CHLOROPLAST ASSIGNMENT

Chloroplasts are organelles involved in photosynthesis, the process of formation of carbohydrates from carbon-di-oxide and water in the presence of sunlight by the plants. 

Chloroplasts were first described as ‘chloroplyllkornern’ (chlorophyll granules) by German botanist Hugo von Mohl in 1837, though because of their large size they were seen long back by Nehemiah Grew who described them in 1682 as green precipitates! It was A.F.W. Schimper who in 1883 introduced the term ‘plastids’ as a substitute for chlorophyll granule. In the same year A. Mayer described ‘grana’ as dense dot like structures embedded in the ‘stroma’ of these plastids.

The terms grana and stroma are still in use but chloroplast has replaced the term plastids for the green organelles of leaves. T.Engelmann a German biologist in 1881 identified the chloroplasts as the sites where photosynthesis occurs. He observed that outside the cells of green alga Spirogyra just close to the large ribbon shaped chloroplasts, bacteria would collect probably to utilize the oxygen being liberated during photosynthesis in the chloroplasts (bacterial chemotaxis).  

                Structure and function

Chloroplasts are large organelles, generally lens shaped in higher plants, of around 2-4 µm wide 5-10µm in long, bound by a double membrane called chloroplast envelope. In addition a third internal membrane called as the thylakoid membrane is also present. The thylakoid membrane forms a network of flattened sac like structures called thylakoids often stacked to form grana (singular granum). The three membranes divide the chloroplast into three compartments: The intermembrane space between the two outer membranes, the stroma and the thylakoid lumen.
Chloroplast structure can be understood under three sub-heads:
1) Envelope
2) Thylakoids
3) Stroma

1) Envelope: Just like nucleus and mitochondria, chloroplasts are bound by a double membrane envelope called outer membrane and inner membrane. Both the membranes are made up of phospholipid bilayer. The space in between the membranes is inter-membrane space. The outer membrane of the chloroplast envelope contains ‘porins’ and so is freely permeable to small molecules. Whereas the inner membrane is impermeable to ions and metabolites and they need specific membrane transporters (integral membrane proteins) to enter the chloroplast. The outer membrane has very less protein content about 30% and have unusually have very little phospholipid. These have a high percentage of galactose containing glycolipids, which have several double bonds in their fatty acids making the membranes highly fluid. This facilitates the lateral diffusion of the protein complexes in the membrane.

2) Thylakoids: The internal membrane system (separate from the double membrane envelope) is organized into disc shaped/flattened membranous structures with empty lumen called thylakoids that are frequently arranged in stacks. Each stack is called granum (plural: grana). Adjacent grana are connected by stromal thylakoid (thylakoids in direct contact with the stroma).

The thylakoid membrane contains the light absorbing pigments, the chain of electron carriers (ETC) and the ATP synthesizing machinery (in this sense it is analogous to the inner membrane of mitochondria). The space inside the thylakoid is the lumen. The protons are pumped across this membrane from the stroma to the thylakoid lumen. This results in formation of an electrochemical gradient which generates ATP when protons are transported back to the stroma.
3) Stroma: The space enclosed by the inner membrane of the chloroplast is called stroma. Stromal matrix is rich in metabolic enzymes and has small double stranded circular DNA molecules, ribosomes (70-S) and plastoglobuli (lipid granules). Dark reaction or the C-3 cycle (Calvin cycle) takes place in stroma. Enzymes for amino acid synthesis and fatty acid synthesis are also present in stroma