Draw a labelled diagram to show the structure of membranes.

Explain passive transport and active transport across membranes.

Award [1] for each structure clearly drawn and correctly labelled.
phospholipid bilayer – with head and tails;
hydrophilic / phosphate / polar heads and hydrophobic / hydrocarbon / fatty acid / non-polar tails labelled;
integral/intrinsic protein – embedded in the phospholipid bilayer;
protein channel – integral protein showing clear channel/pore;
peripheral/extrinsic protein – on the surface;
glycoprotein with carbohydrate attached; (carbohydrate should project outwardly from membrane protein)
cholesterol – shown embedded in bilayer; (must appear in hydrophobic region)
thickness indicated (10 nm); (allow answers in the range of 7 nm to 13 nm)

diffusion/facilitated diffusion and osmosis are passive;
do not require energy/ATP;
diffusion is movement from high to low concentration/down a (concentration) gradient;
facilitated diffusion uses (protein) channels/carrier proteins;
osmosis is water movement from lower to higher solute concentration / from higher to lower water potential/concentration;
across a partially permeable membrane;
active transport/formation of vesicles require energy;
in the form of ATP;
active transport moves materials up/against the (concentration) gradient/from low to high concentration;
protein pumps required;
endocytosis into cells / exocytosis out of cells;

example of active or passive transport; (e.g. sodium potassium pump for active transport / oxygen exchange in alveoli for passive transport)

Candidates knew their hydrophilic heads and hydrophobic tails! Overall performance on this question was good. As noted earlier, improvement was seen in the quality of the drawings. However, some details needed to be more exact: peripheral/extrinsic protein should have appeared on the membrane surface, not fully embedded and flush with the surface; channel proteins, by definition, required a channel or pore.

Explanations of passive and active transport (A.S. 2.4.5, 2.4.6) involved many ideas that candidates seemed to know. Responses were generally well organized. Easy marks were gained for knowing which type of transport required ATP and for knowing details about different concentration gradients. Candidates did stumble when they confused protein pumps needed in active transport for protein channels used in facilitated diffusion. Some candidates also forgot that osmosis involves the movement of water molecules, not just “particles,” from lower to higher solute concentration gradients across semi-permeable membranes. Instead of explaining osmosis in terms of solute concentration, other candidates correctly wrote about movement of water molecules from higher to lower water potential.