Outline four types of membrane transport, including their use of energy.

Draw the structure of a dipeptide.

Explain the action of enzymes in digestion and the different roles of at least two named enzymes that are produced in the pancreas.

a. simple diffusion is passive movement of molecules/ions along a concentration gradient ✔

b. facilitated diffusion is passive movement of molecules/ions along a concentration gradient through a protein channel «without use of energy» ✔

c. osmosis is the passage of water through a membrane from lower solute concentration to higher ✔ OWTTE

d. active transport is movement of molecules/ions against the concentration gradient«through membrane pumps» with the use of ATP/energy ✔ Active transport requires mention of the use of energy.

e. endocytosis is the infolding of membrane/formation of vesicles to bring molecules into cell with use of energy
OR
exocytosis is the infolding of membrane/formation of vesicles to release molecules from cell with use of energy ✔

f. chemiosmosis occurs when protons diffuse through ATP synthase «in membrane» to produce ATP ✔

The description of each type of transport should include the name and brief description.

mpa, mpb and mpc require reference to concentration.

a. two amino acids, one with NH₂/NH₃+end and one with COOH/COO– end ✔

b. peptide bond between C=0 and N—H correctly drawn ✔

c. «chiral» C with H and R group on each amino acid ✔

d. peptide bond labelled/clearly indicated between C terminal of one amino acid and N terminal of the second amino acid ✔

Labels not required for amino group and carboxyl group.

a. enzymes catalyse/speed up chemical reactions/lower the energy needed ✔ OWTTE

b. have specific active sites to which specific substrates bind ✔

c. enzyme catalysis involves molecular motion and the collision of substrates with the active site ✔ OWTTE

d. enzymes break macromolecules into monomers/smaller molecules indigestion ✔

e. smaller molecules/monomers more readily absorbed ✔

f. <<pancreas>> secretes enzymes into the «lumen of» small intestine ✔

g. the small intestine has an alkaline pH ✔

h. enzymes have maximum action at specific pHs
OR
enzymes can be denatured at other pHs ✔

i. amylase breaks down starch into sugars/disaccharides ✔

j. lipase breaks lipids/triglycerides into monoglycerides/fatty acids and glycerol ✔

k. endopeptidase/protease breaks «peptide» bonds in proteins/polypeptides ✔

l. accept any other valid pancreatic enzyme, substrate and product ✔

Award [6 max] if there is no mention of two specific groups of enzymes.

Membrane transport – In spite of the question asking about energy, many lost easy marks by forgetting to refer to it. Others did not mention the gradients. In osmosis, please stress to the students which concentration they are talking about. They should be talking clearly about water potential or solute concentration.

Structure of the dipeptide – candidates who also do chemistry were at an advantage here. Diagrams went from the superb to something Picasso might have produced.

Enzymes in digestion. A disturbing number of candidates saw the word pancreas and wrote about insulin and glucagon. These are hormones, produced by ENdocrine glands not enzymes produced by EXocrine glands. Better candidates scored nearly full marks, by describing enzyme action and correctly describing the three main enzymes from the pancreas. Weaker candidates could not string the correct sequence together and scored very few marks. Better candidates wrote some sort of plan before starting, using the space below the instructions for Section B. This should be encouraged.