CCEA A Level Biology 4.1 Homeostasis Exam Questions

About CCEA A Level Biology 4.1 Homeostasis

Section 4.1 opens Unit A2 1 with homeostasis, and CCEA uses the kidney as its main example. You need the principles of homeostasis (receptor, corrective mechanism and negative feedback), the gross structure of the kidney and the detailed structure of the nephron, including podocytes, the proximal convoluted tubule, the loop of Henle and the collecting duct. You must explain ultrafiltration in terms of pressures, selective reabsorption, how the loop of Henle creates a salt gradient in the medulla, and osmoregulation by ADH. This builds on water potential and active transport from AS 1. Long answer questions on the nephron are very common.

What you need to know

• Principles Homeostasis is keeping the internal environment constant. A receptor detects a change, a corrective mechanism responds, and negative feedback switches off the response as the value returns to normal, preventing over-correction.
• Kidney and nephron Gross structure: cortex, medulla, pyramids, pelvis, ureter, bladder, urethra. Nephron: Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule and collecting duct. The kidney removes urea and creatinine and controls the water potential of body fluids.
• Ultrafiltration The afferent arteriole is wider than the efferent, giving high hydrostatic pressure in the glomerulus. Fluid is forced through the squamous endothelium, basement membrane (the actual filter) and podocytes. This is opposed by the low solute potential of the blood and the back pressure of filtrate. Blood cells and large proteins stay in the blood.
• Selective reabsorption In the proximal tubule, all glucose and amino acids and much salt are reabsorbed by active transport, helped by microvilli, many mitochondria and basal invaginations. Water follows by osmosis, returning to the blood mainly via the vasa recta.
• Loop of Henle The descending limb is permeable to water; the ascending limb is impermeable to water and moves sodium and chloride ions out. This creates a high salt concentration (low water potential) in the medulla, so water can be withdrawn from the collecting duct.
• ADH and feedback Osmoreceptors in the hypothalamus detect low blood water potential. ADH, made in the hypothalamus and released from the posterior pituitary, increases permeability of the distal tubule and collecting duct to water, so more is reabsorbed and urine is concentrated. As water potential returns to normal, ADH release falls.

Common mistakes in 4.1 Homeostasis

• Saying the glomerulus filters by active transport. Ultrafiltration is driven by high hydrostatic blood pressure.
• Saying ADH makes the kidney produce more urine. ADH increases water reabsorption, giving a smaller volume of more concentrated urine.
• Mixing up which limb of the loop of Henle is permeable to water. The descending limb is permeable; the ascending limb is not.
• Calling the basement membrane a layer of cells. It is a non-cellular layer of glycoprotein and is the main filter.
• Saying glucose is filtered out of the blood because it is a waste. Glucose is filtered because it is small, then fully reabsorbed in the proximal tubule.

Try three 4.1 Homeostasis exam-style questions