Edexcel International GCSE Biology 4BI1: Excretion – Study Guide

This study guide teaches 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 be examined on either paper. Points 2.72B–2.79B are bold with a B reference, so they are Paper 2 only. It applies to the June and November series examined under Issue 3.

Use it with the excretion revision notes and the excretion practice questions. The full course is on the Edexcel IGCSE Biology hub, and you can tick off each point on the printable checklist. Earlier parts of Topic 2 are covered in the cell structure and biological molecules guide.

What this sub-topic covers

Spec point	What you must be able to do	Paper
2.70	Explain where carbon dioxide and oxygen come from as waste products in a plant, and how they leave through stomata	1 and 2
2.71	Know the excretory products of the lungs, kidneys and skin	1 and 2
2.72B	Understand how the kidney carries out excretion and osmoregulation	Paper 2 only
2.73B	Describe the urinary system: kidneys, ureters, bladder, urethra	Paper 2 only
2.74B	Describe a nephron: Bowman’s capsule, glomerulus, convoluted tubules, loop of Henle, collecting duct	Paper 2 only
2.75B	Describe ultrafiltration and the composition of glomerular filtrate	Paper 2 only
2.76B	Understand how water is reabsorbed from the collecting duct	Paper 2 only
2.77B	Understand why glucose is selectively reabsorbed at the proximal convoluted tubule	Paper 2 only
2.78B	Describe the role of ADH in regulating the water content of the blood	Paper 2 only
2.79B	Understand that urine contains water, urea and ions	Paper 2 only

Excretion is the removal of waste products of metabolism from the body. Metabolism means the chemical reactions in cells. Do not confuse excretion with egestion: faeces are mostly undigested food that never entered the body’s cells, so passing faeces is egestion.

2.70 Excretion in flowering plants

Plants have no excretory organs. Their two main waste gases are made by the two processes you already know.

• Carbon dioxide is a waste product of respiration. Every living plant cell respires all the time, day and night.
• Oxygen is a waste product of photosynthesis. Only cells with chloroplasts make it, and only in the light.

Both gases leave the leaf by diffusion through the stomata, when their concentration in the leaf air spaces is higher than in the air outside.

What leaves the leaf depends on the balance of the two processes:

• In the dark: only respiration happens. Carbon dioxide builds up in the air spaces and diffuses out. Oxygen diffuses in.
• In bright light: photosynthesis is faster than respiration. The plant uses all its own respiratory carbon dioxide and takes in more, and makes more oxygen than it uses. The extra oxygen diffuses out.

In bright light, respiratory carbon dioxide is still produced. It is used straight away in photosynthesis, so it is not lost.

Worked example 1

A plant is kept in a sealed, transparent container in bright light for 6 hours and then in darkness for 6 hours. Explain how the oxygen concentration in the container changes.

1. In bright light, the rate of photosynthesis is greater than the rate of respiration.
2. So the leaf makes more oxygen than it uses. Oxygen is a waste product here and diffuses out through the stomata. The oxygen concentration rises.
3. In darkness, photosynthesis stops but respiration continues.
4. The leaf takes in oxygen through the stomata and gives out carbon dioxide, a waste product of respiration. The oxygen concentration falls.

Leaf structure and the hydrogen-carbonate indicator practical belong to sub-topic 2(g), Paper 2 only.

2.71 Organs of excretion in humans

Organ	Excretory products	Where the waste comes from
Lungs	Carbon dioxide (and water vapour)	Respiration in all cells
Kidneys	Urea, excess water, excess ions (salts), in urine	Urea is made in the liver from excess amino acids
Skin	Water, ions (salts) and a small amount of urea, in sweat	As above

Urea is carried in the plasma (point 2.60) from the liver to the kidneys. Sweating mainly cools the body, but sweat still removes some waste, so the skin counts as an organ of excretion.

2.73B The urinary system (Paper 2 only)

Follow a molecule of urea from the blood to the outside:

• The renal artery brings blood, including urea, to each kidney.
• Each kidney filters the blood and makes urine. Cleaned blood leaves in the renal vein.
• A ureter carries urine from each kidney to the bladder.
• The bladder stores urine. A ring of muscle (a sphincter) keeps it closed.
• The urethra carries urine from the bladder out of the body.

Ureter (two, kidney to bladder) and urethra (one, bladder to outside) are easy to swap; the wrong word is not credited.

Inside the kidney are an outer cortex, an inner medulla and the pelvis, where urine collects.

2.74B The nephron (Paper 2 only)

Each kidney contains a very large number of tiny tubules called nephrons. In order, filtrate passes through:

1. Bowman’s capsule – a cup-shaped end of the nephron in the cortex. It surrounds the glomerulus and collects the filtrate.
2. Proximal convoluted tubule (first coiled tubule) – reabsorbs all the glucose, plus some water and ions, into the blood.
3. Loop of Henle – dips down into the medulla. It builds up a high concentration of ions (salts) in the tissue of the medulla.
4. Distal convoluted tubule (second coiled tubule) – adjusts the ions reabsorbed.
5. Collecting duct – runs down through the medulla. Water is reabsorbed here, under the control of ADH. Collecting ducts empty urine into the pelvis.

The glomerulus is not part of the tubule itself. It is a knot of blood capillaries inside Bowman’s capsule, supplied by a branch of the renal artery.

