellulose chains.
  • The cellulose chains are linked together by hydrogen bonds to form strong fibres called microfibrils.
  • The strong fibres mean cellulose provides structural support for cells.
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Lipids-the basics.

  • Triglycerides are macromolecules- they are complex molecules with a relatively large molecular mass.
  • Like all lipids, they contain the chemical elements carbon, hydrogen and oxygen.
  • Triglycerides have one molecule of glycerol with three fatty acids attached to it.
  • They have hydrocarbon tails that are hydrophobic. These tails make lipids insoluble in water.

  • The 'R' is a variable group hydrocarbon tail.
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Lipids-the basics.

  • Triglycerides are synthesised by the formation of an ester bond between each fatty acid and the glycerol molecule.
  • Each ester bond is formed by a condensation reaction (in which a water molecule is released).
  • The process in which triglycerides are synthesised is called esterification.
  • Triglycerides break down when the ester bonds are broken. Each ester bond is broken in a hydrolysis reaction (in which a water molecule is used up).
  • There are two kinds of fatty acids: saturated and unsaturated. The difference is in their hydrocarbon tails.

  • Phospholipids are also macromolecules. They are similar to triglycerides, except one fatty acid is replaced by a phosphate group.
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Three Lipids To Know. (Triglycerides And Phospholi


  • In animals and plants, triglycerides are mainly used as energy storage molecules.
  • Some bacteria use triglycerides to store energy and carbon.
  • Triglycerides are good for storage because:
  • The long hydrocarbon tails of the fatty acids contain lots of chemical energy- a load of energy is released when they are broken down. Because of these tails, lipids contain about twice as much energy per gram as carbohydrates.
  • They are insoluble. The triglycerides bond together as insoluble droplets in cells because the fatty acid tails are hydrophobic- the tails face inwards, sheilding themselves from water.


  • They are found in the cell membrane of all cells, they make up the phospholipid bilayer, they control what enters and leaves the cell.
  • The phospholipid heads are hydrophilic and their heads are hydrophobic, so they form a double layer with the heads facing out towards water on either side.
  • They centre is hydrophobic, so water cannot pass through.
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Three Lipids To Know. (Cholesterol)


  • It has a hydrocarbon ring structure attached to a hydrocarbon tail.
  • The ring structure has a polar hydroxyl (OH) group attached to it.
  • In eukaryotic cells, cholesterol molecules help to strengthen the cell membrane by interacting with the phospholipid bilayer.
  • Cholesterol has a small size and flattened shape, this allows cholesterol to fit in between the phospholipid molecules in the membrane.
  • They bind to the hydrophobic tails of the phospholipids causing them to pack more closely together.
  • This helps may the membrane less fluid and more ridgid.
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Proteins- the basics.

  • Proteins are made from long chains, of amino acids.
  • Proteins are polymers.
  • Amino acids are the monomers in proteins.
  • dipeptide is formed when two amino acids join together.
  • polypeptide is formed when more than two amino acids join together.
  • Proteins are made up of one or more polypeptides.
  • All amino acids have the same general structure, a carboxyl group (-COOH) and an amino group (-NH2) attached to a carbon atom. The difference betweem amino acids is the variable (R) group that they contain.


  • All amino acids contain the same chemical elements, carbon, oxygen, hydrogen and nitrogen, some also contain sulfur.
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Proteins- the basics.

  • Amino acids are linked together by peptide bonds to form dipeptides and polypeptides.
  • A molecule of water is released during the reaction- a condensation reaction.
  • The reverse of the reaction adds a molecule of water to break the peptide bond- a hydrolysis reaction.

Primary Structure.

  • This is the sequence of amino acids in the polypeptide chain, different proteins have a different sequence of amino acids in their primary structure.
  • A change in one amino acid can change the structure of a whole protein.

Secondary Structure.

  • Hydrogen bonds form between amino acids in a chain, this makes it coil into a alpha helix or fold into a beta pleated sheet.

Tertiary And Quarternary Structure.

  • The strucure is coiled further and becomes 3D.
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Bonds In Proteins.

  • Primary Structure- held together by peptide bonds in amino acids.
  • Secondary Structure- held together by hydrogen bonds.
  • Tertiary Structure- a few bonds:
  • Ionic interactions- weak attractions between negatively-charged R groups and positively-charged R groups on different parts of the molecules.
  • Disulfide bonds- whenever two molecules of the amino acid cysteine come close together, the sulfur atom in each bond, this forms a disulfide bond.
  • Hydrophobic and hydrophilic interactions- when hydrophobic R groups are close together in the protein, they clump together. This means that hydrophilic R groups are pushed to the outside.
  • Hydrogen bonds- these weak bonds form between positively-charged hydrogen atoms in some R groups and negatively-charged atoms in the other R group, on the polypeptide chain.
  • Quaternary Structure- this is determined by the tertiary structure of the individual polypeptide chains bonded together. This means it can be influenced by all the bonds mentioned above.
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Globular Proteins. (3 types)

