(b) the structure of a chloroplast and the sites of the two main stages of photosynthesis

The components of a chloroplast including outer membrane, lamellae, grana, thylakoid, stroma and DNA

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 called chloroplast envelope

• are filled with a cytosol-like fluid - stroma

  • CO₂, sugars, enzymes and other molecules are dissolved in the stroma

the 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

membrane system of stroma

• consists of a series of flattened fluid-filled sacs known as thylakoids

  • thylakoid membranes contain pigments, enzymes and electron carriers

  • they stack up to form structures known as grana (singular – granum)

• membrane system provides a large number of pigment molecules that ensure as much light as necessary is absorbed

grana

• are connected by membranous channels called stroma lamellae, which ensure the grana are connected but distanced from each other

• The membranes of the grana create a large surface area

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)

Stroma

• 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 in the stroma

Location of stages 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

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 (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)