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

These are condensed revision notes for AQA A-level Biology (7402) sections 3.6.1.1 (Survival and response) and 3.6.1.2 (Receptors), both A-level only, from the AQA AS and A-level Biology specification for first teaching 2015. For full explanations read the study guide, then test yourself with the practice questions. See also the AQA A-level Biology hub, the printable checklist and the free diagnostics.

Key definitions

Term	Meaning
Stimulus	A detectable change in the internal or external environment
Tropism	Directional growth response of a plant to a directional stimulus
IAA	Indoleacetic acid, the growth factor named in the specification
Taxis	Directional movement in response to the direction of a stimulus
Kinesis	Change in speed of movement or rate of turning in response to stimulus intensity; random, not directional
Reflex	Rapid, automatic response that does not need conscious decision
Generator potential	Depolarisation in a receptor’s membrane caused by a stimulus
Visual acuity	Ability to tell two close points apart

Why response matters

Organisms that detect and respond to change are more likely to find food, avoid harm and remain where conditions suit them. Plants respond by growth. Mobile animals and microorganisms respond by movement, and many animals by reflexes. Every response needs a receptor to detect the stimulus and an effector to carry out the response.

IAA and tropisms

IAA moves from growing regions to other tissues and changes cell elongation. The effect depends on the tissue.

	Shoot	Root
High IAA	More elongation	Less elongation
Side with more IAA	Grows faster	Grows slower
Phototropism	Positive: IAA to shaded side, shoot bends to light	Explained using shoots
Gravitropism	Negative: IAA to lower side, tip curves up	Positive: IAA to lower side, root curves down

Method in steps for any tropism answer:

1. State where IAA moves (shaded side or lower side).
2. State its effect on elongation in that organ (shoot or root).
3. State which side grows faster.
4. State the bending and name the response (positive or negative).

Small reminder: shoot sections lengthen 4.0 mm without IAA and 9.8 mm with a high concentration. Root sections lengthen 10.0 mm and 3.5 mm. Shoot: (9.8 − 4.0) / 4.0 × 100 = +145%. Root: (3.5 − 10.0) / 10.0 × 100 = −65%. Same hormone, opposite effects.

Taxis and kinesis

	Taxis	Kinesis
Depends on	Direction of the stimulus	Intensity of the stimulus
Response	Directional movement	Change in speed or turning rate
Movement	Towards (positive) or away (negative)	Random
Example	Positive chemotaxis of bacteria to a nutrient; Euglena to light	Woodlice moving faster in dry air

Both keep a mobile organism in a favourable environment. A kinesis works because the organism leaves unfavourable areas more quickly and stays in favourable ones.

Small reminder: a woodlouse makes 5 turns per minute in damp air and 14 in dry air. Percentage increase = (14 − 5) / 5 × 100 = 180%.

Simple reflex

Pathway: stimulus, receptor, sensory neurone, relay neurone, motor neurone, effector, response.

• Fast: short pathway, few synapses.
• Automatic: no decision needed and no learning needed.
• Protective: pulls the body part away from harm, so less tissue damage.

Diagram in words:

Receptor, then sensory neurone, then relay neurone, then motor neurone, then effector, with a synapse between each pair of neurones.

Small reminder: path length 0.60 m at 60 m s⁻¹ takes 0.60 / 60 = 0.010 s = 10 ms. With two synapses at 0.5 ms each, total = 10 + 1.0 = 11 ms.

Required practical 10 in brief

Item	Notes
Apparatus	Choice chamber (two or more sections) or maze
Animal	Woodlice or fly larvae
Independent variable	One environmental variable, for example humidity or light
Dependent variable	Number of animals in each section (or time to complete the maze)
Controls	Temperature, time, species, age or size, start position, other variable
Reliability	Repeats, enough animals, mean values
Ethics	Gentle handling, short exposure, no harmful extremes, return animals

Chi-squared reminder. Expected = total / number of categories. χ² = Σ (O − E)² / E. Degrees of freedom = categories − 1. Reject the null hypothesis if χ² is greater than the critical value (3.84 for one degree of freedom at p = 0.05).

Worked reminder: 50 larvae, 32 in the dark half and 18 in the light half. E = 25. χ² = 49/25 + 49/25 = 3.92. As 3.92 > 3.84, the difference is significant at p = 0.05, but only just. Say so and suggest more repeats.

