6.1 Response to Stimuli (A Level only)

Plants respond to external stimuli to maximise their growth rate.  Figure 1 below shows how a plant responds to gravity.

Figure 1

Explain what is happening at the point labelled X to cause the response shown. Refer to the diagram in your answer.

Plant growth is regulated by growth factors. These are produced by cells in the growing tip of the shoot before being transported to the roots. Use your knowledge of plant and cell transport to suggest how growth factors might reach cells in the plant roots.

One mechanism that plants could use to sense the direction of gravity is known as the starch-statolith hypothesis, illustrated in Figure 2 below.  

Figure 2

g = direction of gravity, N = nucleus, V = permanent vacuole, A = starch grains, also known as statoliths.

Use Figure 2 to suggest how the starch-statolith hypothesis explains plant cells’ ability to detect the direction of gravity.

In an investigation, scientists grew seedlings with a mutation that prevented them from producing starch grains. The scientists examined the roots and found that the seedlings still curved downwards, but with a reduction in root curvature in comparison to normal seedlings.  State what these results suggest about the starch-statolith hypothesis.

Figure 1 shows the changes in membrane potential taking place inside the axon of a Pacinian corpuscle when it is stimulated with increasing levels of force.  Line A represents the lowest level of force, increasing up to the maximum for line C.

Figure 1

Explain how stimulation of the Pacinian corpuscle leads to the changes seen in line C.

Explain the differences shown in Figure 1.

Pacinian corpuscles are one of several different types of sensory receptors found in the skin. They are an example of what is known as a fast-adapting receptor, while some other types of receptors are described as slow adapting.  Figure 2 below shows the different responses of fast and slow adapting receptors to the same stimulus.

Figure 2

Use Figure 2 to describe the difference in response of Pacinian corpuscles and slow-adapting receptors and suggest the difference between their roles.

Pacinian corpuscles are found in mammals, while a very similar highly-sensitive, fast-adapting receptor known as a Herbst corpuscle is found in birds. Herbst corpuscles are found in high numbers in the wing tips of many species, the bills of ducks, the legs of herons, and the tongues of woodpeckers.  

Suggest how the distribution of these receptors is an advantage to birds.

Figure 1 provides information about the distribution of different photoreceptors across the retina of the human eye.

Figure 1

Use Figure 1 to describe and explain how the colour of an image formed at point A would differ from an image formed at point B.

A rare eye condition known as cone-rod dystrophy is caused by slow degeneration of the photoreceptors in the eye.  In the early stages of the condition, most cell degeneration takes place at point B (see Figure 1) before progressing later to the outer parts of the retina.  Suggest and explain how the symptoms of cone-rod dystrophy might present over time. 

The number of rods cells on the human retina averages at 90,000 cells per mm².  The diameter of the retina is 32mm.  

Use this information and the formula provided to calculate the total number of rod cells found on the retina.  

Area of a circle = πr²

Rod and cone cells are unusual receptors, in that they are in a depolarised state while in the dark, and they become hyperpolarised when stimulated.  The flow diagram in Figure 2 shows the sequence of events that take place when light falls on a rod cell.

Figure 2

Use the information provided in Figure 2 to suggest how this sequence of events leads to the hyperpolarisation of rod cells.

A condition known as sinus node dysfunction can result when a specific type of tissue in the heart is replaced by fibrous connective tissue.  Suggest how this could cause problems for impulse initiation and transmission within the heart.

One symptom of sinus node dysfunction is bradycardia, or a slow heart rate.  One treatment for bradycardia is to take a drug called atropine.  The effects of atropine, along with some other chemicals, on the heart rate of frogs is shown in Figure 1 below.

Figure 1

Describe the effect that atropine has when administered alongside acetylcholine.  Suggest an explanation for this.

A student concluded from the graph that epinephrine would be a better treatment for bradycardia in humans than atropine. Evaluate their conclusion.

Epinephrine (see Figure 1) is also known as adrenaline.  

Explain the role played by adrenaline in the control of heart rate.

A builder accidentally cuts his finger on an exposed nail protruding from a piece of wood. He immediately pulls his finger away. This reaction occurs due to a simple reflex are involving three neurones.

Figure 1 shows the pathway involved in this reaction.

Figure 1

Suggest three advantages of simple reflexes.

Describe and explain two ways in which an axon may be adapted to conduct impulses at a faster rate in the nervous system.

Synapses ensure that nerve impulses travel unidirectionally towards a muscle fibre. Explain how.

Pacinian corpuscles are one of many receptors found in the skin, they respond to changes in pressure. A scientist wanted to research the effects of different pressures on the magnitude of membrane potentials generated. They investigated this effect by connecting multiple microelectrodes to the end of a toe and applying different pressures to the toe. The microelectrodes measured the maximum membrane potential of the pacinian corpuscle and its sensory neurone when different pressures were applied. Figure 1 below shows the structure of the Pacinian corpuscle, along with its sensory neurone and the position of the microelectrodes. 

Figure 1

Table 1 below shows the results.

Table 1

Explain how the sensory neurone maintains a resting potential when no pressure is applied.

Explain the mechanism by which the application of pressure to the Pacinian corpuscle in the toe produces the changes in membrane potential recorded by microelectrode A.

The membrane potential at microelectrode B was identical for both medium and heavy pressure. Explain why.

Demyelination describes the destruction of myelin surrounding axonal fibres. It is caused by diseases that damage the myelin sheath or the cells that form it. Dysmyelination refers to abnormal and defective myelin sheath. It often occurs due to hereditary mutations that affect the synthesis and formation of myelin. Explain how a lack of functional myelin can result in slower responses to stimuli.

A student undertakes some moderate exercise by walking around a football pitch. Explain what causes her heart rate to increase while she walks.

An electrocardiogram (ECG) measures the electrical fluctuation within a cardiac muscle as a heart is beating. Figure 1 below shows an ECG trace for a normal, healthy person and an ECG trace for a person suffering from heart disease.

Figure 1

Describe the path that electrical impulses follow when they are transmitted from the sinoatrial node to the ventricular muscles in a healthy heart.

Explain how the information from Figure 1 suggests that the damage caused to the diseased heart is likely to have affected the sinoatrial node (SAN).

Exercise can temporarily elevate an individual’s heart rate. Name two legal substances that may cause an individual's heart rate to increase for a period of time .

A research scientist wanted to investigate the behaviour of a species of woodlice that lives in soil. She cultured three samples of woodlice in three separate boxes of soil for 4 months. Each box contained an adequate food supply but were kept at a different temperatures.The temperatures used were 15 °C, 20 °C and 25 °C.

The food was removed from the boxes for 6 hours and then the woodlice were transferred onto a glass surface where there was no food. Each surface had a temperature gradient across it. After 1 hour, the research scientist recorded the position of each woodlouse.

Figure 1 below shows the results. ◼️ marks where she released the worms onto the glass surface.

Figure 1

The research scientist concluded that the woodlice demonstrated taxis. How do the results support this conclusion?

Suggest an explanation for the worms’ behaviour on the glass surfaces that lack a food source.

For the entire duration of the experiment, the surfaces were exposed to light that was dim and even, just enough so the research scientist could see where the woodlice went. Apart from seeing where the woodlice moved, suggest three reasons why it was important that the light was both dim and even.

Plants do not possess a nervous system yet they are still able to grow/move in response to certain stimuli. Describe what plants use in place of a nervous system.