Plant cells possess some special organelles called plastids, responsible for synthesis and storage of organic substances. Schimperused the term plastid in the year 1883 of these, the most important is the chloroplast.

The chloroplasts together with the mitochondria form the biochemical machines of energy transformations in living cells. In a chloroplast, the electromagnetic energy contained in the sun light is trapped and converted into the chemical energy by the process ofphotosynthesis.

A comparative account of chloroplasts and mitochondria is presented in the Structure:

The chloroplasts are characterised by the presence of pigments such as chlorophylls and carotenoids. Besides, coloured plastids may be seen in different parts of the plants. For example, the colour of petals is due to chromoplasts. In the embryonic cells, the colourless plastids are called leucoplasts.


The shape and size of chloroplasts may vary in different cells within a species, but number is relatively constant in the cells of the same tissue. In leaves of the higher plants, each: contains a large number of special oval or discoid chloroplasts. Frequently, they are vesic with colourless centres.

The average diameter of the chloroplasts is 4 to 6 pm. However, changes in the vole and shape of chloroplasts occur in the presence or absence of light. This organelle is limited smooth outer membrane, which regulates the transport of materials between the cytoplasm its interior.

The inner membrane is provided by extensive foldings. The inner membrane give to a series of internal parallel membrane sheets called lamellae. Lamellae are suspended fluid matrix called stroma. Stroma contains about 50% of soluble proteins, ribosomes,” and enzymes for protein synthesis.

Most of the lamellae in the chloroplast are organised to form sac like structures c, thylakoids. These flattened vesicles are arranged as a membranous network. The outer us; of the thylakoids is in contact with the stroma and its inner surface encloses intrathylakoid s Thylakoids may stack like a pile of coins called grana. They may be unstacked forming a sy of tubules that are joined to the grana thylakoids.


A typical chloroplast has between 40-60 grana and each granum has 2-10 flatt thylakoids. The thylakoids contain 50 per cent protein and all components of photosynthe

Chemical Composition:

The molecular organization of thylakoids is largely based on the fluid lipid-protein of biological membranes. The main components are chlorophylls, carotenoids, plastoqui and lipids. Chlorophyll is an asymmtrical molecule having a hydrophilic head made up of pyrrole rings bound to each other.

It is otherwise called the porphyrin head. Magnesium forms a complex with four rings. It has a long hydrophobic chain called photo tail attached! Of the rings. In higher plants, there are two types of chlorophylls: chlorophyll a and b. In chloro b, an aldehyde (CHO) group occurs in the second pyrrole ring of the porphyrin head, which chlorophyll a the aldehyde group is replaced by a methyl (CH,) group.


In lower plants like chlorophyll c, d and e and in bacteria, bacteriochlorophyll and bacterioviridin are thei harvesting pigments, besides chlorophyll a and b.


The main function of the chlorophyll is to trap light energy, required for the formation of two important products, ATP and NADPH. This is essential for there assimilation of CO, Chloroplasts, by the process of photosynthesis liberate oxygen and synthesize most of the chemical energy as food, which in turn is used by all living organisms.