Outline the functions of rough endoplasmic reticulum and Golgi apparatus.

Outline the control of metabolism by end-product inhibition.

Explain how hydrophobic and hydrophilic properties contribute to the arrangement of molecules in a membrane.

a. ribosomes on RER synthesize/produce polypeptides/proteins ✔

b. proteins from RER for secretion/export/use outside cell/for lysosomes ✔

c. Golgi alters/modifies proteins/example of modification ✔

d. vesicles budded off Golgi transport proteins «to plasma membrane»
OR
exocytosis/secretion of proteins in vesicles from the Golgi ✔

Accept “for use inside and outside the cell” for mpb.

a. metabolism is chains/web of enzyme-catalyzed reactions
OR
metabolic pathway is a chain of enzyme-catalyzed reactions ✔

b. end product/inhibitor is final product of chain/pathway ✔

c. inhibits/binds to/blocks the first enzyme in chain/pathway ✔

d. non-competitive inhibition ✔

e. end-product/inhibitor binds to an allosteric site/site away from the active site ✔

f. changes the shape of the active site/affinity of the active site «for the substrate» ✔

g. prevents intermediates from building up
OR
prevents formation of excess «end» product/stops production when there is enough
OR
whole metabolic pathway can be switched off ✔

h. negative feedback ✔

i. binding of the end product/inhibitor is reversible
OR
pathway restarts if end product/inhibitor detaches/if end product concentration is low ✔

j. isoleucine inhibits/slows «activity of first enzyme in» threonine to isoleucine pathway ✔

Allow mark points shown in clearly annotated diagrams.

To gain mpd, mpe and mpf the answer must be in the context of end-product inhibition, not enzyme inhibition generally.

a. hydrophilic is attracted to/soluble in water and hydrophobic not attracted/insoluble ✔

b. hydrophilic phosphate/head and hydrophobic hydrocarbon/tail in phospholipids ✔

c. phospholipid bilayer in water/in membranes ✔

d. hydrophilic heads «of phospholipids» face outwards/are on surface ✔

e. hydrophobic tails «of phospholipids» face inwards/are inside/are in core ✔

f. cholesterol is «mainly» hydrophobic/amphipathic so is located among phospholipids/in hydrophobic region of membrane ✔

g. some amino acids are hydrophilic and some are hydrophobic ✔

h. hydrophobic «amino acids/regions of» proteins in phospholipid bilayer «core» ✔

i. hydrophilic «amino acids/regions of» proteins are on the membrane surface ✔

j. integral proteins are embedded in membranes due to hydrophobic properties/region
OR
transmembrane proteins have a hydrophobic middle region and hydrophilic ends ✔

k. peripheral proteins on are on the membrane surface/among phosphate heads due to being «entirely» hydrophilic
OR
«carbohydrate» part of glycoproteins is hydrophilic so is outside the membrane ✔

l. pore of channel proteins is hydrophilic ✔

Allow mark points shown in clearly annotated diagram.

In any part of the answer, accept polar instead of hydrophilic and non-polar or apolar instead of hydrophobic.

Most candidates had some knowledge and some had broad understanding of this topic. A common mistake was to think that the rough endoplasmic reticulum makes ribosomes. Many candidates stated that the Golgi apparatus packages proteins but they did not always mention that the packages are vesicles. If this was all that the Golgi did, vesicles from the rough ER could be used to secrete proteins. The role of the Golgi apparatus therefore involved processing or modification of proteins rather than just packaging of them.

Answers to this question were rather polarised. Candidates who had studied this topic and understood it had no difficulty in scoring high marks but other candidates struggled to include any useful ideas in their answers. Weaker answers tended to exclude the ideas of enzyme catalysis, pathways of reactions and the mechanisms involved in non-competitive enzyme inhibition. Diagrams to illustrate the process were a valuable part of some answers.

Candidates struggled to explain the meaning of the term hydrophobic. Many stated the literal meaning – water-fearing, hence repelled by water. This was not accepted as hydrophobic molecules aren’t repelled by water. They appear to be, because water is more attracted to polar or ionic substances than to apolar/hydrophobic substances, but there is no chemical mechanism for repulsion and of course molecules do not fear each other. Many focused only on phospholipids, but stronger answers also included information on proteins and how the positions they occupy within the membrane depend on their hydrophobic and hydrophilic properties.