Homeostasis

Question 1

Homeostasis definition

Answer 1

Homeostasis is the maintenance of a stable internal environment

Question 2

Negative feedback

Answer 2

Negative feedback means that when there is an increase/decrease from the set point, the opposite effect is instigated- produces a response that returns the value to the norm

Question 3

Positive feedback

Answer 3

occurs when a deviation from an optimum causes changes that result in an even greater deviation from the normal

Question 4

THERMOREGULATION

Answer 4

• If the body temperature is too low, enzyme-controlled reactions take place too slowly (lack of KINETIC ENERGY).
• If the body temperature is too high, enzymes could be denatured
• If blood pH deviates from the normal set point, enzymes and other proteins will be denatured.

Endotherms (mammals and birds)
* Maintain body temperature by both physiological and behavioural means.

Ectotherms (all animals except mammals and birds)
* maintain body temperature by behavioural means only

Question 5

Pancreas hormones

Answer 5

α (alpha) cells- secrete the hormone glucagon
β (beta) cells- secrete the hormone insulin

These two hormones are antagonistic, and have opposite effects on blood glucose:

Insulin & Glucagon levels fluctuate and both can be present in the blood at the same time.

Question 6

Insulin

Answer 6

Insulin is a hormone secreted by the β cells in the Pancreas. The aim of Insulin is to reduce blood glucose levels to the normal range.

1. Insulin binds to complementary receptors on cell surface membrane of target cells.
2. This controls the uptake of glucose by regulating the inclusion (addition) of glucose carrier proteins (GLUT4) in the surface membranes of target cells.
3. Insulin also activates enzymes that stimulate the conversion of glucose to glycogen (glycogenesis). This therefore decreases blood glucose.

Question 7

Glucagon

Answer 7

Glucagon is the hormone secreted by α cells in the Pancreas. The aim of Glucagon is to increase blood glucose levels to a normal range.

1 Glucagon binds to receptors on the cell surface membrane of target cells
2 Activates enzymes involved in the hydrolysis of glycogen to glucose (glycogenolysis)
3 Activates enzymes involved in the conversion of glycerol and amino acids into glucose (gluconeogenesis)

Question 8

Second messenger model of hormone action

Answer 8

Adrenaline is a hormone which also increases blood glucose levels.

1. Adrenaline and glucagon both bind to (specific) transmembrane protein receptors on the surface of the cell membrane – they are called the first messenger
2. A hormone receptor complex is formed (causes the receptor protein to change tertiary structure / shape)
3. The hormone-receptor complex activates adenylate cyclase (an enzyme inside the cell that results in the conversion of ATP into cyclic AMP [cAMP] that acts as a second messenger).
4. The second messenger causes a series of chemical changes that produce the required rapid response.
5. The cAMP is the second messenger and activates protein kinase enzymes which produce a CASCADE of chain of reactions that catalyse the conversion of glycogen to glucose.

Question 9

Glycogenolysis, Glycogenesis, Gluconeogenesis definition

Answer 9

Glycogenolysis - Glycogen hydrolysed to glucose.
Glycogenesis - Glucose to glycogen (condensation).
Gluconeogenesis - Glucose from amino acids and glycerol.

Question 10

TYPE I Diabetes

Answer 10

Insulin-dependent diabetes (early-onset diabetes)

A severe insulin deficiency due to autoimmune killing of β cells (possibly due to a virus), or faulty gene.

Question 11

TYPE II Diabetes

Answer 11

Non-insulin-dependent diabetes (late-onset diabetes)

Insulin is produced, but the insulin receptors in the target cells are unresponsive, so insulin has no effect.

This is sometimes described have having a lack of sensitivity to insulin.

Question 12

symptoms of diabetes

Answer 12

• High thirst due to osmosis of water from cells to the blood, which has a low water potential.
• Large volumes of urine production due to excess water in blood.
• Poor vision due to osmotic loss of water from the eye lens.
• Tiredness due to loss of glucose in urine and poor uptake of glucose by liver and muscle cells.
• Muscle wasting due to gluconeogenesis caused by increased glucagon.

Question 13

Explain the effect of sweating or panting on temperature control.

Answer 13

• Evaporation (of water from lining of mouth or skin);
• Heat transferred from blood;

Question 14

Describe how a change in blood pH or blood pressure can cause a change in heart rate.

Answer 14

Carbon dioxide) detected by chemoreceptors / (pressure) detected by baroreceptors;
* Medulla/cardiac centre involved;
* More impulses to SAN/along sympathetic nerve;

• (Decrease) pH detected by chemoreceptors in carotid artery /aorta;
• Sends (more) impulses to medulla (oblongata);
• More Nerve impulses sent by sympathetic nervous system to SAN;

Question 15

Less CO2 in the blood leads to a reduction in heart rate.

Describe how.

