8.1 Response to the Environment

Neurones can be either myelinated or non-mylinated. A myelinated axon conducts impulses faster than a non-myelinated axon.

Explain why.

There are three types of neurones: sensory neurones, motor neurones and relay neurones, as shown in the diagram below.

Which of the statements about the three neurones is true?

I. Neurone A is the sensory neurone and carries the impulse from the receptors

II. Neurone B is the motor neurone and carries the impulse to the effector

III. Neurone C is the relay neurone and is myelinated

IV. Neurone A and C are myelinated along their axon

The nervous system is different to the endocrine system. 

Contrast the nervous system with the endocrine system. 

Nerve impulses pass through the nervous system along a pathway.

Complete the nervous pathway shown below that leads to a change in the diameter of the pupil in the eye.

(1)

The diagram below shows an eye in two conditions.

State the light condition that would lead to the effect seen in eye A and eye B.

(1)

Describe the sequence of events in the eye that lead to the effect seen in eye A. 

(3)

Body temperature can affect the speed of an action potential. Research has found that reaction time is slower when body temperature falls. This is because nerve impulse conduction is slower. 

Explain how a lower temperature can affect the speed of an action potential.

The permeability of an axon's cell membrane changes during an action potential. 

The graph below shows the changes in membrane permeability, during a single action potential, to both sodium (Na⁺) and potassium (K⁺) ions. 

Explain the steep increase in sodium ion permeability between 0.5 ms and 0.7 ms.

During an action potential, the membrane potential of the axon reaches +40 mV and falls steeply.

Use the information from the graph in part (b) to explain this fall in membrane potential.

After exercise, ATP is required for the resting potential to be re-established in axons. 

Explain how this occurs. 

The diagram shows part of the human eye. 

Explain the following scenarios. 

When one photon of light falls on cell 1 no light is seen. When one photon of light falls on cell 2, and one on cell 3 and one on cell 4 then light is seen. 

(3)

When light hits cells 5 and 6 light is seen in colour.  

(2)

Some mammals are nocturnal and are active at night. The number and distribution of their rod and cone cells within the retina is different to that of a human retina.

Describe and explain this difference. 

Explain the sequence of events that occur in rod cells in both light and dark conditions.

The graph below shows the effect of increasing IAA concentration on the growth of plant roots and shoots.

Describe the effect of increasing IAA concentration on the growth of plant shoots shown in the graph.

Describe how the effect of IAA on plant roots differs from its effect on plant shoots as shown in the graph.

IAA can move within the growing regions of a plant shoot to other tissues via diffusion. When light is shone on the plant shoot, IAA will stimulate growth on the shaded side. 

Explain how growth on the shaded side of a young shoot helps to keep the leaves in a favourable environment for survival.

A herbicide has a similar chemical structure to IAA. Unlike IAA it is not broken down by the plant. When applied to weeds it can be absorbed into all plant tissues. Their growth is very rapid and distorted and the plant will die within days of application.

Explain the rapid and distorted growth of weeds when this herbicide is applied.