2.9.6Structure & Functions in Living Organisms
Nephron Function — Edexcel International GCSE Biology 4BI1 Revision Notes & Topical Questions
5 min read 3 parts Lesson 20 of 21 Unit quiz
Section 1 of 3Ultrafiltration
The first stage happens between the glomerulus and the Bowman's capsule.
1
Blood enters the glomerulus through a wide vessel and leaves through a narrower one, so the blood pressure inside the glomerulus is high.
2
This high pressure forces small molecules, namely water, glucose, urea and ions, through the thin filter membrane into the Bowman's capsule.
3
Large molecules such as proteins, and blood cells, are too big to pass through the membrane, so they stay in the blood.
4
The fluid collected in the Bowman's capsule is called the glomerular filtrate.
5
Filtration is by pressure, so substances are forced out of the blood rather than diffusing out.
Section 2 of 3Selective Reabsorption
Useful substances must be taken back into the blood before the filtrate becomes urine.
Where
- This takes place mainly in the proximal convoluted tubule, which is surrounded by a network of blood capillaries.
Glucose
- All of the glucose is reabsorbed from the filtrate back into the blood by active transport, against its concentration gradient.
Energy
- PCT cells have many mitochondria to supply the ATP needed for active transport.
Urea stays
- Urea is not reabsorbed and so becomes more concentrated in the filtrate as water is later removed.
Section 3 of 3Exam Checklist & Tips
2.9.6 – Key Points for the Exam
- Ultrafiltration Filtering of small molecules from the blood under high pressure.
- Glomerular filtrate The fluid in the Bowman's capsule containing water, glucose, urea and ions.
- Selective reabsorption The return of useful substances such as glucose from the filtrate to the blood.
- Active transport The movement of substances against a concentration gradient using energy.
- Urea The nitrogenous waste that remains in the filtrate and leaves in urine.
Common Mistakes
- Saying that proteins are filtered into the capsule; they are too large and normally stay in the blood.
- Stating that glucose is reabsorbed by diffusion; it is moved by active transport.
- Forgetting that protein or glucose in urine is a sign of a problem, such as high blood pressure or diabetes.
- Saying that most water reabsorption happens in the proximal tubule;
- for this course it is the collecting duct, controlled by ADH.
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