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I have a quick question regarding Vitamin A deficiency.

The photoreceptor molecules in both rods and cones have the same general structure which is retinal which is bound to a protein called opsin and depending on the photoreceptor opsin changes but the retinal is always the same. And it is the retinal that makes the opsin light sensitive and so it is highly important.

My question is, why is it that when researching the symptoms of vitamin A deficiency one of the most common ones is night blindness. I understand that if not enough retinal is formed not enough rhodopsin is formed and so the patient would suffer from night blindness but wouldn't it also mean that the patient wouldn't be able to produce enough of the other photoreceptors as well and so doesn't only suffer from night blindness but from complete blindness?

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Good question! As you write Vitamin A is quite important vision as it is an integral part of the light sensitive rhodopsin protein. Rhodopsin is necessary for a type of photoreceptor, the rod. It is located in outer segment of the photoreceptor. Vitamin A is also required to other opsins that are needed for the cones to function.

When someone has Vitamin A deficiency it does not mean that they have no Vitamin A. For example instead of having i.e. 100 Vitamin A you only have 30 (arbitrary numbers!).

In the retina, you have the very light sensitive rods and the less light sensitive cones. Rods need a lot of rhodopsin (and hence Vitamin A) as they have to detect small levels of lights (it is thought that a rod can detect a single photon!).

Cones, on the other hand are not so sensitive to light. During the day there are tons of photons*. Since you need less Vitamin A for cones to function, you will see the symptoms of Vitamin A deficiency later compared to rods.

Hence, when you still have a bit of Vitamin A but not enough (e.g. 30 instead of 100) you will first notice it in systems that need Vitamin A the most. It turns out that this are the rods. And yes, if you had 0 Vitamin A you would be blind. But since Vitamin A is needed for many other processes, you will probably be dead at that point.

*Personally, I find it hard to grasp just how large the difference in Illuminance (and hence photon numbers) is for example between day and night. See here for a comparison. As you can see, our visual system has to deal with an enormous dynamical range of illuminance. The fact that we usually don't even think about these differences in light intensities in everday live is testament to how fantastic our visual system is in dynamically adapting to the huge differences in input.

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    $\begingroup$ Thank you for your answer! You explained it perfectly, sometimes people explain things in a really complicated way and I need a dictionary just to go through the answer but yours was really easy to understand and I appreciate that. $\endgroup$ – James Jun 11 at 13:41

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