Organismal Biology and Physiology
Organismal Biology and Physiology examines how multicellular organisms maintain internal stability, exchange materials, coordinate responses, and defend themselves. The exam favors questions that trace a pathway or predict the effect of disrupting a feedback loop, so focus on cause-and-effect sequences rather than isolated organ facts.
Homeostasis and feedback
- Negative feedback reverses a deviation and is the dominant regulatory mode.
- Positive feedback amplifies a change and is usually self-limiting, as in childbirth contractions.
- Thermoregulation, blood glucose control, and osmoregulation are standard feedback examples.
Transport and gas exchange
- Mammals use a four-chambered heart; arteries carry blood away from the heart, veins return it.
- Fish gills use countercurrent exchange, where water and blood flow in opposite directions, maximizing oxygen transfer.
- Insects use tracheal systems; plants exchange gases through stomata and move water through xylem and sugars through phloem.
Neural and endocrine control
- A resting potential depends on K⁺ permeability; an action potential results from Na⁺ influx followed by K⁺ efflux.
- Hormones act on target cells with specific receptors; the hypothalamus and pituitary form a key control axis.
Immunity
- Innate immunity is fast and nonspecific; adaptive immunity is slower and specific.
- Antibody-mediated immunity targets extracellular pathogens; cell-mediated immunity targets infected cells.
Worked example
A person loses 1 L of hypotonic sweat. Blood osmolarity rises. Osmoreceptors in the hypothalamus detect the increase, triggering ADH release from the posterior pituitary. ADH increases water reabsorption in the collecting ducts, producing more concentrated urine and restoring osmolarity. Thirst also increases water intake. The response is negative feedback because it reverses the initial rise in osmolarity.
Traps that cost marks
- Reversing xylem and phloem direction; xylem carries water upward, phloem carries sugars source to sink.
- Claiming arteries always carry oxygenated blood; the pulmonary artery carries deoxygenated blood.
- Confusing negative feedback with positive feedback.
- Treating ADH as a salt-regulating hormone; it primarily increases water reabsorption.
- Ignoring that countercurrent exchange requires opposite flow directions.