AQA A-level Biology: Survival, Response and Receptors (7402)

This guide covers AQA A-level Biology (7402) sections 3.6.1.1 (Survival and response) and 3.6.1.2 (Receptors), from the AQA AS and A-level Biology specification for first teaching 2015. Both sections are A-level only. Topic 6 is examined on the A-level papers whose content lists include topics 5 to 8 and topics 1 to 8. Control of heart rate (3.6.1.3) is not covered here. For the other pages see the revision notes and the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. To find weak spots first, try the free diagnostics.

What the specification asks for

Statement	What you must be able to do
3.6.1.1 Survival	Explain that responding to environmental change raises the chance of survival
3.6.1.1 Plant growth factors	Explain how IAA concentration affects cell elongation in roots and shoots, as an explanation of gravitropism and phototropism
3.6.1.1 Taxes and kineses	Describe both as simple responses that keep a mobile organism in a favourable environment
3.6.1.1 Reflex	Explain the protective effect of a three-neurone simple reflex (no spinal cord or root detail needed)
Required practical 10	Investigate an environmental variable and animal movement with a choice chamber or maze
3.6.1.2 Pacinian corpuscle	Use it to show that receptors respond only to specific stimuli and produce a generator potential; know its basic structure and the role of stretch-mediated sodium ion channels
3.6.1.2 Retina	Explain differences in sensitivity to light, sensitivity to colour and visual acuity using the optical pigments of rods and cones and their connections in the optic nerve

Why responding helps survival

An organism that detects a change and responds to it is more likely to find food, avoid harm and stay in conditions where its enzymes work. A stimulus is a detectable change in the environment. A receptor detects it and an effector produces the response. Plants and animals both do this, but by different means.

Growth factors and tropisms (3.6.1.1)

In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli. The growth factor named in the specification is indoleacetic acid (IAA). A directional growth response is a tropism: growth towards the stimulus is positive, away from it is negative.

IAA is made in growing regions such as shoot tips and root tips and spreads to other tissues by diffusion and active transport. Its effect on cell elongation depends on the tissue and the concentration:

Tissue	Higher IAA concentration	Result
Shoot	Stimulates elongation	Side with more IAA grows faster
Root	Inhibits elongation	Side with more IAA grows slower

Phototropism

Light from one side makes IAA move towards the shaded side of the shoot. The shaded side has the higher concentration, so its cells elongate more, and the shoot bends towards the light (positive phototropism). This explanation is for shoots.

Gravitropism

A shoot lying on its side shows IAA moving to the lower side. In the shoot, more IAA means more elongation on the lower side, so the tip curves upward (negative gravitropism). In a root on its side, IAA also collects on the lower side. Here a high concentration inhibits elongation, so the upper side elongates faster and the root curves downward (positive gravitropism).

Worked example 1: interpreting elongation data

Sections of shoot and root were placed in solutions of different IAA concentration for 24 hours and the lengthening measured.

IAA (arbitrary units)	Shoot lengthening (mm)	Root lengthening (mm)
0	4.0	10.0
10	9.8	3.5

Percentage change in shoot = (9.8 − 4.0) / 4.0 × 100 = +145%. Percentage change in root = (3.5 − 10.0) / 10.0 × 100 = −65%.

The same high concentration stimulates elongation in the shoot but inhibits it in the root. This is why one hormone can bend a shoot upward and a root downward.

Taxes and kineses

Both are simple responses of a mobile organism that keep it in a favourable environment.

• A taxis is a directional movement in response to the direction of a stimulus. Movement towards the stimulus is positive and away from it is negative. Example: Euglena swims towards light (positive phototaxis), and bacteria move towards a higher concentration of a nutrient (positive chemotaxis).
• A kinesis is a change in the speed of movement or rate of turning in response to the intensity of a stimulus, not its direction. The movement is random. Example: woodlice move faster and turn more often in dry air, so they are more likely to leave dry areas, and they slow down in damp air, so they tend to stay.

The test is direction. If the animal moves in a particular direction relative to the stimulus, it is a taxis. If its speed or turning changes with stimulus level, it is a kinesis.

Worked example 2: a kinesis calculation

In 60 seconds a woodlouse travels 288 mm in dry air and 114 mm in humid air.

Speed in dry air = 288 / 60 = 4.8 mm s⁻¹. Speed in humid air = 114 / 60 = 1.9 mm s⁻¹. Percentage increase in dry air = (4.8 − 1.9) / 1.9 × 100 = 153% (152.6% to 3 s.f. is 153%).

The faster movement in dry air makes it likely the animal will leave. In humid air it moves slowly and stays. No direction was involved, so this is a kinesis.

The simple reflex

A reflex is a fast, involuntary response that happens without a decision by the brain. The specification uses a three-neurone reflex arc as the example, without spinal cord or root detail.

1. A receptor detects the stimulus and a sensory neurone carries impulses towards the central nervous system.
2. A relay neurone connects the sensory and motor neurones by synapses.
3. A motor neurone carries impulses to an effector, a muscle or gland, which responds.

Protective effect. The pathway is short and has few synapses, so the response is very fast. It is involuntary, so the response happens before you consciously register pain, and it does not have to be learnt. A withdrawal reflex pulls a hand from a hot object and the blink reflex protects the eye. Both reduce tissue damage.

