6 Organisms respond to changes: 16 Homeostasis

Question 1

What does homeostasis in mammals involve?

Answer 1

Physiological control systems that maintain the internal environment within restricted limits.

Question 2

What is the importance of maintaining a stable core temperature?

Answer 2

If body temperature is too high, enzymes may become denatured.
The hydrogen bonds that hold the enzyme in its tertiary structure break so the active site changes shape.
If body temperature is too low, enzyme activity is reduced, slowing the rate of metabolic reactions.
Metabolic reactions are less efficient.

Question 3

What is the importance of maintaining stable blood pH?

Answer 3

If blood pH is too high or too low, enzymes become denatured.
The hydrogen bonds in the tertiary structure break, changing the shape of the active site.
Metabolic reactions are less efficient.

Question 4

What is the importance of maintaining a stable blood glucose concentration?

Answer 4

If blood glucose concentration is too high, the water potential of blood decreases so water moves out of cells into the blood by osmosis. This can cause cells to shrivel up and die.
If blood glucose concentration is too low, there isn’t enough glucose for respiration to produce ATP for processes like muscle contraction.

Question 5

What does negative feedback do?

Answer 5

Restores systems to their original level.

Question 6

How does having multiple negative feedback systems give a greater degree of control?

Answer 6

Having multiple negative feedback mechanisms means you can actively increase or decrease a level, separately, so it returns to normal.
If there was only one negative feedback mechanism, you could only increase or decrease a level, not both.
Multiple negative feedback mechanisms gives you a faster response and more control.

Question 7

What does positive feedback do?

Answer 7

Amplify a change from the normal level to stimulate a faster negative feedback response.

Question 8

What are the factors that influence blood glucose concentration?

Answer 8

Eating and exercise.

Question 9

Describe the role of the liver and the action of insulin in glycogenesis.

Answer 9

When blood glucose concentration is too high:
- insulin is secreted by beta cells (in islets of Langerhans in the pancreas) into the blood
- insulin binds to specific receptors on the cell membranes of target cells (liver and muscle cells)
- it increases the permeability of cell membranes to glucose by increasing the number of channel proteins.
- insulin also activates enzymes that convert glucose to glycogen (glycogenesis).
- the cells store glycogen as an energy source
- insulin also increases the rate of respiration of glucose, especially in muscle cells.

Question 10

Describe the role of the liver and the action of glucagon in glycogenolysis.

Answer 10

When blood glucose concentration is too low:
- glucagon is secreted by alpha cells (in islets of Langerhans in the pancreas) into the blood
- glucagon bins to specific receptors on the cell membranes of liver cells
- it activates enzymes that break down glycogen into glucose (glycogenolysis)
- glucagon decreases the rate of respiration of glucose in cells

Question 11

Describe the action of glucagon in gluconeogenesis.

Answer 11

When blood glucose concentration is too low:
- glucagon is secreted by alpha cells (in islets of Langerhans in the pancreas) into the blood
- it activates enzymes that are involved in the formation of glucose from glycerol and amino acids (gluconeogenesis).

Question 12

Describe the role of adrenaline.

Answer 12

When blood glucose concentration is low, you’re stressed, and during exercise:
- adrenaline is secreted from the adrenal glands
- it binds to receptors in the cell membrane of liver cells
- it activates glycogenolysis
- it inhibits glycogenesis
- it also activates glucagon secretion and inhibits insulin secretion, which increases glucose concentration
- the body has more glucose for muscles to respire

Question 13

Describe the second messenger model of adrenaline and glucagon action.

Answer 13

1. Adrenaline and glucagon bind to their receptors and activate adenylate cyclase.
2. Activated adenylate cyclase converts ATP into a second messenger - cyclic AMP (cAMP).
3. cAMP activates protein kinase which catalyses the conversion of glycogen to glucose.

Question 14

What is the cause and control of Type 1 diabetes?

Answer 14

The immune system attacks the beta cells in the islets of Langerhans so they can’t produce any insulin.
May be due to a genetic predisposition or a viral infection.

