OCR Biology Unit 2 & 3

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  • Created on: 29-04-13 13:15


A dipeptide is two amino acids joined together by a peptide bond. Many amino acids form a polypeptide chain. This forms a protein.

 Primary structure is the unique sequence of different amino acids in a polypeptide chain (different for each protein molecule).

Secondary structure is how the polypeptide chain folds: pleated (beta-sheet) or twisted (alpha-helix) and held by hydrogen bonds.

Tertiary structure is the 3D shape of a protein; quaternary is several polypeptides in the protein molecule (e.g. four in haemoglobin).

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C6H12O6 is the formula for glucose (a and b forms). The six carbon atoms are arranged in a single sugar ring (a monosaccharide).

Two monosaccharides can be joined by a glycosidic bond (in a condensation reaction) to form a disaccharide (maltose, sucrose).

Many monosaccharides form a polysaccharide – e.g. coiled chains in starch (amylose), straight in cellulose and branching in glycogen.

 Starch and glycogen are compact chains of glucose so are good for energy storage. Straight cellulose forms strong fibres (in cell walls).

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Lipids and Identification

Lipids: mainly triglycerides made up of a glycerol head joined to three fatty acid (hydrocarbon) chains; insoluble, good for insulation.

Phospholipids: have one fatty acid replaced by a phosphate group. The phosphate is hydrophilic and the fatty acids are hydrophobic.

Identification: proteins by the biuret test (blue to purple); starch by iodine (brown to blue-black); lipids by emulsion test (goes cloudy).

 Benedict’s test: blue to orange or green for reducing sugars (mono and some disaccharides). Non-reducing sugars (sucrose) are boiled with acid.

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Nucleic Acids

Deoxyribonucleic acid (DNA) is a molecule with two chains (strands) of nucleotides (polynucleotides) twisted in a double helix.

DNA nucleotide: made up of a sugar (deoxyribose); a phosphate group; and a base (either Adenine, Guanine, Cytosine or Thymine).

A on one strand always pairs with T on the opposite strand and G always pairs with C. This is called complementary base pairing.

 The order of bases in a section of DNA codes for the sequence of amino acids in a polypeptide – this DNA section is a gene

  DNA can copy itself (replicate) by using (conserving) each strand to make a new one by base pairing, forming two new DNA molecules.

 RNA is like DNA but is a single strand. Its nucleotides have ribose sugar (not deoxyribose) and Uracil replaces Thymine as a base.

DNA is a store of information (in the nucleus) used to make all the proteins (e.g. enzymes) needed to control cell activities.

Messenger RNA is a copy of the DNA sequence which is used in the cytoplasm (with other types of RNA) to make proteins.

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Enzymes speed up (catalyse) metabolic reactions by lowering activation energy. Some catalyse reactions inside cells (intracellular); others are secreted (extracellular).

They are globular proteins with a unique 3D (tertiary) shape. Part of this is an active site which only fits a specific substrate molecule.

 Lock and key hypothesis: a key fitting a lock (as above). Induced fit hypothesis: the active site changes shape to fit the substrate.

 An enzyme-substrate complex forms. Bonds are made or broken and an enzyme-product complex forms. Product is then released.

 At low temperatures there is less kinetic energy and enzymes are slower. At high temperatures enzymes can be denatured and stop working.

 A too-high or too-low pH alters the enzyme shape and slows reaction; there is an optimum pH and temperature at which an enzyme works best.

Increasing substrate or enzyme concentration speeds up the reaction. It reaches a maximum rate; further increase has no effect.

Competitive inhibitors act like the substrate and slow an enzyme. Non-competitive inhibitors change the shape of an enzyme.

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Diet and Nutrition

 A balanced diet has seven components. If these are unbalanced malnutrition can occur (obesity from too much energy-rich food).

Plant material (fruit, vegetables, rice, cereal) is essential in a diet. All food chains, including human ones, start with plants.

Too much saturated fat (e.g. in meat, dairy foods) can lead to coronary heart disease (coronary arteries blocked by cholesterol).

 Low Density Lipoproteins (small fat/protein drops in blood) carry cholesterol which can stick to artery walls causing blockages.

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Food Production

Selective breeding is only using organisms which have the required characteristics – high milk yield in cows or disease resistance in crops.

Fertilisers improve crop growth. Pesticides improve yield by reducing disease. Antibiotics slow spread of disease (e.g. in cattle).

Microorganisms grown rapidly for food production (e.g. yeast in bread making). Mycoprotein is made from fungus but may be unpalatable.

 Food spoilage by microorganisms can be prevented by heating (cooking, pasteurising), adding sugar/salt, freezing or irradiation.

