homeostasis

Question 1

What is homeostasis?

Answer 1

maintenance of a constant internal environment in the body

Question 2

why is homeostasis necessary?

Answer 2

-To maintain a constant internal environment of blood and tissue fluids within set points, effects:
-Low temperature,
consequence: slowed
metabolism / enzymes
less active
- High temperature,
consequence: enzymes
denatured
- Low water potential,
consequence: water
leaving cells / cells
shrink
- High water potential,
consequence: water
enters cells / cells burst
- Low blood glucose,
consequence: effect
on respiration
- High blood glucose,
consequence: water
leaving cells / cells
shrink
- Control of pH,
consequence: enzymes
become less active

Question 3

Define negative feedback

Answer 3

The mechanism to keep changes to the factor within narrow limits, by increasing or decreasing accordingly during a change in the factor

Question 4

Define positive feedback

Answer 4

enhances or accelerates the output created by an activated stimulus

Question 5

Describe the role of coordination system in homeostasis?

Answer 5

Coordination system receives information about stimuli from receptors and determines what the response should be by sending instructions to effectors

Two coordination systems in mammals:
1) Nervous system, by
electrical impulses
transmitted along
neurones
2) Endocrine system, by
hormones (chemical
messengers) travel in
the blood

Question 6

cells that detect changes in core temperature

Answer 6

thermo-receptors in hypothalamus

Question 7

cells that detect changes in external temperature

Answer 7

thermo-receptors in skin

Question 8

Describe the ways in which the body can conserve heat in a cold environment

Answer 8

1) Vasoconstriction – contraction of the muscles in the walls of the arterioles in skin surface, narrowing the lumens, reducing the supply of blood, hence less heat lost from the blood
2) Shivering – involuntary contraction of the skeletal muscles generate heat, absorbed by the blood
3) Raising body hairs – contraction of the muscles attached to the hairs, increasing the depth of fur and the layer of insulation, trapping air close to the skin 4) Decrease in sweat production – reduces heat loss by evaporation from skin surface
5) Increase secretion of adrenaline – increases the rate of heat production in the liver

Question 9

thermoregulation by adrenal and thyroid gland when temperature decreases

Answer 9

both increase metabolic rate (more heat produced)

when environment temperature decreases gradually:

1) hypothalamus secretes a hormone which activates anterior pituitary gland to release TSH (thyroid stimulating hormone)

2) TSH stimulates thyroid gland to secrete thyroxine into blood

3) thyroxine increases metabolic rate which increases heat production, especially in the liver

4) when temperature starts to increase again, hypothalamus responds by reducing release of TSH by anterior pituitary gland

Question 10

Describe the ways in which the body can rapidly lose heat in a warm environment?

Answer 10

1) Vasodilation – relaxation of the arterioles in skin, hence it widens, more blood flows to the capillaries, heat energy lost
2) Increasing sweat production – sweat glands increase production of sweat which evaporates on the surface of the skin, removing heat from the body
3) Lowering body hairs – relaxation of the muscles attached to the hairs, hence they lie flat, reducing the depth of fur and layer of insulation

Question 11

excretion

Answer 11

removal of unwanted products of metabolism

Question 12

The formation of urea from excess amino acids by liver cells

Answer 12

1) Deamination / removal of amine group and ammonia (NH3) formed, which is then combined with carbon dioxide forming the urea cycle

2) Ammonia is a soluble and toxic compound, hence needed to be converted into urea (main nitrogenous excretory product) – less soluble and less toxic

Question 13

uric acid

Answer 13

nitrogenous waste excreted in the urine formed from the breakdown of purines from nucleotides

Question 14

Structure of kidney:

Answer 14

Each kidney receives blood from a renal artery; return blood via a renal vein

Narrow tube – ureter – carries urine from kidney to bladder

Urethra – single tube – carries urine to the outside of the body

Question 15

What is a nephron?

