Survival, Response To Stimuli And Neurones

Question 1

Describe the effect of IAA on root and shoot growth.

Answer 1

• IAA synthesised in shoots tips
• IAA diffuses into growing region
• light causes movement of IAA from light side to shaded side
• proportionally more IAA on shaded side
• causes directional growth as there is greater cell elongation on shaded side
• shaded side grows faster and causes shoot to bend towards the light
• IAA synthesised in root tips
• IAA diffuses into growing regio
• IAA moves towards underside
• inhibits cell elongation
• uneven growth
• causes directional growth away from light and downwards towards gravity

Question 2

Describe what is meant by the term taxis

Answer 2

• moves towards or away from directional stimulus

Question 3

Describe what is meant by the term kinesis.

Answer 3

• movement is random

Question 4

Compare and contrast taxis and tropism.

Answer 4

• both directional responses to stimuli
• taxis involves the movement of the entire organism whereas tropism is the movement of part of an organism

Question 5

Advantages of simple reflex arcs.

Answer 5

• rapid
• protect against damage to body tissues
• do not have to be learnt
• help escape from predators
• enable homeostatic control

Question 6

Why does the rate of oxygen consumption increase as a neuron transmits an impulse.

Answer 6

• ATP required for active transport
• Na+ moved out only at nodes in myelinated

Question 7

Describe how a resting potential is maintained in a neurone.

Answer 7

• active transport of sodium across membrane
• out of neurone
• differential permeability to K+ and Na+
• membrane more permeable to K+ ions

Question 8

How is the potential across the membrane reversed when an action potential is produced?

Answer 8

• sodium ion gates open
• Na+ rapidly diffuse in

Question 9

Describe the sequence of events leading to the release of acetylcholine and its binding to the postsynaptic membrane.

Answer 9

• depolarisation of presynaptic membrane
• Ca2+ channels open and calcium ions enter
• synaptic vesicles move to fuse with presynaptic membrane and release acetylcholine
• acetylcholine diffuses across synaptic cleft
• binds to receptors on post-synaptic membrane
• sodium ions enter leading to depolarisation

Question 10

How does a nerve impulse arriving at a synapse cause the release of neurotransmitter from vesicles at the pre-synaptic knob.

Answer 10

• depolarisation causes Ca2+ channel to open
• Ca2+ enter by facilitated diffusion
• causes synaptic vesicles to fuse with membrane

Question 11

Give two reasons why transmission across a cholinergic synapse is unidirectional.

Answer 11

• presynaptic neurone has neurotransmitter
• postsynaptic has receptors

Question 12

Binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic.

Explain how this will inhibit transmission of nerve impulses by postsynaptic neurons.

Answer 12

• neurone becomes more negative
• more sodium ions required
• for depolarisation

Question 13

Describe how the speed of the conduction could be increased in a neurone.

Answer 13

• axon is myelinated which provides electrical insulation
• shows saltatory conduction

Question 14

Myelination affects the rate of conduction of a nerve impulse. Explain how.

Answer 14

• myelination provides electrical insulation
• action potentials jump from node of Ranvier to another
• fewer jumps

Question 15

Describe how the inhibition of acetylcholinesterase affects the action of synapses.

Answer 15

• acetylcholine not broken down
• Na+ ions enter / continued depolarisation

Question 16

Describe what is meant by the term refractory period.

Answer 16

• limits number of impulses per second
• maximum frequency of impulse transmission
• period of time between threshold and resting membrane potential
• when maximum frequency reached, no further increase in information