1.2Unit 2 · Transport, Excretion & Coordination
Excretion — Unit 2 — Edexcel IGCSE Biology Modular - UNIT 2 Revision Notes & Topical Questions
10 min read 4 parts Lesson 3 of 4 1 quiz
Lesson summary
Name the excretory products of plants and humans and the organs that remove them.Plants excrete O2 (by day) and CO2 (at night) through stomata by diffusion.Humans excrete CO2 via the lungs, urea via the kidneys and water/salts via the skin.Urea is made in the liver by deamination…
- Excretion in plants
- Excretion in humans
- Two roles of the kidney
- Urinary system
- Nephron structure
- Ultrafiltration
Section 1 of 4Excretion — Unit 2
Excretion in plants:
leaves excrete oxygen produced in photosynthesis and carbon dioxide produced in respiration. These gases diffuse out through the stomata. Which gas is excreted depends on the balance of photosynthesis and respiration.Excretion in humans:
the lungs excrete carbon dioxide from respiration; the kidneys excrete urea, excess water and ions in urine; the skin loses water, salts and small amounts of urea in sweat.Two roles of the kidney:
excretion (removing urea) and osmoregulation (keeping the water and ion content of blood constant so that cells are not damaged by osmosis).Urinary system:
the renal artery brings blood to each kidney and the renal vein takes filtered blood away.- Each kidney has an outer cortex, an inner medulla and a pelvis.
- Urine passes down the ureters to the bladder, which stores it; a sphincter muscle controls release through the urethra.
Nephron structure:
each kidney contains about a million nephrons.- A nephron starts with the Bowman's capsule around the glomerulus, then the proximal convoluted tubule, loop of Henle, distal convoluted tubule and finally the collecting duct, which drains into the pelvis.
Ultrafiltration:
the afferent arteriole entering the glomerulus is wider than the efferent arteriole leaving, which creates high pressure.- Water, glucose, urea, amino acids and ions are forced into the capsule to form the glomerular filtrate.
- Red blood cells and large proteins remain in the blood.
Selective reabsorption:
in the proximal tubule all the glucose is reabsorbed by active transport, using energy from the many mitochondria in the tubule cells,- which also have microvilli to increase surface area.
- Some water and ions are also reabsorbed.
- The loop of Henle sets up a salty medulla so water can later leave the collecting duct by osmosis.
ADH and water balance:
if the blood is too concentrated (e.g. after sweating), the hypothalamus detects this and the pituitary releases more ADH.- ADH makes the collecting duct walls more permeable, so more water is reabsorbed and a small volume of concentrated urine is produced.
- If blood is too dilute, less ADH is released and a large volume of dilute urine results.
Composition of urine:
water, urea and mineral ions. The volume and concentration vary with water intake, temperature and exercise; glucose and protein are normally absent.Why waste must be removed:
If metabolic wastes build up they become toxic.- For example, carbon dioxide dissolves to form an acid that lowers the pH of cells and reduces enzyme activity, and high solute levels make body fluids too concentrated so water leaves cells by osmosis.
Excretion is not egestion:
Faeces are mostly undigested food that never entered body cells, so removing them is egestion, not excretion. Excretion only refers to waste made by metabolism, such as urea and carbon dioxide.Water balance of the body:
Water is gained from food and drink and as a product of aerobic respiration.- It is lost in exhaled air, in sweat and in urine.
- Losses from the lungs and skin cannot be adjusted to water balance, but the kidneys control how much water leaves in urine.
Effects of wrong water balance on cells:
If blood is too dilute, water enters cells by osmosis and they swell and may burst. If blood is too concentrated, cells lose water by osmosis, shrink and cannot work properly. Osmoregulation prevents both.Interpreting nephron data:
In a healthy person there is no protein in the filtrate (too large to be filtered) and no glucose after the proximal tubule.- Glucose in urine suggests diabetes.
- Protein in urine suggests the glomerulus is damaged, e.g. by high blood pressure.
- Urea concentration is highest in the collecting duct because water has been removed.
Urine colour:
A large volume of pale urine contains a lot of water, so urea is dilute. A small volume of dark yellow urine contains little water, so urea is concentrated, e.g. after exercise or on a hot day.
Section 2 of 4Process & sequence
- Blood enters the kidney in the renal artery and passes into the glomerulus.
- High pressure forces water, glucose, urea, salts into the Bowman's capsule (ultrafiltration); cells and proteins stay in blood.
- In the proximal tubule all glucose is reabsorbed into the blood by active transport.
- The loop of Henle concentrates salts in the medulla.
- Water is reabsorbed from the collecting duct, the amount controlled by ADH.
- The remaining fluid (urine) drains through the ureter to the bladder.
- Filtered blood leaves the kidney in the renal vein.
Urine formation in a nephron — write these steps in this order; each step is a marking point.
- Blood enters glomerulus
- Ultrafiltration into Bowman's capsule
- Glucose reabsorbed in proximal tubule
- Loop of Henle concentrates medulla
- Water reabsorbed (ADH) in collecting duct
- Urine to ureter and bladder
Section 3 of 4Compare
| Organism / organ | Waste product | How removed |
|---|---|---|
| Plant leaves | Oxygen (from photosynthesis) | Diffuses out through stomata |
| Plant leaves | Carbon dioxide (from respiration, at night) | Diffuses out through stomata |
| Human lungs | Carbon dioxide (and some water) | Exhaled |
| Human kidneys | Urea, excess water and ions | Urine |
| Human skin | Water, salts, a little urea | Sweat |
| Region | Function |
|---|---|
| Glomerulus and Bowman's capsule | Ultrafiltration of small molecules from blood |
| Proximal convoluted tubule | Selective reabsorption of all glucose (active transport), some water and ions |
| Loop of Henle | Creates high salt concentration in the medulla so water can be reabsorbed |
| Distal convoluted tubule | Adjusts ion balance |
| Collecting duct | Reabsorbs water under control of ADH |
Section 4 of 4Worked examples
A student runs on a hot day and drinks little. Explain how the volume and concentration of urine change.
- Water is lost in sweat, so the blood becomes more concentrated.
- This is detected by the hypothalamus and the pituitary gland releases more ADH into the blood.
- ADH makes the collecting ducts more permeable to water, so more water is reabsorbed into the blood by osmosis.
- The urine produced is smaller in volume and more concentrated.
- Water is lost in sweat, so the blood becomes more concentrated.
Explain why glucose is present in the glomerular filtrate but not in urine of a healthy person.
- Glucose molecules are small enough to be forced out of the glomerulus during ultrafiltration.
- However, in the proximal convoluted tubule all glucose is selectively reabsorbed into the blood by active transport, so none reaches the urine.
- Glucose molecules are small enough to be forced out of the glomerulus during ultrafiltration.
Marked practiceQuizzes for this lesson
Last updated: