Chloroplasts (CIE A Level Biology)

• 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

An electron micrograph showing the structures within a chloroplast