Cambridge AS and A Level Biology (9700) Syllabus

Why are viruses described as acellular and as obligate parasites?

They have no cell structure (no cytoplasm, membranes or organelles) and cannot reproduce on their own; they must use a host cell's machinery to replicate.

What type of genetic material does HIV contain and what enzyme does it carry?

HIV contains RNA and carries the enzyme reverse transcriptase, which makes DNA from its RNA genome.

State the general formula of a carbohydrate.

Cx(H2O)y, i.e. the ratio of hydrogen to oxygen is 2:1 (as in water).

Name the bond formed between two monosaccharides and the reaction that forms it.

A glycosidic bond, formed by a condensation reaction (with the loss of a water molecule).

Give the monomers that make up maltose, sucrose and lactose.

Maltose = glucose + glucose; sucrose = glucose + fructose; lactose = glucose + galactose.

Compare the structure of amylose and amylopectin.

Both are made of alpha-glucose. Amylose has 1,4 glycosidic bonds forming an unbranched helix; amylopectin has 1,4 and 1,6 glycosidic bonds making it branched.

Explain how the structure of glycogen suits its function as a storage molecule.

It is a highly branched polymer of alpha-glucose, giving many ends for rapid hydrolysis to release glucose; it is compact and insoluble so it does not affect water potential.

Describe the structure of cellulose and how it relates to its function.

It is an unbranched polymer of beta-glucose with alternate molecules inverted; chains form straight microfibrils held by many hydrogen bonds, giving high tensile strength for plant cell walls.

What is the difference between alpha-glucose and beta-glucose?

They are isomers differing in the position of the hydroxyl (OH) group on carbon 1: below the ring in alpha-glucose and above the ring in beta-glucose.

Name the components and bond involved in forming a triglyceride.

One glycerol molecule and three fatty acids, joined by three ester bonds formed by condensation reactions.

Distinguish between a saturated and an unsaturated fatty acid.

A saturated fatty acid has no carbon-carbon double bonds (straight chains); an unsaturated fatty acid has one or more C=C double bonds, causing kinks in the chain.

Explain why triglycerides are good energy stores.

They are highly reduced with many C-H bonds, releasing about twice the energy per gram of carbohydrate; they are insoluble so do not affect water potential, and yield metabolic water on oxidation.

Describe the structure of a phospholipid and how it behaves in water.

It has a glycerol backbone with two fatty acid tails (hydrophobic) and a phosphate-containing head (hydrophilic). In water it forms a bilayer with heads facing outward and tails inward.

State the general structure of an amino acid.

A central carbon bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable R (side) group.

Name the bond joining amino acids and the reaction that forms it.

A peptide bond, formed by a condensation reaction between the amino group of one amino acid and the carboxyl group of another, releasing water.

Define the primary structure of a protein.

The specific sequence and number of amino acids in a polypeptide chain, held together by peptide bonds.

Describe the secondary structure of a protein.

Regular folding of the polypeptide into alpha-helices or beta-pleated sheets, held in place by hydrogen bonds between the amino and carboxyl groups of the backbone.

List the four types of bond that stabilise the tertiary structure of a protein.

Hydrogen bonds, ionic bonds, disulfide bonds (covalent), and hydrophobic/hydrophilic interactions between R groups.

Compare a globular and a fibrous protein, giving one example of each.

Globular proteins are roughly spherical, soluble and metabolically active (e.g. haemoglobin, enzymes); fibrous proteins are long, insoluble and structural (e.g. collagen, keratin).

Describe the structure of haemoglobin.

A globular protein with quaternary structure made of four polypeptide chains (two alpha and two beta), each with a haem prosthetic group containing iron that binds one oxygen molecule.

Describe the structure of collagen and how it suits its function.

Three polypeptide chains wound into a triple helix held by hydrogen bonds, with staggered fibrils cross-linked covalently, giving high tensile strength for support.

Explain why water is described as a polar molecule.

Oxygen attracts the shared electrons more strongly than hydrogen, giving the oxygen a slightly negative charge and the hydrogens slightly positive charges, creating a dipole.

