The site of photosynthesis of the plant is chloroplast. All the plants which can undergo through the process of photosynthesis must have the chloroplast in their body. A chloroplast is an organelle unique to plant cells that contains chlorophyll and is responsible for enabling photosynthesis to occur, so that plants can convert sunlight into chemical energy. So basically, without chloroplasts, plants could not create energy and food as well. So the chloroplast may be called as the kitchen of the plants.
The whole photosynthetic machinery lies in chloroplast, the green plastid. Chloroplasts differ in size, shape and chemical organization. In higher plants, chloroplasts are mostly present in palisade and spongy parenchyma tissues of leaves and cortical cells of young stems. Chloroplasts are disc-shaped structures, measuring 4-10 um in length and 1-3 um in breadth (where, 1 um = 1000th of a millimeter).The number of chloroplasts per cell indifferent groups of plants is relatively constant. In higher plants, there are 20-40 chloroplasts per cell.
Each chloroplast is enclosed by a double membrane of lipo-protein which is 40 to 60 A thick (where, lA=1000th of a jam).The space enclosed by the double membrane envelope is said to be stroma. It is filled with a proteinaceous matrix and contains a complex inner membrane system. The stroma also contains starch grains and osmiophilic droplets and is the site of complex dark reaction in photosynthesis.
By the study with electron microscopic have revealed that the inner membrane of the chloroplast extends at several points across it to form a lamellar system . The lamellae are stacked one above other. These stacks, known as grana, are the centres of light reaction of photosynthesis. There are usually 40-60 grana in a mature chloroplast. These grana are interrelated by stroma lamellae or intergrana lamellae which are not stacked. Each granum is composed of 5-25 discs and each disc encloses a space said to be thylakoid. The lamellae of the disc are composed of alternating layers of lipid and aqueous protein. These layers are divided by monomolecular layers of chlorophyll and carotenoid molecules .
On the inner jurface ofthgjarnellarjriembrane, small sphericaljtructures are present. These structural known as quantasomes, are the structyral-and functional units of chloroplast. Each quantasome is capableofl absorbing a mole quantum of light.
The chloroplast contains about 30-35% proteins, 20-30% phospholipids, 5-10% chlorophylls; 4-5% carotenoids. In addition, ribosomes, DNA and starch grains are also present in the stroma.
Chloroplast ribosomes are smaller than those of the cytoplasmic ribosomes and are about 70 s in diameter.
Chloroplasts develop from proplastids, the double membrane bound undifferentiated bodies found in the growing regions of the green plant. In light, proplastids develop into normal chloroplast. The in limiting membrane invigilates at several places and is subsequently organized into functional. There is also synthesis of chlorophyll and rest of the photosynthetic machinery.
So we can say, the plant cells are remarkable in that they have two organelles specialized for energy production: chloroplasts, which create energy via photosynthesis, and mitochondria, which produce energy through respiration, a particularly important process when light is unavailable. Like the mitochondrion, the chloroplast is different from most other organelles because it has its own DNA and reproduces separately of the cell in which it is found; an apparent case of endosymbiosis. According to DNA proof, the eukaryotic organisms that later became plants likely added the photosynthetic corridor in this way, by acquiring a photosynthetic bacterium as an endosymbiont.