Topic 6C: Homeostasis

Question 1

What is homeostatis

Answer 1

The maintenance of a stable internal environment, which is vital for the normal functioning of cells and preventing damage to the body.

Question 2

Problems caused by high blood glucose concentration

Answer 2

Water potential is reduced, meaning water molecules diffuse out of the cells and into the blood via osmosis, causing the cells to shrivel up and die

Question 3

Problems caused by low blood glucose concentration

Answer 3

Cells cannot respire as there isn’t enough glucose available, so they cannot carry out normal functions

Question 4

Negative feedback

Answer 4

The mechanism which restores the level to normal.

Normal level
⬇️
Level changes from normal
⬇️
Receptors detect a change
⬇️
Communication via Nervous or hormonal systems
⬇️
Effectors respond
⬇️
(Back to start)

Multiple negative feedback systems help to control levels.

Question 5

Positive feedback

Answer 5

The mechanism which amplifies a change away from the normal.

Normal level
⬇️
Level changes from normal
⬇️
Receptors detect a change
⬇️
Communication via Nervous or hormonal systems
⬇️
Effectors respond
⬇️
(Back to level changes from normal)

Question 6

Islet of langerhan

Answer 6

A cluster of cells in the pancreas, containing Beta cells which secrete insulin into the blood and Alpha cells which secrete glucagon into the blood.

Question 7

The role of insulin in controlling blood glucose concentration

Answer 7

1) Insulin lowers the blood glucose concentration by binding to specific receptors on cell membranes of muscle and liver cells.
2) This activates enzymes that carry out glycogenesis to convert glucose into glycogen.
3) It also increases the permeability of these membranes so the cell takes in more glucose from the blood.
4) Insulin increases the rate of respiration of glucose.

Question 8

The role of Glucagon in controlling blood glucose concentration

Answer 8

1) Glucagon raises blood glucose concentration by binding to specific receptors in the cell membranes of muscle and liver cells.
2) This activates enzymes that catalyse glycogenolysis and gluconeogenesis reactions to produce glucose.
3) Glucagon also decreases the rate of respiration of glucose.

Question 9

Blood glucose concentration reactions

Answer 9

Glycogenesis:
Glucose -> Glycogen

Glycogenolysis:
Glycogen -> Glucose

Gluconeogenesis:
Glycerol + amino acids -> Glucose

Question 10

Hormonal Vs Nervous response

Answer 10

Hormonal:
Slower.
Can affect whole body.
Longer lasting.

Nervous:
Faster.
Are very localised.
Shorter lasting.

Question 11

Negative feedback to control a rise in blood glucose levels

Answer 11

1) Normal blood glucose concentration.
2) Rise in blood glucose concentration.
3) Pancreas detects change.
4) Pancreas secretes Insulin and stops Glucagon secretion.
5) Respiration increases, glycogenesis activated, cells take up more glucose.

Question 12

Negative feedback to control a fall in blood glucose levels

Answer 12

1) Normal blood glucose concentration.
2) Fall in blood glucose concentration.
3) Pancreas detects change.
4) Pancreas stops Insulin secretion and secretes Glucagon.
5) Respiration decreases, glycogenolysis activated, gluconeogenesis activated.

Question 13

What is a Glucose transporter

Answer 13

Glucose transporters are channel proteins which allow glucose to be transported across a cell membrane.

Question 14

How do Glucose transporters work

Answer 14

1) When Insulin levels are low GLUT 4 is stored in vesicles in the cytoplasm of cells.
2) However, when Insulin levels rise, Insulin binds to receptors on the cell surface membrane.
3) This triggers the movement of GLUT4 to the membrane, where it allows glucose to transport through via facilitated diffusion.

Question 15

Adrenaline

Answer 15

Adrenaline increases the blood glucose concentration by binding to receptors on liver cells which:

• Activates glycogenolysis.
• Inhibits glycogenesis.
• Activates glucagon secretion and inhibits Insulin secretion.

Question 16

What are second messengers

Answer 16

1) The binding of a hormone to cell receptors activated an enzyme on the inside of the cell membrane, which produces a chemical known as a second messenger.
2) This activates other enzymes inside the cell to bring about a response.

