Chloroplasts (Edexcel IAL Biology)

Chloroplasts: Structure & Function

Spec point — 5.5

• 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