The structure of the nephron and its role in: the formation of glomerular…

The idea

High hydrostatic pressure in the glomerulus forces water and small solutes into the Bowman's capsule to form filtrate. In the proximal convoluted tubule, sodium-potassium pumps create a gradient for the co-transport of sodium ions and glucose into epithelial cells, followed by water via osmosis. The loop of Henle acts as a counter-current multiplier maintaining a sodium ion gradient in the medulla. Its ascending limb actively transports sodium ions out but is impermeable to water, lowering the medulla's water potential. The descending limb is permeable to water, which exits by osmosis. Finally, water is reabsorbed by osmosis from the distal convoluted tubule and collecting duct down this water potential gradient, producing concentrated urine.

What earns the marks

• High hydrostatic pressure in the glomerulus forces small molecules through the basement membrane to form glomerular filtrate.
• Glucose is reabsorbed in the proximal convoluted tubule by co-transport with sodium ions.
• The ascending limb of the loop of Henle is impermeable to water and actively transports sodium ions into the medulla, lowering its water potential.
• The loop of Henle acts as a counter-current multiplier, maintaining a steep water potential gradient in the medulla.
• Water leaves the distal convoluted tubule and collecting duct by osmosis down a water potential gradient.

Common mistakes and how to put them right

• Writing 'sodium' rather than 'sodium ions' when describing the gradient in the medulla.

  Put it right: Specifying ions is necessary for scientific accuracy.

• Stating glucose is actively transported into the blood.

  Put it right: The active step is the sodium-potassium pump, while glucose enters by co-transport.

• Claiming water is pumped out of the collecting duct instead of moving passively by osmosis down a water potential gradient.

• Forgetting to state that the ascending limb of the loop of Henle is impermeable to water, which is crucial for establishing the medullary concentration gradient.

Exam tips

• Learn the sequence of the nephron (Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct) to accurately locate processes.
• When explaining the loop of Henle, explicitly use the term 'counter-current multiplier' and state that the ascending limb is impermeable to water.
• When explaining ADH action, detail the sequence: aquaporins inserted into the membrane, increased permeability, then more water reabsorbed by osmosis.