Controlled by insulin therapy, such as insulin injections or an insulin pump.
Eating regularly and controlling carbohydrate intake can help avoid a sudden rise in glucose.

Question 15

What is the cause and control of Type 2 diabetes?

Answer 15

Often linked with obesity, family history, lack of exercise, age, and poor diet.
Occurs when the beta cells don’t produce enough insulin or when the body’s cells don’t respond properly to insulin. The insulin receptors on target cells are faulty or not there.

Can be controlled by eating a healthy diet, losing weight, and regular exercise. Glucose-lowering medication can be taken and insulin injections may be needed.

Question 16

Evaluate the positions of health advisors and the food industry in relation to the increased incidence of Type 2 diabetes.

Answer 16

Health advisors recommend a healthy diet, regular exercise, and weight loss. They aim to educate people on how to have a healthier lifestyle to reduce their risk of developing Type 2 diabetes. They would like the food industry to reduce advertising of junk food, to improve the nutritional value of their products, and to use clearer labelling so consumers can make healthier choices.

The food industry have used sugar alternatives in their products and reduced the sugar, fat, and salt content. But they still need to increase profit. When public perception of healthy eating changes, the industry will respond long-term.

Question 17

What is osmoregulation?

Answer 17

The control of the water potential of the blood.

Question 18

Describe the role of hormones in osmoregulation.

Answer 18

When blood water potential is too low:
- osmoreceptors in the hypothalamus detect the low water potential
- an impulse is sent to the posterior pituitary gland which is stimulated to release more antidiuretic hormone (ADH) into the blood
- more ADH makes the walls of the distal convoluted tubule and collecting duct more permeable to water.
- more water is reabsorbed into the medulla and blood by osmosis
- a small volume of concentrated urine is produced and less water is lost from the body

Question 19

Describe the formation of glomerular filtrate.

Answer 19

1. Blood from the renal artery enters the afferent arteriole into the glomerulus.
2. Filtered blood leaves the glomerulus throught the efferent arteriole which is smaller in diameter so the blood in the glomerulus is under high pressure.
3. The high pressure forces liquid and small molecules (not proteins or cells) out of the glomerulus and into the Bowman’s capsule, forming glomerular filtrate.

Question 19

Describe the structure of the nephron.

Answer 19

• afferent arteriole
• glomerulus
• efferent arteriole
• Bowman’s capsule
• proximal convoluted tubule
• loop of Henle
• distal convoluted tubule
• collecting duct

Question 20

Describe the reabsorption of glucose and water in the nephron.

Answer 20

1. The glomerular filtrate passes through the proximal convoluted tubule.
2. The epithelium of the proximal convoluted tubule has microvilli to provide a large surface area for reabsorption of useful substances into the blood.
3. Glucose is reabsorbed by active transport and facilitated diffusion (co-transport).
4. Water moves into the blood by osmosis down the water potential gradient.

Question 21

Describe the maintenance of a gradient of Na+ ions in the medulla.

Answer 21

The loop of Henle maintains a Na+ ion gradient:
1. Near the top of the ascending limb, Na+ ions are pumped out into the medulla by active transport. The ascending limb is impermeable to water so water stays inside. This creates a low water potential in the medulla as there’s a high concentration of ions.
2. Water moves out of the descending limb into the medulla by osmosis down the established water potential gradient. This makes the filtrate more concentrated. The water in the medulla is reabsorbed into the blood.
3. Near the bottom of the ascending limb, Na+ ions diffuse out into the medulla, further lowering the water potential in the medulla.

Question 22

Describe the reabsorption of water by the distal convoluted tubule and collecting duct.

Answer 22

• water moves out of the distal convoluted tubule by osmosis and is reabsorbed into the blood
• there’s counter-current flow between the descending limb of the loop of Henle and the collecting duct
• the filtrate in the collecting duct has a low water potential but the interstitial fluid has a lower water potential.
• so the water potential gradient is maintained down the whole length of the collecting duct so water moves out by osmosis throughout