Answer 15

A nephron is one functional unit of kidney

Question 16

Structure of a nephron

Answer 16

1) bowman’s capsula- at cortex
2) Proximal convoluted tubule- at cortex
3) Loop of henle- at medulla
4) distal convoluted tubule- at cortex
5) collecting duct- at medulla
6) ureter- at pelvis

Question 17

Outline how a negative feedback mechanism works. [4]

Answer 17

1) change in factor away from set-point
2) detected by receptor
3) hormone released or nerve impulse sent
4) hormone / impulse reaches effector
5) effector performs corrective action
6) factor returns to set-point

Question 18

The kidney makes urine in a two-stage process

Answer 18

1) Ultrafiltration – filtering of small molecules including urea into the Bownman’s capsule from the blood

2) Selective reabsorption – taking back useful molecules from the fluid in the nephron as it flows along

Question 19

factors affecting water potential

Answer 19

solute potential (water potential is lowered) and pressure potential (water potential is raised)

Question 20

Describe the process of ultrafiltration
(pp)

Answer 20

1) afferent arteriole has wider lumen than efferent arteriole
2) this causes high blood pressure in glomerulus
3) hence plasma passes through the pores between the endothelial cells of the capillaries
4) however, red and white blood cells / large proteins (plasma proteins) / molecules greater than 68 000(MM), cannot pass through
5) due to the basement membrane which acts as a selective barrier
6) filtrate through the basement membrane can freely pass through the podocytes due to its alit pores and forced into the Bowman’s capsule (renal capsule)

Question 21

substances that have to be re-absorbed

Answer 21

1) all glucose
2) vitamins
3) much of water
4) some inorganic ions e.g. Na and Cl ions
5) amino acids

Question 22

In which part of the nephron does selective reabsorption occurs?

Answer 22

In the proximal convoluted tubule. Water, glucose and ions are reabsorbed back into the blood

Question 23

Describe the process of selective reabsorption of glucose
(pp)

Answer 23

1) Na⁺/K⁺ pumps in basal membranes of cells lining proximal convoluted tubule
2) pump out Na⁺ out of cell via active transport into the blood
3) this lowers Na⁺ inside the cell
4) so that more Na⁺ diffuses from fluid in lumen of the tubule
5) via co-transporter molecules in the membrane
6) passive movement of Na⁺ provides energy to move glucose (even against a concentration gradient); example of indirect/secondary active transport
4) once glucose is inside the cell, it diffuses via GLUT proteins in the basal membrane into the blood
5) removal of solutes increases water potential of filtrate; a water potential gradient is established so water moves down this gradient

Question 24

Adaptations of PCT cells for selective reabsorption
(pp)

Answer 24

1) Microvilli- increase the surface area of the inner surface facing the lumen to increase absorption of Na+ / glucose / amino acids
2) Tight junctions to hold adjacent cells together so that fluid cannot pass between the cells
3) Many mitochondria to provide ATP for sodium-potassium (Na+-K+) pump
4) Many co-transporter proteins in the membrane facing the lumen
6) Folded basal membrane to increase surface area to increase sodium potassium pumps to move Na+ into the blood
7) More ER for increase in protein synthesis

Question 25

What happens in the loop of henle in the nephron

Answer 25

located at medulla
1) Na+ & Cl- are actively transported out of higher end of ascending limb into the tissue fluid

2) This increases concentration of ions in tissue fluid

3) Water is therefore lost from the descending limb

4) Loss of water concentrates Na+ and Cl− along the descending limb.

6) This concentrates the fluid inside the loop, so ions passively move down their concentration gradient, into the tissue fluid

Question 26

selective reabsorption at Distal Convoluted Tubule (DCT)

Answer 26

located at cortex
1) 1st part of DCT= similar to LOH (Ascending limb)
Na+ ions are actively pumped from the fluid in the tubule into the tissue fluid, into the blood
2) 2nd part of DCT = similar to collecting duct
K+ ions are actively transported into the tubule, where the rate of transfer of the two ions are variable, helps regulate the concentration of these ions in the blood

Question 27

Selective reabsorption at collecting duct

Answer 27

located at medulla
1) tissue fluid at medullas has high concentration of solutes
2) so water moves out of collecting duct
3) high reabsorption of water back into blood causes formation of urine

Question 28

What is meant by osmoregulation?

Answer 28

the control of the water potential of body fluids

Question 29

monitoring water potential of blood

Answer 29

1) water potential of blood is constantly monitored by osmoreceptors in hypothalamus
2) when a decrease is detected, nerve impulses are sent along neurones to the posterior pituitary gland
3) they stimulate the release of ADH
4) ADH reduces water loss in urine by making kidneys absorb as much as possible

Question 30

Where is ADH released into the blood?

Answer 30

ADH from the hypothalamus is transferred to the posterior pituitary gland where it is secreted into the blood

Question 31

Explain how ADH increases the water potential in blood

Answer 31

1) Osmoreceptors detect and send impulses to the posterior pituitary gland to secrete antidiuretic hormone (ADH)
2) ADH in the blood binds to receptors on the cells of collecting duct, activating intracellular enzymes
3) Vesicles that contain aquaporin in the cell are stimulated to fuse to membrane
4) This causes duct to become permeable to water hence water moves out, down its conc. gradient
- water moves out of
tubule into tissue fluid
- as tissue fluid in medulla
has very low pH
- volume of urine is
smaller and more
concentrated

Question 32

Explain what happens when water potential in blood is increased?

