Chloroplasts are the organelles in which photosynthesis takes place. They are found mainly in the palisade mesophyll cells of leaves, and are usually disc-shaped, about 2 to 10 µm across.
• Envelope: a double membrane that surrounds the chloroplast and controls what enters and leaves.
• Thylakoids: flattened membrane sacs. Their membranes hold the photosynthetic pigments (in photosystems), electron carriers and ATP synthase, so they are the site of the light-dependent reactions.
• Grana (singular granum): stacks of thylakoids, which give a very large surface area for absorbing light.
• Intergranal lamellae: thylakoids that join one granum to another.
• Stroma: the fluid around the thylakoids. It contains the enzymes of the light-independent reactions, including RUBISCO, so it is the site of the Calvin cycle.
• Starch grains and lipid droplets: stores of excess products.
• Circular DNA and 70S ribosomes: chloroplasts can make some of their own proteins.The arrangement is efficient. ATP and reduced NADP are made on the thylakoid membranes and released straight into the stroma, where they are used. The thylakoid membranes are impermeable to protons except through ATP synthase, so a proton gradient can form.Like mitochondria, chloroplasts have DNA and ribosomes similar to those of prokaryotes, supporting the idea that they evolved from photosynthetic bacteria.
• Envelope: a double membrane that surrounds the chloroplast and controls what enters and leaves.
• Thylakoids: flattened membrane sacs. Their membranes hold the photosynthetic pigments (in photosystems), electron carriers and ATP synthase, so they are the site of the light-dependent reactions.
• Grana (singular granum): stacks of thylakoids, which give a very large surface area for absorbing light.
• Intergranal lamellae: thylakoids that join one granum to another.
• Stroma: the fluid around the thylakoids. It contains the enzymes of the light-independent reactions, including RUBISCO, so it is the site of the Calvin cycle.
• Starch grains and lipid droplets: stores of excess products.
• Circular DNA and 70S ribosomes: chloroplasts can make some of their own proteins.The arrangement is efficient. ATP and reduced NADP are made on the thylakoid membranes and released straight into the stroma, where they are used. The thylakoid membranes are impermeable to protons except through ATP synthase, so a proton gradient can form.Like mitochondria, chloroplasts have DNA and ribosomes similar to those of prokaryotes, supporting the idea that they evolved from photosynthetic bacteria.