Homeostasis

Question 1

Homeostasis definition

Answer 1

physiological control systems maintaining an internal environment within restricted limits in organisms.

Question 2

Importance of homeostasis - general

Answer 2

cells of the body are in an environment that suits their requirements and means they can function normally despite external change. There are constant fluctuations around the optimum point.

Question 3

Why is maintaining a constant body temperature important?

Answer 3

Low temp - a decrease in enzyme activity and the rate of biochemical reactions as molecules have less KE. Eg low respiratory rate - ATP needed by cells cannot be produced fast enough.
High temp - enzymes and proteins may not function as efficiently of be denatured, so metabolic reactions cannot be catalysed.

Question 4

Importance of maintaining a constant pH

Answer 4

Enzymes and proteins may function less efficiently or become denatured if pH fluctuates far away from the optimum.

Question 5

Importance of maintaining a constant blood water potential

Answer 5

Changes to blood water potential can cause cells to shrink or expand/ burst by water entering or leaving by osmosis. This means the cell can’t function normally. Maintaining a constant blood glucose concentration is important in keeping WP constant, as well as for ensuring cells have a reliable glucose source for respiration.

Question 6

Why is maintaining a constant BGC important?

Answer 6

For keeping WP constant, as well as for ensuring cells have a reliable glucose source for respiration.

Question 7

Positive feedback

Answer 7

Deviation from the optimum point causes changes that lead to even greater deviation.

Question 8

Examples of positive feedback

Answer 8

Stimulation of a generator potential and action potential in neurones, hyper/ hypothermia

Question 9

Negative feedback

Answer 9

Deviation from the optimum triggers a mechanism to return it to the optimum. When this happens, corrective mechanisms are switched off.

Question 10

The role of the pancreas in regulating BGC

Answer 10

The pancreas is a gland which produces the hormones insulin and glucagon. Groups of hormone producing cells = islets of Langerhans, including α cells which produce glucagon and β cells which produce insulin.

Question 11

Where do the hormones produced by the pancreas have their effect?

Answer 11

The liver

Question 12

3 important processes involved in BGC which occur in the liver

Answer 12

Glycogenesis, glycogenolysis, gluconeogenesis

Question 13

What is glycogenesis?

Answer 13

conversion of glucose to glycogen, this occurs when BGC is high. Glucose is removed from the blood by the liver, then converted.

Question 14

What is glycogenolysis?

Answer 14

the breakdown of glycogen to form glucose. This occurs when BGC is low - the liver converts glycogen back into glucose which diffuses into the blood to raise BGC.

Question 15

What is gluconeogenesis?

Answer 15

The production of glucose from sources other than carbohydrates, eg glycerol and amino acids. This occurs when the liver’s glycogen supply runs out.

Question 16

What is the advantage of having separate negative feedback mechanisms to control departure from the optimum in both directions?
BGC example

Answer 16

There are positive actions in both directions, giving a greater degree of homeostatic control.

eg if glucagon raises BGC above the optimum, it would take a while to fall if it could only be lowered through metabolic activity. However, insulin allows BGC to be lowered and returned to the optimum much more rapidly.

Question 17

When is insulin secreted?

Answer 17

when BGC rises

Question 18

The action of insulin

3

Answer 18

• attaches to receptors on the surfaces of target cells
• controls the uptake of glucose by regulating the inclusion of channel proteins in the surface membranes of target cells (by making vesicles containing the protein channels in their membrane fuse with the CSM)
• activates enzymes involved in the conversion of glucose to glycogen

Question 19

How is BGC returned to the opt when it increases?

Answer 19

increase is detected by receptor cells in the pancreas, which produce insulin. Insulin causes an increase in the rate of glucose uptake by cells and glycogenesis, eg in the liver, to BGC returns to opt.
Lowering BGC causes β cells to secrete less insulin - this is negative feedback.

Question 20

How does BGC fall without the action of insulin (eg in diabetics)

Answer 20

Glucose is taken into cells to be used for respiration and is secreted in urine

Question 21

When is glucagon released

Answer 21

when a fall in BGC is detected

Question 22

What is the effect of insulin

Answer 22

To decrease BGC

Question 23

The action of glucagon

Answer 23

• Attaches to receptors on the CSM of target cells (eg liver cells).
• Activates enzymes that convert glycogen to glucose. (glycogenolysis)
• Activating enzymes involved in gluconeogenesis - conversion of amino acids and glycerol to glucose.
  (gluconeogenesis)

Question 24

Negative feedback with hormones

Answer 24

the secretion of a hormone results in a reduction in its own secretion.

Question 25

The role of adrenaline

Answer 25

Adrenaline is a hormone involved in the ‘fight-or-flight’ response. It is produced by the adrenal glands at times of stress, excitement or danger. Adrenaline raises BGC: - Attaching to protein receptors on the CSM of target cells - Activating enzymes involved in converting glycogen to glucose.

