The Kidney - Selective Reabsorption

Question 1

What structures are involved in selective reabsorption

Answer 1

Proximal Convoluted Tubule (PCT)
Distil Convoluted Tubule (DCT)

Question 2

What is the purpose of selective reabsorption

Answer 2

Reabsorbs substances into the blood as the filtrate passes along the nephron, most importantly filtering glucose out of the proximal tubule into the bloodstream

Question 3

Where does most reabsoprtion occur

Answer 3

Proximal convoluted tube with epithelial cells

Question 4

How are epithelial cells adapted

Answer 4

Microvilli = increase SA
Co-transporter proteins = transports specific solute across luminal membrane
High number of mitochondria = provide energy for sodium potassium pump
Tightly packed cells = no fluid can pass between cells

Question 5

How do the sodium ions move out of the ascending loop of henle and what does this result in for water

Answer 5

Ascending is impermeable to water. In the basal membranes of the tubules there are sodium potassium pumps that pump sodium ions out of epithelial cells and into the blood, lowering water potential in the cortex. Osmosis is moving water molecules form an area of high to low concentration so water diffuses by osmosis into the cortex back into the blood stream.

Question 6

What is permeable to water in the loop of henle

Answer 6

The descending loop of henle is permeable to water (can leave using osmosis)
Ascending is impermeable to water

Question 7

How does the loop of henle filter out water (simply)

Answer 7

1) Na+ actively transport out of ascending loop of Henle
2) cortex water potential drops
3) Water comes out of descending loop by osmosis to maintain the water potential gradient
4) Water diffuses out of the tubule and back into the blood stream

Question 8

How does water leave the loop of henle

Answer 8

Water potential is higher in descending loop of henle and low in cortex (because Na+ have been actively transported out)
Water tries to reach an equilibrium due to the water potential differences and so moves via osmosis from an area of high concentration (tubule) to an area of low (cortex)

Question 9

Where does the loop of henle extend to and from

Answer 9

From cortex to medulla

Question 10

What is the loop of henle and collecting duct involved in

Answer 10

Osmoregulation
Loop of henle = salt gradient
Collecting duct = water retention

Question 11

How does the proximal convoluted tubule work

Answer 11

1) Sodium potassium pumps use ATP from mitochondria (in the basement membrane of epithelial cells) to pump Na+ out of the tubule into the blood by active transport, lowering the Na+ conc
2) Na+ diffuse down a conc gradient from the lumen of the pct into the epithelial lining cells through special carrier proteins by facilitates diffusion
4) The molecules have been co-transported into the cells of the pct then diffuse into the blood

Question 12

Adaptations of selective reabsorption

Answer 12

Many microvilli on luminal membrane = increases surface area of reabsorption
Many co-transporter proteins in luminal membrane = each type of co-transporter protein transports a specific solute across the luminal membrane
Many mitochondria = provides ATP for sodium-potassium pump
Cells packed close together = no fluid can pass between cells (has to be reabsorbed through cells)

Question 13

How does the proximal convoluted tubule work

Answer 13

1) Sodium-potassium pumps in the basal membrane move Na+ out of the epithelial cells and into the blood using ATP from mitochondria
2) Na+ move down the conc gradient from filtrate into epithelial cells using co transporters which also bring glucose and amino acids in
3) Co transport proteins in the basal membrane allow solutes to diffuse down the conc gradient from epithelial cells (high) into the blood (low)

Question 14

What structures do ions move through during the proximal convoluted tubule

Answer 14

1) lumen through sodium ions and transport proteins
2) Enter epithelial cell
3) Endothelium
4) Capillary

Question 15

How do sodium ions move into the capillary during the proximal convoluted tubule selective reabsorption

Answer 15

1) Na+ in epithelial cell and tubule lumen
2) Na+ in epithelial cell move into capillary by active transport
3) Creates concentration gradient between lumen (high) and epithelial cell (low)
4) Na+ move into the epithelial cell using cotransport proteins by facilitated diffusion

Question 16

How are other molecules transported into the capillary in the proximal convoluted tubule using Na+

Answer 16

1) Co transport proteins can transport Na+ and other molecules such as glucose, each co-transport protein in specific to a type of molecule
2) Glucose and Na+ together are transported by facilitated diffusion and co transport through co transport proteins from the lumen into the epithelial cell
3) They diffuse down the conc gradient into the capillary from the epithelial cell

Question 17

How is water affected in the proximal convoluted tubule

Answer 17

1) When Na+ and other molecules (e.g. gluose) enters the epithelial cell, the water potential gradient in the epithelial cell decreases and increases in the lumen
2) Water potential gradient therefore forms
3) ALlowing water molecules to move by osmosis from the lumen (high) to the epithelial cell (low)
4) Also moves from the epithelial cell into the capillary by osmosis in the same way

Question 18

How is water affected in the proximal convoluted tubule

Answer 18

1) When Na+ and other molecules (e.g. gluose) enters the epithelial cell, the water potential gradient in the epithelial cell decreases and increases in the lumen
2) Water potential gradient therefore forms
3) ALlowing water molecules to move by osmosis from the lumen (high) to the epithelial cell (low)
4) Also moves from the epithelial cell into the capillary by osmosis in the same way

Question 19

How does ADH increase conc of urine in the collecting duct ( 2 marker )

Answer 19

Conc rises in collecting duct because water is lost by osmosis
ADH increases permeability of walls to water

Question 20

Key words for answering questions

Answer 20

ADH increases permeability of DCT and CD
Water moves out by OSMOSIS
Water potential