Edexcel International GCSE Biology 4BI1: Excretion – Revision Notes

These revision notes cover sub-topic 2(i), Excretion (specification points 2.70–2.79), of the Pearson Edexcel International GCSE Biology (4BI1) specification, Issue 3. The qualification is untiered. Points 2.70 and 2.71 can appear on either paper; points 2.72B–2.79B are bold with a B reference and are Paper 2 only. They apply to the June and November series examined under Issue 3.

For full explanations and worked examples, read the excretion study guide first. Then test yourself with the excretion practice questions. The course hub is Edexcel IGCSE Biology and the full point-by-point list is on the printable checklist. For earlier Topic 2 content, see the cell structure revision notes.

Key definitions

• Excretion: removal of the waste products of metabolism (chemical reactions in cells) from the body.
• Egestion: removal of undigested food as faeces. Not excretion.
• Osmoregulation: control of the water content (and ion content) of the blood.
• Urea: a nitrogen-containing waste made in the liver from excess amino acids, carried in plasma.
• Urine: the liquid made by the kidneys – water, urea and ions.
• Ultrafiltration: filtration of small molecules out of the blood under high pressure, in the glomerulus.
• Selective reabsorption: taking particular useful substances, such as glucose, back into the blood from the nephron.
• Glomerular filtrate: the liquid that enters Bowman’s capsule.
• ADH: antidiuretic hormone, released by the pituitary gland; makes the collecting duct more permeable to water.

2.70 Plants: waste gases

Gas	Waste product of	When it leaves the leaf	Route out
Carbon dioxide	Respiration	In the dark (or dim light), when respiration is faster than photosynthesis	Diffusion through stomata
Oxygen	Photosynthesis	In bright light, when photosynthesis is faster than respiration	Diffusion through stomata

Respiration never stops. In bright light its carbon dioxide is used in photosynthesis instead of leaving.

2.71 Organs of excretion

Organ	Excretes
Lungs	Carbon dioxide (and water vapour)
Kidneys	Urea, excess water, excess ions – as urine
Skin	Water, ions and a little urea – as sweat

2.73B Urinary system (Paper 2 only)

Renal artery → kidney → ureter → bladder (stores urine) → urethra → outside. Cleaned blood leaves the kidney in the renal vein.

Memory hook: the ureter comes first (kidney to bladder); the urethra is the exit.

2.74B Nephron structure and jobs (Paper 2 only)

Part	Where	Job
Glomerulus	Cortex, inside the capsule	Knot of capillaries; blood at high pressure
Bowman’s capsule	Cortex	Collects glomerular filtrate (ultrafiltration)
Proximal convoluted tubule	Cortex	Reabsorbs all glucose, plus some water and ions
Loop of Henle	Dips into medulla	Makes medulla tissue concentrated (high ion concentration)
Distal convoluted tubule	Cortex	Adjusts ions reabsorbed
Collecting duct	Through the medulla	Water reabsorbed by osmosis; controlled by ADH

2.75B Ultrafiltration in steps (Paper 2 only)

1. Vessel into the glomerulus is wider than the vessel out.
2. So blood pressure in the glomerulus is high.
3. Small molecules are forced out through the capillary wall and capsule wall.
4. They form the glomerular filtrate in Bowman’s capsule.

In the filtrate	Not in the filtrate (too large)
Water, glucose, urea, ions, amino acids	Red and white blood cells, platelets, plasma proteins

2.77B Glucose reabsorption (Paper 2 only)

• Why: glucose is needed for respiration, so the body must not lose it.
• Where: proximal convoluted tubule. All of it, normally.
• How: including active transport, using energy from respiration – so the tubule cells have many mitochondria (and microvilli for surface area).
• Glucose in urine means the filtrate held more glucose than the tubule could take back, for example when blood glucose is very high in untreated diabetes.

2.76B and 2.78B Water reabsorption and ADH (Paper 2 only)

Method in steps – “explain how water is reabsorbed”:

1. Loop of Henle makes the medulla tissue concentrated.
2. Collecting duct passes through the medulla.
3. Water leaves the duct by osmosis into the tissue, then into the blood.
4. ADH sets how permeable the duct walls are, and so how much water leaves.

Blood water content	ADH	Collecting duct walls	Urine
Low (sweating, not drinking, salty food)	More	More permeable	Less volume, more concentrated
High (drinking a lot)	Less	Less permeable	More volume, more dilute

This is negative feedback, and body water content is an example of homeostasis (2.81).

2.72B and 2.79B Summary (Paper 2 only)

• Excretion role: removes urea (filtered, not reabsorbed).
• Osmoregulation role: controls blood water content by varying water reabsorption at the collecting duct.
• Urine contains: water, urea, ions. Healthy urine has no glucose and no protein.

Two calculations to know

Concentration factor: concentration in urine ÷ concentration in filtrate. It is bigger than 1 for urea because water is reabsorbed but urea is not.