  • In globular proteins, the hydrophilic R groups on the amino acids tend to be pushed to the outside of the molecule. This is caused by the hydrophobic and hydrophilic interactions in the proteins tertiary structure.
  • This means they are soluble, and are easily transported in fluids.
  • Globular proteins have a range of functions:
  • Haemoglobin- a globular protein that carries oxygen around the body in red blood cells. It is a conjugated protein, this means it is a protein with a non-protein group attached. The non-protein part is called a prosthetic group. Each of the four polypeptide chains in haemoglobin has a prosthetic group called haem. A haem group contains iron, which oxygen binds to.
  • Insulin- a hormone secreted by the pancreas. It regulates the blood glucose level. It is soluble, as it can be transported in blood to the tissue. An insulin molecule consists of two polypeptide chains, held together by disulfide bonds.
  • Amylase- an enzyme, catalyses the breakdown of starch in the digestive system. It is made of a single chain of amino acids. It secondary structure contains both alpha-helix and beta-pleated sheet sections. Most enzymes are globular proteins.
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Fibrous Proteins. (3 types)

  • Fibrous proteins are insoluble and strong. They are structural proteins and are fairly unreactive.
  • Collagen- found in animal connective tissues, such as bone, skin and muscle. It is a very strong molecule. Minerals can bind to the protein to increase its rigidity, e.g. in bone.
  • Keratin- found in many of the external structures of animals, such as skin, hair, nails, feathers and horn. It can be either flexible, or hard and tough.
  • Elastin- found in elastic connective tissues, such as skin, large blood vessels, and some ligaments. It is elastic, so it allows tissues to return to their original shape after they have been stretched.
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Inorganic Ions.

  • An ion is an atom that has an electric charge.
  • An ion with a positive charge is called a cation.
  • An ion with a negative charge is called a anion.
  • An inorganic ion is one that does not contain carbon, they are very important, you must know about these few:
  • Calcium (Ca2+) - involves transmission of nerve impulses and the release of insulin from the pancreas. It acts as a cofactor for many enzymes.
  • Sodium (Na+) - important for generating nerve impulses, for muscle contraction, and for regulating fluid balance.
  • Potassium (K+) - important for generating nerve impulses, muscle contraction, and regulating fluid balance. Activates essential enzymes.
  • Hydrogen (H+) - affects the pH of substances. And, photosynthesis.
  • Ammonium (NH4+) - absorbed from soil by plants, important source of nitrogen.
  • Nitrate (NO3-) - absorbed from soil by plants, important source of nitrogen.
  • Hydrogencarbonate (HCO3-) - acts as a buffer, which maintains the pH in the blood.
  • Chloride (Cl-) - involved in the 'chloride shift' and maintains blood pH, cofactor.
  • Phosphate (PO43-) - involved in photosynthesis and respiration, synthesises.
  • Hydroxide (OH-) - affects the pH of substances.
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Biochemical Tests For Molecules.

Benedicts Test For Reducing Sugars.

  • Reducing sugars include all monosaccharides e.g. glucose and some disaccharides e.g. maltose.
  • You add benedicts reagent (which is blue) to a sample, and heat it in a water bath that has been brought to the boil.
  • The colour of the precipitate changes from:
  • Blue - Green - Yellow - Orange - Brick red.
  • If the test is positive, it will form a coloured precipitate.
  • The higher the concentration of reducing sugar, the further the colour change.

Benedicts Test For Non-Reducing Sugars.

  • If the result of the reducing sugars is negative, there could still be non-reducing sugars present. Firstly, these must be broken down into monosaccharides.
  • This is done by getting a new sample, adding dilute hydrochloric acid and heating it in a water bath. It is then neutralised by using sodium hydrogencarbonate. Then carry out the benedicts test.
  • If the solution is positive, it forms a coloured precipitate.
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Biochemical Tests For Molecules Part 2.

Test Strips For Glucose.

  • Test strips are coated in reagent, they are dipped in a test solution and change colour if glucose is present. 
  • The colour change can be compared to a chart to see how much glucose is present.
  • These can be used to test urine, to check for diabetes.

Iodine Test For Starch.

  • Add iodine dissolved in potassium iodide solution to the test sample.
  • If starch is present, the sample changes from browny-orange to a dark, blue-black.
  • If theres no starch, it stays browny-orange.

Biuret Test For Proteins.

  • The test solution needs to be alkaline, so a few drops of sodium hydroxide added.
  • Then you add some copper (II) sulfate solution.
  • If protein is present, the solution turns purple.
  • If there no protein, the solution turns blue.
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Biochemical Tests For Molecules Part 3.

Emulsion Test For Lipids.

  • Shake the substance with ethanol, for about a minute and pour the solution into water.
  • If a lipid is present, the solution will turn milky.
  • The more lipid there is, the more noticable the milky colour will be.
  • If there is no lipid, the solution will stay clear.


  • Benedicts reagent can be used and a colorimeter to get a quantitative estimate of how much glucose there is in a solution.
  • colorimeter is a device that measures the strength of a coloured solution by seeing how much light passes through it.
  • A colorimeter measure the absorbance. The more concentrated the colour of the solution, the higher the absorbance is.
  • It is easier to measure the concentration of the blue benedicts solution thats left after the test. So, the higher the glucose concentration, the lower the absorbance of solution.
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Biochemical Tests And Separating Molecules.

  • Biosensors are devices that use a biological molecule, such as an enzyme to detect a chemical.
  • The biological molecule produces a signal, which is converted into an electrical signal by a transucer.
  • This electrical signal is then processed and can be used to work out information.


