Homeostasis is the maintenance of a stable internal environment

Question 1

Importance of homeostasis

Answer 1

Changes in external environment can affect your internal environment

Homeostasis involves control systems that keep your internal environment roughly constant

Question 2

Importance of maintaining stable core temperature

Answer 2

Enzymes work at an optimum temp

If body temperature is too low, there will not be enough Ek, so fewer successful collisions/enzyme-substrate complexes formed

If body temperature is too high enzymes denature as H-bonds in tertiary structure break and the active site changes shape, no longer complementary to substrate so fewer successful collisions/enzyme-substrate complexes formed

Question 3

Importance of maintaining stable blood pH

Answer 3

Enzymes work at an optimum pH

If the blood pH is too low or high, the enzymes will denature as ionic bonds in tertiary structure break

The active site changes shape so that it is no longer complementary to substrate

Therefore there are fewer successful collisions and enzyme-substrate complexes formed

Question 4

Importance of maintaining a stable blood glucose concentration

Answer 4

If blood glucose concentration too low (hypoglycaemia) there will not be enough glucose for respiration (particularly brain + nervous system) so less ATP is produced and active transport etc. can’t happen

If blood glucose concentration too high (hyperglycaemia) blood will have a low water potential (as increased solute)

Water will be lost from tissue to blood via osmosis and kidneys can’t absorb all glucose so more water lost in urine, causing dehydration

Question 5

Negative feedback

Answer 5

Receptors detect levels that are too low/high

Effectors respond to counteract change

Restores levels to normal/original

e.g. regulation of body temperature

Question 6

Positive feedback

Answer 6

Amplifies a change from the normal level as effectors respond to further increase the level away from normal level

Advantage – rapidly activate something
e.g. blood clot or hypothermia

Not involved in homeostatic system

Question 7

Role of multiple negative feedback mechanisms in homeostasis

Answer 7

More control over changes in internal environment

Controls departures in different directions from the original state/actively increase or decrease a level to normal

Faster response and greater control with multiple feedback mechanisms

Question 8

Factors that influence blood
glucose concentration

Answer 8

Eating food containing carbohydrates increases amount of glucose being absorbed from the intestine to the blood (blood glucose concentration rises)

Exercise increases rate of respiration of glucose (blood glucose concentration falls)

Question 9

Action of insulin in blood glucose concentration

Answer 9

Insulin lowers blood glucose concentration when it is too high

Secreted by beta cells in islets of Langerhans in pancreas

Insulin binds to specific receptors on cell surface membranes of liver/muscle cells (target cells/effectors)

Increases permeability of muscle cell membrane to glucose by increasing number of channel proteins
 (GLUT4) in cell surface membrane and therefore cells can uptake more glucose from blood by facilitated diffusion

Insulin also activates enzymes in liver/muscle cells that convert glucose to glycogen

The cells are also able to store glycogen in their cytoplasm as an energy source and the process of forming glycogen from glucose is called glycogenesis

Insulin also increases the rate of respiration of glucose

Question 10

Action of glucagon in blood glucose concentration

Answer 10

Glucagon raises blood glucose concentration when it is too low

Secreted by alpha cells in islets of Langerhans in pancreas

Glucagon binds to specific receptors on cell surface membranes of liver cells (target cells)

Activates enzymes involved in the breakdown of glycogen to glucose (glycogenolysis)

Activates enzymes involved in the conversion of glycerol/amino acids to glucose (gluconeogenesis)

Glucagon decreases the rate of respiration of glucose in cells

Question 11

Role of adrenaline in blood glucose concentration

Answer 11

Hormone secreted by adrenal glands (above kidneys) when blood glucose concentration is low, or when the body is stressed/exercising

Binds to specific receptors on cell surface membranes of liver cells (target cells)

Activates enzymes involved in the conversion of glycogen to glucose (glycogenolysis)

Inhibits glycogenesis (synthesis of glycogen from glucose)

It also activates secretion of glucagon and inhibits the secretion of insulin

Therefore it increases blood glucose concentration (more glucose for respiration)

