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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.
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Chromoplasts:
The chromoplasts may be further divided on the basis of colour of the pigment and these are of the following types —
1. Chloroplasts:
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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:
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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.
1. Carotenoids and xanthophylls:
Carotene protects against photo-oxidation. These are red-yellow or brown in colour. These give colour to flowers and fruits.
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2. Leucoplasts:
These plastids are devoid of pigment and are membranous structures. They serve to store starches, oils and proteins. These are of the following tyes—
(a) Amyloplasts:
These are food storage cells and store starch. These are generally found in storage tubers, cotyledons and endosperm. These are found in regions of little or no illumination. Amyloplasts have nucleoids and ribosomes.
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(b) Elaioplasts:
These are found in certain monocotyledons and their function is to store oils.
(c) Proteinoplasts (aleuroneplasts):
These are found in seeds of Ricinus and Brazilnut, and store proteins. Epidermal cells of Helleborus also possess proteinoplasts.
Plastid differentiation depends upon the metabolic requirements of the cell. The chloroplasts may develop from leucoplasts, and chromoplasts, which are considered end forms of plastid differentiation, may develop from either leucoplasts or chloroplasts.
Proplastids can differentiate into one of three types of plastids and since, in certain cases, one type of plastid can differentiate into another, it has been generally assumed that all plastids are essentially the same in structure, having the ability to differentiate in various ways, depending upon the requirements of the cells.
