CIE A Level Biology (9700) 2019-2021

Revision Notes

13.1.2 Thylakoids & the Stroma

Thylakoids & the Stroma

  • Plant cells contain chloroplasts which is the site of photosynthesis
  • Chloroplasts are filled with a fluid known as the stroma
  • The system of membranes found in the stroma of the chloroplast consists of a series of flattened fluid-filled sacs known as thylakoids
  • In places, these thylakoids stack up to form structures known as grana (singular – granum)
  • The light-dependent stage of photosynthesis occurs in the thylakoid membranes and the thylakoid spaces (the spaces inside the thylakoids)
  • The thylakoid membranes contain the pigments, enzymes and electron carriers required for the light-dependent reactions
  • 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 pigment molecules are arranged in light-harvesting clusters known as photosystems
  • In a photosystem, the different pigment molecules are arranged in funnel-like structures 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)
  • The stroma is the fluid that fills the chloroplasts and surrounds thylakoids
  • CO2, sugars, enzymes and other molecules are dissolved in the stroma
  • The stroma is the site of the light-independent stage of photosynthesis

Exam Tip

Don’t get confused between the light-dependent and light-independent reactions – you need to know where each of these sets of reactions occurs. The photosynthetic pigments required to absorb light energy are only found in the thylakoid membranes, meaning that the reactions that occur here are dependent on light (light-dependent)!


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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