Explain the types of plastids in brief
Answers
The two important types of Plastids are as follows:
The plastids contain the pigments which may be of various types.
Sometimes pigments are absent. On the basis of presence of pigments, the plastids are of two types:
(i) the chromoplasts (chromatophores) having pigment, and
(ii) the leucoplasts which are colourless plastids.
Chromolasts:
The chromoplasts may be further divided on the basis of colour of the pigment and these are of the following types —
1. Chloroplasts:
It is the most common plastid (Gr. Chlor, green, plast, and living) which contains chlorophyll a and b pigments, and DNA and RNA. Chloroplasts are found mainly in the cells of the leaves of higher plants and algae. It is the most biologically important plastid. By the process of photosynthesis, they produce oxygen and the most of the chemical energy used by living organisms.
2. Phaeoplast:
These are yellow or brown plastids found in brown algae, diatoms and dinoflagellates. Fucoxanthin is a carotenoid pigment which masks the colour of chlorophyll a, which is also present. It also absorbs light and transfer the energy to chlorophyll a.
3. Rhodoplasts:
These are red coloured plastids. It is found in red algae and its red colour is due to phycoerythrin. It also absorbs light.
4. Chromatophores:
These are present in the blue-green algae. The term chromatophore is used instead of plastid, since the pigments are not organized within a discrete plastid body but are often arranged on lamellar structures in concentric rings or plates within algal cell. Blue-green colour of this algae is due to phycocyanin and phycobilins. These accessory pigments do not participate in photosynthesis.
5. Purple photosynthetic bacteria have a chlorophyll, bacteriochlorophyll. Their purple colour is due to special carotenoids.
Non-photosynthetic chromoplasts:
A variety of accessory pigments is also found which do not appear to be directly involved in photosynthesis or energy transfer.
Chromoplasts may develop from chloroplasts by accumulation of nonphotosynthetic pigments, e.g., red carotenoid, lycopin in tomatoes. Genes for synthesis of pigments lie in the nucleus.
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