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C15.1 Neurones and nervous coordination AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 85
- Hormone response is slow.
- Widespread and long-lasting.
- Nervous response is rapid, localised and short-lived.
15.2 The nerve impulse AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page no. 89
1 Active transport of sodium ions out of the axon by sodium-potassium pumps is faster than active transport of potassium ions into the axon. Potassium ions diffuse out of the axon but few, if any, sodium ions diffuse into the axon because the sodium ·gates’ are closed. Overall, there are more positive ions outside than inside and therefore the outside is positive relative to the inside.
2 A = closed B = open C = closed D = closed E =closed F =open
15.3 Passage of an action potential AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 92
1 a Node of Ranvier
b Because the remainder or the axon is covered by a myelin sheath that prevents ions being exchanged I prevents a potential difference being set up.
c lt moves along in a series of jumps from one node of Ranvier to the next.
d Saltatory (conduction)
e It is faster than in an unmyelinated axon.
2 It remains the same does not change.
15.4 Speed of the nerve impulse AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 95
1 During the refractory period the sodium voltage-gated channels arc closed so no sodium ions can move inwards and no action potential is possible. This means there must be an interval between one impulse and the next.
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All-or-nothing principle -There is a particular level or stimulus that triggers an action potential. At any level above this threshold, a stimulus will trigger an action potential that is the same regardless of the size of the stimulus (the ‘all’ part). Below the threshold, no action potential is triggered (the ‘nothing’ part).
3 Mammals have myelinated neurons and so have saltatory conduction. Mammals arc endothermic and their constant, usually higher, body temperature increases the rate of diffusion of ions across the axon membrane and hence the speed of conduction or the action potential.
15.5 Structure and function of synapses AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 99
1 It possesses many mitochondria and large amounts of endoplasmic reticulum. 2 Explain how the postsynaptic neurone is adapted to receive the neurotransmitter. lt has receptor molecules for neurotransmitters e.g., acetylcholine, on its membrane.
3 Neurotransmitter is released from vesicles in the presynaptic neurone into the synaptic cleft when an action potential reaches the synaptic knob. The neurotransmitter diffuses across the synapse to receptor molecules on the postsynaptic neurone to which it binds, thereby se11ing up a new action potential.
4 Only one end can produce neurotransmitter and so this end alone can create a new action potential in the neurone on the opposite side of the synapse. At the other end there is no neurotransmitter that can be released to pass across the synapse and so no new action potential can be set up.
5 a The relatively quiet background noise of traffic produces a low-level frequency of action potentials in the sensory neurones from the ear. The amount of neurotransmitter released into the synapse is insufficient to exceed the threshold in the postsynaptic neurone and to trigger an action potential and so the noise is ‘filtered out’ I ignored. Louder noise secrete a higher frequency and the amount of neurotransmitter released is sufficient to trigger an action potential in the postsynaptic neurone and so there is a response. This is an example of temporal summation. (Note: An explanation in terms of spatial summation is also valid: many sound receptors with a range of thresholds~ more receptors respond to the louder noise ~ more neurotransmitter~ response.)
b Reacting LO low-level stimuli (background traffic noise) that present little danger can overload the (central) nervous system and so organisms may fail to respond to more important stimuli. High-level stimuli (sound of horn) need a response because they are more likely to represent a danger.
6 a Increase in speed 64 -40 = 24 ms
-1 Percentage increase 24/40 x 100 =37.5%
b Reflex arcs allow rapid responses to potentially harmful situations. Information passes across synapses relatively slowly compared to the speed it passes along an axon. The fewer synapses there are, the shorter the overall time taken to respond 10 a stimulus -an advantage where a rapid response is required.
15.6 Transmission across a synapse AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 101
1 a Sodium ions
b Acetylcholine
c ATP
d Calcium ions
2 To recycle the choline and ethanoic acid; to prevent acetylcholine £rom continuously generating a new action potential in the postsynaptic neurone.
15.7 Structure of skeletal muscle AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 106
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Muscles require much energy for contraction. Most of this energy is released during the Krebs cycle and electron transport chain in respiration. Both these take place in mitochondria.
2 A =Z-line B =H-zone C =I-band (isotropic band) D = A-band (anisotropic band).
3 The actin and myosin filaments lie side by side in a myofibril and overlap at the edges where they meet. lf cut where they overlap, both filaments can be seen. lf cut where they do not overlap, we see one or other filament only.
4 Slow-twitch fibres contract more slowly and provide less powerful contractions over a longer period. Fast-twitch fibres contract more rapidly and produce powerful contractions but only for a short duration.