Worked example 3: reflex time

A reflex arc is 0.90 m long overall and impulses travel at 50 m s⁻¹. There are two synapses, each adding 1.0 ms.

Conduction time = 0.90 / 50 = 0.018 s = 18 ms. Synaptic delay = 2 × 1.0 = 2 ms. Total = 18 + 2 = 20 ms.

A voluntary response would add further synapses and processing in the brain, so it would take longer.

Required practical 10: animal movement

You investigate the effect of an environmental variable on the movement of an animal using either a choice chamber or a maze. Typical animals are woodlice or fly larvae. A choice chamber has two or more linked sections with different conditions.

• Variable: for example humidity (calcium chloride or silica gel for dry, damp filter paper for humid) or light (one half covered in black card or foil).
• Method: put a set number of animals, such as 10, in the centre. Count how many are in each section at fixed intervals. Repeat several times, using a fresh group each time.
• Control variables: temperature, time allowed, animal species and size, starting position, and the same lighting or humidity in the other variable.
• Reliability: repeat, use many animals and calculate a mean. Let the animals settle before counting.
• Ethics: handle gently, avoid harmful extremes of dryness or light, keep the exposure short, and return them to where they were collected.

Worked example 4: chi-squared on choice chamber data

Fifty fly larvae start in the middle of a chamber with a light half and a dark half. After 10 minutes, 33 are in the dark half and 17 in the light half.

Null hypothesis: the larvae distribute equally between dark and light, so any difference is due to chance. Expected = 50 / 2 = 25 in each half.

χ² = Σ (O − E)² / E = (33 − 25)² / 25 + (17 − 25)² / 25 = 2.56 + 2.56 = 5.12.

Degrees of freedom = number of categories − 1 = 1. The critical value at p = 0.05 is 3.84. As 5.12 > 3.84, you reject the null hypothesis. There is a significant difference: the larvae prefer the dark. Writing “significant at the 5% level” is better than stating a fact about all larvae.

Receptors and the Pacinian corpuscle (3.6.1.2)

A receptor responds only to a specific stimulus. Photoreceptors respond to light and not to pressure. A Pacinian corpuscle is a pressure (mechanical) receptor in the skin. Stimulation of a receptor leads to a generator potential, a change in the membrane potential of the sensory neurone ending.

Structure

The ending of a sensory neurone sits at the centre of the corpuscle. It is wrapped in many layers of connective tissue called lamellae, separated by gel. The neurone membrane at the ending contains stretch-mediated sodium ion channels.

How a generator potential forms

1. Pressure deforms the lamellae.
2. The membrane of the neurone ending stretches and the stretch-mediated sodium ion channels widen.
3. Sodium ions diffuse in through the channels.
4. The membrane depolarises. This depolarisation is the generator potential.
5. If the generator potential reaches the threshold, the sensory neurone then fires an action potential.

A bigger stimulus opens more channels and gives a larger generator potential. If the threshold is not reached, no action potential is sent.

Worked example 5: generator potential and threshold

Resting potential is −70 mV and threshold is −55 mV. A light touch gives −62 mV and a firm press gives −52 mV.

Light touch: change = −62 − (−70) = +8 mV, which is below the 15 mV needed. No action potential. Firm press: change = −52 − (−70) = +18 mV, above 15 mV. An action potential is triggered.

The human retina

Light reaches the photoreceptors in the retina after passing through the layers in front of them. Rods and cones are connected to bipolar neurones, which connect to the sensory neurones of the optic nerve.

Feature	Rods	Cones
Pigment	Rhodopsin	Three kinds of iodopsin (red, green, blue sensitive)
Sensitivity to light	High: work in dim light	Low: need brighter light
Connections	Many rods share one bipolar neurone	One cone connects to one bipolar neurone
Visual acuity	Low	High
Colour	Monochromatic	Colour vision
Location	More numerous at the edges	Concentrated at the fovea

• Sensitivity. Rhodopsin breaks down in low-intensity light. Many rods connect to one bipolar neurone, so their generator potentials add together (summation) and reach the threshold.
• Acuity. Two nearby points of light stimulate separate cones with separate bipolar neurones, so two sets of impulses go to the brain and they are seen as separate. Several rods share one neurone, so two points are seen as one.
• Colour. The three cone types absorb different wavelengths. The brain compares how strongly each type is stimulated. Rods have one pigment and give no colour discrimination.

Worked example 6: convergence

Suppose 120 rods connect to 4 bipolar neurones. Average = 120 / 4 = 30 rods per neurone. A foveal region with 1 cone per bipolar neurone has a ratio of 1. Thirty rods contribute to each impulse pathway, giving sensitivity but poor acuity.

Common errors

• Calling a kinesis “movement towards” something; it is a change in speed or turning.
• Saying IAA always promotes growth. In roots a high concentration inhibits elongation.
• Saying the Pacinian corpuscle produces an action potential directly; the generator potential comes first.
• Writing “more light” for rods without saying that summation brings the bipolar neurone to threshold.
• Forgetting that a receptor responds to a specific stimulus.

Official syllabus

AQA AS and A-level Biology specification for first teaching 2015, published by AQA (sections 3.6.1.1 and 3.6.1.2).