A Level Prep Revision

Question 1

Describe translocation in plants (5)

Answer 1

1. Sucrose moves into companion cells from source/leaf cells by facilitated diffusion
2. Sucrose cotransported from companion cells into sieve tube elements coupled by H+ ions
3. Lowers water potential in sieve tube and water enters by osmosis from xylem
4. Creates high hydrostatic pressure causing mass movement of sucrose towards sink/root down hydrostatic gradient
5. Where sucrose at sink used for respiration or converted to starch for storage

Question 2

formation of an action potential (8)

Answer 2

1. Energy of stimulus causes Na+ voltage gated channels to open
2. Na+ influx into axoplasm down electrochemical gradient cause reversal in potential difference across membrane
3. As Na+ diffuse into axon, more Na+ channels open
4. Once +40mV action potential established, voltage gated Na+ channel close
5. Voltage gated K+ channels open and K+ diffuse out
6. Causes more K+ channels open starting repolarisation of axon
7. Outward diffusion of K+ causes temporary overshoot (hyperpolarisation) and voltage gated K+ channels close
8. Na-K pump re-establishes resting potential

Question 3

Muscle contraction (7)

Answer 3

1. Ca2+ diffuse into myofibril (sarcoplasm) from sarcoplasmic reticulum down conc. gradient
2. Ca2+ cause tropomyosin to change shape and move away from actin binding sites
3. Myosin heads with ADP attached bind to actin binding sites forming cross bridge
4. Myosin heads change angle/bend (to low-energy configuration) pulling actin filament (to centre of sarcomere/m-line) and releasing ADP
5. ATP attaches to myosin head causing it to detach from actin
6. Ca2+ activate ATPase which hydrolyses ATP to ADP + Pi, releasing energy for myosin head to return to ‘high- energy configuration’
7. Myosin head with ADP reattaches to binding site further along actin and cycle repeats

Question 4

How is resting potential maintained (4)

Answer 4

1. Na+ actively transported out of axon and K+ into axon by Na-K pump
2. For every 3 Na+ out of axon, 2 K+ move
3. Most K+ channels open but Na+ channels closed
4. So greater conc. Na+ in surrounding tissue fluid and greater conc. K+ in axon and overall more positive charge surroundings creates electrochemical gradient

Question 5

Transmission across synapse (8)

Answer 5

1. Depolarisation/arrival of action potential to presynaptic membrane
2. Cause Ca2+ channels to open & Ca2+ enters synaptic knob (by facilitated diffusion)
3. Ca2+ cause synaptic vesicles to migrate to & fuse with presynaptic membrane
4. Releasing acetylcholine that diffuses across synaptic cleft (quickly because diffusion pathway short)
5. Ach then binds to receptor sites on Na+ protein channels on postsynaptic membrane causing them to open
6. Influx of Na+ generates new action potential in postsynaptic neurone
7. Acetylcholinesterase hydrolyses Ach to choline + Ethanoic acid which diffuse back across synaptic cleft into presynaptic neurone (recycled)
   (rapid breakdown of Ach also prevents continuously generating action potential so discrete transfer of info across synapse)
8. ATP from mitochondria recombines choline + ethanoic acid into Ach, stored in synaptic vesicles and Na+ channels close in absence of Ach

Question 6

Ultrafiltration in Bowman’s capsule (4)

Answer 6

1. Afferent arteriole wider than efferent so build-up of hydrostatic pressure in glomerulus
2. Filtrate can pass beneath and between gaps in podocytes and gaps between endothelial cells of capillary (reduce resistance)
3. Filtrate passes through (capillary) basement membrane
4. Small substances pass (eg. Amino acids, glucose, urea, Na+ ions) but large substances cannot (eg. Proteins, red blood cells)

Question 7

General process of osmoregulation…

Answer 7

1. Osmoreceptors in hypothalamus detect fall in water potential
2. When water potential of blood low, water lost from osmoreceptor cells by osmosis (to blood)
3. Osmoreceptor cells shrink causing hypothalamus to produce ADH
4. ADH passes to posterior pituitary gland where it is secreted into capillaries
5. ADH passes to kidneys where increases permeability to water of cell membrane of cells lining wall of distal convoluted tubule and collecting duct
6. More water leaves collecting duct by osmosis down water potential gradient, and re-enters blood, only prevents water potential of blood from getting lower
7. Osmoreceptors send nerve impulse to thirst centre of brain