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Biology SL/HL (Option E): Differences between rod and cone cells in the retina?

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SL/HL Biology (Option E): What are three main differences between rod and cone cells in the retina of the human eye?

There are three main differences you need to be able to name between the rod and cone cells of the eye:

1. LIGHT INTENSITY - Cones are used to see in bright light and Rods are used to see in dim light.

2. WAVELENGTH SENSITIVITIES - There are three types of Cone which are sensitive to red, blue and green light respectively, and which are used for your colour vision. Conversely, Rods have only one type which is sensitive to all visible wavelengths.

You don't need to be able to state it in the exam, but rods are basically for your black/white vision in poor light and cones are for colour vision in full light. In full light, your rods are more or less useless and all your vision is based off signals coming into the brain from your cone cells.

3. RATIO OF CELLS:NERVE FIBRES - each Cone cell connects up to a single nerve fibre that then transmits signals through the optic nerve to the relevant part of the brain. Rod cells on the other hand have several rods all connecting up to one nerve fibre (i.e. there isn't a 1:1 ratio of cell:nerve fibre).

You also need to know a fourth fact relevant to this:

4. LOCATION - Cone cells are concentrated in the fovea (an area of the retina which forms the 'centre' of our visual field and gives us the most acute vision). Rods are distributed all over the retina.

Revision Tip!

Roddim = rods are used in dim light.

C for Cone, C for colour vision, C for concentrated in the fovea

(Thanks to Hus for this.)

It also helps to just think through logically. If you know rods are a rubbish fall-back for when light conditions are bad you can imagine that all the facts seem equally 'rubbish' - only one type of rod, not concentrated in any special area, don't even have a single dedicated nerve but have to share nerve fibres with other rod cells.

On the other hand, cones are responsible for our whole colour vision! So we get to have three types of them, have them concentrated in a particular spot where most of our vision happens, we need bright light (obviously - to see!) and because we need it to be highly accurate, we don't want a general picture we want specific feedback, so each cone cell should have one dedicated nerve.

Good luck! As usual, any questions please ask either here or in the Biology forum.

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