OxfordAQA International AS and A-level Biology: Reflexes, taxes and kineses, and sensory receptors (9610) – Revision Notes

Condensed recall for sections 3.4.1, 3.4.2.1 and 3.4.2.2 of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. Unit 4 is International A-level only. For full explanations, use the study guide. Then practise with the practice questions, tick off topics on the checklist, find your course on the hub, and try the free diagnostics.

3.4.1 Reflex arc

Route: stimulus, then receptor, then sensory neurone, relay neurone and motor neurone, then effector and response.

Point	Detail
Neurones	Three, with two synapses between them
Effector	A muscle (contracts) or a gland (secretes)
Speed	Fast: short pathway, few synapses
Control	Involuntary and automatic; no conscious decision needed
Protective role	Limits damage to the body before you are aware of the stimulus

Examples that fit: withdrawing a hand from a hot object; blinking when an object approaches the eye.

Reflex time: method

1. Conduction time = path length ÷ speed. Keep units consistent (m and m s⁻¹ give seconds).
2. Synaptic delay = number of synapses × delay at each.
3. Total = conduction time + synaptic delay. Convert to ms if asked.

Reminder: a 0.90 m pathway at 45 m s⁻¹ takes 0.90 ÷ 45 = 0.020 s = 20 ms. Two synapses at 0.5 ms add 1.0 ms. Total 21 ms.

Taxes and kineses

	Taxis	Kinesis
Movement is	Directed relative to the stimulus	Random
Changes with	Direction of the stimulus	Intensity of the stimulus
What changes	Direction of travel	Speed or rate of turning
Positive / negative	Towards / away from the stimulus	Not used
Examples	Chlamydomonas swimming to light; blowfly larvae moving away from light; bacteria moving up a nutrient gradient	Faster and more frequent turning in an unfavourable region, slower in a favourable one

Survival value: both keep an organism in, or take it to, a favourable environment, so it is more likely to find food, light or shelter and avoid harm.

Kinesis argument in four steps: (1) speed or turning is higher in the unfavourable condition; (2) so the animal is likely to move out of it by chance; (3) in the favourable condition it slows and turns less; (4) so it stays there.

Worked reminder: 54 mm in 30 s is 54 ÷ 30 = 1.8 mm s⁻¹. 15 turns in 30 s is 30 turns per minute.

Worked reminder (taxis): 58 of 120 cells in the section nearest a lamp is 48.3%. A high, steadily falling percentage with distance from the lamp is a positive phototaxis.

3.4.2.1 Pacinian corpuscle

Specific stimulus: a receptor responds only to its own kind of stimulus. The Pacinian corpuscle responds to mechanical pressure, not to light. A receptor converts the energy of the stimulus into a change in membrane potential.

Structure (basic):

• sensory neurone ending in the centre;
• concentric layers (lamellae) of connective tissue around it;
• viscous gel between the layers;
• stretch-mediated sodium ion channels in the neurone ending membrane.

Pressure to impulse: chain

1. Pressure deforms the lamellae.
2. The neurone ending membrane is stretched.
3. Stretch-mediated sodium ion channels widen.
4. Sodium ions diffuse in.
5. Membrane depolarises: a generator potential.
6. If the generator potential reaches the threshold, an action potential is set up in the sensory neurone.

Generator potential	Action potential
Graded: larger stimulus, larger change	Occurs or does not occur
Stays in the receptor region	Travels along the neurone
Set up by stretch-mediated channels	Set up by voltage-gated channels once threshold is reached

Worked reminder: resting potential −70 mV, threshold −55 mV, so 15 mV of depolarisation is needed. A stimulus that takes the potential to −57 mV gives 13 mV, which is 2 mV short, so no action potential.

3.4.2.2 Rods and cones

	Rods	Cones
Pigment	Rhodopsin (one type)	Three types, each absorbing different wavelengths
Light intensity	Respond in dim light	Need bright light
Colour	Do not distinguish colours	Distinguish colours
Connections	Many rods to one bipolar neurone	Usually one cone to one bipolar neurone
Result	High sensitivity, low acuity	Low sensitivity, high acuity

Linking the three differences

• Sensitivity (rods). Pigment is changed by low-intensity light, and summation of the generator potentials of many rods brings the shared bipolar neurone to threshold.
• Acuity (cones). Two points on separate cones, each with its own neurone, send separate impulses. Rods that share a neurone send one set, so the points merge.
• Colour (cones). The brain compares how strongly the three cone types are stimulated. Rods have one pigment, so there is no comparison to make.

