5.6.2 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 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)

• 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 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 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)