  • Chromatography is used to seperate stuff in a mixture- once it is seperated components can be identified.
  • You must know about paper chromatography and thin-layer chromatography.
  • They both have the same basic principle: 
  • A mobile phase, where the molecules can move. In both paper and TLC the mobile phase is a liquid solvent.
  • A stationary phase, where the molecules cant move. In paper chromatography, the stationary phase is a piece of chromatography paper. In TLC, the stationary phase is a thin layer of solid, on a glass or plastic plate.
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Chromatography- extra information.

  • The mobile phase moves through or over the stationary phase.
  • The components in the mixture spend different amounts of time in the mobile phase and stationary phase.
  • The components that spend longer in the mobile phase travel faster or further.
  • The time spent in the different phases is what seperates out the components of the mixture.

Process of paper chromatography.

  • Draw a pencil line near the bottom of a piece of chromatography paper, and put a concentrated spot of the mixture of amino acids on it. 
  • Add a small amount of prepared solvent to a beaker and dip the bottom of the paper into it. This should be done in a fume cupboard, and covered with a lid.
  • As the solvent spreads up the paper, the different amino acids move with it, but at different rates, so they seperate out.
  • When the solvents nearly reached the top, take the paper out and mark the solvent front with a pencil. Then leave it to dry, and analyse it.
  • Amino acids are not coloured, so they cannot be seen. Therefore, they must be sprayed with ninhydrin solution.
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Nucleotides- the basics.

  • nucleotide is a type of biological molecule, that is made from a pentose sugar (a sugar with five carbon atoms), a nitrogenous base (one containing nitrogen) and a phosphate group.
  • All nucleotides contain the elements carbon, hydrogen, oxygen, nitrogen and phosphorus.
  • Nucleotides are important, they are momomers that make up DNA and RNA. These are both types of nucleic acid. DNA is used to store genetic information and RNA is used to make proteins.
  • ADP and ATP are types of nucleotides and are used to store and transport energy in cells.
  • The pentose sugar in a DNA neucleotide is called deoxyribose.
  • Each DNA nucleotide has the same sugar and a phosphate group, the base on each nucleotide can vary.
  • There are four possible bases, adenine (A), thymine (T), cytosine (C) and guanine (G).
  • Adenine and guanine are a type of base called purine.
  • Cytosine and thymine are a type of base called a pyrimidine.
  • A purine base contains two carbon-nitrogen rings joined together, and a pyrimidine base has one carbon-nitrogen ring.
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Nucleotides- continued.

  • RNA contains nucleotides with a ribose sugar.
  • Like DNA, RNA nucleotide has a phosphate grou and one of the four different bases.
  • In RNA, uracil replaces thymine as a base.
  • An RNA molecule is made up of a single polynucleotide chain.
  • To phosphorylate a nucleotide, you add one or more phosphate groups to it
  • ADP contains the base adenine, the sugar ribose and two phosphate groups.
  • ATP contains the base adenine, the sugar ribose and three phosphate groups.
  • ATP provides energy for chemical reactions in the cell.
  • ATP is synthesised from ADP and inorganic phosphate using the energy from an energy-releasing reaction. The ADP is phosphorylated to form ATP and a phosphate bond is formed.
  • Energy is stored in the phosphate bond. When this energy is needed by a cell, ATP is broken back down into ADP and an inorganic phosphate. Energy is released from the phosphate bond and used by the cell.
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Polynucleotides And DNA.

  • The nucleotides join up between the phosphate group of one nucleotide and the sugar of another.
  • The bond between the phosphate group and the sugar is called a phosphodiester bond, and the chain of sugars and phosphates is known as the sugar-phosphate backbone.
  • Polynucleotides can be broken down into nucleotides again, by breaking the phosphodiester bond.
  • Two DNA polynucleotide strands join together by hydrogen bonding between the bases.
  • Each base can only join with one particular partner- this is called complementary base pairing.
  • Adenine always pairs with thymine (A-T), and cytosine always pairs with guanine (C-G).
  • A purine (A or G) always pairs with pyrimidine (T or C).
  • Two hydrogen bonds from between A and T, and three hydrogen bonds form between C and G.
  • Two antiparallel polynucleotide strands twist to form the DNA double-helix.
  • DNA can be purified using a precipitation reaction, it is a procedure that can be viewed on page 36-37 on the revision guide.
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DNA In Self-Replication.

  • DNA helicase (an enzyme) breaks the hydrogen bonds between the two polynucleotide DNA strands. The helix then unzips, to form two single strands.
  • Each original single strand acts as a template for a new strand. Free- floating DNA nucleotides join to the exposed bases on each original template strand by complementary base pairing. (A-C) (T-G).
  • The nucleotides of the new strand are joined together by the enzyme DNA polymerase. This forms the sugar-phosphate backbone. Hydrogen bonds form between the bases on the original and new strands. The strands twist to form a double-helix.
  • Each new DNA molecule contains one strand from the original DNA molecule and one new strand.
  • This type of copying is called semi-conservative replication because half of the strands in each new DNA molrcule are from the original DNA.
  • DNA replication is very accurate- it has to be, so that genetic information is conserved each time the cell is replicated.
  • Every so often a mutation occurs, a mutation is any change to the DNA base sequence. They can often alter the sequence of amino acids in a protein.
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Genes And Protein Synthesis.

