Survival and Response and the Nervous System

Question 1

What is a stimulus?

Answer 1

Any detectable change in the internal or external environment of an organism that produces a response in that organism.

Question 2

Describe what is meant by the term taxis.

Answer 2

Directional movement towards or away from a stimulus.

Question 3

Describe what is meant by the term kinesis.

Answer 3

Movement is random/non-directional.

Question 4

Compare and contrast taxis and kinesis.

Answer 4

• Both are movements of the whole organism in response to a stimulus.
• Taxis is directional movement towards or away from the stimulus, kinesis is random movement.

Question 5

What is positive taxis?

Answer 5

Movement towards the stimulus.

Question 6

What is negative taxis?

Answer 6

Movement away from the stimulus.

Question 7

What is a tropism?

Answer 7

A growth movement of part of a plant in response to a directional stimulus.

Question 8

What is positive tropism?

Answer 8

Growth towards a stimulus.

Question 9

What is a negative tropism?

Answer 9

Growth away from a stimulus.

Question 10

Advantage of plant shoots growing towards light (positive phototropism)?

Answer 10

Increases chance of light falling on leaves, increases rate of photosynthesis.

Question 11

Advantage of plant roots growing away from light (negative phototropism)?

Answer 11

Roots grow into soil, more likely to absorb water and mineral ions.

Question 12

Compare and contrast taxis and tropism.

Answer 12

• Both are directional responses to stimuli.

- Taxis involves the movement of the entire organism whereas tropism is the movement of part of an organism.

Question 13

Describe the effect of IAA on shoot growth.

Answer 13

• Synthesised in shoots tips
• Diffuses into elongating region
• Light causes IAA to move from light side to shaded side
• More IAA on shaded side
• Causes directional growth as there is greater elongation on shaded side
• Shaded side grows faster, causes shoot to bend towards light

Question 14

Describe the effect of IAA on root growth.

Answer 14

• IAA synthesised in root tips
• IAA diffuses into elongating region
• IAA moves towards shaded side
• Inhibits cell elongation
• Uneven growth (more elongation on side with less IAA)
• Causes directional growth away from light and downwards towards gravity

Question 15

Describe how a resting potential is maintained in a neurone.

Answer 15

• Active transport of sodium ions across membrane
• Out of neurone/higher concentration outside
• Differential permeability to K+ and Na+
• Membrane more permeable to K+ ions

Question 16

The potential across the membrane is reversed when an action potential is produced. Describe how.

Answer 16

• Sodium ion gates / channel proteins open

- Na+ rapidly diffuse in

Question 17

Explain the advantages of simple reflex arcs.

Answer 17

• Rapid
• Protect against damage to body tissues
• Do not have to be learnt
• Help escape from predators
• Enable homeostatic control

Question 18

As a neurone transmits an impulse, its rate of oxygen consumption increases.
Explain why.

Answer 18

• ATP required for active transport
• Na+ (actively) moved out only at nodes in myelinated / Na+ (actively) moved out along whole length of axon in non-myelinated

Question 19

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

Answer 19

• Impulse jumps from node of Ranvier to node of Ranvier / depolarisation only at node of Ranvier
• Fewer jumps / depolarisations to travel length of axon;

Question 20

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

Answer 20

-Myelinate axon so shows saltatory conduction/impulses jump from node of Ranvier to node of Ranvier
OR
-Increase diameter of axon so less resistance to flow of ions

Question 21

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

Answer 21

• Depolarisation of presynaptic membrane
• Ca2+ channels open and calcium ions enter (synaptic knob)
• This causes synaptic vesicles to move and fuse with presynaptic membrane and release acetylcholine
• Acetylcholine diffuses across synaptic cleft
• Acetylcholine binds to receptors on the postsynaptic membrane
• Sodium ions enter postsynaptic neurone leading to depolarisation

Question 22

When a nerve impulse arrives at a synapse, it causes the release of neurotransmitter from vesicles in the presynaptic knob. Describe how.

Answer 22

• Nerve impulse/depolarisation of membrane causes Ca2+ channel proteins to open
• Ca2+ enter by facilitated diffusion
• Causes synaptic vesicles to fuse with presynaptic membrane;

Question 23

The binding of GABA to receptors on postsynaptic membranes causes negatively charged chloride ions to enter postsynaptic neurones.
Explain how this will inhibit transmission of nerve impulses by postsynaptic neurones.

Answer 23

• Inside of postsynaptic neurone becomes more negative/hyperpolarised
• More sodium ions required to reach threshold OR not enough sodium ions enter to reach threshold
• For depolarisation/action potential

Question 24

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

Answer 24

• Acetylcholine not broken down and stays bound to receptor

- Na+ ions continue to enter, Na+ channels kept open, action potentials continue to be produced.

Question 25

Describe what is meant by the term refractory period.

Answer 25

The period of inexcitability following the transmission of an impulse, where no new action potentials can be produced.

Question 26

Outline the All or Nothing Law.

Answer 26

A principle that states that all stimuli above a certain threshold value will generate the same size of action potential, regardless of the strength of the stimulus.