DP IB Biology: HL

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First teaching 2014

Last exams 2024

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8.3.5 Chloroplast

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Chloroplast Structure & Function

Structure

  • Chloroplasts are the organelles in plant cells where photosynthesis occurs
  • These organelles are roughly 2 - 10 μm in diameter (they are larger than mitochondria)
  • Each chloroplast is surrounded by a double-membrane envelope
    • Each of the envelope membranes is a phospholipid bilayer
    • The outer membrane is permeable to a range of ions and small molecules
    • The inner membrane contains transport proteins that only allow certain molecules or ions to enter or leave the chloroplast
  • Chloroplasts are filled with a cytosol-like fluid known as the stroma
    • CO2, sugars, enzymes and other molecules are dissolved in the stroma
    • 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
    • The thylakoid membranes contain pigments, enzymes and electron carriers
    • These thylakoids stack up to form structures known as grana (singular – granum)
    • Grana are connected by membranous channels called stroma lamellae, which ensure the stacks of sacs are connected but distanced from each other

Cell components_Chloroplast Chloroplast structure

  • The membrane system provides a large number of pigment molecules that ensure as much light as necessary is absorbed
  • The pigment molecules are arranged in light-harvesting clusters known as photosystems
    • In a photosystem, the different pigment molecules are arranged in funnel-like structures in the thylakoid membrane
    • Each pigment molecule passes energy down to the next pigment molecule in the cluster until it reaches the primary pigment reaction centre

Adaptations of chloroplasts to photosynthesis

  • Stroma:
    • The gel-like fluid contains enzymes that catalyse the reactions of the light-independent stage
    • The stroma surrounds the grana and membranes, making the transport of products from the light-dependent stage into the stroma rapid
  • Grana:
    • The granal stacks create a large surface area for the presence of many photosystems which allows for the maximum absorption of light
    • It also provides more membrane space for electron carriers and ATP synthase enzymes
  • DNA:
    • The chloroplast DNA contains genes that code for some of the proteins and enzymes used in photosynthesis
  • Ribosomes:
    • The presence of ribosomes allows for the translation of proteins coded by the chloroplast DNA
  • Inner membrane of chloroplast envelope:
    • The selective transport proteins present in the inner membrane control the flow of molecules between the stroma and cytosol (the cytoplasm of the plant cell)
  • Thylakoid space:
    • This is where a proton gradient develops (to generate ATP)
    • The space has a very small volume so a proton gradient can develop very quickly

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Cara Head

Author: Cara Head

Cara graduated from the University of Exeter in 2005 with a degree in Biological Sciences. She has fifteen years of experience teaching the Sciences at KS3 to KS5, and Psychology at A-Level. Cara has taught in a range of secondary schools across the South West of England before joining the team at SME. Cara is passionate about Biology and creating resources that bring the subject alive and deepen students' understanding