6.4 Homeostasis

Question 1

What is homeostasis?

Answer 1

The maintenance of the internal environment within an optimum range.

Question 2

What factors is homeostasis necessary to control?

Answer 2

• pH
• Temperature
• Blood glucose levels

Question 3

What is negative feedback?

Answer 3

When a change is responded to in order to counteract the effects of the change

Question 4

Why must the blood glucose concentration not get too high?

Answer 4

Glucose can affect the water potential of the blood.
An increase in blood glucose concentration will decrease the water potential of the blood.
Water will move out of tissues into the blood by osmosis. This causes dehydration of the cells and the cells will die.

Question 5

Why must the blood glucose concentration not get too low?

Answer 5

Glucose is a respiratory substrate.
There must be enough glucose in the blood to meet the demands of respiring cells.
If glucose levels are too low, respiration rate will slow.

Question 6

What two external factors affect blood glucose levels?

Answer 6

• Eating
• Exercising

Question 7

What is glycogenesis?

Answer 7

The making of glycogen from glucose

Question 8

What is glycogenolysis?

Answer 8

The hydrolysis of glycogen into glucose

Question 9

What is gluconeogenesis?

Answer 9

When glycerol and amino acids are used to make glucose

Question 10

What happens in response to high blood glucose concentration?

Answer 10

• Beta cells in the pancreas detect the change and secrete insulin into the blood
• Insulin binds to receptors on the muscle cell membranes.
• More glucose channel proteins are formed in the cell membrane. This causes:
  The rate of uptake of glucose by muscle cells to increase.
  The rate of respiration in the muscle cells to increase.
• Insulin binds to receptors on the liver cell membranes.
• Enzymes in the liver cells that convert glucose to glycogen (glycogenesis) are activated

Question 11

What happens when blood glucose concentration gets too low?

Answer 11

• Alpha cells in the pancreas detect the change and secrete glucagon into the blood
• Glucagon travels to the liver cells and binds to receptors on the cell membranes
• The liver cells produce enzymes that convert glycogen to glucose (glycogenolysis).
• Binding of glucagon to liver cell membranes also causes the release of enzymes that form glucose from glycerol and amino acids (gluconeogenesis)

Question 12

Describe the steps of the adrenaline response

Answer 12

• Adrenaline is secreted from the adrenal gland in response to low blood glucose concentration, exercise and stress.
• Adrenaline binds to receptors on the liver cell membrane.
• Adrenaline induces two reactions in the liver cells:
  Activation of glycogenolysis (glycogen → glucose).
  Inhibition of glycogenesis (glucose → glycogen).
• Adrenaline also promotes secretion of glucagon from the pancreas and inhibits secretion of insulin.

Question 13

What are primary messengers?

Answer 13

Primary messengers are messengers that do not enter a cell.
Primary messengers exert an action on the cell membrane by binding to receptors and triggering a change within the cell.

Question 14

What are secondary messengers?

Answer 14

Secondary messengers initiate and coordinate responses that take place inside a cell.
Secondary messengers are usually activated by the binding of a primary messenger to a cell surface receptor.

Question 15

Explain cAMP’s role in the control of blood glucose

Answer 15

Adrenaline or glucagon (primary messengers) bind to receptors on the cell membranes of liver cells. Binding of adrenaline or glucagon activates an enzyme called adenylate cyclase. Adenylate cyclase converts ATP to cyclic AMP (cAMP). cAMP activates an enzyme called protein kinase A.
Protein kinase A triggers a cascade of reactions that result in glycogenolysis.

Question 16

Which type of diabetes is diabetes mellitus?

Answer 16

Both type 1 and 2

Question 17

Cause and treatment for type 1 diabetes

Answer 17

• Type I diabetes is caused when the beta cells in the pancreas are attacked by the immune system.
• The beta cells become damaged and can no longer produce insulin.
• Insulin is injected regularly during the day or an insulin pump can be used continuously.

