Question

In a chloroplast, what is the ratio of chl a: chl b and carotenoids: xanthophylls respectively?
A. 3:1 and 2:1
B. 2:1 and 3:1
C. 3:2 and 1:1
D. 1:1 and 3:2

Answer 1

Hint: Chlorophyll, carotenoids, xanthophylls and phycobilins are the photosynthetic pigments. All types of photosynthetic pigments are located in plastids - chloroplast or chromoplasts. Chlorophyll is present in abundance and carotenes are more in ratio than xanthophylls.

Complete answer:
All photosynthetic organisms absorb solar radiation which is a source of energy that initiates the process. The light is absorbed through the agency of certain molecules called pigments. The role of pigments is to convert the absorbed solar energy into chemical energy. These pigments are located in plastids- chloroplast or chromoplasts. Embedded in the matrix of the chloroplast in a membrane system which is the site of the light reactions in photosynthesis.

Photosynthetic pigments are as follows-
Chlorophylls (green pigments)-
The chlorophylls are the main pigments which plants use in photosynthesis. They mainly absorb red and blue-violet light, and reflect most of the blue-green, green and yellow light. There are at least seven types of chlorophylls- chl a, chl b, chl c, chl d, chl e, bacterio-chlorophyll and bacterioviridin.
Of these chlorophyll a is the primary pigments m involved in photosynthesis. Light energy absorbed by all other types of Chlorophylls and accessory pigments is ultimately transferred to Chlorophyll a. The amount of chlorophyll b is around one-fourth of total chlorophylls whereas chlorophyll a is abundantly present as it is the primary pigment. Hence the ratio between chlorophyll a : chlorophyll b is 3:1.

Carotenoid pigments-
Carotenoids occur in the thylakoids of chloroplasts and chromoplasts along with chlorophyll molecules. It is of two types- carotenes and xanthophylls
A. Carotenes- It is orange-yellow in colour. Beta carotene is most widespread and associated with the chlorophyll in all photosynthetic organisms.
B. Xanthophylls- It is a yellow-brown carotenoid responsible for autumn colours of leaves. The ratio of Carotenes: xanthophylls are 2:1.
phycobilins- This is an accessory pigment found in red and blue-green algae.

Hence, the correct answer is option (A).

Note: All photosynthetic pigments are present in chloroplast. Chlorophyll is of seven types among them chlorophyll a is the main pigments whereas chlorophyll b is one-fourth of all chlorophylls and hence the ratio between a and b is 3:1. Carotenoids are of two types: carotene and xanthophylls. This shows a ratio of 2:1.