CIE A Level Biology Topic 14: Homeostasis Exam Questions

About CIE A Level Biology Topic 14: Homeostasis

Homeostasis in the Cambridge 9700 syllabus explains how internal conditions are kept within narrow limits by negative feedback. In mammals you need to know the formation of urea in the liver, the structure of the kidney and nephron, ultrafiltration in the glomerulus, selective reabsorption in the proximal convoluted tubule, the role of the loop of Henle and the control of water potential by ADH and aquaporins. Blood glucose control by insulin and glucagon, including the second messenger cyclic AMP, and the use of test strips and biosensors are also included. In plants you need to explain how guard cells open and close stomata and the role of abscisic acid.

What you need to know

• Negative feedback A receptor detects a change from the set point, a coordination centre processes the information and effectors bring about a response that returns the factor to the set point.
• Excretion Excess amino acids are deaminated in the liver; the amino group forms ammonia, which is converted to urea in the ornithine cycle. Urea is removed by the kidneys.
• Ultrafiltration and reabsorption High hydrostatic pressure in the glomerulus forces small molecules through the endothelium, basement membrane and podocytes into the Bowman's capsule. In the proximal convoluted tubule, glucose, amino acids and most water are reabsorbed using cotransport and microvilli.
• Loop of Henle and ADH The loop of Henle creates a low water potential in the medulla. ADH is released from the posterior pituitary when blood water potential falls; it causes insertion of aquaporins into collecting duct cells so more water is reabsorbed.
• Blood glucose Insulin from beta cells increases glucose uptake via GLUT4 and glycogenesis. Glucagon from alpha cells binds receptors on liver cells, activating cyclic AMP and an enzyme cascade that causes glycogenolysis and gluconeogenesis.
• Stomata Guard cells open stomata when K+ is pumped in, lowering water potential so water enters and the cells become turgid. Abscisic acid causes stomatal closure during water stress.

Common mistakes in Topic 14: Homeostasis

• Saying the kidney filters out urea only; ultrafiltration removes all small molecules, and useful ones are reabsorbed.
• Writing that ADH makes the kidney produce more urine; ADH increases water reabsorption so less, more concentrated urine is produced.
• Confusing glucagon with glycogen; glucagon is the hormone, glycogen is the storage polysaccharide.
• Stating that insulin converts glucose to glycogen itself; insulin activates enzymes in liver cells that do this.
• Describing negative feedback without returning the factor to the set point.

Try three Topic 14: Homeostasis exam-style questions