Edexcel A Level (IAL) Biology -8.8 Detection- Study Notes- New Syllabus
Edexcel A Level (IAL) Biology -8.8 Detection- Study Notes- New syllabus
Edexcel A Level (IAL) Biology -8.8 Detection- Study Notes -Edexcel A level Biology – per latest Syllabus.
Key Concepts:
- 8.8 understand how the nervous systems of organisms can detect stimuli with reference to rods in the retina of mammals, the roles of rhodopsin, opsin, retinal, sodium ions, cation channels and hyperpolarisation of rod cells in forming action potentials in the optic neurones
Detection of Stimuli by Rod Cells in Mammals
🌱 Introduction
Rod cells in the retina detect low light levels. They convert light stimuli into electrical signals (action potentials) in optic neurones.
🔍 Structure of Rod Cells
- Outer segment: contains stacked membranes with rhodopsin.
- Inner segment: contains nucleus and mitochondria for energy.
- Synaptic terminal: connects to bipolar cells, then optic neurones.
🔹 Key Molecules
| Molecule | Role |
|---|---|
| Rhodopsin | Photopigment in rods; absorbs light |
| Opsin | Protein component of rhodopsin |
| Retinal | Light-sensitive molecule (changes shape when light hits) |
| Sodium ions (Na⁺) | Maintain dark current through cation channels |
| Cation channels | Let Na⁺ flow in darkness, keeping rods depolarised |
| Hyperpolarisation | Response to light, reduces neurotransmitter release |
🧬 How Rod Cells Work
1. In the Dark
- Rhodopsin inactive, cation channels open.
Na⁺ ions enter rod cell continuously → depolarised (~-40 mV).- Rod releases neurotransmitter (glutamate) at synapse → inhibits bipolar cells.
- No signal sent to brain.
2. In the Light
- Light hits retinal, causing it to change shape (cis → trans).
- Activates opsin → triggers cascade → cGMP broken down.
- Cation channels close → Na⁺ entry stops.
- Rod hyperpolarises (~-70 mV).
- Glutamate release decreases → bipolar cells activated → action potentials in optic neurones.
- Signal transmitted to brain → perception of light.
🔹 Key Points
- Rods are more sensitive to low light than cones.
- Hyperpolarisation, not depolarisation, signals light detection.
- Na⁺ ion flow controls the “dark current”.
- Rods communicate with bipolar cells, which then activate optic neurones.
📊 Summary Table
| Condition | Rhodopsin | Cation Channels | Membrane Potential | Neurotransmitter |
|---|---|---|---|---|
| Dark | Inactive | Open | Depolarised (~-40 mV) | Glutamate released (inhibits bipolar cells) |
| Light | Activated | Closed | Hyperpolarised (~-70 mV) | Glutamate release decreases → bipolar cells activated |
📦 Quick Recap
Rods detect dim light, rhodopsin = opsin + retinal.
Dark: Na⁺ enters → depolarised → neurotransmitter inhibits bipolar cells.
Light: Retinal changes shape → opsin activated → cation channels close → hyperpolarisation.
Bipolar cells activated → action potential in optic neurones → brain detects light.
Rods detect dim light, rhodopsin = opsin + retinal.
Dark: Na⁺ enters → depolarised → neurotransmitter inhibits bipolar cells.
Light: Retinal changes shape → opsin activated → cation channels close → hyperpolarisation.
Bipolar cells activated → action potential in optic neurones → brain detects light.