Thylakoids and the Stroma

• plant cells contain chloroplasts which are the site of photosynthesis
• chloroplasts are fulled with fluid known as stroma
• system of membranes found in stroma consists of a series of flattened, fluid filled sacs known as thylakoids
• in places, thylakoids stack up to form structures known as grana (single: Granum)
• light-dependent stage of photosynthesis occurs in the thylakoid membrane and thylakoid spaces
• thylakoids have two main functions - they act as a surface area where light energy is absorbed by pigments and converted into chemical energy; and they also store this chemical energy in ATP molecules
• thylakoids contain the pigments, enzymes and electron carriers required for light - dependent reactions
• membranes of the grana create large surface area to increase number of light dependent reactions that can occur
• membrane system provides a large number of pigment molecules in an arrangement that ensures as much light as necessary is captured
• pigment molecules are arranged in a light-harvesting clusters known as photosystems
• in photosystems, the different pigment molecules are arranged in a funnel-like structures that the thylakoid membrane (each pigment molecule passes energy down to the next in the cluster until it reaches primary pigment reaction center)
• Photosystem 1 and 2 arranged in order of discovery, however photosystem 2 is the light harvesting pigments while photosystem 1 is the reaction center
• Diagram

  
• Stroma is fluid that fills the chloroplasts that SURROUNDS thylakoids
• CO2, sugars, enzymes and other molecules are dissolved in the stroma fluid
• stroma is the site of the light independent stage of photosynthesis