Chloroplast is an elongated or disc-shaped double membrane cell structure found in plants. It is a type of organelle known as a plastid, characterized by its high concentration of chlorophyll. In animal cells, the mitochondria produces the majority of the cell’s energy from food. It does not have the same function in plant cells. Plant cells use sunlight as their energy source; the sunlight must be converted into energy inside the cell in a process called photosynthesis. Chloroplasts are the structures that perform this function with the help of chlorophyll, which is a green pigment that absorbs energy from sunlight to make food for the plants by converting light energy into chemical energy. Chloroplasts also carry out a number of other functions, including fatty acid synthesis, much amino acid synthesis, and the immune response in plants.
Chloroplasts are highly dynamic that they circulate and are moved around within plant cells, and occasionally pinch in two to reproduce. Their behavior is strongly influenced by environmental factors like light color and intensity. Chloroplasts, like mitochondria, contain their own DNA (ctDNA or cpDNA), and it is also known as the plastome. Chloroplasts are thought to be inherited from their ancestor in a way that a photosynthetic cyanobacterium was engulfed by an early eukaryotic cell and can be traced back to a single endosymbiotic event.
All chloroplasts have at least three membrane systems—the outer chloroplast membrane, the inner chloroplast membrane, and the thylakoid system. Chloroplasts that are the product of secondary endosymbiosis may have additional membranes surrounding these three.
The outer and the inner membrane each being a phospholipid bilayer form the double membrane with an intermembrane space between them. The inner membrane borders the stroma and regulates passage of materials in and out of the chloroplast. Between the inner membrane and the thylakoid space lies stroma, otherwise called the chloroplast matrix which is an alkaline aqueous fluid rich in proteins. It contains those enzymes that catalyze the light-independent reactions of photosynthesis.
Thylakoid membranes are also referred to as thylakoid. They are disc-shaped structures that are the sites of light absorption at which the light-dependent reactions of photosynthesis take place. The region within the membrane forming each thylakoid (by enclosing the contents of the thylakoid) is called the lumen of the thylakoid. Chlorophyll molecules, accessory pigments, enzymes and electron transport systems forms the thylakoids that are either present on the surface or embedded within the thylakoids. These thylakoids are arranged in stacks called grana. A single granum is a stack of several thylakoids one on top of another and there are many such grana within each chloroplast. Two or more grana are connected to each other by stromal lamellae. Hence the lamellae act as a “skeleton” of the chloroplast, maintaining efficient distances between the grana, thereby maximizing the overall efficiency of the chloroplast.
Starch also exists in chloroplasts in the form of tiny lumps called “granules” or sometimes “grains”. These are present because they are the (insoluble) storage carbohydrate product of photosynthesis.