  • A gene is a sequence of DNA nucleotides that code for a sequence of amino acids in a polypeptide.
  • Different proteins have a different number and order of amino acids.
  • Its the order of nucleotide bases in a gene that determines the order of amino acids in a protein.
  • Each amino acid is coded for by a sequence of three bases (triplet) in a gene.
  • Different sequences of bases code for different amino acids, so the sequence of bases in a section of DNA is a template that is used for proteins during synthesis.
  • DNA molecules are found in the nucleus, but the organisms that make proteins are found in the cytoplasm.
  • DNA is too large to be moved out the nucleus, so a section is copied into mRNA, in a process called transcription.
  • The mRNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it is used to synthesise a protein, in a process called translation.
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Three Types Of RNA (mRNA, tRNA, rRNA)

Messenger RNA (mRNA)

  • Made in the nucleus.
  • It has three adjacent bases, called a codon.
  • It carries the genetic code from the DNA in the nucleus to the cytoplasm, where it is used to make a protein during translation.

Transfer RNA (tRNA)

  • Found in the cytoplasm.
  • It has an amino acid binding site at one end and a sequence of three bases at the other end called an anticodon.
  • It carries the amino acids that are used to make proteins to the ribosomes during translation.

Ribosomal RNA (rRNA)

  • Forns two sub units in a ribosome, and moves along the mRNA strand in synthesis.
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The Genetic Code.

  • The genetic code is the sequence of base triplets (codons) in DNA or mRNA, which codes for specific amino acids.
  • In the genetic code, each base triplet is read in sequence, seperate from the triplet before it and after it, the code is non-overlapping.
  • The genetic code is degeneraye- there are more possible combinations of triplets than there are amino acids (20:64). This means that some amino acids are coded for by more than one base triplet.
  • Some triplets are used to tell the cell when to start and stop production of the protein- these are called start and stop signals, and are found at the beginning and end of the gene.
  • The genetic code is also universal- the same specific base triplets code for the same amino acids in all living things.
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Protein Synthesis. (Transcription)

  • Transcription starts when RNA polymerase (enzyme) attaches to the DNA double-helix at the beginning of a gene.
  • The hydrogen bonds between the two DNA strands in the gene break, seperating the strands, and the DNA molecule uncoils.
  • One of the strands is then used as a template to make an mRNA copy.
  • The RNA polymerase then lines up free RNA nucleotides alongside the template strand. Complementary base pairing means that the mRNA strand ends up being a complementary copy of the DNA template strand.
  • Once the RNA nucleotides have paired up with their specific bases on the DNA strand, they are joined together, to form an mRNA molecule.
  • The RNA polymerase moves along the DNA, seperating the strands and assembling the mRNA strand.
  • The hydrogen bonds between the uncoiled strands of DNA re-form once the RNA polymerase has passed by and the strands coil back into a double-helix.
  • When RNA polymerase reaches a stop codon, it stops making mRNA and detaches from the DNA.
  • It then moves out of the nucleus, and attaches to a ribosome in the cytoplasm.
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Protein Synthesis. (Translation)

  • It occurs at the ribosomes in the cytoplasm.
  • The mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to the ribosome.
  • A tRNA molecule, with an anticodon thats complementary to the start codon on the mRNA, attaches itself to this mRNA by complementary base pairing.
  • A second tRNA molecule attaches itself to the next codon on the mRNA in the same way.
  • rRNA in ribosomes catalyses the formation of a peptide bond between the two amino acids attached to the tRNA molecules. This joins the amino acids together. The first tRNA molecule moves away, leaving its amino acid behind.
  • A third tRNA molecule binds to the next codon on the mRNA. Its amino acid binds to the first two and the second tRNA molecule moves away.
  • This process continues, producing a chain of linked amino acids, until a stop codon appears on the mRNA molecule.
  • The polypeptide chain moves away from the ribosome, and translation is complete.
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Enzymes- the basics.

  • Enzymes speed up chemical reactions by acting as biological catalysts. They catalyse metabolic reactions at a cellular level and for an organism as a whole.
  • Enzymes can affect the structures in an organism, as well as the functions.
  • One example of an intracellular enzyme is hydrogen peroxide, it is the toxic by-product of several cellular reactions. If it is left to build up, it can kill cells.
  • One example of an extracellular enzyme is amylase and trypsin, these both work outside cells in the human digestive system.
  • Enzymes are globular proteins.
  • Enzymes have an active site, which has a specific shape. The active site is the part of the enzyme that the substrate molecules bind to.
  • The specific shape of the active site is determined by the enzymes tertiary structure.
  • For the enzyme to work, the substrate has to fit into the active site. If the substrate shape does not match, the reaction will not be catalysed.
  • Activation energy the minimum amount of kinetic energy that needs to be supplied for a reaction to take place.
  • Enzymes speed up the rate of reaction, by making the reaction require a lower activation energy to occur.
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Enzymes-the basics 2.

