13.1.3 Thylakoids & the Stroma

• Plant cells contain chloroplasts which is the site of photosynthesis
• Chloroplasts are filled with a fluid known as the stroma
• The system of membranes found in the stroma of the chloroplast consists of a series of flattened fluid-filled sacs known as thylakoids
• In places, these thylakoids stack up to form structures known as grana (singular – granum)
• The light-dependent stage of photosynthesis occurs in the thylakoid membranes and the thylakoid spaces (the spaces inside the thylakoids)
• The thylakoid membranes contain the pigments, enzymes and electron carriers required for the light-dependent reactions
• 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 pigment molecules are arranged in light-harvesting clusters known as photosystems
• In a photosystem, the different pigment molecules are arranged in funnel-like structures 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)

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

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