Excretion
BiologyΒ· 2.70β2.79BΒ· 25 min read
1. Excretion in Flowering Plants (Core)β β ββββ± 5 min
Excretion
Removal of waste products formed from metabolic reactions inside cells, which are toxic if allowed to build up.
Example:
Carbon dioxide produced during aerobic respiration is an excretory product.
Flowering plants produce two main metabolic waste products excreted via stomata on the underside of leaves:
- Carbon dioxide: produced as waste of aerobic respiration in all plant cells, excreted when light levels are low (respiration rate > photosynthesis rate)
- Oxygen: produced as waste of photosynthesis in palisade cells, excreted when light levels are high (photosynthesis rate > respiration rate)
A potted plant is kept in a dark cupboard for 24 hours. Name the main excretory product it releases during this period, and state how it leaves the plant.
- 1
Step 1: Recall photosynthesis stops in the dark, but respiration continues to release energy for cell processes.
- 2
Step 2: Identify the waste product of aerobic respiration: carbon dioxide.
- 3
Step 3: Recall gaseous waste in plants exits via diffusion through open leaf stomata.
Exam tip:
Distinguish waste products released in light vs dark: oxygen is only excreted when photosynthesis is faster than respiration.
2. Human Excretory Organs & Waste Products (Core)β β ββββ± 5 min
Humans have three main excretory organs that remove metabolic waste from the body. Note that egestion of faeces (undigested food) is NOT excretion, as faeces is not a product of cellular metabolism.
Excretory Organ | Waste Products Excreted |
|---|---|
Lungs | Carbon dioxide, small amounts of water vapour |
Kidneys | Urea, excess water, excess mineral ions (collectively urine) |
Skin (sweat glands) | Water, excess mineral ions, small amounts of urea (collectively sweat) |
Name the excretory organ responsible for removing carbon dioxide from the body, and state one other waste product it excretes.
- 1
Step 1: Match carbon dioxide to its excretory organ: the lungs.
- 2
Step 2: Recall the secondary waste product released by lungs: water vapour, released when you breathe out.
Exam tip:
Mark schemes strictly pair organs and products: never list urea as a lung product or carbon dioxide as a kidney product, you will lose marks.
3. Urinary System & Nephron Structure (Biology-only, Paper 2)β β β ββA2 onlyβ± 5 min
Nephron
Microscopic functional unit of the kidney, responsible for filtering blood and producing urine.
The human urinary system filters blood, produces urine, and excretes waste. Its key structures are:
- Kidneys: two bean-shaped organs that filter blood and produce urine
- Ureters: two tubes that carry urine from kidneys to the bladder
- Bladder: muscular sac that stores urine temporarily
- Urethra: tube that carries urine from the bladder out of the body
- Bowman's capsule: cup-shaped structure surrounding the glomerulus, collects filtered fluid
- Glomerulus: knot of capillaries where ultrafiltration occurs
- Proximal convoluted tubule: first coiled section where selective reabsorption occurs
- Loop of Henle: U-shaped section supporting water balance (no detailed mechanism required for this spec)
- Collecting duct: final section that carries urine to the ureter and controls water reabsorption
Name the structure that carries urine from the kidney to the bladder, and name the functional unit of the kidney where urine is formed.
- 1
Step 1: Identify the tube connecting kidney to bladder: ureter.
- 2
Step 2: Recall the functional unit of the kidney: nephron.
Exam tip:
You will often be asked to label diagrams: make sure you can distinguish the ureter (kidney to bladder) from the urethra (bladder out of body), and identify all key nephron structures.
4. Kidney Function: Ultrafiltration & Selective Reabsorption (Biology-only, Paper 2)β β β β βA2 onlyβ± 5 min
Kidneys carry out two key functions: excretion of metabolic waste (urea) and osmoregulation (control of blood water and ion balance). These processes happen via two core steps in the nephron: ultrafiltration and selective reabsorption.
Ultrafiltration
Process where high pressure in the glomerulus forces small soluble molecules out of blood into the Bowman's capsule.
Example:
Glomerular filtrate (fluid entering Bowman's capsule) contains water, glucose, urea, and mineral ions, but no blood cells or large proteins, which are too large to pass through capillary walls.
After ultrafiltration, useful molecules are reabsorbed into the blood via selective reabsorption:
- All glucose is reabsorbed from the proximal convoluted tubule via active transport, so no glucose is present in normal urine
- Useful amounts of water and mineral ions are reabsorbed along the tubule and collecting duct as required
- Urea is not reabsorbed, so it remains in the filtrate to be excreted in urine
Explain why large protein molecules are not found in the glomerular filtrate of a healthy person.
- 1
Step 1: Recall ultrafiltration relies on high pressure forcing small molecules out of the glomerulus.
- 2
Step 2: State that large protein molecules are too big to fit through gaps in the glomerular capillary walls and Bowman's capsule membrane.
- 3
Step 3: Conclude that proteins remain in the blood and do not enter the filtrate.
