A leaf appears green, but most of the cells and cell material are colorless. The green color comes from the chlorophyll molecules situated in the chloroplasts. Chloroplasts are the green colored plastids first reported by Sachs. The term chloroplast was coined by Schimper. Chloroplasts are reported to be found in all green parts of the plants like the leaves, young branches, stems, sepals, unripe fruits and also in roots of certain plants like Taeniophyllum, Tinospora etc.
The chloroplasts are distributed homogeneously in the cytoplasm of the plant cells. But in certain plant cells they may remain concentrated around the nucleus. They have definite orientation in the cell cytoplasm. As chloroplasts are motile organelles they show both passive and active movements.
The chloroplasts are located in the mesophyll region, between the upper and the lower epidermis. It consists of several layers of loosely arranged spongy parenchyma cells with intercellular spaces. Due to the presence of chloroplasts, mesophyll region is the chief photosynthetic tissue of the leaf. The number of chloroplast in a single mesophyll cell ranges from 1 to 50. This number may vary from cell to cell depending on plant species, age, and health of the cell.
Shape: Chloroplasts are spherical or oval or discoid or biconcave in shape in higher plants. The shape of the chloroplasts many vary in different plant cells. They are vesicular and have a colorless centre.
Size: The size of chloroplast may vary among different plant species. Generally the chloroplast measures 2-3 µm in thickness and 5-10 µm in diameter. The chloroplasts of the polyploid plant cells are larger than that of the diploid plant cells.
The chloroplasts of the plants which grow in shade are larger with chlorophyll than those of the plants grown in sunlight.
Number: The number of chloroplasts may vary from cell to cell and also from species to species. Also the physiological state of the cell also plays an important role in determining the number of chloroplasts present in the cell.
When the number of chloroplasts is inadequate, it is increased by division on the other hand if the number of chloroplasts is excessive, it is reduced by degeneration.
A chloroplast comprises of three main components namely Envelope, Stroma and Thylakoids.
Envelope: The chloroplast is bound by an envelope which is made up of two unit membranes. These unit membranes are separated by periplastidal space of 10nm. Exchange of molecules between chloroplast and cytosol occurs across this double membrane envelope.
The inner envelope acts like a control gate and regulator, controlling the flow of necessary material and particles into, and out of, the chloroplast. The small, simple, and important molecules like water (H2O), carbon dioxide (CO2), and oxygen (O2) can pass through this membrane. Isolated membranes of envelope do not have chlorophyll pigment and cytochromes. The isolated membranes appear yellow due to the presence of small amounts of carotenoids.
Stroma: The inner space of chloroplast is filled with a colorless matrix known as stroma. Stroma fills most of the volume of the chloroplast. Stroma surrounds the thylakoids and contains almost 50% of the proteins of the chloroplast. The proteins present in the stroma are soluble in nature. A large number of lipoprotein membranes extend from one end to the other called as stroma lamellae.
Stroma is also found to contain 70s ribosomes and DNA molecules. These ribosomes and DNA molecules synthesize some of the structural proteins of the chloroplast. Carbon fixation, synthesis of sugars, starch and fatty acids also takes place inside the stroma.
Thylakoids: Thylakoids consists of closed and flattened vesicles. These vesicles are arranged as a membranous network. The outer surface of the thylakoid is in contact with the stroma and the inner surface encloses an intra-thylakoid space. Thylakoids are found to be stacked one on other like the pile of coins. This stack of thylakoids is called as grana. The number of grana in the matrix of a chloroplast may be 40 to 80.
The thylakoids present in the granum are called as granum thylakoids. The space within the thylakoid is called as lumen. The photosynthetic pigments like chlorophylls and carotenoids are present in the thylakoid membrane.
The presence of DNA in the chloroplasts helps in self-duplication. Hence chloroplasts are also called as semi-autonomous organelles.
The chloroplasts are chiefly concerned with assimilation of food materials by photosynthesis. This process of photosynthesis occurs in two stages:
Light dependent reaction: The first part of the photosynthetic process, the light-dependent reaction, takes place in the thylakoids. The thylakoids are stacked and are connected to one another by the stroma lamellae. The outer surface of thylakoid is called the thylakoid membrane.
The pigments and ATP synthetase molecules which help in photosynthesis are present are on the thylakoid membrane and so this membrane is also called as photosynthetic membrane.
The pigments present on the thylakoid membrane are called as solar collector pigments as they act as the antennae to capture the solar energy. The captured solar energy "knocks loose" an electron which provides the energy to other molecules to start the process of photosynthesis. Special enzymes break the water molecules apart through the process called as photolysis of water. Water molecules are broken down into oxygen and hydrogen atoms. The hydrogen atoms are further broken down into a hydrogen ions and an electron.
Electrons go to the chlorophyll and other pigment molecules where they replace the missing electrons "knocked loose" by photons of light. The hydrogen ions flow through special channels in the membrane and their movement provides the energy for the formation of ATP molecules. Oxygen atoms link up with other oxygens to form O2, and then they flow out into the air. ATP and NADPH from step 1 are used to provide energy for the making of carbohydrates in step 2.
These pigments are generously distributed throughout the chloroplast. The light dependent reactions, take place in the thylakoid space and the thylakoid membrane while the light independent reactions, occur in the stroma surrounding the thylakoids.
Light independent reaction: This reaction takes place in the stroma. In this stage, carbon dioxide is fixed. A bunch of enzymes use the fixed carbon dioxide molecules and hydrogen ions to assemble sugar fragments. These sugar fragments have 3-carbon fragments and are called half-glucose molecules.
Finally half-glucose molecules pass through outer membrane of the chloroplast into the cell. In the cell more enzymes join the 3-carbon fragments to produce a glucose molecule. Now the glucose becomes the basic building block for a bunch of other carbohydrates, such as sucrose, lactose, ribose, cellulose and starch.
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