Question 26

Why is it important that adrenaline increases BGC

Answer 26

muscles need more glucose for stronger contractions under the influence of adrenaline. - fight or flight response.

Question 27

The second messenger model of glucagon/ adrenaline action

Answer 27

1. Adrenaline/ glucagon binds to a transmembrane protein receptor on a liver cell CSM. 2. This binding causes the protein to change shape on the inside of the membrane. 3. The shape change leads to activation of an enzyme called adenylate cyclase. 4. Adenylate cyclase converts ATP to cyclic AMP (cAMP). 5. cAMP acts as a second messenger, binding to protein kinase enzyme, changing its shape and activating it. 6. Active protein kinase enzyme catalyses the conversion of glycogen to glucose, which leaves liver cells by facilitated diffusion into the blood.

Question 28

The effect of glucagon

Answer 28

an increase in BGC, which returns to its optimum. This causes α cells to secrete less glucagon. (negative feedback)

Question 29

How is type 1 diabetes caused?

Answer 29

the body cannot produce insulin. This could be due to an autoimmune response where the body attacks its own β cells. Type I diabetes normally begins in childhood, develops quickly and has obvious symptoms.

Question 30

Which type of diabetes is 'insulin dependent' and which is not?

Answer 30

Type 1 is insulin dependent.

Question 31

How is type 1 diabetes managed?

Answer 31

by injecting insulin. The dose of insulin must be matched to the glucose intake, so BGC is monitored using biosensors. Carbohydrate intake and exercise must also be managed.

Question 32

How is type 2 diabetes caused?

Answer 32

glycoprotein receptors on body cells become lost or unresponsive to insulin, or the supply of insulin from the pancreas is inadequate. Type II generally develops slowly in middle-aged people and has less severe symptoms. People who are overweight or have a poor diet are more likely to develop type II diabetes.

Question 33

How is type 2 diabetes controlled?

Answer 33

by carefully managing carbohydrate intake and the amount of exercise taken. Drugs which stimulate insulin production may also be taken.

Question 34

Where are receptors which detect changes in BGC levels located?

Answer 34

The pancreas

Question 35

Where are receptors which detect changes in blood temperature located?

Answer 35

The hypothalamus

Question 36

Name 1 organ in the human body containing cells with glucagon receptors.

Answer 36

The liver

Question 37

Name the processes stimulated to occur when glucagon is secreted

Answer 37

Gluconeogenesis, glycogenolysis

Question 38

How is high BGC returned to normal?

Answer 38

High BGC detected by the pancreas, stimulates B cells to release insulin. Insulin causes increased glucose uptake, eg glucose to glycogen by the liver, uptake by muscles.

Question 39

Which cells have large glycogen stores?

Answer 39

liver and muscle

Question 40

Which cell types produce insulin?

Answer 40

Beta cells in the pancreas/ islets of Langerhans

Question 41

Which cell types produce glucagon?

Answer 41

Alpha cells in the pancreas/ islets of Langerhans

Question 42

Why is osmoregulation important?

Answer 42

An optimum concentration of water and salts needs to be maintained in tissue fluid and blood plasma, as changes to water potential of these fluids can cause osmotic problems - cells expand or shrink and cannot function properly.

Question 43

Nephron structure

Answer 43

Nephron structure Renal (Bowman’s) capsule: cup-shaped, surrounds the glomerulus. The inner layer is formed of specialised cells called podocytes. Proximal convoluted tubule: loops surrounded by blood capillaries, epithelial cell walls with microvilli. Loop of Henle: long, hairpin look extending from the cortex into the medulla and back. Surrounded by blood capillaries. Distal convoluted tubule: series of loops surrounded by (fewer) blood capillaries with epithelial cell walls. Collecting duct: tube into which several distal convoluted tubules connect. It becomes wider as it empties into the renal pelvis.

Question 44

Why does ultrafiltration occur in the glomerulus?

Answer 44

The diameter of the afferent arteriole is larger than that of the efferent arteriole, so hydrostatic pressure increases in the glomerulus. This means small molecules such as water and mineral ions are forced out of the capillary and into the renal capsule to form glomerular filtrate. Blood cells and proteins are too large to be forced out.

Question 45

Where in the nephron does ultrafiltration occur?

Answer 45

the glomerulus

Question 46

Where does reabsorption of water and glucose occur?

