7.3.5Unit 6: Practical Skills in Biology II
Selective Reabsorption in the Kidney — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 32 of 48 Unit quiz
Section 1 of 4The Proximal Convoluted Tubule
Microvilli
- Finger-like projections on the epithelial cells increase the surface area for reabsorption.
Mitochondria
- Many mitochondria supply the ATP required for active transport of sodium ions.
Co-transporters
- Carrier proteins bring sodium ions in together with glucose or amino acids from the filtrate.
Tight packing
- Closely packed cells make sure that substances pass through the cells, not between them.
Blood supply
- A dense capillary network carries away the reabsorbed substances and maintains concentration gradients.
Section 2 of 4Reabsorption Process
1
Sodium ions are actively transported out of the tubule cells into the surrounding tissue fluid, lowering their concentration inside the cells.
2
Sodium ions then diffuse into the cells from the filtrate, carrying glucose and amino acids with them through co-transporter proteins.
3
Glucose and amino acids move from the cells into the blood by facilitated diffusion, and chloride ions follow the positive charge.
4
The reduced water potential in the tissue fluid causes water to move out of the filtrate by osmosis, and some urea diffuses out.
5
All glucose is normally reabsorbed, so none appears in the urine of a healthy person.
Section 3 of 4The Loop of Henle
The Loop of Henle
Aim
- To make urine more concentrated than blood, so that water is conserved.
Ascending limb
- Sodium and chloride ions leave the filtrate into the medulla, but the wall is impermeable to water.
Medulla
- The accumulating ions give the medulla a very low water potential.
Descending limb
- Permeable to water, so water leaves by osmosis into the medulla and the filtrate becomes more concentrated.
Countercurrent multiplier
- Flow in opposite directions in the two limbs builds up a steep water potential gradient across the medulla.
Section 4 of 4Exam Checklist & Tips
7.3.5 – Key Points for the Exam
- Selective reabsorption Return of useful substances from the filtrate to the blood.
- Co-transporter A protein that moves two substances across a membrane together.
- Countercurrent multiplier The arrangement in the loop of Henle that creates a steep concentration gradient.
- Proximal convoluted tubule The first tubule after the Bowman's capsule, where most reabsorption occurs.
- Water potential A measure of the tendency of water to move from one place to another.
Common Mistakes
- Saying glucose is reabsorbed by osmosis; it uses co-transport and facilitated diffusion.
- Stating that the ascending limb is permeable to water.
- Forgetting that the active transport in the PCT is of sodium ions.
- Describing reabsorption as taking place in the glomerulus.
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