  • Enzymes only work with the substrates that fit their active sites. Early scientists studying the action of enzymes came up with the 'lock and key' model. This is where the substrate fits into the enzyme in the same way a ket fits into a lock.
  • Scientists soon realised that the lock and key model was not fully accurate. The enzyme and substrate do have to fit together in the first place, but new evidence showed that the enzyme-substrate complex changed shape slightly to complete this fit. This locks the substrate even more tightly to the enzyme.
  • Therefore, scientists re modified the old lock and key model, and came up with the 'induced fit' model.
  • The 'induced fit' model helps to explain why enzymes are so specific and only bond to one particular substrate.
  • The substrate does not only have to be the right shape to fit the active site, it has to make the active site change shape the right way. This is a prime example of how a widely accepted theory can change when new evidence comes along.
  • The 'induced fit' model is now widely accepted.
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Factors Affecting Enzyme Activity. (Temperature)


  • The rate of an enzyme-controlled reaction increases when the temperature is increased.
  • This is because more heat, means more kinetic energy, so molecules move faster.
  • This makes the enzymes more likely to collide with the substrate molecules.
  • The energy of this collision also increases, which means each collision is more likely to result in a reaction. However, if the temperature gets too high, the reaction stops.
  • The rise in temperature makes the enzymes molecules vibrate more. And, if the temperature goes above a certain level, this vibration breaks some bonds that hold the enzyme in shape. Also, the active site changes shape and the enzyme and substrate can no longer fit together. Furthermore, at this point the enzyme is denatured and no longer functions as a catalyst.
  • The temperature co-efficent or Q10 value for a reaction shows how much the rate of a reaction changes when the temperature is raised by 10 degrees celsius.
  • At temperatures before the optimum, a Q10 value of 2 means that the rate doubles when the temp is raised by 10 degrees celsius, a Q10 value of 3 would mean that the rate trebles. (the value is normally 2).
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Factors Affecting Enzyme Activity. (pH And Enzyme


  • All enzymes have a optimum pH value. Most human enzymes work best at a value of 7, but there are acceptions to this. For example, the stomach enzyme pepsin woks best at a acidic pH 2.
  • Above and below the optimum, the ions found in acids and alkalis can mess up ionic bonds and hydrogen bonds holding an enzymes tertiary structure together. This causes the active site to change shape, meaning the enzyme is denatured.

Enzyme Concentration.

  • The more enzyme molecules there is in a solution, the more likely a substrate molecule is to collide with one and form an enzyme- substrate complex. So increasing the concentration of the enzyme, increases the rate of reaction.
  • But, if the amount of substrate is limited, there is a point where there is more than enough enzyme molecules to deal with available substrates, so adding more enzyme at this point has no effect.
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Factors Affecting Enzyme Activity. (Substrate Conc

  • The higer the substrate concentration, the faster the reaction- more substrate molecules means a collision between substrate and enzyme is more likely, so more active sites will be used.
  • This is only true until a 'saturation' point though. After that, there are so many substrate molecules that the enzyme have as much as they can cope with, and adding more substrate makes no difference to the rate of reaction.
  • Substrate concentration decreases with time during a reaction, so if no other variables are changed, the rate of reaction will decrease over time. This makes the initial rate of reaction the highest rate of reaction.


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Cofactors, Coenzymes, And Inhibition.

  • Some enzymes will only work if there is another non-protein substance bound to them. These are called cofactors.
  • Some cofactors are inorganic molecules or ions. They work by helping the enzyme and substrate bind together. They do not directly participate in the reaction so are not used up or changed.
  • Some cofactors are organic molecules- these ones are called coenzymes. They participate in the reaction, and are changed by it, like a second substrate. They can often be used as carriers, moving chemical groups between different enzymes. They are continually recycled during this process.
  • If a cofactor is tightly bound to the enzyme, it is known as a prosthetic group.
  • Enzyme activity can be prevented by enzyme inhibitors- molecules that bind to the enzyme that they inhibit. Inhibition can be competitive or non-competitive.
  • Competitive inhibitor molecules have a similar shape to that of substrate molecules.
  • They compete with the substrate molecules to bind to the active site, but no reaction takes place. Instead they block the active site, so no substrate molecules can fit in.
  • If the inihibitor is in high concentration, it will take up nearly all the active sites.
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Non-Competitive Inhibitors And Uses Of Inhibitors.

  • Non-competitive inhibitor molecules bind to the enzyme away from its active site. They bind to the enzymes allosteric site.
  • This causes the active site to change shape, so the substrate molecules can no longer bind to it.
  • They do not compete with the substrate molecule to bind to the active site, because they have a different shape all together.
  • Increasing the concentration of a substrate will not make a difference to the reaction rate- enzyme activity will still be inhibited.
  • If they are strong, covalent bonds, the inhibitor cannot be removed easily and the inhibition is irreversible.
  • If they are weaker, hydrogen bonds or weak ionic bonds, the inhibitor can be removed and the inhibition is reversible.
  • Some antiviral drugs e.g. reverse transcriptase inhibitors inhibit the enzyme reverse transcriptase, which catalyses the replication of viral DNA. This prevents the virus from replicating.
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Metabolic Pathways And More Enzyme Inhibition.

  • A metabolic pathway is a series of connected metabolic reactions. The product of the first reaction takes part in the second reaction. Each reaction is catalysed by a different enzyme.
  • Many enzyme are inhibited by the product of the reaction they catalyse, this is known as product inhibition.
  • End-product inhibition is when the final product in a metabolic pathway inhibits an enzyme that acts earlier on in the pathway.
  • End-product inhibition is a way of regulating and controlling the amount of end-product that gets made.
  • Both product and end-product inhibition are reversible. So when the level of product starts to drop, the level of inhibition will start to fall and the enzyme can function again- this means more product can be made.
  • Enzymes are often synthesised as inactive precursors in metabolic pathways to prevent them causing damage to cells.
  • Part of the precursor molecule inhibits its action as an enzyme, once this part is removed,the enzyme becomes active again.
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Cell Membranes-the basics.

Membranes at the surface of cells (PLASMA membranes).

