CIE A Level Biology (9700) 2019-2021

Revision Notes

13.3.1 Chloroplasts

Chloroplast Structures & their Functions

  • Chloroplasts are the organelles in plant cells where photosynthesis occurs
  • Each chloroplast is surrounded by a double-membrane envelope
    • Each of the envelope membranes is a phospholipid bilayer
  • Chloroplasts are filled with a fluid known as the stroma
    • The stroma is the site of the light-independent stage of photosynthesis
  • A separate system of membranes is found in the stroma
    • This membrane system is the site of the light-dependent stage of photosynthesis
    • The membrane contains the pigments, enzymes and electron carriers required for the light-dependent reactions
    • This membrane system consists of a series of flattened fluid-filled sacs known as thylakoids
    • 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
    • The membranes of the grana create a large surface area to increase the number of light-dependent reactions that can occur
    • This membrane system provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is absorbed
  • The stroma also contains small (70S) ribosomes, a loop of DNA and starch grains:
    • The loop of DNA codes for some of the chloroplast proteins (other chloroplast proteins are coded for by the DNA in the plant cell nucleus)
    • The proteins coded for by this loop of chloroplast DNA are produced at the 70S ribosomes
    • Sugars formed during photosynthesis are stored as starch inside starch grains

Exam Tip

Make sure you can identify the structures of a chloroplast on a diagram AND that you can explain the function of each of these structures.

Author:

Alistair graduated from Oxford University in 2014 with a degree in Biological Sciences. He has taught GCSE/IGCSE Biology, as well as Biology and Environmental Systems & Societies for the International Baccalaureate Diploma Programme. While teaching in Oxford, Alistair completed his MA Education as Head of Department for Environmental Systems and Societies.
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