5.1.1 Chloroplast Structure and Function

Structures and Functions

• 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%%