Outline four different processes, with examples, that allow substances to pass through the plasma membrane.

Humans need to balance water and solute concentrations and also excrete nitrogenous wastes. Explain how the different parts of the kidney carry out these processes.

Describe adaptations in mammals living in desert ecosystems to maintain osmolarity in their bodies.

1. simple/passive diffusion down a concentration gradient / from high concentration to low concentration (without the use of channels/proteins); (e.g., CO₂ / O₂ / H₂O / steroid hormones)
2. osmosis is the diffusion of water from an area of high water potential / low solute concentration to low water potential / high solute concentration;
3. facilitated diffusion is passive transport/diffusion through a protein channel; (e.g., glucose)
4. active transport requires energy/ATP to move the molecules through a protein channel (e.g., Na-K pump / sodium potassium pump) against a concentration gradient/from low solute concentration to high concentration;
5. endocytosis is the infolding of membranes to form a vesicle and take in a large molecule; (e.g., macrophages engulfing pathogens)
6. exocytosis is the fusion of vesicles with membranes to release a large molecule; (e.g. neurotransmitters)

1. humans are osmoregulators/maintain the internal concentrations of the blood/osmolarity within specific/ limited range / OWTTE;
2. glomerulus / Bowman’s capsule (in the nephron) carry out ultrafiltration;
3. proximal convoluted tubule selectively reabsorbs glucose/solute/salts/amino acids;
4. loop of Henle maintains hypertonic conditions in the medulla/absorbs salts (by active transport);
5. loop of Henle reabsorbs water (by osmosis);
6. (osmoreceptors in the hypothalamus) cause production of ADH if the blood is too concentrated / person is dehydrated / OWTTE;
7. ADH causes more uptake of water/increases permeability in the collecting duct;
8. resulting in a more concentrated urine / lower volume of urine;
9. excess amino acids are broken down producing nitrogenous waste / ammonia / urea as a result;
10. ammonia is toxic and is converted into non-toxic urea;
11. urea is eliminated in the urine;

Marks can be awarded to clearly annotated diagrams.

1. behavioural adaptations to avoid over-heating / hiding in burrows/out of sun during hot period of day / active at cooler times of the day/nocturnal animals / panting;
2. adaptations for heat exchange such as large ears;
3. may have longer loop of Henle (to reabsorb more water);
4. may produce more ADH (according to osmotic concentrations of the blood) / produce concentrated urine / lower volume of urine;
5. camel humps that store fat that releases (metabolic) water when broken down;
6. reduced sweat;
7. any other valid adaptation; (e.g., light coloured coats)

This question in section B was the second most commonly selected, but in performance, tended to do more poorly than the other questions.

Most candidates could list the processes that allow passage across the plasma membrane, but many answers lacked some of the necessary elements, most commonly leaving out examples (e.g. an example for gradient).

Generally, there were good accounts of the functioning of the nephron, but some elements were missing such as which substances are reabsorbed in the proximal tubule and which are absorbed in the loop of Henle, although permeability was mentioned. Discussion about the role of ADH was well done. The discussion of the management of nitrogenous wastes was least well done in this question.

Long loops of Henle was the most common desert adaptation discussed. There were common misconceptions about camels’ humps being water storage organs rather than the production of metabolic water.