StemUp: AQA A level Biology 3.6.4 Homeostasis

What does homeostasis involve? (3)

- Involves physiological control systems

- That maintain the internal ‏‏‎ ‏‏‎environment with restricted limits

- I.e. maintains a relatively constant internal ‏‏‎‎‏‏‎environment

Why is it important that we maintain a stable core temperature? (3)

- To provide an optimum temperature for enzyme activity

- Low temperatures will not provide sufficient kinetic energy for optimum enzyme activity

- If temperatures are too high, enzymes will denature

Why is it important that we maintain a stable blood pH? (2)

- To provide an optimum pH for enzyme activity

- Extreme changes in pH from the optimum will cause enzymes to denature

Why is it important that we maintain a stable blood glucose concentration? (2)

- To provide a sufficient substrate for respiration

- To release energy for metabolic processes that occur in the body

Why is it important that we maintain a stable water potential of blood within restricted limits? (2)

- So that excess water does not enter or leave body cells by osmosis

- Which will cause cells to burst or dehydrate and shrivel

What is a negative feedback system? (3)

- Restores systems to their original level

- Deviations from the norm (original level)

- Results in corrective processes being stimulated to restore them to their norm

What are two factors that affect blood glucose concentration? (2)

- Consumption of carbohydrates (glucose is absorbed)

- Rate of respiration of glucose - this uses up glucose rapidly e.g. during exercise due to muscle contraction

What is the role of the liver in glycogenesis? (1)

Converts glucose to glycogen

What is the role of the liver in glycogenolysis? (1)

Converts glycogen to glucose

What is the role of the liver in gluconeogenesis? (1)

Converts amino acids and/or glycerol into glucose

Describe the action of insulin in decreasing blood glucose concentration (6)

1. Insulin attaches to specific receptors on the cell surface membrane of target cells, e.g., liver and muscle cells

2. This causes more glucose channel proteins to be incorporated into the cell surface membrane

3. This increases the membrane's permeability to glucose, allowing more glucose to enter cells by facilitated diffusion

4. Insulin activates enzymes involved in the conversion of glucose to glycogen (glycogenesis)

5. This lowers the glucose concentration in the blood

6. Glucose enters cells by facilitated diffusion down a concentration gradient

Describe the action of glucagon in increasing blood glucose concentration (3)

1. Glucagon attaches to specific receptors on cell surface membranes of target cells eg. liver

2. Glucagon activates the enzymes that are involved in glycogenolysis (hydrolysis of glycogen to glucose)

3. It also activates enzymes that are involved in gluconeogenesis (the conversion of glycerol / amino acids into glucose)

Describe the action of adrenaline in increasing blood glucose concentration (2)

1. Adrenaline attaches to specific receptors on cell surface membranes of target cells eg. liver

2. Adrenaline activates enzymes involved in glycogenolysis (hydrolysis of glycogen to glucose)

Describe the secondary messenger model of adrenaline and glucagon action (5)

1. Adrenaline / glucagon ('first messenger') attaches to specific receptors on cell membrane

2. This activates enzyme adenylate cyclase

3. This enzyme converts many ATP to many cyclic AMP (cAMP)

4. cAMP acts as the second messenger that activates protein kinase enzymes

5. Protein kinases activates the enzymes that are required to break down glycogen to glucose

What is the cause of type 1 diabetes? (2)

- Occurs when the cells in the pancreas that is responsible for for the production of insulin

- Are destroyed by the immune system

- Usually due to genetics that increase the chances of development

What is an example of a treatment used for type 1 diabetes? (2)

Daily injection of insulin

What is the cause of type 2 diabetes? (2)

Often occurs because:

- Cells have fewer insulin receptors

- Their receptors are faulty and can no longer respond to insulin

What is a significant risk factor for type 2 diabetes? (1)

Long term obesity

What is an example of a treatment used for type 2 diabetes? (3)

- Controlling diet

- E.g. avoiding foods that cause a rapid surge in blood glucose

- By regular exercise

Draw the structure of a single nephron (11)

How is the glomerular filtrate formed? (5)

1. There is high hydrostatic pressure in the glomerulus

2. This is because the diameter of the afferent arteriole (in) is wider than that of the efferent arteriole (out)

3. Small substances, e.g., water, glucose, ions, and urea, are forced into the glomerular filtrate

4. These substances are then filtered through the pores between capillary endothelial cells, the capillary basement membrane, and podocytes

5. Large proteins and blood cells remain in the blood.

Describe how glucose is reabsorbed by the proximal convoluted tubule (4)

1. Sodium ions are being actively transported out of epithelial cells into the capillary

2. Sodium ions are then moved by facilitated diffusion from the lumen in the proximal convoluted tubule into the epithelial cells

3. This brings glucose into the epithelial cells against its concentration gradient

4. Glucose will then move into the capillary by facilitated diffusion down its concentration gradient

How is water reabsorbed by the proximal convoluted tubule? (2)

1. Glucose in the capillaries lowers water potential

2. Water will move by osmosis from a higher water potential in the PCT into the capillaries

What is the role of the loop of henle in maintaining the gradient of sodium ions in the ascending limb? (3)

1. Sodium ions are actively transported out of the ascending limb (so filtrate concentration decreases) and water remains as ascending limb is impermeable to water

2. This increases the concentration of sodium ions in medulla (surrounding tissue around the loop of henle)

3. Which lowers the water potential of the medulla

What is the role of the loop of henle in maintaining the gradient of sodium ions in the descending limb? (3)

1. Water moves out of the descending limb by osmosis

2. This allows water to be reabsorbed by capillaries (so filtrate concentration increases)

3. So sodium ions can diffuse back into the descending limb of the loop of henle

Describe the reabsorption of water by the distal convoluted tubule and collecting ducts (3)

1. Water moves out of the distal convoluted tubule and collecting duct by osmosis, down a water potential gradient

2. This is controlled by ADH

3. Which increases the permeability of the collecting duct

What is meant by osmoregulation? (1)

The control of the water potential of blood via negative feedback

How does our body respond to an increase in water potential? (5)

1. Osmoreceptors detect an increase in the water potential of blood

2. So the hypothalamus produces less ADH

3. Posterior pituitary gland secretes less ADH into blood

4. ADH attaches to receptors on the collecting duct and decreases permeability of cells to water

5. So less water is reabsorbed from the DCT / collecting duct by osmosis

6. Urine will have a larger volume and be less concentrated

How does our body respond to a decrease in water potential? (5)

1. Osmoreceptors detect decrease in water potential

2. So the hypothalamus produces more ADH

3. Posterior pituitary gland secretes more ADH into blood

4. ADH attaches to receptors on collecting duct and increases permeability of cells to water (aquaporins join cell surface membrane)

5. So more water reabsorbed from DCT / collecting duct by osmosis

6. Urine will have a smaller volume and be more concentrated