AQA A-Level Biology: Osmoregulation and Nephron Function — mark scheme explained

The question

Explain how the hypothalamus and posterior pituitary gland work together to maintain the water potential of blood. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Step 1: Identify the role of osmoreceptors in the hypothalamus.

• S2

  Osmoreceptors detect changes in the water potential of the blood.

• S3

  Step 2: Describe what happens when the blood becomes too concentrated (low water potential).

• S4

  When the blood is too concentrated, osmoreceptors are activated, triggering the release of ADH from the posterior pituitary gland.

• S5

  Step 3: Explain the effect of ADH on the kidney.

• S6

  ADH acts on the distal convoluted tubule and collecting ducts to increase water reabsorption, diluting the blood and restoring its water potential to normal levels.

• S7

  Step 4: Describe what happens when the blood is too dilute (high water potential).

• S8

  When the blood is too dilute, less ADH is released, leading to decreased water reabsorption and more dilute urine.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Step 1: Identify the role of osmoreceptors in the hypothalamus.

2. S2

   Osmoreceptors detect changes in the water potential of the blood.

3. S3

   Step 2: Describe what happens when the blood becomes too concentrated (low water potential).

4. S4

   When the blood is too concentrated, osmoreceptors are activated, triggering the release of ADH from the posterior pituitary gland.

5. S5

   Step 3: Explain the effect of ADH on the kidney.

6. S6

   ADH acts on the distal convoluted tubule and collecting ducts to increase water reabsorption, diluting the blood and restoring its water potential to normal levels.

7. S7

   Step 4: Describe what happens when the blood is too dilute (high water potential).

8. S8

   When the blood is too dilute, less ADH is released, leading to decreased water reabsorption and more dilute urine.

9. Final answer: The hypothalamus contains osmoreceptors that detect changes in blood water potential. When the blood becomes too concentrated, these receptors trigger the release of ADH from the posterior pituitary gland. ADH increases water reabsorption in the kidney, diluting the blood and restoring its water potential to normal levels. Conversely, when the blood is too dilute, less ADH is released, leading to decreased water reabsorption and more dilute urine.

Common mistakes

• Confusing the roles of the hypothalamus and posterior pituitary gland in osmoregulation. — Remember that the hypothalamus contains osmoreceptors that detect changes in blood water potential, while the posterior pituitary gland releases ADH in response to these changes.
• Misunderstanding how ADH affects water reabsorption in the kidney. — ADH increases the permeability of the distal convoluted tubule and collecting ducts to water, leading to more water reabsorption and more concentrated urine.
• Confusing the functions of the descending and ascending limbs of the loop of Henle. — The descending limb is permeable to water but not solutes, while the ascending limb is permeable to solutes but not water. This creates a concentration gradient in the medulla.
• Failing to explain how the loop of Henle contributes to the concentration gradient in the medulla. — Practice explaining that the descending limb reabsorbs water, making the filtrate more concentrated, while the ascending limb actively transports solutes out of the filtrate into the medulla, further concentrating the filtrate.
• Misidentifying the key structures of the nephron and their functions. — Review the structure of the nephron, including the glomerulus, Bowman's capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting ducts. Understand the specific functions of each part.
• Failing to describe the process of glomerular filtration in detail. — Practice explaining that blood enters the glomerulus under high pressure, forcing water and small molecules out into Bowman's capsule. Large molecules like proteins and cells are retained in the blood.

Where the marks go

• Full worked solution (all marking points)4 marks

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