Organismal Biology
Organismal Biology tests how whole organisms maintain a suitable internal environment: transport, gas exchange, excretion, control and reproduction in plants and mammals. Marks come from linking each structure explicitly to its function rather than describing anatomy alone.
Transport in plants and mammals
- Root hair cells give a large surface area; water moves by osmosis through the cortex via the apoplast and symplast, and the Casparian strip forces the symplastic route.
- Cohesion–tension: transpiration pull, cohesion between water molecules and adhesion to lignified xylem walls move water and mineral ions upward.
- Xylem is dead, lignified, with no end walls; phloem has sieve tube elements and companion cells and translocates sucrose from source to sink by mass flow.
- Mammals have double circulation; the left ventricle wall is thicker; the SA node paces the heart; arteries, veins and capillaries each suit their role.
Gas exchange and excretion
- Alveoli: large total surface area, thin walls, moist surface, rich blood supply; ventilation maintains the diffusion gradient and surfactant prevents collapse.
- Fish gills use countercurrent flow, keeping a diffusion gradient along the whole gill.
- Kidney: ultrafiltration at the glomerulus under pressure; selective reabsorption of glucose, amino acids, most water and ions in the proximal convoluted tubule; the loop of Henle builds a salt gradient; ADH raises collecting-duct permeability to water.
Control and reproduction
- Nervous control: action potential, depolarisation, synaptic transmission by neurotransmitters, reflex arc.
- Hormonal control by negative feedback, for example insulin lowering and glucagon raising blood glucose.
- Menstrual cycle: FSH stimulates the follicle, oestrogen triggers the LH surge and ovulation, progesterone maintains the uterine lining.
Worked example
After a long match a player sweats heavily, losing water but little salt, so blood water potential falls. Osmoreceptors in the hypothalamus detect this; the posterior pituitary releases more ADH; collecting ducts become more permeable to water; more water is reabsorbed into the blood; urine volume falls and urine is more concentrated; blood water potential returns to normal by negative feedback. Drinking two litres of water reverses the sequence, so urine output rises.
Traps
- Saying xylem carries sucrose or phloem carries water.
- Describing transpiration as active transport.
- Confusing ADH with aldosterone, or omitting the direction of change.
- Forgetting that the loop of Henle creates the medullary salt gradient.