Question 12

Secondary messenger model

Answer 12

Adrenaline and glucagon demonstrate the secondary messenger model because they cause glycogenolysis to occur inside cell even though they bind to receptors on the outside of the cell

1. Receptors for adrenaline/glucagon have specific tertiary structures that bind to specific complementary receptors on cell membrane
2. Activate the enzyme called adenylate cyclase
3. Adenylate cyclase converts ATP into a chemical signal called a ‘second messenger’ i.e. cyclic AMP or cAMP
4. cAMP activates an enzyme called protein kinase A
5. Protein kinase A activates a cascade to break down glycogen to glucose (glycogenolysis)

Question 13

Glucose transporters in the control of blood glucose concentration

Answer 13

Skeletal and cardiac muscle cells contain a channel protein called GLUT4

Glucose transporter that is stored in vesicles in the cytoplasm of cells whilst insulin levels are low

When insulin binds to receptors on the cell-surface membrane, it triggers the movement of GLUT4 to the membrane

Glucose can then be transported into the cell through the GLUT4 protein via facilitated diffusion

Question 14

Diabetes what is it

Answer 14

Diabetes is where blood glucose concentration can’t be controlled properly.

Blood glucose concentration peaks higher and takes longer to decrease and remains high after a meal

Question 15

Type 1 diabetes cause and effects

Answer 15

Caused by a gene mutation which causes an autoimmune response on beta cells in the islets on Langerhans so the body can’t produce insulin

After eating, blood glucose level rises and stays high (hyperglycaemia)

Question 16

Type 1 diabetes control: insulin therapy

Answer 16

Injections of insulin (not by mouth as protein is digested) or use of insulin pump

Dose of insulin matched to glucose intake/use biosensors

If too much insulin administered, produces a dangerous drop in glucose levels (hypoglycaemia)

Question 17

Type 1 diabetes control: diet manipulation

Answer 17

Eating regularly, control carbohydrate intake e.g. carbs which are broken down/absorbed slower in order to avoid a sudden rise in glucose

Question 18

Type 2 diabetes cause and effects

Answer 18

Caused by a poor diet/lack of exercise/obesity

It occurs when beta cells do not produce enough insulin or when glycoprotein/receptors on cell membranes lose responsiveness to insulin (faulty) and so since the cells are less responsive to insulin, they don’t take up enough glucose

Question 19

Type 2 diabetes control: insulin therapy

Answer 19

Use of drugs which target insulin receptors/use of insulin so that there is more glucose uptake by cells/tissues

Question 20

Type 2 diabetes control: diet manipulation

Answer 20

Reduced sugar intake (carbs) in diet/eat food with low glycaemic index so that less sugar is absorbed into blood

Reduced fat intake so that less fat is converted to glucose

More (regular) exercise since it uses glucose/fats by increasing respiration

Lose weight for increased sensitivity of cells to insulin and an increased uptake of glucose by cells

Question 21

The structure of the nephron

Answer 21

Long tubules (along with the bundle of capillaries) where the blood is filtered are called nephrons

Blood from renal artery enters smaller arterioles in the cortex of the kidney

Each arteriole splits into a glomerulus, a bundle of capillaries looped inside the Bowmans capsule (ultrafiltration takes place here)

Arteriole that takes blood into each glomerulus is called the afferent arteriole (smaller so higher pressure) whereas the arteriole that takes filtered blood away is called the efferent arteriole

High pressure in glomerulus forces liquid and small molecules in the blood out of the capillary and past three layers into the Bowmans capsule

Liquid and small molecules enter the nephron tubules which consists of the capillary wall, a membrane (basement membrane) and the epithelium of the Bowmans capsule

Substances that enter the Bowmans capsule are known as the glomerular filtrate, whereas larger molecules like proteins and blood cells cant pass through, so they stay in the blood

Glomerular filtrate passes along nephron and useful substances are reabsorbed

Finally filtrate flows through the collecting duct and passes out of the kidney along the ureter

Question 22

Osmoregulation

Answer 22

Osmoregulation is the control of water and salt levels in the body

Controlled by hormones e.g. antidiuretic hormone (ADH) which affect the distal convoluted tubule and collecting duct