Answer 15

• (less CO2 in blood) Detected by chemoreceptors;
• (Chemoreceptors) located in aorta
• Fewer impulses to cardiac centre / medulla (oblongata);
• (More) impulses along parasympathetic nerve
• (To) SAN;

Question 16

Describe the role of glycogen formation and its role in lowering blood glucose levels.

Answer 16

• Glucose concentration in cell/liver falls;
• Below that in blood (plasma)/ higher in blood;
• Creates/maintains glucose concentration/diffusion gradient;
• Glucose enters cell/leaves blood by facilitated diffusion/via carrier(protein)/channel (protein);

Question 17

Describe how blood glucose levels can be increased using hormones

Answer 17

• Release of glucagon;
• Leads to formation of glucose in liver (cells);
• From amino acids/fatty acids;

Question 18

Osmoregulation

Answer 18

Control of water potential of the blood

• Osmoreceptor cells in the hypothalamus of the brain detect the fall in water potential
• ADH is secreted into the capillaries by the posterior pituitary gland.
• ADH travels to kidneys in blood and increases the permeability of the cells of the distal convoluted tubule and collecting duct to water
• ADH works by causing vesicles bound with aquaporin proteins, to fuse with the cell surface membranes of cells of the distal convoluted tubule and colleting duct.

Question 19

Describe how ultrafiltration occurs in a glomerulus.

Answer 19

• High blood/hydrostatic pressure;
• Two named small substances pass out eg water, glucose, ions, urea;
• (Through small) gaps/pores/fenestrations in (capillary) endothelium;
• (And) through (capillary) basement membrane;

Question 20

How is urea removed from the blood?

Answer 20

1. Hydrostatic pressure / description of pressure / description of how pressure generated; (FENESTRATIONS of capillary walls)
2. Causes ultrafiltration (Allow description of ultrafiltration) at Bowman’s capsule / glomeruli / renal capsule;
3. Through basement membrane/connective tissue;
4. Enabled by small size of urea molecule;

Question 21

Reabsorption of glucose and water by proximal convoluted tubule

Answer 21

85% of filtrate returns to blood here.
* Cell of the proximal convoluted tubule (PCT) have microvilli and contain lots of mitochondria to provide ATP for active transport.

• Sodium ions are actively transported out of the cells lining the PCT into blood capillaries to reduce their sodium ion concentration.
• This allows sodium ions from the filtrate to move into the surrounding cells via facilitated diffusion, but they can only come in via co-transport with another molecule (e.g. glucose, amino acids etc).
• This allows useful molecules to be reabsorbed back into the blood and ensures they do not leave the body via urine.

Question 22

Loop of Henle – Descending limb and ascending limb (Further reabsorption of water)

Answer 22

Descending limb is narrow and highly permeable to water
Ascending limb is wider and impermeable to water

• Sodium ions are actively transported out of the ascending limb using ATP, the Na+ ions RE-ENTER the DESCENDING LIMB.
• This lowers the water potential between the two limbs, drawing water out of the permeable descending limb by osmosis into the medulla region.
• As the ascending limb is impermeable to water then little or no water leaves that part of the loop.
• The water from the descending limb then enters capillaries returning to the blood.

Question 23

Convoluted tubule

Answer 23

In the interstitial space, there is a gradient of water potential, with the highest in the cortex area and lowest further down medulla.

The collecting duct is permeable to water and so water continues to move out by osmosis as there is always a water potential gradient along the entire collecting duct.

All water that leaves the collecting duct moves into the interstitial fluid and then into the blood via the surrounding capillaries. The remaining filtrate becomes urine, which then leaves the body

Desert animals have longer loops of Henle and collecting ducts to reabsorb more water

Question 24

Explain how urea is concentrated in the filtrate

Answer 24

1. Reabsorption of water / by osmosis;
2. At the PCT / descending LoH;
3. At the DCT / CD;
4. Active transport of ions / glucose creates gradient (in context);

Question 25

Explain how ADH causes movement of water from the lumen to the collecting duct

Answer 25

1. ADH causes vesicles containing aquaporins to be inserted into membrane
2. water enters cell through aquaporins;
3. by osmosis down a water potential gradient;
4. (from cell) to capillary;
5. via interstitial fluid / tissue fluid;

Question 26

Explain why a thicker medulla leads to more concentrated urine.

Answer 26

• Thicker medulla means a longer loop of Henle;
• increase in sodium ion concentration (in medulla) so sodium ion gradient maintained for longer (in medulla);
• water potential gradient maintained (for longer), so more water reabsorbed (from loop and collecting duct) by osmosis;

Question 27

Describe the action of ADH in the kidney

Answer 27

• Permeability of membrane is increased;
• More water absorbed from collecting duct;
• Smaller volume of urine;
• Urine becomes more concentrated;