Eating carrots won’t improve your eyesight.
That is, unless you have vitamin A deficiency which causes a condition known as nyctalopia (or night blindness). People suffering from this condition are less able to see in low light levels. In extreme cases it may lead to poor vision in the light as well. Nyctalopia is not only caused by vitamin A deficiency. It is most commonly caused by a genetic problem that causes the rods in your eyes to become less responsive to light.
Your eyes have rods and cones that detect light. Rods function largely in black and white but are very numerous (about 100 million per eye); whilst cones are colour sensitive and concentrated in smaller numbers (about 5 million) right in the centre of the retina in an area called the fovea. Rods can be triggered by just a few photons whilst cones require many more to get a response. This is why colour vision is so poor in low light levels.
There are three kinds of cones; one each responsible for long, medium and short wavelengths of light. These wavelengths correspond to red, green and blue light.
The colours we see are your brain’s interpretation of the different frequencies striking your light sensitive cells (photoreceptors).
So now to carrots; what have they to do with anything?
Carrots contain an orange pigment called β-carotene (pronounced beta carotene). This can be converted in human cells to vitamin A, which in turn leads to the production of rhodopsin. Rhodopsin enables photoreceptors to detect light. Without it, rods will not work, which is why vitamin A deficiency leads to an inability to see well in low light levels. It is very unusual for anyone in the western world to develop vitamin A deficiency because there is plenty of it in our diet, but it can be a problem in developing countries where the diet is less varied.
Eating more carrots cannot make your eyesight better but, were you suffering from vitamin A deficiency, eating carrots could cure this and therefore restore your damaged eyesight to its normal level.
The octopus has a very similar but also subtly different structure to their eyes. The basic parts are there, with a retina to receive the focused image. They also have a lens that they can change with ciliary muscles – this is how the image is focused. There is also an iris that can control the amount of light entering the lens. It is remarkable given how different our evolutionary paths have been.
Questions…
- Suggest two other vitamins and the conditions caused by their deficiency.
- What technique would you use to separate some β-carotene from a sample of carrot juice?
- Why do you think that we do not see colour well in our peripheral vision?
- To what phylum of living things does an octopus belong?
- Identify a significant difference in the structure of human and octopus eyes.
1. Vitamin C deficiency causes scurvy and Vitamin D deficiency causes rickets.
2. Distillation
3. There are less cones and they are located in the fovea which is in the centre of the retina so they are not triggered.
4. Mollusca
5. The nerve fibres are on the outside of the photoreceptor cells in the octopus.
Q1 – excellent
Q2 – for distillation to work you have to know about the various boiling points of the various substances. All you know about β-carotene is that it is orange. Does that trigger any thoughts?
Q3 – well done (fewer cones – not less!)
Q4 – yes it’s a mollusc like snails and slugs
Q5 – indeed, it is a much cleaner and more efficient arrangement