Chloroplasts contain several important membranes, vital for their function. Like mitochondria, chloroplasts have a double-membrane envelope, called the chloroplast envelope. Each membrane is a phospholipid bilayer, between 6 and 8 nm thick, and the two are separated by a gap of 10-20nm, called the intermembrane space. The outer membrane is permeable to most ions and metabolites, but the inner membrane is highly specialised with transport proteins.
The origin of chloroplasts is now largely accepted by the botany community as occurring via endosymbiosis on an ancestral basis with the engulfment of photosynthetic bacterium within the eukaryotic cell. Over millions of years the endosymbiotic cyanobacterium evolved structurally and functionally, retaining its own DNA and cellular mitosis capabilities but losing its ablility to live outside of the host cell.
Within the inner membrane, in the region called the stroma, there is a system of interconnecting flattened membrane compartments, called the thylakoids. These are the sites of light absorption and ATP synthesis, and contain many proteins, including those involved in the electron transport chain. Photosynthetic pigments such as chlorophyll α and B, and some others e.g. xanthophylls and carotenoids are also located within this space. These are responsible for the conversion of light energy to chemical energy as described below:
Functions of Thylakoids
The membranes of the chloroplasts contain photosystems I and II which harvest solar energy in order to excite electrons which travel down the electron transport chain. This exergonic fall in potential energy along the way is used to pump H+ ions from the stroma into the thylakoid space. A concentration gradient is formed, which allows chemiosmosis to occur, where the protein ATP synthase harvests the potential energy of the Hydrogen ions and uses it to combine ADP and a phosphate group to form ATP.
Experiments have shown that the the pH within the stroma is about 7.8, while that of the thylakoid space is about 5. This corresponds to a thousandfold difference in concentration of H- ions.
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