Question 18

Cause and treatment for type 2 diabetes

Answer 18

• Type 2 diabetes is correlated with obesity, lack of exercise, age and family history.
• Type 2 diabetes develops when the beta cells in the pancreas no longer produce enough insulin or when the muscle and liver cells stop responding to insulin.
• Type 2 diabetes is treated by eating a healthy diet and exercising.

Question 19

Where are beta and alpha cells located in the pancreas?

Answer 19

Islets of Langerhans

Question 20

What organ controls water potential?

Answer 20

Kidneys

Question 21

What is osmoregulation?

Answer 21

Osmoregulation is the control of the water potential in the blood.

Question 22

Where in the kidney does osmoregulation take place?

Answer 22

The nephrons

Question 23

Explain the process of ultrafiltration in the kidney

Answer 23

• The branch of capillary that enters the glomerulus is much wider than the branch that exits the glomerulus. This creates a high blood pressure in the glomerulus. (Enters through afferent arteriole leaves through efferent arteriole).
• The high blood pressure causes the fluid and its solutes (e.g. glucose, amino acids) in the blood to be forced out of the capillary. This is called pressure filtration.
• The liquid and small molecules pass through three layers to get into the bowman’s capsule and enter the nephron tubules.
• Larger molecules like proteins and blood cells can’t pass through so stay in the blood.

Question 24

What are the substances that enter the bowman’s capsule known as?

Answer 24

The glomerular filtrate

Question 25

What are the three membranes that molecules pass through during ultrafiltration?

Answer 25

- Capillary endothelium - fluid flows through pores - Basement membrane - slit pores - Bowman’s capsule epithelium (podocytes) - The epithelial cells, called podocytes, have finger-like projections that the substances can flow between.

Question 26

Explain the process of selective reabsorption at the proximal convoluted tubule (PCT)

Answer 26

- Na+ ions are actively transported out of the epithelial cells surrounding the PCT and into the blood by sodium-potassium pumps. K+ ions are also transported into the epithelium. - Na+ ions in the filtrate diffuse from the PCT into the epithelial cells (down their concentration gradient) through co-transporter proteins. Co-transporter proteins allow glucose and amino acids to be transported into the epithelial cells along with the Na+ ions. - These molecules can then diffuse from the epithelial cells into the blood. - The movement of Na+ ions, glucose and amino acids into the bloodstream causes the water potential to decrease in the blood and increase in the PCT. Water in the PCT diffuses into the blood through osmosis.

Question 27

Explain osmoregulation at the loop of henle, DCT and collecting duct

Answer 27

- Na+ ions are actively transported out of the top of the ascending limb into the surrounding tissue fluid in the medulla. This causes the water potential of the medulla to decrease. The ascending limb is impermeable to water so water cannot diffuse out. - The descending limb is permeable to water. This means that water inside the tubule can diffuse out because there is a lower water potential in the medulla. The water is reabsorbed by the bloodstream. - Na+ ions diffuse out of the bottom of the ascending limb into the medulla. This further increases the solute concentration of medulla. - Water diffuses out of the distal convoluted tubules (DCT) and collecting duct by osmosis and is reabsorbed into the blood.

Question 28

What receptors detect changes in blood water potential?

Answer 28

Osmoreceptors

Question 29

What hormone controls osmoregulation?

Answer 29

Antidiuretic hormone (ADH)

Question 30

What section of the brain detects changes in blood water potential?

Answer 30

Hypothalamus

Question 31

Explain how decrease in blood water potential is detected and responded to

Answer 31

- If the water potential decreases, water diffuses out of the osmoreceptor cells and the cells shrink - When osmoreceptors shrink, this is detected by the posterior pituitary gland. - The posterior pituitary gland then releases ADH into the blood. - ADH is a hormone that binds to receptors on the cell membrane of epithelial cells of the distal convoluted tubule (DCT) and the collecting duct. - When ADH binds, vesicles containing aquaporins fuse with the cell membrane. Aquaporins are protein channels for water. Aquaporins increase the permeability of the DCT and collecting duct. This means that more water is reabsorbed into the blood by osmosis.