Answer 32

Osmoreceptors no longer stimulate ADH production, so aquaporins moved back into cytoplasm as vesicles, making cells impermeable to water again

Question 33

What are the effects of hypoglycemia and hyperglycemia?

Answer 33

• hypoglycemia: not enough glucose for respiration. the cells cannot function properly
• hyperglycemia: the high glucose concentration in the blood decreases the water potential. water leaves the cell, prevent normal functioning

Question 34

⍺-cells of islets of Langerhans

Answer 34

secrete glucagon when blood glucose concentration is low

Question 35

β-cells of islets of Langerhans

Answer 35

secrete insulin when blood glucose concentration is high

Question 36

GLUT 4

Answer 36

muscle and adipose

Question 37

GLUT 2

Answer 37

liver cells (always have GLUT proteins present in their cell surface membranes)

Question 38

GLUT 1

Answer 38

brain cells (always have GLUT proteins present in their cell surface membranes)

Question 39

Difference between gluconeogenesis, glycogenesis and glycogenolysis

Answer 39

• gluconeogenesis: proteins and fats to glucose
• glycogenesis: glucose to glycogen
• glycogenolysis: glycogen to glucose

Question 40

Describe the role played by insulin in the control of blood glucose concentration. [5]

Answer 40

1) insulin binds to receptors on X cells
2) GLUT(X) added to membrane of cells
3) glucokinase phosphorylates glucose in liver cells
4) faster respiration of glucose
5) activates glycogen synthase
6) causes lipid synthesis
7) blood glucose concentration decreases

Question 41

effects of insulin [4]

Answer 41

1) stimulates cells with receptors for it to increase glucose absorption rate, convert it to glycogen and use it in respiration

2) when insulin binds to receptors on muscle cells, vesicles with GLUT4 proteins are moved to the cell surface membrane and fuse with it

3) stimulates the activation of enzyme glucokinase which phosphorylates glucose (traps it inside as phosphorylated glucose can’t pass through transporters in membrane)

4) stimulates enzymes phosphofructokinase and glycogen synthase which add glucose molecules to glycogen, increasing their size

Question 42

how glucagon increases blood glucose [9]

Answer 42

1) ⍺-cells respond by secreting glucagon, β-cells stop insulin secretion
2) glycogen binds to receptor molecules in cell surface membrane of liver cells
3) receptor changes conformation
4) G-protein is activated (because of binding)
5) adenylyl cyclase is activated (enzyme that catalyses conversion of ATP to cyclic AMP)
6) cyclic AMP is second messenger
7) cyclic AMP binds to kinase enzymes that activate other enzymes by phosphorylating them
8) this enzyme cascade amplifies original signal from glucagon
9) glucose phosphorylase (at the end of cascade) is activated
10) it catalyses breakdown of glycogen to glucose by removing glucose from 1,6 branches
11) concentration of glucose increases in cell so it diffuses out via GLUT2 into blood
12) can stimulate gluconeogenesis (producing glucose from amino acids and lipids)

Question 43

cell that doesn’t have glucagon receptors

Answer 43

muscle cells; they’re stimulated by adrenaline which increases glucose by activating the same enzyme cascade as glucagon

*glucose produced remains in muscle cell where it’s required for respiration

Question 44

Effects of Adrenaline on liver cells
(pp)

Answer 44

1) Adrenaline binds to receptors in the cell surface membrane and Receptor changes shape
2) G proteins activated
3) Adenylyl cyclase activated
4) resulting to ATP converted to cyclic AMP
5) cAMP is a second messenger
6) Cyclic AMP activates kinase protein
7) which activates enzymes through phosphorylation
8) resulting to enzyme cascade
9) hydrolysis of glycogen is stimulates
10) Glucose diffuses out of liver cell through GLUT2 transporter proteins into the blood
11) Increase in blood glucose concentration

Question 45

The presence of glucose and ketones in urine is indicative of which disease?

