From the earliest days of plant cytology it was known that cells which are capable of photosynthesis contain chloroplasts and organelles which contain a green pigment, chlorophyll. Choroplasts are relatively large, disc-shaped organelles in the cell; they are easily visible under light microscope even in unstained preparations because of their green colour. Chloroplasts show considerable variations in shape, size, and number in various species of plants. Chloroplasts absorb light in the violet and blue wave lengths, and also in the red regions of the visible spectrum of light. This portion ofthe spectrum between 400nm and 700nm is also referred to as photosynthetically active radiation (PAR). Chemically a typical chloroplast is composed of 50 to 60 per cent proteins, 25 to 35 per cent lipids, 5 to 10 per cent chlorophyll and small amounts of RNA and DNA.
Electron Microscopic View of a Section of a Chloroplast
The chloroplast is enclosed in two smooth membranes, an outer and an inner membrane. The two membranes are separated by periplastidial space. The inner membrane is elaborated into an intricate system of lamellae.
The inner side of the plastids is clearly dividable into
Stacks of Thylakoids forming Three Grana
- A colourless ground substance called stroma and
- A membrane system consisting of closed flattened sacs called thylakoids. Thylakoids are piled up one on top of the other like piles of coins and are packed in definite areas called grana. There may be 40-50 grana per chloroplast and each granum may consist of 2-100 thylakoids. The arrangement and appearance of thylakoids show variations in different species of plants. The lamellae of the grana contain the light absorbing pigments. The inner surfaces of the two layers of membranes are granular due to the presence of small spherical bodies (spheroids) which are called quantasomes. The quantasomes are the smallest units capable of carrying out photochemical reactions. Each quantasome contains about 200 chlorophyll molecules.