Question 23

Roles of hypothalamus, posterior pituitary and antidiuretic hormone (ADH) in osmoregulation

How the body responds to a decrease in water potential…

Answer 23

1. The water content of the blood drops so its water potential drops. This is detected by osmoreceptors in hypothalamus
2. Hypothalamus produces more ADH (antidiuretic hormone) via the posterior pituitary gland
3. ADH travels in blood to kidney and attaches to receptors on collecting duct/DCT (distal convoluted tubule) of kidney
4. ADH increases permeability of cells/walls of the DCT/collecting duct (more aquaporins fuse with cell membrane) to water so more water absorbed is reabsorbed into the blood via osmosis and leaves the DCT/collecting duct
5. A small amount of highly concentrated urine is produced and less water is lost

Question 24

Roles of hypothalamus, posterior pituitary and antidiuretic hormone (ADH) in osmoregulation

How the body responds to an increase in water potential…

Answer 24

1. The water content of the blood rises, so its water potential rises. This is detected by osmoreceptors in the hypothalamus
2. Hypothalamus produces less ADH as posterior pituitary gland secretes less ADH into blood
3. Less ADH travels in blood to kidney and attaches to receptors of collecting duct/ DCT of kidney
4. ADH decreases permeability of cells/walls of the DCT/collecting duct to water and urea to water so less water is reabsorbed into the blood via osmosis and leaves the DCT/collecting dust
5. A large amount of dilute urine is produced and more water is lost

Question 25

Role of nephron in osmoregulation

Answer 25

Selective reabsorption takes place after the formation of glomerular filtrate

Flows along the proximal convoluted tubule (PCT), through the loop of Henle, and along the distal convoluted tubule (DCT)

Reabsorption of glucose and water from the glomerular filtrate occurs at the PCT into the blood

Maintenance of a gradient of sodium ions in the medulla by the loop of Henle

Reabsorption of water by the DCT and collecting duct

Question 26

Formation of glomerular filtrate

Answer 26

Diameter of efferent arteriole smaller than afferent arteriole so that the glomerulus is under higher pressure

Build-up of hydrostatic pressure in glomerulus

Water, glucose, mineral ions squeezed out of capillary/glomerulus into the Bowman’s capsule to form glomerular
 filtrate

Liquid and small molecules pass through 3 layers to reach the Bowmans capsule; pores in the capillary endothelium, basement membrane and podocytes which act as a filter

Large proteins/blood cells aren’t passed through as too large

Question 27

Reabsorption of glucose and water by the proximal convoluted tubule (PCT)

Answer 27

1. Sodium ions actively transported out of epithelial cell to capillary, lowering the concentration of Na+ in the epithelial cell
2. Na+ moves via facilitated diffusion from PCT into epithelial cell down concentration gradient via co-transporting glucose/amino acids/Cl-, which increases concentration of glucose etc. in epithelial cell
3. Glucose/amino acids/Cl- move into capillary via facilitated diffusion down concentration gradient (reabsorbed) which lowers the water potential in the capillary
4. Water moves via osmosis down the water potential gradient into capillary (reabsorbed)

Question 28

Maintaining a gradient of sodium ions in the medulla by the loop of Henle

Answer 28

1. Loop of Henle acts as a counter current multiplier which maintains the water potential gradient and water can leave collecting duct/DCT by osmosis
2. Na+ actively transported out of ascending limb. Ascending limb is impermeable to water so water remains, increasing the concentration of Na+ in medulla, lowering the water potential
3. Water moves out of descending limbs/collecting duct by osmosis into medulla and is reabsorbed by capillaries, so the filtrate is more concentrated as it moves down the ‘hairpin’
4. Na+ diffuses into descending limb, recycling Na+ in loop of Henle and reduces water potential further

Question 29

Reabsorption of water by the distal convoluted tubule and collecting ducts

Answer 29

Water moves out of the DCT and collecting duct by osmosis down a water potential gradient

Controlled by ADH which changes their permeability