Percentage of filtrate reabsorbed: (volume of filtrate − volume of urine) ÷ volume of filtrate × 100. Use the same units and the same time period for both volumes.

Small reminder: if filtrate is formed at 110 cm³ per minute and urine at 2.2 cm³ per minute, the percentage reabsorbed is (110 − 2.2) ÷ 110 × 100 = 98%.

Method in steps: reading a plasma, filtrate and urine table

1. Same in plasma and filtrate? The substance is small enough to be filtered.
2. Zero in filtrate? Too large to pass out of the glomerulus (proteins, cells).
3. In filtrate, zero in urine? It is completely reabsorbed (glucose, at the proximal convoluted tubule).
4. Higher in urine than filtrate? Water has been reabsorbed but the substance has not (urea).
5. Quote figures from the table with units when you describe a change, and give a cause for each difference.

Answer frame: “Explain how the kidney produces urine”

Use the order of the nephron and give one cause for each step:

• high pressure in the glomerulus → ultrafiltration into Bowman’s capsule;
• filtrate contains water, glucose, urea, ions – no proteins or cells;
• all glucose reabsorbed in the proximal convoluted tubule, by active transport;
• water reabsorbed by osmosis from the collecting duct, more when ADH is high;
• urea stays in the tubule, so urine holds water, urea and ions;
• urine passes along the ureter to the bladder.

Links to other sub-topics

• 2.60: plasma carries urea from the liver to the kidneys.
• 2.69: the renal artery and renal vein are part of the circulation you must know.
• 2.81: body water content is an example of homeostasis.
• 2.95B: ADH’s source, role and effects can be asked under co-ordination too.
• 2(g): leaf gas exchange (Paper 2 only) builds on point 2.70.

Must-know distinctions

• Excretion vs egestion: waste of metabolism vs undigested food.
• Urea vs urine: the waste chemical vs the whole liquid.
• Ureter vs urethra: kidney to bladder vs bladder to outside.
• Ultrafiltration vs selective reabsorption: non-selective, by pressure, at the glomerulus vs chosen substances back into blood, at the tubules.
• Proximal vs distal convoluted tubule: first coil (glucose back) vs second coil (ions adjusted).
• Oxygen vs carbon dioxide in plants: waste of photosynthesis vs waste of respiration.
• More ADH vs less ADH: less, concentrated urine vs more, dilute urine.

Quick self-test

1. Name the process that produces carbon dioxide as a waste product in a leaf.
2. Through which structures do waste gases leave a leaf?
3. Name two excretory products of the skin.
4. (Paper 2 only) Name the tube that carries urine from the bladder to the outside.
5. (Paper 2 only) Name the part of the nephron where ultrafiltration happens.
6. (Paper 2 only) Give two substances present in blood plasma but absent from glomerular filtrate.
7. (Paper 2 only) Why is all the glucose reabsorbed?
8. (Paper 2 only) By what process does water leave the collecting duct?
9. (Paper 2 only) Which gland releases ADH?
10. (Paper 2 only) After a salty meal, does ADH release rise or fall, and what happens to urine volume?
11. (Paper 2 only) Filtrate is formed at 130 cm³ per minute and urine at 1.3 cm³ per minute. Calculate the percentage of filtrate reabsorbed.
12. (Paper 2 only) Urea is 0.25 g per dm³ in filtrate and 16 g per dm³ in urine. How many times more concentrated is it in urine?

Answers

1. Respiration.
2. Stomata (by diffusion).
3. Any two of: water, ions (salts), urea.
4. Urethra.
5. Glomerulus / Bowman’s capsule.
6. Any two of: plasma proteins, red blood cells, white blood cells, platelets.
7. It is needed for respiration (as an energy source).
8. Osmosis.
9. Pituitary gland.
10. ADH rises; urine volume falls (urine is more concentrated).
11. (130 − 1.3) ÷ 130 × 100 = 99.0%.
12. 16 ÷ 0.25 = 64 times.

Where marks are usually lost

• Saying oxygen is “made by respiration” or that plants only respire at night.
• Writing “the stomata excrete” without saying the gas diffuses out through them.
• Listing faeces or bile as excretory products of the organs in 2.71.
• Swapping ureter and urethra, or writing “urea” when the question asks about urine.
• Saying ultrafiltration “selects” urea: all small molecules enter the filtrate.
• Giving “glucose is filtered” as the reason there is no glucose in urine – the mark is for reabsorption in the proximal convoluted tubule.
• Saying ADH “reabsorbs water” – ADH changes the permeability of the collecting duct; water moves by osmosis.
• Saying “more ADH, more urine” – it is the reverse.
• Dividing by the urine volume instead of the filtrate volume in a percentage reabsorbed calculation.
• Calling the urea concentration rise “urea being added”: urea is concentrated because water is removed.

Official syllabus

Pearson Edexcel International GCSE in Biology (4BI1) specification, Issue 3 (first assessment June 2019), published by Pearson Education Limited. These notes cover Topic 2, sub-topic (i) Excretion, points 2.70–2.79B.