8.1.8Unit 6: Practical Skills in Biology II
Detection of Stimuli — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 42 of 48 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4Structure of the Eye and Retina
The eye focuses light onto receptor cells that convert it into nerve impulses.
Iris
- Iris Muscles in the iris control the size of the pupil and so how much light enters the eye.
Lens
- Lens Ciliary muscles, attached by suspensory ligaments, change the lens shape to focus objects at different distances.
Retina
- Retina The light-sensitive layer at the back of the eye contains the photoreceptors called rods and cones.
Fovea
- Fovea Light is focused here, and this region contains many photoreceptors, mostly cones.
Optic nerve
- Optic nerve Impulses leave through this nerve, which exits at the blind spot where there are no photoreceptors.
Part 2 of 4Rod Cells and Cone Cells
The two types of photoreceptor give us vision in dim light and in colour.
Rod Cells and Cone Cells
Rods
- Rods They are mainly in the outer retina and respond to light intensity, giving a black and white image.
Cones
- Cones They are concentrated in the fovea and respond to wavelength, so they allow colour vision.
Colour detection
- Colour detection Cones are red-, green- or blue-sensitive, and the mix of cones stimulated determines the colour seen.
Pigment
- Pigment Rod cells contain rhodopsin, which breaks into retinal and opsin when light is absorbed;
- this is bleaching.
| Feature | Rod cells | Cone cells |
|---|---|---|
| Location | Mainly outer retina | Mainly fovea |
| Responds to | Light intensity | Wavelength of light |
| Image produced | Black and white | Colour |
Part 3 of 4How a Rod Cell Generates an Impulse
A rod cell works in an unusual way: it is depolarised in the dark and hyperpolarised in the light.
1
In the dark, sodium ions are pumped out, but they diffuse back in through open cation channels, so the cell stays depolarised.
2
The depolarised rod cell releases an inhibitory neurotransmitter that stops the bipolar neurone from generating an action potential.
3
In the light, rhodopsin is bleached as cis retinal changes to trans retinal, and the sodium channels in the rod membrane close.
4
Sodium ions are still pumped out but cannot return, so the inside of the cell becomes more negative: it is hyperpolarised.
5
The hyperpolarised rod stops releasing inhibitory neurotransmitter, so the bipolar neurone generates an action potential.
6
The impulse passes along the bipolar neurone to the optic nerve and then to the brain.
Part 4 of 4Exam Checklist & Tips
8.1.8 – Key Points for the Exam
- Photoreceptor A receptor cell that generates an electrical response when stimulated by light.
- Fovea The region of the retina with the most cones, where light is focused for sharpest colour vision.
- Rhodopsin The light-sensitive pigment of rod cells that splits into retinal and opsin in light.
- Hyperpolarisation A change that makes the inside of a cell more negative than its normal resting potential.
- Bipolar neurone The neurone that carries impulses from rod and cone cells to the optic nerve.
Common Mistakes
- Saying rod cells depolarise in light; they are depolarised in the dark and hyperpolarised in the light.
- Forgetting that rods release an inhibitory neurotransmitter in the dark, which is why the bipolar neurone stays silent.
- Placing the blind spot at the fovea;
- the blind spot is where the optic nerve leaves and has no photoreceptors.
- Stating that cones give black and white vision; rods give black and white and cones give colour.
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