Syllabus Edition

First teaching 2023

First exams 2025

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Chloroplasts (CIE A Level Biology)

Revision Note

Phil

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Phil

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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 captured

  • 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

Chloroplast Structure Diagram

Chloroplast structure

An electron micrograph showing the structures within a chloroplast

Examiner 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. It is also important to be able to describe the compartmentalization within a chloroplast and how that separates the reactions of the light-dependent and the light-independent stages.

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Phil

Author: Phil

Expertise: Biology

Phil has a BSc in Biochemistry from the University of Birmingham, followed by an MBA from Manchester Business School. He has 15 years of teaching and tutoring experience, teaching Biology in schools before becoming director of a growing tuition agency. He has also examined Biology for one of the leading UK exam boards. Phil has a particular passion for empowering students to overcome their fear of numbers in a scientific context.