OxfordAQA International A Level Biology
OxfordAQA International A Level Biology 3.4.2 Receptors Exam Questions
Free OxfordAQA International A Level Biology revision for 3.4.2 Receptors: what you need to know, common mistakes and exam-style questions with answers. Covers receptor specificity, the Pacinian corpuscle and generator potentials, and rods, cones and optical pigments in the human retina.
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About OxfordAQA International A Level Biology 3.4.2 Receptors
Receptors in Unit 4 uses two examples to show how stimuli are converted into nerve impulses. For the Pacinian corpuscle you need to describe its basic structure, explain why receptors respond only to specific stimuli, and explain how deformation opens stretch-mediated sodium ion channels to produce a generator potential. For the human retina you need to explain how the different optical pigments and the neural connections of rods and cones give differences in sensitivity to light, sensitivity to colour and visual acuity. It links to nerve impulses and synaptic transmission, and summation at synapses is a key idea for explaining rod sensitivity.
What you need to know
- Receptor specificity Each type of receptor responds to a specific stimulus. The Pacinian corpuscle responds only to mechanical pressure.
- Pacinian corpuscle The corpuscle has a sensory neurone ending surrounded by layers of connective tissue (lamellae). Pressure deforms the lamellae and the neurone membrane, opening stretch-mediated sodium ion channels.
- Generator potential Sodium ions diffuse into the neurone ending, depolarising it and producing a generator potential. If the generator potential reaches the threshold, an action potential is produced.
- Rods Rods contain rhodopsin and are very sensitive to light because several rods connect to one bipolar neurone, so their generator potentials combine (summation) to reach threshold. This gives low visual acuity.
- Cones There are three types of cone with different iodopsin pigments sensitive to different wavelengths, giving colour vision. Each cone connects to its own bipolar neurone, so cones give high visual acuity but need bright light.
Common mistakes in 3.4.2 Receptors
- Saying rods give colour vision. Cones give colour vision; rods give black and white vision in dim light.
- Explaining high acuity in cones by saying they are more sensitive. Acuity is high because each cone has its own connection to a bipolar neurone, so the brain can distinguish points close together.
- Saying pressure directly causes an action potential. Pressure produces a generator potential, which must reach threshold to cause an action potential.
- Writing that sodium ions are pumped into the neurone. They diffuse in through open channels.
Try three 3.4.2 Receptors exam-style questions
1. Explain how pressure on a Pacinian corpuscle leads to a generator potential. (3 marks)
Show the mark scheme answer
Pressure deforms the lamellae and the neurone membrane; stretch-mediated sodium ion channels open; sodium ions diffuse in, depolarising the membrane.
2. Explain why rods allow us to see in dim light. (3 marks)
Show the mark scheme answer
Several rods connect to a single bipolar neurone; the small generator potentials from each rod add together (summation); enough to reach threshold and produce an action potential.
3. Explain why cones give higher visual acuity than rods. (2 marks)
Show the mark scheme answer
Each cone is connected to its own bipolar neurone; so separate impulses are sent to the brain from points close together.
3.4.2 Receptors FAQs
What does OxfordAQA International A Level Biology 3.4.2 Receptors cover?
It covers receptor specificity, the Pacinian corpuscle and generator potentials, and rods, cones and optical pigments in the human retina.
Which unit is 3.4.2 Receptors examined in?
It is part of Unit 4, Control, which is an International A2 paper. The Unit 5 synoptic paper can also draw on this content.
What is the difference between sensitivity and acuity?
Sensitivity is the ability to detect low light intensity, which rods are good at because of summation. Visual acuity is the ability to tell apart points that are close together, which cones are good at because each has its own neurone.
