Chloroplast -The site of photosynthesis : The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplast. Chloroplast are present in all the green parts of plants and leaves. There may be over half a million chloroplasts per square millimetre of leaf surface. In higher plants, the chloroplasts are discoid or lens-shaped. They are usually 4-10micromt in diameter and 1-3micromt in thickness.

These are double membrane-bound organelles in the cytoplasm of green plant cells. Chloroplast has two unit membranes made up of lipoprotein. Outer membrane of chloroplast is permeable and an inner one impermeable to protons. Inside the membranes is the proteinaceous ground substance called stroma, which contain a variety of particles, osmiophilic droplets, dissolved salts, small double stranded circular DNA molecules and 70S type ribosomes along with various enzymes. Inside the stroma is found a system of chlorophyll bearing double-membraned sacs thylakoids or lamellae.

Thylakoids are flattened sacs arranged like the stacks of coins. One stack of thylakoids is called granum. Different grana are connected with the help of tubular connections called stroma lamellae or frets. Grana are the sites for light reaction of photosynthesis and consist of photosynthetic unit 'quantasomes' (Found in surface of thylakoids). Photosynthetic unit can be defined as number of pigment molecules required to affect a photochemical act, that is the release of a molecule of oxygen. Park and Biggins (1964) gave the term quantasome for photosynthetic units is equivalent to 230 chlorophyll molecules.