3. Chloroplasts
The chloroplast (chlor=green; plast=living) is most widely occurring chromoplast of the plants. It occurs mostly in the green algae and higher plants. The chloroplast contains the pigment chlorophyll a and chlorophyll b and DNA and RNA.

Chloroplasts:
Chloroplasts were described as early as seventeenth century by Nehemiah Grew and Antonie van Leeuwenhoek.

Distribution:
The chloroplasts remain distributed homogeneously in the cytoplasm of plant cells. But in certain
cells, the chloroplasts become concentrated around the nucleus or just beneath the plasma membrane. The chloroplasts have a definite orientation in the cell cytoplasm. Chloroplasts are motile organelles, and show passive and active movements.

Morphology:
Shape: Higher plant chloroplasts are generally biconvex or plano-convex. However, in different plant cells, chloroplasts may have various shapes, viz., filamentous, saucer-shaped, spheroid, ovoid, discoid or club-shaped. They are vesicular and have a colourless centre.

Size: The size of the chloroplasts varies from species to species. They generally measure 2–3μm in thickness and 5–10μm in diameter (Chlamydomonas). The chloroplasts of polyploid plant cells are comparatively larger than those of the diploid counterparts. Generally, chloroplasts of plants grown in the shade are larger and contain more chlorophyll than those of plants grown in sunlight.

Number: The number of the chloroplasts varies from cell to cell and from species to species and is related with the physiological state of the cell, but it usually remains constant for a particular plant cell. Algae usually have a single huge chloroplast. The cells of the higher plants have 20 to 40 chloroplasts. According to a calculation, the leaf of Ricinus communis contains about 400,000 chloroplasts per square millimeter of surface area. The chloroplasts are composed of the carbohydrates, lipids, proteins, chlorophyll, carotenoids (carotene and xanthophylls), DNA, RNA and certain enzymes and coenzymes. The chloroplasts also contain some metallic atoms as Fe, Cu, Mn and Zn. Chloroplasts have very low percentage of carbohydrate. They contain 20–30 per cent lipids on dry weight basis. The most common alcohols of the lipids are the choline, inositol, glycerol, ethanolamine. The proteins constitute 35 to 55 per cent of the chloroplast. Chlorophyll is the green pigment of the chloroplasts. It is an asymmetrical molecule which has hydrophilic head of four rings of the pyrols and hydrophobic tail of phytol. Chemically the chlorophyll is a porphyrin like the animal pigment haemoglobin and cytochromes except besides the iron (Fe), it contains Mg atom in between the rings of the pyrols which remain connected with each other by the methyl groups. The chlorophyll consists of 75 per cent chlorophyll a and 25 per cent chlorophyll b.
The carotenoids are carotenes and xanthophylls, both of which are related to vitamin A. The carotenes have hydrophobic chains of unsaturated hydrocarbons in their molecules. The xanthophylls contain many hydroxy groups in their molecules. Chloroplast have their own genetic material which is circular like that of bacterial chromosome.

Isolation:
Chloroplasts are routinely isolated from plant tissues by differential centrifugation following the
disruption of the cells.

Ultrastructure:
Chloroplast comprises of three main components:

1. Envelope
The entire chloroplast is bounded by a double unit membrane. Across this double membrane envelope occurs exchange of molecules between chloroplast and cytosol. Isolated membranes of envelope of chloroplast lack chlorophyll pigment and cytochromes but have a yellow colour due to the presence of small amounts of carotenoids. They contain only 1 to 2 per cent of the total protein of the chloroplast.

2. Stroma
The matrix or stroma fills most of the volume of the chloroplasts and is a kind of gel-fluid phase
that surrounds the thylakoids (grana). It contains about 50 per cent of the proteins of the chloroplast, most of which are soluble type. The stroma also contains ribosomes and DNA molecules both of which are involved in the synthesis of some of the structural proteins of the chloroplast. The stroma is the place where CO₂ fixation occurs and where the synthesis of sugars, starch, fatty acids and some proteins takes place.