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Health and Disease

Parasite: lives in/on a host and causes harm (e.g. Plasmodium). Pathogen: a disease-causing organism (e.g. Mycobacterium, HIV).

Malaria: caused by Plasmodium and spread by mosquitoes (the vector). TB: caused by Mycobacterium and spread by coughs.

HIV/AIDS: caused by Human Immunodeficiency Virus and spread by exchange of body fluids (e.g. during unprotected sex, sharing needles).

 These diseases have global impact (they affect millions of people).  Malaria affects over 200 million people; HIV and TB are spreading.

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Smoking and Disease

Tar from smoking: narrows airways; paralyses cilia; causes inflammation and coughing (bronchitis); and increases risk of infection.

Alveoli lose elasticity, burst and surface area is reduced (emphysema). Carcinogenic chemicals in tar cause lung cancer.

Nicotine in blood causes clots and stroke. CO causes atherosclerosis (damage to arteries) and coronary heart disease (CHD).

 Tropical plants (high biodiversity), and using DNA to show how bacteria cause disease, may result in the development of new medicines.

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The immune response recognises a pathogen by its surface molecules (antigens) and makes specific antibodies.

Antibodies are large blood proteins with a specific ’Y’ shape. Part of this fits antigen molecules of a specific pathogen and attaches.

Attached antibodies can neutralise (orkill) pathogens, e.g. by causing bacteria to clump together (agglutination).

 Antibodies are produced by B-cell lymphocytes (white blood cells). T-cell lymphocytes attack pathogens directly.

 Memory cells remain in the blood after an infection. They recognise the pathogen if it re-infects for a faster immune response.

 Other white blood cells, phagocytes, kill pathogens by engulfing and ingesting them (phagocytosis).

Artificial immunity: injection of a weakened pathogen to produce memory response; or injecting antibodies (no memory cells).

Natural immunity: infection by pathogen (causes immune memory to protect next time); or a mother can transfer antibodies to a baby.

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A species is a group of organisms with similar characteristics which can interbreed and produce fertile offspring.

A habitat is the place where an organism lives. Biodiversity is the different habitats, species and genetic variation in an ecosystem.

Biodiversity can be measured by random sampling – counting the number of species using quadrats instead of in the whole habitat.

 Recording the number of species is known as species richness. Recording how many of each species is species evenness.

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Classification: sorting organisms into groups (e.g. phylum, class).

Taxonomy: studying characteristics of organisms so they can be classified.

Phylogeny: how organisms are related in evolution.

1 Prokaryotes: single celled; have no nucleus (naked DNA).

2 Fungi: cells walls of chitin; have hyphae which form a mycelium.

 3 Plants: all multicellular; photosynthesise; cell wall of cellulose.

4 Animals: multicellular; heterotrophic; can locomote.

5 Protoctists: organisms that don’t fit into above groups (e.g. algae).

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Variation is the differences between individual organisms. Variation occurs between different species and within the same species.

Continuous variation is when there are two extremes and a full range of values in between (e.g. weight or height in humans).

Discontinuous variation is when there are distinct types with no values in between (e.g. male or female, early- or late-flowering).

Darwin observed: offspring are similar to parents; no two individuals are identical; organisms can overproduce; population size is stable.

Darwin proposed: there is a struggle to survive; organisms with the best adaptations survive better and pass on their characteristics.

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The frequency (percentage) of these adaptations (variants) in a population increase – they have been selected (natural selection).

In separate populations selected adaptations may be so different that they cannot interbreed forming a new species (speciation).

Insects and bacteria reproduce rapidly; the fast pesticide- and drug-resistance evolution makes it hard to find new pesticides or drugs.

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Extinction is when a species ceases to exist. Human activities have increased extinction (e.g. due to hunting and habitat destruction).

Species conservation allows us to learn new technology from how organisms function. It is important to the environment.

Climate change can alter rainfall patterns. This could affect crop growth or cause diseases (e.g. malaria) to occur in new areas.

 Biodiversity allows the breeding or cultivating of new varieties of plants/animals which can survive diseases or changing habitats.

Conservation in situ is conserving a species in its habitat. This also protects the habitat but humans may want to use the land (conflict).

 Conservation of endangered species ex situ is conserving a species outside its habitat (e.g. breeding endangered species in zoos, collecting seeds of rare/extinct plant species in seed banks).

 Although breeding species in zoos protects from extinction, they are not in their own habitat and might not survive in the wild.

Nations cooperate with laws about conservation (e.g. CITES (Convention in Trade in Endangered Species).

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