Edexcel International A Level Biology

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5.5 Chloroplasts

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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

Cell components_Chloroplast

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

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Naomi graduated from the University of Oxford with a degree in Biological Sciences. She has 8 years of classroom experience teaching Key Stage 3 up to A-Level biology, and is currently a tutor and A-Level examiner. Naomi especially enjoys creating resources that enable students to build a solid understanding of subject content, while also connecting their knowledge with biology’s exciting, real-world applications.