Exam tip:
When explaining ultrafiltration, always mention high pressure in the glomerulus and size of molecules: this is required for full marks.
5. Osmoregulation & Role of ADH (Biology-only, Paper 2)β β β β βA2 onlyβ± 5 min
Osmoregulation is the control of water and salt balance in the blood to maintain constant water potential, so cells do not shrink or swell. It is controlled by antidiuretic hormone (ADH), released by the pituitary gland in the brain, via negative feedback.
If blood is too concentrated (e.g. after heavy sweating, not drinking enough water): pituitary releases MORE ADH β collecting duct becomes more permeable to water β more water reabsorbed into blood β small volume of concentrated urine produced, blood water levels return to normal.
If blood is too dilute (e.g. after drinking large amounts of water): pituitary releases LESS ADH β collecting duct becomes less permeable to water β less water reabsorbed β large volume of dilute urine produced, blood water levels return to normal.
A person drinks 2 litres of water in 10 minutes. Explain how their body responds to regulate blood water content.
- 1
Step 1: State that drinking large amounts of water makes the blood too dilute.
- 2
Step 2: Explain that the pituitary gland will release less ADH.
- 3
Step 3: State that this makes the collecting duct less permeable to water, so less water is reabsorbed into the blood.
- 4
Step 4: Conclude that a large volume of dilute urine is produced, returning blood water concentration to normal.
Exam tip:
Never state that ADH 'makes you urinate less' β always explain its effect on collecting duct permeability and water reabsorption to get full marks.
6. Common Pitfalls
Wrong move:
Calling faeces an excretory product
Why:
Faeces is undigested food that never entered cells for metabolic reactions, so its removal is egestion, not excretion.
Correct move:
Only list metabolic waste products (CO2, urea, plant oxygen) as excretory products, and explicitly distinguish excretion from egestion if asked.
Wrong move:
Stating that the kidney produces urea
Why:
Urea is produced in the liver from excess amino acids via deamination; the kidney only removes urea from the blood.
Correct move:
Always link urea production to the liver, and urea excretion to the kidney.
Wrong move:
Including proteins or blood cells in glomerular filtrate or normal urine
Why:
Large proteins and blood cells are too big to pass through the glomerulus membrane during ultrafiltration, so they remain in the blood.
Correct move:
State that glomerular filtrate contains only water, glucose, ions, urea, and normal urine contains only water, ions, urea.
Wrong move:
Stating ADH directly reduces urine volume without explaining mechanism
Why:
Mark schemes require you to link ADH levels to collecting duct permeability and water reabsorption for full marks.
Correct move:
Explain that more ADH increases collecting duct permeability, so more water is reabsorbed, leading to smaller volumes of concentrated urine.
Wrong move:
Mixing up excretory organ and waste product pairs
Why:
Mark schemes strictly pair organs and products, so incorrect pairs lose marks automatically.
Correct move:
Memorise the standard pairs: lungs = CO2 + water vapour; kidneys = urine (urea, water, ions); skin = sweat (water, ions, small urea amounts).
7. Quick Reference Cheatsheet
Topic | Key Exam Facts |
|---|---|
Plant Excretion | CO2 (respiration, dark) and O2 (photosynthesis, light) excreted via stomata |
Human Excretory Organs | Lungs: CO2 + water vapour; Kidneys: urine; Skin: sweat |
Urinary System Flow | Kidney β ureter β bladder β urethra |
Nephron Core Structures | Bowman's capsule, glomerulus, proximal convoluted tubule, loop of Henle, collecting duct |
Ultrafiltration | High pressure filters small molecules into Bowman's capsule; no proteins/cells in filtrate |
Selective Reabsorption | All glucose reabsorbed in proximal tubule; no glucose in normal urine |
ADH Action | More ADH = more permeable collecting duct = more water reabsorbed = concentrated urine |
Normal Urine | Only contains water, urea, and excess mineral ions |
8. Frequently Asked
Is faeces an excretory product?
No, faeces is undigested food that never entered cells for metabolism, so its removal is called egestion, not excretion.
Why is there no glucose in normal urine?
All glucose is selectively reabsorbed into the blood from the proximal convoluted tubule via active transport. Glucose in urine is a common sign of diabetes.
What happens if ADH levels are low?
The collecting duct becomes less permeable to water, so less water is reabsorbed into the blood, leading to large volumes of dilute urine.
Going deeper
- official_specEdexcel IGCSE Biology 4BI1 SpecificationSee section 2.70β2.79B for full excretion content
What's Next
Now that you have mastered excretion content for Edexcel IGCSE Biology, move on to related topics that build on this knowledge. Next, study Homeostasis (including skin temperature regulation) to connect osmoregulation to the body's wider control systems, and revise Gas Exchange in Plants to consolidate your understanding of stomatal function. For Paper 2 Biology students, practice past paper questions on kidney function and ADH to test your extended content knowledge, and practice labelling diagrams of the urinary system and nephron to avoid easy marking point losses in your exam.