Answer 46

the proximal convoluted tubule

Question 47

Adaptations of PCT cells for reabsorption of water and glucose

Answer 47

microvilli provide LSA mitochondria provide ATP for active transport Carrier proteins for active transport Channel proteins for facilitated diffusion

Question 48

Substances present in glomerular filtrate

Answer 48

``` water glucose urea amino acids/ fatty acids ions, eg sodium ```

Question 49

substances not present in glomerular filtrate

Answer 49

blood cells, large proteins, platelets

Question 50

How is glucose reabsorbed in the PCT?

Answer 50

co-transport

Question 51

Describe the reabsorption of glucose in the PCT by co-transport

Answer 51

1. Sodium ions are actively transported out of PCT cells into blood capillaries which carry them away, hence lowering sodium ion conc. in these cells. 2. Sodium ions diffuse down a concentration gradient from the PCT lumen into epithelial lining cells via special facilitated diffusion carrier proteins. 3. These carrier proteins also carry another molecule (glucose, amino acids, chloride ions ... ) with the sodium ions - this is cotransport. 4. The cotransported molecules then diffuse into the blood.

Question 52

How is the renal capsule adapted for ultrafiltration?

Answer 52

Podocytes, the specialised cells lining the renal capsule, have spaces between them to allow filtrate to pass through. (capillaries in glomerulus also have spaces between endothelial cells)

Question 53

What is the role of the loop of Henle?

Answer 53

To maintain a gradient of sodium ions in the medulla

Question 54

Describe the gradient of sodium ions/ water potential in the medulla maintained by the loop of Henle

Answer 54

Sodium ion concentration increases and water potential decreases the further into the medulla you go,

Question 55

Describe the permeabilities of the ascending and descending limbs of the loop of Henle.

Answer 55

Descending - thin walls which are permeable to water | Ascending - wider with thicker walls which are impermeable to water

Question 56

Explain the role of the loop of Henle in the absorption of water from the filtrate

Answer 56

- sodium ions are actively removed from the ascending limb - the ascending limb is impermeable to water - water moves out of the descending limb - the water potential in the medulla becomes more negative - the longer the loop, the greater the osmotic gradient, so more water is reabsorbed - water leaves the collecting duct by osmosis

Question 57

Explain the role of ADH in the production of concentrated urine

Answer 57

- If the water potential of the blood is too low, this is detected by osmoreceptors in the hypothalamus. - The hypothalamus stimulates the pituitary gland to release more ADH - ADH increases the permeability of the collecting duct to water by increasing the number of aquaporins - more water is reabsorbed by osmosis, from the nephron into the blood

Question 58

Where is ADH produced and released into the blood?

Answer 58

produced in the hypothalamus and released at the pituitary gland

Question 59

What does ADH stand for?

Answer 59

antidiuretic hormone

Question 60

How does the binding of ADH to receptors in the kidney lead to an increase in blood WP?

Answer 60

ADH binds to specific protein receptors on the CSM of collecting duct and DCT cells, leading to the activation of the enzyme phosphorylase, which causes vesicles in the cell which contain aquaporins in their membrane to fuse with the CSM. The increase in numbers of aquaporins makes the membrane more permeable to water. water passes out of CD/DCT by osmosis and is reabsorbed by the blood.

Question 61

What happened once ADH has caused a rise in blood WP?

Answer 61

The rise in WP is detected by osmoreceptor cells, which send fewer impulses to the pituitary. Less ADH is released and the permeability of collecting ducts returns to normal. This is negative feedback.

Question 62

Compare and contrast the features of hormonal and nervous transmission

Answer 62

Hormones - have slow, long-lasting and widespread effects | Nerve impulses - rapid, short-lived and localised

Question 63

Why do hormones only have an effect on certain cells?

Answer 63

as only the target cells have the specific protein receptors which are complementary to the shape of the specific hormone

Question 64

How do hormones reach their target cells?

Answer 64

they travel in blood plasma

Question 65

What happens if blood WP rises above the optimum?

Answer 65

hypothalamus detects this, stimulating pituitary to release less ADH, so collecting duct becomes less permeable to water etc.

Question 66

Importance of the counter-current multiplier

Answer 66

needed to make urine which is more concentrated than blood plasma. Counter-current flow between fluid in the descending and ascending limbs increases/ multiplies the osmotic gradient between fluid in the tubules and the medulla. Counter-current flow between fluid in the collecting duct and ascending limb means that filtrate in the CD meets fluid in the medulla with an even lower WP - a WP gradient exists for the whole length of the collecting duct.

Question 67

Name the blood vessels associated with the nephron

Answer 67

Afferent arteriole, glomerulus, efferent arteriole, blood capillaries

Question 68

Role of the afferent and efferent arterioles in the nephron

Answer 68

Afferent arteriole - brings blood to the glomerulus Efferent arteriole - recombined glomerulus - takes blood away from the renal capsule and glomerulus, has a smaller diameter than the afferent arteriole - this means blood in the glomerulus is under high pressure and small molecules are forced out. It then branches to form blood capillaries.

Question 69

Role of the glomerulus in the nephron

Answer 69

Glomerulus - branched knot of capillaries in the renal capsule from which fluid is forced out of the blood. (ultrafiltration). Glomerular filtrate is formed.

Question 70

Role of the blood capillaries in the nephron

Answer 70

a network of capillaries surrounding PCT, loop of Henle and DCT. This is where reabsorption of mineral salts, glucose and water occurs. Blood capillaries re-merge to form venules, which merge to form the renal vein.