OCR A Level Biology

Revision Notes

5.6.2 Chloroplast Structure & Function

Chloroplast Structure & Function

  • 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
  • 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
    • The membranes of the grana create a large surface area
    • This 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 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 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

Thylakoid membrane photosystem, downloadable AS & A Level Biology revision notes

An example of a photosystem in a chloroplast thylakoid membrane: a light-harvesting cluster of photosynthetic pigments involved in the light-dependent stage of photosynthesis.

Location of the stages of photosynthesis

  • The process of photosynthesis is made up of two stages, the light-dependent stage and the light-independent stage
  • These stages take place in specific locations within the chloroplast
  • The first stage, the light-dependent stage takes place on the thylakoid membranes of the grana
    • Light becomes trapped within the reaction centres of the grana
  • The light-independent stage takes place in the stroma

The two stages of photosynthesis, downloadable AS & A Level Biology revision notes
The location of the different stages of photosynthesis. The products of the light-dependent reaction are used in the light-independent reaction.

Adaptations of chloroplasts to photosynthesis

  • Chloroplasts are specialised organelles that are adapted to carry out 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 (cpDNA) 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 cpDNA
  • 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)

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.

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