3.6.4 Homeostasis

Question 1

What’s homeostasis

Answer 1

Maintenance of an organisms internal environment within restricted limits

Question 2

What maintains homeostasis in mammals

Answer 2

Physiological control systems

Question 3

Why is it important to maintain a stable core temp and blood pH (enzymes)

Answer 3

Enzymes controlling biochemical reactions in the cell and channel proteins are sensitive to changes in pH and temp, changing these factors reduces the rate of reaction or stops them working, for example denaturing enzymes

Question 4

Stages in a control mechanism

Answer 4

Optimum point, system operates best
Receptor, detects stimulus
Coordinator, sends instructions
Effector, muscle or gland creating change
Feedback mechanism, receptor responds to stimulus

Question 5

What’s negative feedback

Answer 5

Change produced by control system,
leads to change in stimulus detected by receptor,turning the system off

Question 6

What’s positive feedback

Answer 6

Deviation from the optimum causes changes resulting in a greater deviation from the normal

Question 7

What’s beneficial about having seperate mechanism for departures in opposite directions

Answer 7

Greater degree of control
Eg blood glucose conc
Both mechanisms (hormones) are highly sensitive

Question 8

Factors influenced blood glucose concentration

Answer 8

Diet, carbs like starch hydrolysed into glucose
Glycogenolysis, hydrolysis of glycogen in small intestine
Gluconeogenesis, glucose produced from amino acids and glycerol

Question 9

What’s glycogenesis

Answer 9

Conversion of glucose into glycogen
Blood glucose conc is higher than normal, liver removes glucose from blood and converts it to glycogen

Question 10

Glycogenolysis

Answer 10

Break down of glycogen to glucose
Blood glucose conc is lower than normal, liver converts stored glycogen into glucose, glucose diffuses into blood restoring normal blood glucose conc

Question 11

What’s gluconeogenesis

Answer 11

Glucose production from amino acids and glycerol when glycogen supply is finished

Question 12

Action of insulin

Answer 12

Beta cells in pancreas have receptors that detect a stimulus which is the rise in blood glucose conc, response is secreting hormone insuline into blood plasma
Body cells have glycoprotein on their cell surface membrane which bind specifically with insulin molecules this

Changes the tertiary structure of glucose transport carrier proteins, they change shape and open, more glucose moves into cell by facilitated diffusion
More carrier proteins can transport glucose, the protein used to make these channels are part of the membrane of vesicles, when insulin conc increases vesicles fuse with cell surface membrane increasing number of glucose protein carriers
Activates enzymes to convert glucose to glycogen and fat
(Insulin helps to move glucose out of blood into cells)
Blood glucose conc is lowered by increasing rate of absorption into muscle cell
Increasing respiratory rate if cells
Increasing the rate of conversion of glucose in glycogen (glycogenesis)
Increasing rate of conversion of glucose into fat
Glucose is therefore removed from the blood returning the glucose conc to optimum, beta cells reduce their secretion of insulin (negative feedback)

Question 13

Action of glucagon

Answer 13

Alpha cells in pancreas detect fall in blood glucose conc, respond by secreting hormone glucagon into blood plasma
Glucagon attaches to specific protein receptors present on the livers cell surface membrane
This activated enzymes which convert glycogen to glucose
Enzymes which convert amino acids and glycerol into glucose (gluconeogenesis) are also activated
Overall glucose concentration in blood is increased until optimum conc is reached, alpha cells reduce secretion of glucagon (negative feedback)

Question 14

Role of adrenaline

Answer 14

Increases blood glucose concentration
Produced by adrenal glands

Adrenaline attaches to protein receptors on the cell surface membrane of target cells, activates enzymes causing the breakdown of glycogen to glucose in the liver

Question 15

What’s the secondary messenger model

Answer 15

A mechanism of hormone action for adrenaline and glucagon

Question 16

Describe what happens in the second messenger model for adrenaline

Answer 16

Adrenaline binds to transmembrane protein receptor within the cell surface membrane of a liver cell, protein on the inside of the membrane changes shape, enzyme adenyl cyclase is activated, this activated enzyme converts ATP to cyclic AMP (cAMP), cAMP acts as a second messenger as it binds to protein kinase changing its shape activating it also, active protein kinase enzyme catalyses conversion of glycogen to glucose, moves out liver by facilitated diffusion and a channel protein

Question 17

What’s type 1 diabetes and it’s causes

Answer 17

Body unable to produce insulin, due to autoimmune condition where body attacks Beta cells in the liver

Question 18

What’s type 2 diabetes and it’s causes

Answer 18

Glycoprotein receptors lost responsiveness to insulin or due to an inadequate supply from pancreas, causes by age and obesity

Question 19

How is type 1 and 2 diabetes controlled

Answer 19

Type 1, insulin injections depending on blood glucose concentration

Type 2, regulating intake of carbohydrates depending on exercise

Question 20

What’s osmoregulation

Answer 20

Control of water potential in the blood

Question 21

Structure of a kidney (not needed for exam but helps with nephron)

Answer 21

Fibrous capsule, protects kidney
Cortex, outer region, contains renal capsule convoluted tubules and blood vessels
Medulla, inner region, contains loops of henle collecting ducts and blood vessels
Renal pelvis, cavity collecting urine
Ureter, tube carrying urine to bladder
Renal artery, supplies kidney with blood from aorta
Renal vein, returns blood to heart using vena cava

Question 22

What’s a nephron

Answer 22

Functional unit of the kidney
Many nephrons make up the kidney

Question 23

Structure of nephron

Answer 23

Renal capsule- start of nephron, cup surrounding capillaries called glomerulus
Proximal convoluted tubule- loops surrounded by capillaries
Loop of henle- loop extending from cortex to medulla and then from medulla to cortex, surrounded by capillaries
Distal convoluted tubule- looped surrounded by blood vessels but fewer than proximal convoluted tubule
Collecting duct- distal convoluted tubules empty into pelvis

