Coordination in Life Processes
This domain tests how digestion, respiration, transport and excretion are coordinated, and how nerves and hormones control the body through reflex arcs and feedback loops.
Digestive coordination
Digestion is enzymatic and pH-dependent. Salivary amylase acts on starch at about pH 6.8; stomach pepsin needs the acidic conditions provided by HCl; pancreatic trypsin and lipase need the alkaline conditions provided by bile and pancreatic bicarbonate. Peristalsis moves food along, villi of the small intestine give a huge surface for absorption, and the large intestine reabsorbs water.
Transport, respiration and excretion
Alveoli provide a thin, moist, richly supplied surface for gas exchange. The human heart has four chambers; arteries carry blood away from the heart, veins towards it, and valves prevent backflow. The nephron filters blood and reabsorbs useful materials, leaving urine.
Nervous and hormonal control
A neuron carries impulses, and at a synapse a chemical transmitter passes the signal to the next neuron. A reflex arc - receptor, sensory neuron, spinal cord, relay neuron, motor neuron, effector - gives a quick response without conscious thought. Hormones act more slowly: insulin from the pancreas lowers blood glucose, glucagon raises it, and thyroxine controls metabolic rate. Feedback keeps levels steady.
Worked example
After a rice meal, blood glucose rises. The pancreas detects this and secretes insulin, which increases glucose uptake by cells and glycogen storage in the liver, so blood glucose falls back to normal. This is negative feedback: the response cancels the initial change.
Traps
Pepsin does not work in alkaline conditions; a reflex action is not decided by the brain, though the brain is informed; hormones travel in the blood, not along nerves; enzymes are specific to their substrates.