2.75B Ultrafiltration (Paper 2 only)

Ultrafiltration is filtration under high pressure in the glomerulus.

• The vessel taking blood into the glomerulus is wider than the vessel taking blood out.
• This builds up a high blood pressure in the glomerulus capillaries.
• Small molecules are forced out of the blood, through the capillary wall and the wall of Bowman’s capsule, into the capsule.
• The liquid in the capsule is the glomerular filtrate.

Composition of glomerular filtrate: water, glucose, urea, ions (salts) and amino acids – at about the same concentrations as in blood plasma.

Not in the filtrate: red blood cells, white blood cells, platelets and plasma proteins. They are too large to pass through the filter, so they stay in the blood.

2.77B Selective reabsorption of glucose (Paper 2 only)

The filtrate contains useful substances as well as waste. Glucose is the key example.

• Why it is reabsorbed: glucose is needed by body cells for respiration. Losing it in urine would waste an energy source.
• Where: the proximal convoluted tubule. All of the glucose is normally reabsorbed here, so a healthy person’s urine contains no glucose.
• How: partly by active transport, which moves glucose against its concentration gradient and uses energy from respiration. The tubule cells have many mitochondria to supply this energy, and microvilli that give a large surface area.

It is called selective because the tubule takes back particular substances, such as glucose, but leaves urea in the tubule.

If the blood glucose concentration is very high (for example in untreated diabetes), there is more glucose in the filtrate than the tubule can reabsorb, and some glucose appears in the urine.

Worked example 2

The table gives example concentrations, in g per dm³, in one person.

Substance	Blood plasma	Glomerular filtrate	Urine
Protein	80	0	0
Glucose	1.0	1.0	0
Urea	0.30	0.30	20

(a) Explain the figures for protein. (b) Explain the figures for glucose. (c) Calculate how many times more concentrated urea is in urine than in the filtrate, and explain the difference.

(a) Protein molecules are too large to be forced out of the glomerulus, so there is none in the filtrate or the urine.

(b) Glucose is a small molecule, so it passes into the filtrate at the same concentration as in plasma. All of it is reabsorbed in the proximal convoluted tubule, so there is none in urine.

(c) 20 ÷ 0.30 = 66.7 times. Urea is not reabsorbed, but most of the water in the filtrate is reabsorbed. The same mass of urea ends up in a much smaller volume of water, so its concentration rises.

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

Water reabsorption from the collecting duct

• The loop of Henle gives the tissue of the medulla a high concentration of ions.
• The collecting duct carries filtrate down through this concentrated medulla.
• Water moves out of the collecting duct by osmosis, from the dilute filtrate into the concentrated tissue, and then into the blood capillaries.
• How much water leaves depends on how permeable the walls of the collecting duct are. ADH controls this.

The role of ADH

ADH (antidiuretic hormone) is released by the pituitary gland in the brain into the blood. Its target is the collecting duct.

Situation	Blood water content	ADH released	Collecting duct	Urine
Sweating heavily, little to drink, salty meal	Too low	More	More permeable to water	Small volume, concentrated
Drinking a lot of water	Too high	Less	Less permeable to water	Large volume, dilute

This is negative feedback: the response reverses the change. ADH also appears under co-ordination (points 2.81 and 2.95B).

Worked example 3

In one person, filtrate is formed at 125 cm³ per minute and urine at 1.0 cm³ per minute. Calculate (a) the volume of filtrate formed per day in dm³ and (b) the percentage of the filtrate volume that is reabsorbed.

(a) 125 × 60 × 24 = 180 000 cm³. Divide by 1000: 180 dm³ per day.

(b) Volume reabsorbed = 125 − 1.0 = 124 cm³ per minute. Percentage = 124 ÷ 125 × 100 = 99.2%.

Almost all the filtrate goes back into the blood.

2.72B and 2.79B Putting it together (Paper 2 only)

The kidney has two roles.

• Excretion: it removes urea (from the breakdown of excess amino acids in the liver), so urea does not build up in the blood. Urea is filtered and not reabsorbed.
• Osmoregulation: it controls the water content (and ion content) of the blood by changing how much water is reabsorbed in the collecting duct, under the control of ADH.

Urine therefore contains water, urea and ions (2.79B). A healthy person’s urine contains no glucose and no protein.

To answer a “how does the kidney…” question, use this order: ultrafiltration in the glomerulus → selective reabsorption of glucose in the proximal convoluted tubule → water reabsorption from the collecting duct, controlled by ADH → urine to the ureter, bladder and urethra.

Common errors

• Writing that oxygen is a waste product of respiration, or carbon dioxide a waste product of photosynthesis. Match each gas to the process that makes it.
• Calling faeces an excretory product. That is egestion.
• Confusing urea (the waste chemical) with urine (the liquid), and ureter with urethra.
• Saying the glomerulus “filters out urea”. Ultrafiltration is not selective: every small molecule passes into the filtrate.
• Saying “glucose is filtered out so it is not in the blood”. Glucose is filtered and then reabsorbed.
• Writing “ADH makes the kidney reabsorb more water” without naming the collecting duct and its permeability.
• Saying more ADH makes more urine. More ADH means less, more concentrated urine.

Next steps

Go on to the excretion revision notes and practice questions. For more of Topic 2, see the cell structure revision notes and cell structure practice. Find your weaker topics with the free diagnostics.

Official syllabus

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