  • They are a barrier between the cell and its environment, this controls what substances enter and leave the cell.
  • They are partially permeable, this means they let some molecules enter, but not others.
  • Substances can move across the membrane by diffusion, osmosis or active transport.
  • They allow recognition by other cells, e.g. the immune system cells.
  • They allow cell communication, e.g. cell signalling.

Membranes within cells.

  • The membranes around organelles divide the cell into different compartments- acting as a barrier between the organelle and the cytoplasm. This makes a lot of functions more efficient.
  • They can form vesicles to transport substances between different areas of the cell.
  • They can control which substances enter and leave the organelle, partially permeable.
  • They can also be the site of chemical reactions.
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The Cell Membrane Structure- basic.

  • In 1992, the fluid mosaic model was created to describe the arrangement of molecules in the membrane.
  • In this model, phospholipid molecules form a continous double layer (bilayer).
  • This bilayer is 'fluid' because the phospholipids are constantly moving.
  • Cholesterol molecules are present within the bilayer.
  • Protein molecules are scattered through the bilayer, like tiles in a mosaic.
  • Some proteins have a polysaccharide chain attached- these are glycoproteins.
  • Some lipids also have a polysaccharide chain attached- these are glycolipids.

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Cell Membrane Components.


  • They have a 'head' and a 'tail'.
  • The head is hydrophilic- meaning it attracts water.
  • The head is hydrophobic- meaning it repels water.
  • The molecules arrange themselves into a bilayer- the heads face out towards water on either side of the membrane.
  • The centre of the bilayer is hydrophobic, so the membrane does not allow water-soluble substances through it- it acts as a barrier to these dissolved substances.


  • Cholesterol is a type of lipid.
  • It is present in all cell membranes (except bacterial cell membranes).
  • Cholesterol molecules fit between the phospholipids. They bind to the hydrophobic tails of the phospholipids, causing them to pack more closely together. This makes the membrane less fluid and more ridgid.
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Cell Membrane Components Part 2.


  • Some proteins form channels in the membrane- these allow small or charged particles through.
  • Other proteins (carrier proteins) transport molecules and ions across the membrane by active transport and facilitated diffusion.
  • Proteins act as receptors for molecules in cell signalling. When a molecule binds to the protein, a chemical reaction is triggered inside the cell.

Glycolipids and glycoproteins.

  • Glycolipids and glycoproteins stabilise the membrane by forming hydrogen bonds with surrounding water molecules.
  • They are also sites where drugs, hormones and antibodies bind.
  • They act as receptors for cell signalling.
  • They are also antigens- cell surface molecules involved in the immune response.
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Cell Signalling.

  • Cells need to communicate with each other to control processes inside the body, and to respond to changes in the environment.
  • Cells communicate with eachother using messenger molecules:
  • One cell releases a messenger molecule, e.g. hormones.
  • This molecule travels to another cell.
  • The messenger molecule is detected by the cell because it binds to a receptor.
  • Proteins in the cell membrane act as receptors for messenger molecules, these are called 'membrane-bound receptors'.
  • Receptor proteins have specific shapes- only messenger molecules with a complementary shape can bind to them.
  • Different cells have different types of receptors- they respond to different messenger molecules.
  • A cell that responds to a particular messenger molecule is called a target cell.
  • Many drugs work by binding to receptors in cell membranes.
  • They either trigger a response in the cell, or block the receptor and prevent it form working.
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Temperature Affecting Cell Membranes.


  • Below 0 celsius- phospholipids don't have much energy, so they cannot move very much. They are packed closely together and the membrane is ridgid. But, channel proteins and carrier proteins in the membrane deform, increasing the permeability of the membrane. Ice crystals may form and pierce the membrane, making it highly permeable when it thaws.
  • Between 0-45 celsius- phospholipids can move around and aren't packed as tightly together- the membrane is partially permeable. As the temperature increases the phospholipids move more because they have more energy- increasing the permeability of the membrane.
  • Above 45 celsius- phospholipid bilayer starts to melt and the membrane becomes permeable. Water inside the cell expands, putting pressure on the membrane. Channel proteins and carrier proteins deform, so they cannot control what enters or leaves the cell- this increases the permeability of the membrane.
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Solvent Changing Affecting Cell Membranes.

  • Surrounding cells in a solvent increases the permeability of their cell membranes.
  • This is because solvents dissolve the lipids in a cell membrane, so the membrane loses its structure.
  • Some solvents increase cell permeability more than others, e.g. ethanol increases it more then methanol.
  • You could investigate the effects of different solvents by doing an experiment using beetroot.
  • Increasing the concentration of the solvent will also increase membrane permeability. 
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  • Diffusionthe net movement of particles, molecules, or ions from an area of a higher concentration to an area of a lower concentration.
  • Molecules can diffuse both ways, but the net movement will be to the area of a lower concentration. This continues until particles are evenly distributed throughout the liquid or gas.
  • The concentration gradient, is the path from an area of higher concentration to an area of lower concentration. Particles diffuse down a concentration gradient.
  • Diffusion is a passive process- no energy is needed for it to happen.
  • Small, non-polar molecules e.g. oxygen are able to diffuse easily through spaces between phospholipids.
  • Water is also small enough to fit between phospholipids, so it is able to diffuse across plasma membranes, even though it is polar. The diffusion of water molecules like this is called osmosis.
  • The rate of diffusion is dependent on several factors:
  • Concentration gradient- the higher it is, the faster the rate of diffusion.
  • Thickness of the exchange surface- the thinner the surface, faster the diffusion rate.
  • The surface area- the larger the surface area, the faster diffusion rate.
  • Temperature- the warmer it is, the faster diffusion rate is.
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Facilitated Diffusion.