Answer 45

Diabetes mellitus

Question 46

Difference between Type 1 and Type 2 diabetes

Answer 46

1) Type 1: insulin dependent
Type 2: non insulin
dependent
2) Type 1: usually occurs
during childhood
Type: usually occurs
during adulthood
3) Type 1: body does not
produce sufficient insulin
Type 2: body does not
respond to insulin
production
4) Type 1: caused by the
destruction of B cells
(autoimmune)
Type 2: caused by the
down regulation of the
insulin receptors
5) Type 1: required insulin
injections to regulate
blood glucose
Type 2: controlled by
managing diet and
lifestyle

Question 47

Symptoms of diabetes mellitus

Answer 47

1) high glucose concentrations in blood and urine (as glucose is not taken up by cells so less glucose converted to glycogen. not all glucose reabsorbed in kidneys)

2) decrease in water potential of blood ( water and salts move out of cells down the concentration gradient. leads to dehydration, production of dilute urine, loss of salts and cramps. detected by osmoreceptors in hypothalamus)

3) fat and proteins used in respiration ( fat and proteins used instead of glucose so leads to weight loss. build up of ketones in blood lowers blood pH)

Question 48

presence of glucose and ketones in urine

Answer 48

indicates that a person may have diabetes. If the concentration for these rises above the renal threshold, then not all glucose has been absorbed from the filtrate in the proximal convoluted tubule –> so will be present in the urine

Question 49

What may be the cause of protein in the urine?

Answer 49

1) disease affecting the glomeruli
2) an infection of the kidney and urinary tract
3) congestive heart failure

Question 50

Explain how dip sticks function to test for glucose in a sample of urine. [8]

Answer 50

1) stick has a pad containing the immobilised enzymes
2) glucose oxidase and
3) peroxidase
4) stick dipped in urine
5) glucose reacts to give hydrogen peroxide (catalysed by glucose oxidase)
6) hydrogen peroxide reacts with a colourless substance (chromogen)
7) to give a coloured substance
8) compare with colour chart
9) more glucose gives darker colour

Question 51

Outline how a glucose biosensor works. [3]

Answer 51

1) pad contains glucose oxidase
2) glucose oxidase reacts with glucose in the blood
3) oxygen detected
4) electric current generated
5) detected by electrode
6) gives numerical value of blood glucose concentration

Question 52

stomata open due to

Answer 52

1) increasing light intensity
2) low carbon dioxide concentrations in the air spaces within the leaf

Question 53

stomata close due to

Answer 53

1) darkness
2) high carbon dioxide concentrations in the air spaces in the leaf
3) low humidity
4) high temperature
5) water stress, when the supply of water from the roots is limited and/or there are high rates of transpiration.

Question 54

explain how stomata have daily rhythms of opening and closing

Answer 54

Opening during the day maintains the inward diffusion of carbon dioxide and the outward diffusion of oxygen. The closure of stomata at night when photosynthesis cannot occur reduces rates of transpiration and conserves water.

Question 55

explain the mechanism by which guard cells open and close stomata
(pp)

Answer 55

1) ATP-powered proton pumps in the cell surface
membrane
2) H+ out of the guard cell
3) using ATP
4) the low H+ concentration and negative charge inside
5) the cell causes K+ channels to open
6) K+ diffuses into the cell by facilitated diffusion
7) Cl- ions move into the cell
8) the high concentration of K+ inside the guard cell lowers the water potential
9) water moves in by osmosis, down a water potential gradient through aquaporins
10) The vacuole volume increases so guard cells become turgid
11) the inner cell wall of guard cells becomes thick, so the cells curve apart

Question 56

structure of guard cells

Answer 56

• wall adjacent to the pore is very thick
• the wall furthest from the pore is thin
• bundles of cellulose microfibrils are arranged as hoops around the cells so that, as the cell becomes turgid, these hoops ensure that the cell mostly increases in length and not diameter
• the thin outer walls bend more readily than the thick inner walls, the guard cells become curved, opening the pore between two cells

Question 57

abscisic acid (ABA)

Answer 57

a stress hormone that is released if a plant is subjected to difficult environmental conditions (e.g., very high temperatures, reduced water supplies) and triggers the closure of stomata to reduce transpiration and prevent water loss

Question 58

role of abscisic acid (ABA) in the closure of stomata during times of water stress

Answer 58

1) ABA binds to cell surface receptors on guard cells
2) Ca2+ inhibits the proton pumps to stop hydrogen ions being pumped out
3) ABA also stimulates the movement of Ca2+ ions into the cytoplasm through the cell surface membrane
4) Ca2+ acts as a second messenger to activate
5) this inhibits K+ influx and promote K+ efflux by opening the channel proteins
6) water potential of guard cell increases due to loss of ions
7) water leaves guard cells by osmosis
8) the guard cells become flaccid and the stomata close