Homeostasis

Question 1

Blood Glucose Regulation

Answer 1

Maintained by negative feedback involving the pancreas, specifically the Islets of Langerhans:
• Alpha cells → secrete glucagon
• Beta cells → secrete insulin

Question 2

🔽 When blood glucose levels are too high (e.g. after a meal):

Answer 2

1. Beta cells in the pancreas detect the rise in blood glucose.
   
   2. They secrete insulin into the bloodstream.
   3. Insulin binds to specific receptors on liver, muscle, and fat cells.
   4. This triggers:
      • Increased permeability of cells to glucose (via more glucose transporter proteins – GLUT4 moved to membrane in muscle cells).
      • Increased rate of glucose uptake by cells.
      • Activation of enzymes converting glucose to glycogen (glycogenesis).
      • Increased glucose respiration in cells.
   5. Result: blood glucose level falls back to normal.

Question 3

🔼 When blood glucose levels are too low (e.g. between meals, exercise):

Answer 3

1. Alpha cells in the pancreas detect the fall in blood glucose.
   
   2. They secrete glucagon into the blood.
   3. Glucagon binds to receptors on liver cells only (not muscle).
   4. This triggers:
      • Glycogenolysis: breakdown of glycogen → glucose.
      • Gluconeogenesis: conversion of amino acids/lipids → glucose.
      • Reduced glucose uptake by cells.
   5. Result: blood glucose level rises back to normal.

Question 4

⚡️ Adrenaline (e.g. during stress or exercise)

Answer 4

1. Released from adrenal glands.
   
   2. Binds to receptors on liver cells.
   3. Activates second messenger system (cyclic AMP).
   4. Stimulates glycogenolysis and inhibits glycogenesis.
   5. Helps raise blood glucose quickly.

Question 5

🔽 When water potential is too low (e.g. dehydration, sweating):

Answer 5

1. Detected by osmoreceptors in the hypothalamus (they shrink due to water loss).
   
   2. Sends nerve impulses to the posterior pituitary gland.
   3. More ADH is released into the blood.
   4. ADH binds to receptors on cells of the collecting duct and distal convoluted tubule (in the nephron).
   5. This triggers:
      • Activation of enzymes that insert aquaporins (water channels) into the membranes.
      • Increased permeability to water.
      • More water reabsorbed into the blood by osmosis from the filtrate.
   6. Urine becomes more concentrated and smaller in volume.
   7. Water potential of blood returns to normal = negative feedback.

Question 6

💧 Water Balance & Osmoregulation – AQA A-level Biology

🔁 Definition

Answer 6

Osmoregulation is the control of water potential of the blood, keeping it within a narrow range, mainly by the kidneys and the hormone ADH (antidiuretic hormone).

Question 7

🔼 When water potential is too high (e.g. after drinking lots):

Answer 7

1. Osmoreceptors detect the rise in blood water potential (they swell).
   
   2. Less ADH released from the posterior pituitary.
   3. Fewer aquaporins inserted into collecting duct/DCT membranes.
   4. Permeability to water decreases.
   5. Less water reabsorbed, more lost in urine.
   6. Urine is dilute and high in volume.
   7. Blood water potential falls back to normal = negative feedback.

Question 8

Ultrafiltration in the Glomerulus – AQA A-Level Biology

Answer 8

1. Blood enters the glomerulus via the afferent arteriole which is wider than the efferent arteriole.
   
   2. This creates high hydrostatic pressure inside the glomerulus capillaries.
   3. The pressure forces blood plasma and small molecules (like water, glucose, amino acids, ions, urea) out of the capillaries.
   4. The fluid passes through three layers forming the filtration barrier:
      • Endothelium of capillaries (has small fenestrations/pores).
      • Basement membrane (acts as a fine filter, stopping large molecules like plasma proteins and blood cells).
      • Podocytes — specialised epithelial cells with foot processes that create slits, allowing fluid through but blocking larger cells.
   5. The resulting fluid, called glomerular filtrate, enters the Bowman’s capsule.
   6. Blood cells and large proteins remain in the blood and exit via the efferent arteriole.

Question 9

Selective reabsorption in the proximal convoluted tubule

Answer 9

1. Glomerular filtrate enters the PCT from Bowman’s capsule; contains water, glucose, amino acids, ions, urea.
   
   2. The PCT wall is made of cuboidal epithelial cells with microvilli, increasing surface area for absorption.
   3. Glucose and amino acids are reabsorbed by co-transport with sodium ions (Na⁺):
      • Sodium ions are actively transported out of the epithelial cells into the blood by the sodium-potassium pump (requires ATP).
      • This maintains a low sodium concentration inside the cells.
      • Sodium ions then move from the filtrate into the epithelial cells by facilitated diffusion, bringing glucose and amino acids along (co-transport).
   4. Glucose and amino acids move into the blood by facilitated diffusion.
   5. Mineral ions (Na⁺, Cl⁻, etc.) are also reabsorbed by active transport and diffusion.
   6. Water follows by osmosis due to the lowered water potential in the blood vessels.
   7. Most of the water, all glucose, most mineral ions, and amino acids are reabsorbed in the PCT.
   8. Urea is partially reabsorbed by diffusion (down its concentration gradient).
   9. The filtrate leaving the PCT has less glucose, amino acids, ions, and less water.

Question 10

Loop of Henle Function
– Countercurrent Multiplier System (AQA A-Level Biology)

Purpose:

Answer 10

• To create a concentration gradient in the medulla of the kidney.
• This gradient allows the kidney to reabsorb water efficiently, producing urine that’s more concentrated than blood — saving water.

Question 11

Loop of Henle structure

Answer 11

Structure:
• The loop has two limbs running in opposite directions:
• Descending limb – permeable to water, impermeable to ions.
• Ascending limb – impermeable to water, actively pumps out ions.

Question 12

Loop of Henley process

Answer 12

1. In the thick ascending limb:
   • Sodium (Na⁺) and chloride (Cl⁻) ions are actively pumped out into the medulla’s interstitial fluid.
   • This lowers the water potential in the medulla, making the tissue fluid very salty.
   • The ascending limb is impermeable to water, so water stays inside the tubule.
   
   2. In the descending limb:
      • The descending limb is permeable to water but not to ions.
      • Water moves out of the descending limb by osmosis into the salty medulla (where ions have been pumped out).
      • This causes the filtrate inside the descending limb to become more concentrated as it descends.
   3. Countercurrent flow:
      • Because the filtrate flows down the descending limb and up the ascending limb in opposite directions, a steep concentration gradient is maintained along the whole length of the loop.
      • This multiplies the concentration gradient, making the medulla’s interstitial fluid very salty.

Question 13

Loop of Henley result

Answer 13

• The kidney can produce urine that is more concentrated than blood plasma.
• The salty medulla allows water reabsorption in the collecting duct via osmosis (regulated by ADH).
• Helps conserve water when the body needs it.