Chloroplasts are the organelles in which photosynthesis takes place. They are found in plant cells of green tissues, especially palisade mesophyll cells in leaves, and in some protoctists (algae). They are usually disc- or lens-shaped, about 2 to 10 µm long.
• Chloroplast envelope: a double membrane surrounding the organelle, which controls what enters and leaves.
• Thylakoids: flattened membrane sacs containing the photosynthetic pigments, such as chlorophyll, and the proteins of the electron transport chain and ATP synthase. Stacks of thylakoids are called grana (singular: granum).
• Intergranal lamellae: thylakoid membranes that link one granum to another.
• Stroma: the fluid surrounding the thylakoids, containing the enzymes of the light-independent reactions (Calvin cycle).
• Starch grains and lipid droplets in the stroma.
• Circular DNA and 70S ribosomes, so the chloroplast can make some of its own proteins and divide.Structure and function
The light-dependent reactions happen on the thylakoid membranes. The stacking of thylakoids into grana gives a very large surface area for pigments to absorb light, and for the electron carriers and ATP synthase. The light-independent reactions happen in the stroma, which is close to the thylakoids, so the ATP and reduced NADP they produce reach it quickly.Like mitochondria, chloroplasts have their own circular DNA and 70S ribosomes, evidence that they evolved from photosynthetic bacteria (endosymbiosis).
• Chloroplast envelope: a double membrane surrounding the organelle, which controls what enters and leaves.
• Thylakoids: flattened membrane sacs containing the photosynthetic pigments, such as chlorophyll, and the proteins of the electron transport chain and ATP synthase. Stacks of thylakoids are called grana (singular: granum).
• Intergranal lamellae: thylakoid membranes that link one granum to another.
• Stroma: the fluid surrounding the thylakoids, containing the enzymes of the light-independent reactions (Calvin cycle).
• Starch grains and lipid droplets in the stroma.
• Circular DNA and 70S ribosomes, so the chloroplast can make some of its own proteins and divide.Structure and function
The light-dependent reactions happen on the thylakoid membranes. The stacking of thylakoids into grana gives a very large surface area for pigments to absorb light, and for the electron carriers and ATP synthase. The light-independent reactions happen in the stroma, which is close to the thylakoids, so the ATP and reduced NADP they produce reach it quickly.Like mitochondria, chloroplasts have their own circular DNA and 70S ribosomes, evidence that they evolved from photosynthetic bacteria (endosymbiosis).