Question 24

Blood vessels in the nephron

Answer 24

Afferent arteriole- from renal artery to renal capsule, supplies nephron with blood
Glomerulus- lots of capillaries, fluid forced out of blood
Efferent arteriole- leaves renal capsule, high pressure smaller diameter
Blood capillaries- surround proximal convoluted tubule, loop of henle, distal convoluted tubule, re absorbing mineral salts, glucose and water, merge into venules

Question 25

How is glomerular filtrate produced by ultrafiltration

Answer 25

Blood enters through afferent arteriole which is wider it splits into capillaries forming glomerulus, causes high hydrostatic pressure as efferent arteriole is narrow

Water and small molecules (glucose, mineral ions) forced out of capillaries through podocytes and form glomerulus filtrate, ultrafiltration removes small molecules whether they are useful or not

Large proteins and blood cells are too large to fit through gaps in capillary endothelium, remain in blood, blood leaves through efferent arteriole

Question 26

What are podocyte cells

Answer 26

Cells wrapped around capillary, filtrate passes through the spaces

Question 27

How is glucose and water reabsorbed in the proximal convoluted tubule

Answer 27

Na+ actively transported out of proximal convoluted tubule into blood, Na+ conc in the proximal convoluted tubule is lowered
Na+ diffuse down a conc gradient via carrier proteins from the lumen of the proximal convoluted tubule to epithelial cells lining the proximal convoluted tubule
There are different carrier proteins which carry Na+ and Cl-/amino acids-glucose, co transport
Molecules co transported into epithelial cells in the proximal convoluted tubule diffuse into blood, glucose/water reabsorbed

Question 28

Adaptations of proximal convoluted tubule for reabsorption

Answer 28

Microvilli, large surface area to reabsorb substances from filtrate
Infolding at the base, large surface area to transfer reabsorbed molecules into blood
Lots of mitochondria, provide ATP for active transport

Question 29

How is the NA+ gradient maintained in the medulla using loop of henle

Answer 29

Na+ actively transported out of ascending limb of loop of henle using ATP
Low water potential (high ion conc) in the medulla, thick walls of ascending limb are impermeable, water doesn’t escape
Walls of descending limb are permeable to water, water moves via osmosis to interstitial space, then into capillaries via osmosis
Less water in interstitial space, lowest water potential at the tip of hairpin
Na+ diffuse and actively pumped out of ascending limb, filtrate in interstitial space has higher water potential
Interstitial space has a water potential gradient, highest at cortex, lowest at medulla

Question 30

How does the distal convoluted tubule and collecting ducts reabsorb water

Answer 30

Collecting duct is permeable to water, filtrate moves down it, water leaves via osmosis into blood vessel through channel proteins specific to water, the number of these channels is altered by ADH
Water potential in filtrate is lowered, water potential in interstitial space is also lowered, water continues to move out of collecting duct via osmosis as the counter current multiplier ensures water potential gradient is maintained

Question 31

Two regions in the loop of henle

Answer 31

Descending limb
Narrow thin walls highly permeable to water

Ascending limb
Wider thicker walls impermeable to water

Question 32

Role of the distal convoluted tubule

Answer 32

Cell walls contain microvilli and lots of mitochondria for rapid reabsorption by active transport

Reabsorbed salt and water
Controls blood pH by selecting which ions are reabsorbed, permeability of walls are changed by hormones

Question 33

What’s the counter current multiplier in the loop of henle

Answer 33

Two liquids flow in opposite directions
Filtrate in collecting duct (low water potential) meets interstitial fluid (even lower water potential), small water potential difference but occurs across whole length of collecting duct, steady flow of water into interstitial fluid, more water enters blood

Question 34

How is low water potential of the blood regulated

Answer 34

Osmoreceptors in hypothalamus detect low water potential, osmoreceptor cells lose water and shrink, produces ADH
ADH passes to posterior pituitary gland, then secreted into capillaries
ADH passes through blood to kidney, increases permeability to water of cell surface membrane of distal convoluted tubule and collecting duct
Specific protein receptors on the cell surface membrane bind to ADH, enzyme phosphorylase is activated
Vesicle containing water channel proteins, found within cell moves and fuses with its distal convoluted tubule cell surface membrane, increases water channels, cell surface membrane more permeable to water
ADH increases permeability of collecting duct to urea, urea passes out, water potential of fluid around duct is lowered, more water leaves collecting duct via osmosis down a water potential gradient and reenters blood
THIS PREVENTS WATER POTENTIAL OF BLOOD BECOMING LOWER as water came from blood initially
Osmoreceptors also send nerve impulses to thirst centre in the brain to encourage water consumption
Osmoreceptors in hypothalamus detect water potential rise, less impulses sent to pituitary gland, it reduces the release of ADH, permeability of collecting ducts and reverts to normal

Question 35

How is high water potential in the blood regulated

Answer 35

Osmoreceptors in hypothalamus detect rise in water potential, increase frequency of nerve impulses to pituitary gland to reduce ADH release
Less ADH via blood reduces permeability of collecting ducts to water and urea
Less water reabsorbed into blood from collecting duct
More dilute urine, water potential of blood falls, when it returns to normal Osmoreceptors in hypothalamus cause pituitary gland to raise ADH levels to normal

Question 36

What can effect water potential of the blood

Answer 36

Drinking too much/little water
Lots of sweating
Lots of salt being taken in in diet or being used and not replaced