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ICSE Question Bank
2020Q3(b)(iii)Sense Organs
ReasoningModerate1 mark
3(b)

Give the biological reasons for the following statements:

We cannot distinguish colours in dim light.

Topic: Colour vision in dim light

Previous-year question practice database

Answer

Exam answer

Colour vision is the function of the CONES of the retina, and cones are stimulated only by BRIGHT light. In dim light the cones do not respond, and we see by means of the RODS, which contain rhodopsin and are sensitive to dim light but give only black-and-white vision.

Explanation

The answer must name BOTH receptors and contrast them, since the question is about why one fails.

CONES — need BRIGHT light; give COLOUR vision; contain IODOPSIN RODS — work in DIM light; give BLACK-AND-WHITE vision; contain RHODOPSIN

A way to keep them paired: Cones for Colour, Rods for the dark — and Rhodopsin with Rods, both beginning with R.

Rhodopsin is made from VITAMIN A, which is why a deficiency of it causes NIGHT BLINDNESS — the rods can no longer make enough pigment.

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