State three properties of water that result from hydrogen bonding and their biological importance.

High specific heat capacity (stable temperatures), high latent heat of vaporisation (cooling by evaporation), and cohesion/high surface tension (water transport in xylem).

Why is water described as a good solvent and why is this important?

Its polarity lets it dissolve ionic and polar substances, so it acts as a transport medium and a site for metabolic reactions in cells.

State one biological role each for the ions calcium (Ca2+), nitrate (NO3-) and magnesium (Mg2+).

Ca2+: cell wall formation/nerve and muscle function; nitrate: source of nitrogen for amino acids and proteins; Mg2+: component of chlorophyll.

Describe the biochemical test for reducing sugars and the positive result.

Add Benedict's solution and heat. A positive result changes the colour from blue to green/yellow/orange/brick-red precipitate.

How do you test for a non-reducing sugar such as sucrose?

First boil with dilute hydrochloric acid to hydrolyse it, neutralise with sodium hydrogencarbonate, then heat with Benedict's solution; a colour change indicates a non-reducing sugar was present.

State the test for starch, lipids and proteins with their positive results.

Starch: iodine solution turns blue-black. Lipids: emulsion test gives a white emulsion. Proteins: biuret test turns from blue to purple/lilac.

Explain the lock-and-key model of enzyme action.

The substrate has a complementary shape to the enzyme's active site and fits exactly like a key in a lock, forming an enzyme-substrate complex so the reaction can occur.

Explain the induced-fit model of enzyme action.

The active site is not initially an exact fit; as the substrate binds, the active site changes shape slightly to mould around the substrate, straining bonds and lowering activation energy.

How do enzymes speed up reactions?

They lower the activation energy needed for the reaction to proceed, by forming enzyme-substrate complexes.

Describe the effect of increasing temperature on enzyme activity.

Rate increases with temperature up to an optimum (more kinetic energy and collisions); above the optimum, hydrogen and other bonds break, the active site changes shape, the enzyme denatures and activity falls.

Describe the effect of pH on enzyme activity.

Each enzyme has an optimum pH; moving away from it alters charges and breaks bonds maintaining tertiary structure, changing the active site shape and reducing activity, eventually denaturing the enzyme.

Explain the effect of substrate concentration on the rate of an enzyme-controlled reaction.

Rate increases with substrate concentration until all active sites are occupied; beyond this point enzyme concentration becomes limiting and the rate plateaus.

Distinguish between a competitive and a non-competitive enzyme inhibitor.

A competitive inhibitor has a shape similar to the substrate and binds to the active site, blocking the substrate. A non-competitive inhibitor binds elsewhere (allosteric site), changing the active site's shape so the substrate no longer fits.

How can the effect of a competitive inhibitor be reduced, and why not for a non-competitive inhibitor?

Increasing substrate concentration outcompetes a competitive inhibitor and restores rate; for a non-competitive inhibitor increasing substrate has no effect because it does not bind at the active site.

What is end-product (feedback) inhibition?

The final product of a metabolic pathway acts as an inhibitor of an enzyme earlier in the pathway, switching off its own production to control reaction rate.

Describe the fluid mosaic model of the cell surface membrane.

A phospholipid bilayer in which proteins are scattered like a mosaic; it is 'fluid' because phospholipids and proteins can move within the layer.

State the functions of cholesterol in the cell surface membrane.

It fits between phospholipids to regulate membrane fluidity and stability, preventing it from becoming too fluid at high temperatures or too rigid at low temperatures, and reduces permeability to ions.

Distinguish between intrinsic (integral) and extrinsic (peripheral) membrane proteins.

Intrinsic proteins span the whole bilayer (e.g. channel and carrier proteins for transport); extrinsic proteins are on one surface only (e.g. receptors, enzymes, structural support).

State the roles of glycoproteins and glycolipids in the cell membrane.

Their carbohydrate chains act as receptors for cell signalling and as antigens for cell recognition, and help cells adhere to one another.