2.9Structures and Functions in Living Organisms
Excretion — Edexcel Modular - OL Revision Notes & Topical Questions
10 min read 12 parts Lesson 9 of 12 1 quiz
Excretion Edexcel O Level Modular Biology notes: understand the origin of carbon dioxide and oxygen as waste products of metabolism and their loss from the stomata of a leaf HumansDiabetes type 1no insulin; type 2: insulin resistance.Negative feedback returns values to the set po…
- Excretion Edexcel O Level Modular Biology: what to do next
- Homeostasis
- Give / State / Name
- Evaluate / Assess
- Keywords the mark scheme looks for in this lesson
- How to get full marks (examiner guidance)
Section 1 of 12Process & sequence
Section 2 of 12Keep going
Excretion Edexcel O Level Modular Biology: what to do next
These Excretion Edexcel O Level Modular Biology notes covered the key ideas, worked examples and exam technique for this topic. Check your Excretion Edexcel O Level Modular Biology understanding with the practice questions below, then reread any Excretion Edexcel O Level Modular Biology point you were unsure about before moving on.
Section 3 of 12Why excretion is needed
The body constantly produces metabolic waste such as urea. The kidneys filter blood to remove waste products, forming urine. But useful substances such as glucose and too much water should not end up in the urine. Three main processes in the nephron form urine: ultrafiltration, selective reabsorption and water reabsorption.
Section 4 of 12Processes in the nephron
| Process | Where | What happens | Substances |
|---|---|---|---|
| Ultrafiltration | Glomerulus and Bowman's capsule | High blood pressure forces small molecules out of the blood into the capsule | Water, glucose, urea, salts |
| Selective reabsorption | Proximal convoluted tubule (PCT) | All glucose is reabsorbed into the blood by active transport | Glucose (also salts) |
| Water reabsorption | Collecting duct (and loop of Henle) | Water is reabsorbed by osmosis, controlled by ADH | Water |
It is called selective reabsorption because glucose, a useful substance, moves from the filtrate back into the blood while waste products like urea stay in the filtrate. The cells of the PCT have many mitochondria to provide ATP for active transport. When describing movements, state clearly from where to where, for example glucose moves from the filtrate into the blood.
Section 5 of 12Control of water: osmoregulation
After the PCT the filtrate passes through the loop of Henle, distal convoluted tubule and collecting duct. The amount of water reabsorbed is controlled by ADH (antidiuretic hormone) from the pituitary gland, which changes the permeability of the collecting ducts. Maintaining water levels is an example of negative feedback.
If the blood water level is high, less ADH is released, the ducts are less permeable and a large volume of dilute urine is produced. Urine contains water, urea and salts. Exam tables may show the relative concentrations of substances in the filtrate at different places in the nephron, or substances in urine, so be able to interpret them.
Section 6 of 12Compare
| Variable | Receptor | Control centre | Effector | Hormone / nerve | Response |
|---|---|---|---|---|---|
| Blood glucose (high) | Pancreas | Pancreas | Liver, muscles | Insulin | Glucose → glycogen |
| Blood glucose (low) | Pancreas | Pancreas | Liver | Glucagon | Glycogen → glucose |
| Body temperature (high) | Thermoreceptors | Hypothalamus | Sweat glands, vessels | Nerves | Sweating, vasodilation |
| Body temperature (low) | Thermoreceptors | Hypothalamus | Muscles, vessels | Nerves | Shivering, vasoconstriction |
| Water (low) | Osmoreceptors | Hypothalamus | Kidney collecting duct | ADH | More water reabsorbed |
Section 7 of 12Worked table analysis and kidney problems
Table question
- a table shows the concentration of substances in blood plasma, filtrate and urine (in g per 100 cm3): protein 8 / 0 / 0;
- glucose 0.10 / 0.10 / 0;
- urea 0.03 / 0.03 / 2.0.
- Interpretation: protein is absent from the filtrate because its molecules are too large to be filtered;
- glucose is present in the filtrate but not urine because all of it is reabsorbed in the PCT;
- urea concentration increases about 60 times in urine because water is reabsorbed while urea stays.
- Reading data this way is a common test of understanding.
If the kidneys are damaged by high blood pressure, infection or diabetes, protein or blood may appear in the urine.
- which is a basis for the test strip used in diagnosis.
Section 8 of 12Worked examples
Explain why glucose is found in the filtrate but not in the urine. [3]3 marks
It is small enough to be filtered, but all of it is reabsorbed by active transport in the PCT.
Describe the role of ADH. [3]3 marks
It makes the collecting duct more permeable to water so more water is reabsorbed into the blood, giving concentrated urine.
Why do PCT cells have many mitochondria? [2]2 marks
They release energy by respiration for active transport.
Section 9 of 12Key terms
| Term | Meaning |
|---|---|
| Ultrafiltration | Filtering of small molecules out of blood under pressure. |
| Selective reabsorption | Reabsorption of useful substances. |
| ADH | Hormone controlling reabsorption of water. |
| Osmoregulation | Control of water levels. |
| Filtrate | Fluid in the nephron. |
Section 10 of 12Syllabus checklist
Section 11 of 12Command terms
Section 12 of 12Exam technique
Marked practiceQuizzes for this lesson
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