  • Some larger molecules e.g. amino acids, ions and polar molecules do not diffuse directly through the phospholipid bilayer of the cell membrane.
  • Instead they diffuse through carrier proteins or channel proteins in the cell membrane- this is called facilitated diffusion.
  • Like diffusion, facilitated diffusion moves particles down a concentration gradient, from a higher to a lower concentration.
  • It is also a passive process- that doesnt use energy.
  • Carrier proteins move large molecules into or out of the cell, down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules.
  • First, a large molecule attaches to a carrier protein in the membrane.
  • Then, the protein changes shape.
  • This releases the molecule on the opposite side of the membrane.
  • Channel proteins form pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles.
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Active Transport.

  • Active transport uses energy to move molecules and ions across plasma membranes, against a concentration gradient.
  • This process involves carrier proteins.
  • The process is pretty similar to facilitated diffusion- a molecule attaches to the carrier protein, the protein changes shape and this moves the molecule across the membrane, releasing it on the other side.
  • The only difference is that energy is used (ATP), to move the solute against its concentration gradient.


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Endocytosis And Exocytosis.


  • Some molecules are way too large to be taken into a cell by carrier proteins.
  • Instead, a cell can surround a substance with a section of its plasma membrane.
  • The membrane then pinches off to form a vesicle inside the cell containing the ingested substance- this is endocytosis.
  • Some cells also take in much larger objects by endocytosis.
  • Like active transport, this process also uses ATP for energy.


  • Some substances produced by the cell need to be released from the cell- this is done by exocytosis.
  • Vesicles containing these substances pinch off from the sacs of the golgi apparatus and move towards the plasma membrane.
  • The vesicles fuse with the plasma membrane and release their contents outside the cell.
  • Some substances arent released outside the cell- they can be inserted into it.
  • Exocytosis uses ATP as an energy source.
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  • Osmosis- the diffusion of water molecules across a partially permeable membrane down a water potential gradient. This means water molecules move from an area of higher water potential to an area of lower water potential.
  • Water potential- the potential (likelihood) of water molecules to diffuse out of or into a solution.
  • Pure water has the highest water potential, all other solutions are lower.
  • If a plant cell is placed in a solution of lower Water Potential, water will diffuse out. This causes the Cytoplasm to shrink and become Flaccid. If enough water leaves, the Cytoplasm will pull away from the cell wall. The cell will become Plasmolysed.
  • Animal cells will also expand when they are placed in a solution of higher Water Potential. Since animal cells do not have cell walls, if this happensexcessively the cell will burst open and become Haemolysed.
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The Cell Cycle.


  • G1- the cell grows and new organelles and proteins are made.
  • G1 Checkpoint- the cell checks that the chemicals needed for replication are present and for any damage to the DNA before entering S-phase.
  • Synthesis- cell replicates its DNA ready to divide by mitosis.
  • G2- cell keeps growing and proteins needed for cell division are made.
  • G2 Checkpoint- the cell checks whether all the DNA has been replicated without ant damage, if it has it can enter mitosis.
  • M phase- mitosis and cytokinesis.
  • Interphase is subdivided into three seperate growth stages. These are called G1, S and G2.
  • The cell cycle is regulated by checkpoints. These occur at key points of the process to make sure it is okay for the cycle to continue.
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  • Mitosis is needed for the growth of multicellular organisms and for repairing damaged tissues. It is also a method of asexual reproduction.
  • Mitosis is one continous process, but is described as a series of division stages- prophase, metaphase, anaphase and telophase.
  • Interphase comes before mitosis. Its when cells grow and replicate for division.
  • The process::


  • The cell carries out its normal functions, but also prepares to divide.
  • The cell's DNA is unravelled and replicated, to double its genetic content.
  • The organelles are also replicated so it has spare ones, and ATP is increased.


  • The chromosomes condense, getting shorter and fatter.
  • Tiny bundles of protein called centrioles start moving to opposite ends of the cell, forming a network of protein fibres across it called the spindle.
  • The nuclear envelope breaks down, and chromosomes lie free in cytoplasm.
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Mitosis Continued.


  • The chromosomes line up along the middle of the cell, and become attached to the spindle by their centromere.
  • At the metaphase checkpoint, the cell checks that all chromosomes are attached to the spindle before mitosis can continue.


  • The centromeres divide, seperating each pair of sister chromatids.
  • The spindles contract, pulling chromatids to opposite ends of the cell, centromere first.


  • The chromatids reach the opposite poles on the spindle.
  • They uncoil and become long and thin again.
  • A nuclear envelope forms around each group of chromosomes, so there are two nuclei.
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Cytokinesis And Meiosis Intro.


  • The cytoplasm divides.
  • In animals, a cleavage furrow forms to divide the cell membrane. This leaves two daughter cells that are genetically identical to the original cell and to each other.
  • Cytokinesis usually begins in anaphase and ends in telophase.
  • It is a seperate process to mitosis.


  • In sexual reproduction, two gametes join together at fertilisation to form a zygote, this zygote then divides and develops into a new organism.
  • Meiosis is a type of cell division that happens in the reproductive organisms to produce gametes.
  • Meiosis involves a reduction division, cells beginning in meiosis have the full number of chromosomes, but the cells formed have half, these are called haploid cells.
  • Cells formed by meiosis are genetically different because each new cell ends up with a different combination of chromosomes.
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Meiosis Divisions.

  • The first division in meiosis is the reduction division, halving the chromosomes.
  • It also begins with interphase, in which the DNA unravels and replicates to produce sister chromatids.
  • Prophase I.
  • Chromosomes condense, getting shorter and fatter.
  • They then arrange themselves into homologous pairs and crossing over occurs.
  • Centrioles start moving to opposite cell ends, forming the spindle fibres.
  • Metaphase I.
  • Homologous pairs line up across centre, attach to spindle fibres by centromeres.
  • Anaphase I.
  • Spindles contract, seperating homologous pairs-one chromosome at each cell end.
  • Telophase I.
  • A nuclear envelope forms around each group of chromosomes.
  • Cytokinesis.
  • Occurs and two haploid daughter cells are produced.
  • Meiosis II.
  • Undergo stages similar to mitosis, anaphase II sister chromatids are seperated.
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Meiosis Conclusion.

  • During prophase I and meiosis I, homologous pairs of chromosomes come together and pair up.
  • The chromatids twist around each other and bits of chromatids swap over.
  • They still contain the same genes, but different allele combinations.
  • Crossing over of chromatids: means that each of the four daughter cells formed from meiosis contains chromatids with different alleles.
  • Independent assortment of chromosomes: each homologous pair is made up of one chromosome from each parent.
  • When pairs line up in metaphase I and are seperated in anaphase I, it is random which chromosome from which pair ends up in which daughter cell.
  • So the four daughter cells produced by meiosis have different combinations of maternal and paternal chromosomes.
  • This is independent assortment of chromosomes.
  • This 'shuffling' leads to genetic variation in any potential offspring.
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Stem Cells- Specialised And Unspecialised Cells.

  • Multicellular organisms are made up of many cells that are specialised for their function.
  • These specialised types orginally came from stem cells.
  • Stem cells are unspecialised cells- they can develop into different types of cell.
  • All multicellular organisms have some form of a stem cell.
  • In humans, they are found in early embryos and in adults.
  • Stem cells divide to become new cells, which then become specialised.
  • The process by which a cell becomes specialised for its job is differentiation.
  • In animals, adult cells are used to replace damaged cells.
  • Plants are always growing, so stem cells are needed to make new shoots and roots.
  • Stem cells are also able to divide to produce more undifferentiated stem cells.
  • Cells in the bone marrow differentiate into blood cells.
  • Cells in the meristems differentiate into xylem and phloem.
  • Stem cells can develop into different specialised cell types, so scientists think they could be used to replace damaged tissue in a range of diseases e.g. parkinsons.
  • Stem cells are also used by scientists researching developmental biology, they hope this will help them understand things like developmental disorders and cancer.
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Animal And Plant Cells To Know.

  • Neutrophils defend the body against disease. Their flexible shape allows them to engulf foreign particles or pathogens. The many lysosomes in their cytoplasm contain digestive enzymes to break down engulfed particles.
  • Erythrocytes carry oxygen in the blood. The biconcave shape provides a large surface area for gas exchange. They have no nucleus, so more room for haemoglobin.
  • Epithelial cells cover the surface of organs. They are joined by interlinking cell membranes and a membrane at their base. Some have cilia that beat to move particles away. Some are very thin to allow for efficient gas diffusion.
  • Sperm cells have a flagellum so they can swim to the egg. They also have lots of mitochondria to provide energy to swim, it also has digestive enzymes to penetrate the surface of eggs.
  • Palisade mesophyll cells contain chloroplasts, so absorb a lot of sunlight. They also have thin walls, so carbon dioxide can easily diffuse into them.
  • Root hair cells absorb water and mineral ions from soil. Also, have a large surface area for absorbtion and a thin permeable cell wall, for entry of water and ions.
  • Guard cells take up water and become turgid. They have thin outer walls and thicker inner walls, which force them to bend outwards, opening the stomata.
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Animal And Plant Tissues To Know.

  • tissue is a group of cells that are specialised to work together to carry out a particular function. A tissue can contain more than one cell type.

Squamous Epithelium a single layer of flat cells that line the surface. They are found primarily on the alveoli in the lungs.

Ciliated Epithelium covered in cilia, found on the surface where things are moved.

Muscle Tissue is made up of bundles of elongated cells called muscle fibres. There are three types: smooth, cardiac, and skeletal. 

Cartilage a connective tissue found in the joints. It is used for shape and support.

Xylem Tissue is a plant tissue with two jobs, it transports water around the plant and supports it. It contains hollow xylem vessels, which are dead, and living parenchyma cells.

Phloem Tissue transports sugars around the plant. Its arranged in tubes and is made up of sieve cells, companoon cells, and some ordinary plant cells. 

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  • organ is a group of different tissues that work together to perform a particular function.
  • The lungs- they contain squamous epithelial tissue and ciliated epithelial tissue. they also have elastic connective tissue and vascular tissue.
  • The leaves- they contain palisade tissue, as well as epidermal tissue, and xylem and phloem.
  • Organs work together to form an organ system- each system has a particular function.
  • The respiratory system is made up of all the organs, tissues and cells involved in breathing.
  • The circulatory system is made up of the organs involved in blood supply.
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