8.3.5 Chloroplast

Structure

• 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
  • If the chloroplast has been photosynthesising there may be starch grains or lipid droplets 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

Chloroplast structure

• The 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 in 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

Adaptations of chloroplasts to 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 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 the chloroplast DNA
• 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)
• Thylakoid space:
  • This is where a proton gradient develops (to generate ATP)
  • The space has a very small volume so a proton gradient can develop very quickly