Excretion questions

Excretion syllabus

Questions on excretion can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

• 2.70 explain the origin of carbon dioxide and oxygen as metabolic waste products in plants, and their loss through leaf stomata.
• 2.71 know the excretory products of the lungs, kidneys and skin.
• 2.72B explain the kidney’s roles in excretion and osmoregulation.
• 2.73B describe the kidneys, ureters, bladder and urethra.
• 2.74B describe a nephron: Bowman’s capsule, glomerulus, convoluted tubules, loop of Henle and collecting duct.
• 2.75B explain ultrafiltration in Bowman’s capsule and describe the glomerular filtrate.
• 2.76B explain reabsorption of water from the collecting duct.
• 2.77B explain selective reabsorption of glucose at the proximal convoluted tubule.
• 2.78B describe how ADH regulates the water content of the blood.
• 2.79B know that urine contains water, urea and ions.

How to answer excretion questions

1. Trace the named substance through the relevant organs, tissues and cells in the correct order.
2. Link structural adaptations to diffusion distance, surface area, concentration gradient or transport rate.
3. Use balanced biological equations where required and distinguish respiration from breathing.
4. When explaining a data trend, quote evidence and connect it to the underlying biological process.

Excretion example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Reading the number of glomeruli from a graph

Adapted from June 2019 (R), Paper 2B · Question 3(b)

Question 1

The diagram shows part of a kidney nephron.

The graph shows the effect of age on the number of glomeruli in the kidneys.

Using the graph, determine the age of a person with 1 600 000 glomeruli in their kidneys.

age = years

Final answer

Age = 65 years.

Mark scheme points

1. M1 Read an age between 64 and 66 years from the graph.

Explanation

Find 1.6 million on the glomeruli axis, then read across to the curve and down to the age axis. This gives approximately 65 years.

Common mistakes

• The examiner report noted that almost all candidates read the graph correctly as 65 years. To check your reading, use 1.6 million on the glomeruli axis first, then read the corresponding age on the horizontal axis.

Example 2: Explaining a diuretic’s effect on urine production

Adapted from June 2016 (R), Paper 1B · Question 6(b)

Question 2

The photograph shows a flower called a dandelion.

If a person picks this flower and then licks their fingers, they will want to urinate. This is because the plant produces a chemical called a diuretic that affects the regulation of the water content of the blood.

Suggest how this diuretic causes more urine to be produced.

Final answer

The diuretic enters the blood and affects the hypothalamus or pituitary, causing less ADH to be released. The collecting ducts then become less permeable to water, so less water is reabsorbed into the blood and more urine is produced.

Mark scheme points

1. M1 The diuretic enters or travels in the blood.
2. M2 It affects the hypothalamus or pituitary, resulting in less ADH.
3. M3 The collecting duct is affected.
4. M4 The collecting duct becomes less permeable to water.
5. M5 Less water is reabsorbed, so less water enters the blood.

Explanation

The diuretic must reach the blood to affect the control of water balance. It reduces ADH release from the hypothalamus–pituitary system. ADH normally increases the collecting ducts’ permeability to water. With less ADH, less water passes from the collecting ducts back into the blood, so more water is excreted in the urine.

Common mistakes

• Do not reverse the effect: less ADH makes the collecting ducts less permeable, not more permeable.
• Saying only that more urine is produced does not explain the mechanism; references to urine alone are ignored by the mark scheme.

Practise Excretion questions

Build a focused practice set from this part of the Edexcel IGCSE Biology syllabus.