Worked reminder (summation): a single rod gives 0.8 mV in dim light and the threshold is 3.0 mV. Four rods give 4 × 0.8 = 3.2 mV, which is enough. Convergence of 90 rods onto 5 bipolar neurones is 90 ÷ 5 = 18 rods for each.

Must-know distinctions

• Taxis or kinesis? Look for direction. Directed movement is a taxis; changes in speed or turning with intensity are a kinesis.
• Generator or action potential? The generator potential is graded and stays in the receptor. The action potential happens only at or above threshold.
• Sensitivity or acuity? Sensitivity is about responding in dim light (rods, summation). Acuity is about telling two points apart (cones, separate neurones).
• Receptor or effector? A receptor detects the stimulus. An effector produces the response.

Answering data questions on this unit

• Behaviour data (distance, turns, counts in zones): calculate speeds or percentages first, then state the pattern, then name the response and link it to survival.
• Membrane potential data: subtract the resting potential to get the depolarisation, compare it with the threshold gap, then comment on whether an action potential results. Quote values from the table.
• Retina data: pick out the ratio of photoreceptors to neurones, then link a high ratio to sensitivity and a ratio near one to acuity.
• Explain questions usually need a feature, a mechanism and a result, in that order. For example: many rods share a neurone (feature), their generator potentials sum (mechanism), so threshold is reached in dim light (result).

Quick self-test

1. Name the three neurones of a simple reflex arc in order.
2. Give two reasons a reflex protects the body.
3. A pathway is 1.2 m long, impulses travel at 60 m s⁻¹, and the two synapses each add 0.5 ms. Calculate the response time in ms.
4. Distinguish a taxis from a kinesis.
5. A flatworm moves 36 mm in 30 s in strong light and 12 mm in 30 s in dim light, and turns more often in strong light. Calculate each speed and name the type of response.
6. State what the Pacinian corpuscle responds to, and why it does not respond to light.
7. Name the channels that open in the Pacinian corpuscle when it is deformed, and the ion that moves.
8. Resting potential is −70 mV and a pressure gives −61 mV. If the threshold is −55 mV, does an action potential occur? Give the depolarisation.
9. Explain why rods allow vision in dim light.
10. Explain why cones give better acuity than rods.
11. Explain why objects look grey in very dim light.

Answers

1. Sensory neurone, relay neurone, motor neurone.
2. It is fast, because the pathway is short with few synapses, and automatic, with no thinking needed. Both limit damage.
3. 1.2 ÷ 60 = 0.020 s = 20 ms; plus 2 × 0.5 = 1.0 ms; total 21 ms.
4. A taxis is directed movement towards or away from a stimulus; a kinesis is speed or turning varies as the stimulus gets stronger or weaker, with no direction involved.
5. Strong light 36 ÷ 30 = 1.2 mm s⁻¹; dim light 12 ÷ 30 = 0.40 mm s⁻¹. This is a kinesis.
6. Pressure. Its membrane has stretch-mediated sodium ion channels that respond to deformation, and it contains nothing that is changed by light.
7. Stretch-mediated sodium ion channels; sodium ions (Na⁺) diffuse in.
8. Depolarisation = −61 − (−70) = 9 mV. The potential of −61 mV is below the −55 mV threshold, so no action potential.
9. Rhodopsin is changed by low-intensity light, and many rods share one bipolar neurone, so their generator potentials sum to reach the threshold.
10. Each cone has its own bipolar neurone, so two points stimulating different cones give two separate sets of impulses.
11. Only rods respond. They have one pigment, so the brain cannot compare responses of different types.

Where marks are usually lost

• Writing that a kinesis moves an organism “towards” or “away from” something.
• Leaving out the word “random” or the link to stimulus intensity.
• Calling the generator potential an action potential.
• Not saying that sodium ions move through channels by diffusion, and that the channels open because the membrane is stretched.
• Saying a receptor “senses” a stimulus without naming the stimulus it responds to.
• Explaining rod sensitivity without the word summation, or with no reference to many rods sharing a neurone.
• Explaining acuity without saying that cones have separate connections.
• Slipping on units: m, ms and s mixed together in reflex-time calculations.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, from OxfordAQA, covering specification content 3.4.1, 3.4.2.1 and 3.4.2.2.