Chloroplasts: Structure & Function
- Chloroplasts are the organelles in plant cells where photosynthesis occurs
- Each chloroplast is surrounded by a double-membrane known as the chloroplast envelope
- Each of the envelope membranes is a phospholipid bilayer
- Chloroplasts are filled with a cytoplasm-like fluid known as the stroma
- The stroma contains enzymes and sugars, as well as ribosomes and chloroplast DNA
- If the chloroplast has been photosynthesising there may be starch grains or lipid droplets in the stroma
- A separate system of membranes is found in the stroma
- This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids, each surrounded by a thylakoid membrane
- Thylakoids stack up to form structures known as grana (singular granum)
- Grana are connected by membranous channels called lamellae (singular lamella), which ensure the stacks of sacs are connected but distanced from each other
- Several components that are essential for photosynthesis are embedded in the thylakoid membranes, including:
- ATP synthase enzymes
- Proteins called photosystems that contain photosynthetic pigments such as chlorophyll a, chlorophyll b, and carotene
Chloroplasts are the site of photosynthesis
Chloroplast structure is related to function
- Chloroplast envelope
- The double membrane encloses the chloroplast, keeping all of the components needed for photosynthesis close to each other
- The transport proteins present in the inner membrane control the flow of molecules between the stroma and cytoplasm
- Stroma
- The gel-like fluid contains enzymes that catalyse the reactions of photosynthesis
- DNA
- The chloroplast DNA contains genes that code for some of the proteins used in photosynthesis
- Ribosomes
- Ribosomes enable the translation of proteins coded by the chloroplast DNA
- Thylakoid membrane
- There is a space between the two thylakoid membranes known as the thylakoid space, in which conditions can differ from the stroma e.g. a proton gradient can be established between the thylakoid space and the stroma
- The space has a very small volume so a proton gradient can develop very quickly
- Grana
- The grana create a large surface area, maximising the number of photosystems and allowing maximum light absorption
- Grana also provide more membrane area for proteins such as electron carriers and ATP synthase enzymes, which together enable the production of ATP
- Photosystems
- There are two types of photosystems; photosystem I and photosystem II, containing different combinations of photosynthetic pigments such as chlorophyll a, chlorophyll b, and carotene
- Each photosystem absorbs light of a different wavelength, maximising light absorption e.g. photosystem I absorbs light at a wavelength of 700 nm while photosystem II absorbs light at a wavelength of 680 nm