Question 17

Second messengers in blood glucose control

Answer 17

1) Adrenaline and glucagon bind to receptors and activate an enzyme called adenylate cyclase.
2) This converts ATP into cyclic AMP (cAMP) which is a second messenger.
3) cAMP activates an enzyme called protein kinase A which activates a cascade that breaks down glycogen into glucose (glycogenolysis).

Question 18

Type 1 diabetes

Answer 18

• The immune system attacks Beta cells in the islets of Langerhans meaning they are unable to produce insulin.
• This can be caused by genetics, and means it is very hard to control blood glucose levels.

Question 19

Type 2 diabetes

Answer 19

• The Beta cells do not produce enough insulin, or insulin receptors don’t work properly, meaning cells don’t take up enough glucose.
• It is caused by obesity and other lifestyle factors, and can be helped by exercise as it increases the rate of respiration of glucose.

Question 20

The kidneys

Answer 20

1) As blood passed through the capillaries in the cortex, ultrafiltration occurs, filtering out substances from the blood.
2) Then the useful substances are reabsorbed through selective reabsorption, and waste substances are passed to the bladder where they are excreted as urine.

Question 21

Ultrafiltration

Answer 21

1) The efferent arteriolar is smaller in diameter than the afferent arteriole meaning the blood in the Glomerulus is under high pressure.
2) Therefore liquid and small molecules are forced out of the blood in the capillary and into the Bozeman’s capsule, where they are known as the Glomerular filtrate and are passed along the rest of the Nephron.
3) However, large molecules (like RBC’s and proteins) are too big to pass through the membrane so remain in the blood.

Question 22

Selective reabsorption

Answer 22

The epithelium of the wall of the proximal convoluted tubule has microvilli to provide a large SA for the reabsorption of useful materials from the Glomerular filtrate.

1) Useful solutes (like glucose) are reabsorbed along the proximal convoluted tubule by active transport and facilitated diffusion.

2) Water enters the blood via osmosis because the water potential of the blood is lower than that of the filtrate. However, it is then reabsorbed into the:
- proximal convoluted tubule
- distal convoluted tubule
- loop of Henle
- collecting duct

3) This only leaves urine in the filtrate which passes along the ureter to the bladder.

Question 23

What is urine

Answer 23

Made up of water, dissolved salts, urea and other substances.

Question 24

Regulation of water content

Answer 24

Osmoregulation: the control of water potential of the blood carried out by the kidneys.

1) If water potential of the blood is low, more water is reabsorbed into the blood from the nephron tubules via osmosis. Therefore, urine is more concentrated, so less water is lost during excretion.
1) If water potential of the blood is high, less water is reabsorbed into the blood from the nephron tubules via osmosis. Therefore, urine is less concentrated, so more water is lost during excretion.

Question 25

The loop of henle

Answer 25

1) At the top of the descending limb, sodium ions are actively pumped into the medulla, lowering the water potential in the medulla.
2) This produces a water potential gradient, causing water to move out of the descending limb and into the medulla via osmosis, making the glomerular filtrate more concentrated.
3) At the bottom of the ascending limb, sodium ions diffuse out into the medulla, further lowering the water potential in the medulla.
4) Therefore water moves out of the collecting duct and into the medulla via osmosis.
5) The water in the medulla is then reabsorbed into the blood through the capillary network and distal convoluted tubules via osmosis.

Question 26

How is ADH secreted

Answer 26

1) When the water potential of the blood decreases, water moves out of the osmoreceptors (found in the hypothalamus) and into the blood via osmosis.
2) This causes the volume of the osmoreceptors to decrease which sends a signal to the posterior pituitary gland.
3) This causes them to release ADH into the blood.

Question 27

How do Antidiuretic hormones (ADH) work

Answer 27

1) ADH molecules bind to receptors on the plasma membranes of cells in the distal convoluted tubule and collecting duct.
2) This causes aquaporins to be inserted into the plasma membrane, making it more permeable to water.
3) Therefore more water is reabsorbed into the medulla and blood via osmosis.
4) This means only a small amount of concentrated urine is produced, so less water is lost from the body.

Question 28

How does the body control dehydration and hydration?

Answer 28

Dehydration = the water potential of the blood decreases so more ADH is released.

Hydration = the water potential of the blood increases so less ADH is released.