Topics in Depth
The Theme of Chloroplast Membranes in Cells
Just like in mitochondria, the membranes of the chloroplast provide the basis for chloroplast function.
Unlike mitochondria, chloroplasts actually have three phospholipid bilayers. And you thought a double bilayer was too complicated! The outer two membranes are similar in structure and orientation to the nuclear membranes: there is an outer membrane, an inner membrane, and a very small space between them called the…you guessed it…intermembrane space. The third chloroplast membrane is arranged in discs called thylakoids. These discs are stacked on top of each other in structures called grana (singular granum) that look a lot like little stacks of green casino chips. Not that you spend any time in casinos, or gambling, for that matter.
Each chloroplast contains many, many grana. The space between the inner chloroplast membrane and the grana is called the stroma. The space inside the thylakoid discs is called the lumen, or, more specifically, the thylakoid lumen. The work of the chloroplast takes place in the stroma, the lumen, and, most importantly, in the thylakoid membrane itself. Here, the light-capturing green pigment chlorophyll is held in place by membrane proteins. Chlorophyll converts the energy from the sun into electrical energy. This electrical energy is then passed from one membrane protein to another, providing the power to pump protons from the stroma into the thylakoid lumen.
Just like in the mitochondria, the protons (hydrogen ions, or H+) rush back across the membrane into the stroma, and ATP synthase, an enzyme, converts the generated energy into ATP. At this point, ATP and other products produced by the thylakoid membrane proteins are combined with molecules of carbon dioxide (CO2) in the stroma to make glucose. The whole elaborate process, from sunlight to glucose, is called photosynthesis. In plant mitochondria, the glucose made in photosynthesis is converted into ATP, which plant cells use to grow, survive, and reproduce. Ultimately, the energy contained in this glucose is used by animals who eat plants, or who eat animals that eat plants, who then convert it back into CO2 where plants can turn it back into glucose and so on. Forget The Lion King: the real circle of life starts and ends in the membranes of chloroplasts.
There are many photosynthetic plants, but have you heard of a photosynthetic animal? Here is one that steals and uses the chloroplasts of the algae it consumes. Read about it here.
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