5 Slow-twitch fibres have myoglobin to store oxygen, much glycogen to provide a source of metabolic energy, a rich supply of blood vessels to deliver glucose and oxygen, and numerous mitochondria to produce ATP. Fast-twitch fibres have thicker and more numerous myosin filaments, a high concentration of enzymes involved in anaerobic respiration and a store of phosphocreatine to rapidly generate ATP from ADP in anaerobic conditions. Banner 2
15.8 Contraction of skeletal muscle AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 111
1 Myosin is made of two proteins. The fibrous protein is long and thin in shape, which enables it to combine with others to form a Jong thick filament along which the act in filament can move. The globular protein forms two bulbous structures (the head) at the end of a filament (the tail). This shape allows it to exactly fit recesses in the actin molecule, to which it can become attached. Its shape also means it can be moved at an angle. This allows it to change its angle when attached to actin and so move it along, causing the muscle to contract.
2 Phosphocreatine stores the phosphate that is used to generate ATP from ADP in anaerobic conditions. A sprinter’s muscles often work so strenuously that the oxygen supply cannot meet the demand. The supply or ATP from mitochondria during aerobic respiration therefore ceases. Sprinters with the most phosphocreatine have an advantage because ATP can be supplied to their muscles for longer, and so they perform better.
3 A single ATP molecule is enough to move an actin filament a distance of 40 nm. Total distance moved by act in filament = 0.8 μm (= 800 nm). Number of ATP molecules required = 800 ~ 40 = 20.
4 One role or ATP in muscle contraction is to attach to the myosin heads, thereby causing them to detach from the actin filament and making the muscle relax. As no ATP is produced after death, there is none to attach to the myosin, which therefore remains attached to act.in, leaving the muscle in a contracted state, i.e. rigor mortis. Banner 3
Practice questions: Chapter 15 AQA A2 Biology B15 Nervous Coordination And Muscles Kerboodle Answers : Page No. 112-113
1 (a) (Ion) channel proteins open. Sodium in. Changes membrane potential/makes inside of axon less negative/positive/depolarisation/ reaches threshold. More channels open/positive feedback
(b) (i) Potassium channels open. Potassium out. Sodium channels close.
(ii) 200 / s.
(iii) Rate with only absolute refractory period is 1000 / s; percentage increase = (800/200) × 100=400%
(c) Pump/active transport/transport against concentration gradient; Of sodium from axon/sodium out/of potassium in.
2 (a) - Causes sodium ion channels to open.
- Sodium ions enter (cell and cause depolarisation).
(b) - (If not removed) keeps binding (to receptors).
- Keeps causing action potentials/depolarisation (in postsynaptic membrane).
- Prevents information being carried across synapse/described consequence
(c) 1. Movement in all groups (about) same before MDMA.
- MDMA increases movement in Group L.
- Group K shows MDMA causes movement.
- No/little increase in mice without receptor/Group M.
3 (a) (i) - Moves out of the way when calcium ions bind.
- Allowing myosin to bind (to actin)/crossbridge formation.
(ii) - Head (of myosin) binds to actin andmoves/pulls/slides actin past.
- (Myosin) detaches from actin and re-sets/moves further along (actin)
- This uses ATP.
(b) (i) 1. (Glycogen broken down) gives (lots of) glucose for glycolysis/anaerobic respiration.
- Glycolysis/anaerobic respiration not very efficient/only yields 2 ATP per glucose.
(ii) - (Many capillaries) give high concentration/lots of oxygen/ shorter diffusion pathway for oxygen/large surface area for oxygen exchange/diffusion.
- Good glucose supply with little glycogen present.
- Allows high rate of/more aerobic respiration OR prevents build-up of lactic acid/(muscle) fatigue.
4 (a) (i) the I-band (1 mark) Decreases.
(ii) Nothing / stays the same length / does not change.
(b) 29545-30455;
(c) - Attachment/cross bridges between actin and myosin.
- ‘Power stroke’ / movement of myosin heads / pulling of actin.
- Detachment of myosin heads.
- Myosin heads move back/to original position / ‘recovery stroke’.
5 a i Contains more/large amount of succinic dehydrogenase. (Slow fibres) have lots of mitochondria/ (slow fibres) respire aerobically.
ii Near edge/outside. Short distance for diffusion of oxygen/Allows rapid diffusion/more diffusion of oxygen; Oxygen used by mitochondria/electron transfer system in mitochondria.
b i Measure with graticule/eyepiece scale. Calibrate against something of known size. OR Estimate/measure field diameter with a scale. Estimate number of fibres to cover diameter.
(ii) Equivalent measurements taken. At random to avoid bias/avoid choice of particular fibres. Large number to be representative/minimise effect of extremes/of anomalies. Banner 4
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