chloroplasts: structure and function

• chloroplasts: 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

  • 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 eachother
• several components that are essential for photosynthesis are embedded in the thylakoid membranes

  • ATP synthase enzymes
  • proteins called photosystems contain photosynthetic pigments such as chlorophyll a, chlorophyll b and carotene
• chloroplast envelope

  • the double membrane encloses the chloroplast, keeping all of the components needed for photosynthesis close to eachother
  • 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

  contains genes that code for some of the proteins used in photosynthesis
• ribosomes

  enables the translation of proteins coded by the chloroplast DNA
• thylakoid membrane

  • thylakoid space in between the 2 membranes of this double membrane 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 creates 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

  • photosystem I, 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