Pacinian corpuscle

• Responds only to pressure, a specific stimulus.
• Structure: sensory neurone ending wrapped in layers of connective tissue (lamellae) with gel between.
• Pressure deforms the lamellae and the neurone membrane.
• Stretch-mediated sodium ion channels widen and Na⁺ enter by diffusion.
• Depolarisation is the generator potential.
• If threshold is reached, an action potential travels along the sensory neurone.
• Bigger stimulus gives bigger generator potential; below threshold gives no action potential.

Small reminder: resting −70 mV, threshold −55 mV. A potential of −58 mV is a change of +12 mV, so no action potential. A potential of −50 mV is +20 mV, so an action potential.

The retina in brief: light passes through the layers of neurones to reach the photoreceptors. Rods and cones pass signals to bipolar neurones, then to the sensory neurones of the optic nerve. The differences between rods and cones come from their pigments and their connections.

Rods and cones

Method in steps for a retina explanation:

1. Name the feature being explained (sensitivity, acuity or colour).
2. Name the pigment involved.
3. Describe the connection to the bipolar neurone.
4. Link the connection to the effect on impulses sent to the brain.

Feature	Rods	Cones
Pigment	Rhodopsin	Iodopsin (three types)
Sensitivity	High; dim light	Low; need bright light
Connection	Many rods to one bipolar neurone	One cone to one bipolar neurone
Acuity	Low	High
Colour	None	Colour vision from three cone types
Why	Summation of generator potentials reaches threshold	Separate pathways keep points of light separate

Exam wording to use: for sensitivity, write “summation”; for acuity, write “separate pathways” or “one-to-one connection”; for colour, write “three types of cone with different pigments”.

Must-know distinctions

• Taxis is directional; kinesis is not.
• Tropism is plant growth; taxis and kinesis are movement of whole organisms.
• Generator potential is graded; an action potential is all-or-nothing.
• Sensitivity is about detecting dim light; acuity is about detail.
• High IAA stimulates shoot elongation but inhibits root elongation.

Quick self-test

1. State the difference between a taxis and a kinesis.
2. Which side of a horizontal shoot has more IAA, and what does it do?
3. Why does a root grow downward when placed horizontally?
4. Name the three neurones in a simple reflex arc, in order.
5. Give two reasons why a reflex protects the body.
6. A woodlouse moves at 2.0 mm s⁻¹ in damp air and 6.0 mm s⁻¹ in dry air. Calculate the percentage increase in speed.
7. State what is meant by a generator potential.
8. Describe how pressure produces a generator potential in a Pacinian corpuscle.
9. Explain why cones give greater visual acuity than rods.
10. Explain why rods work in dim light but cones do not.

Answers

1. A taxis is directional movement in response to the direction of a stimulus; a kinesis is a change in speed or turning in response to stimulus intensity.
2. The lower side. IAA stimulates elongation there, so the tip curves upward.
3. IAA collects on the lower side and inhibits elongation, so the upper side elongates faster and the root curves down.
4. Sensory neurone, relay neurone, motor neurone.
5. It is fast (short pathway, few synapses) and automatic (no decision or learning), so the body part is moved away before more damage occurs.
6. (6.0 − 2.0) / 2.0 × 100 = 200%.
7. A depolarisation of the receptor membrane produced by the stimulus.
8. Pressure deforms the lamellae and membrane, stretch-mediated sodium ion channels widen, Na⁺ diffuse in, and the membrane depolarises.
9. Each cone has its own bipolar neurone, so adjacent points of light send separate impulses to the brain.
10. Many rods share a bipolar neurone, so summation of generator potentials reaches the threshold in dim light. Cones need more light to be stimulated enough.

Where marks are usually lost

• Describing a kinesis as movement towards or away from a stimulus.
• Giving IAA the same effect in root and shoot.
• Writing “IAA makes the shoot grow towards light” without saying that it moves to the shaded side and increases elongation there.
• Missing that the generator potential comes before the action potential in a Pacinian corpuscle.
• Writing “sodium channels open” without saying they are stretch-mediated and that deformation of the membrane causes this.
• Saying rods are “more sensitive” without linking this to rhodopsin and to summation.
• Describing acuity with no mention of the one-to-one connection of cones.
• Giving a chi-squared conclusion as proof instead of “significant at p = 0.05”.
• Missing control variables in required practical 10 answers, or forgetting humane treatment of the animals.

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).