Photosynthesis: Light-dependent reaction

where does the light dependent reaction take place?

• The light-dependent reaction takes place in the grana of the chloroplast

What are the products and reactants of the LDR

• The light-dependent reaction uses water and light energy

• It produces ATP, reduced NADP (or NADPH) which is used in the light-independent reaction and oxygen as a by-product. 

The key stages in the LDR

• Absorption of light energy and photoionisation

• Electron transfer along the electron transport chain

• Photolysis of water

• Chemiosmosis

Stage 1 of the LDR

• Chlorophyll is a green pigment that is found embedded in the membrane of the grana. When light hits a chlorophyll molecule, this causes one of its electrons to absorb some of the light energy and move to a higher energy level.

  • The electron has become excited. This is called photoionisation

Stage 2 of LDR

The high energy electrons pass to a carrier protein in the membrane. From there the electron passes through a series of protein carrier molecules in the membrane, called an electron transfer chain

Stage 3 of LDR

• As the electrons move down this electron transfer chain, energy is transferred allowing protons (H+) to be pumped inside the thylakoid lumen. A proton gradient builds up and these protons then move from a high concentration to a lower concentration outside (in the stroma) via the ATP synthase enzyme which is embedded in the thylakoid membrane

  • This produces ATP from ADP and Pi

  • This is called a chemiosmosis; described by the chemiosmosis theory

Stage 4 of the LDR

• The electron that has been lost by the chlorophyll molecule needs to be replaced. A process called photolysis occurs

  • Light energy splits water into oxygen (which is given off as a waste product), electrons, which replace those lost by chlorophyll,  and H+ ions (or protons)

Stage 5 of LDR

The H+ ions join up with the electrons that have passed through the electron transfer chain to become hydrogen atoms, and they join with a coenzyme called NADP to become NADPH which is a reduced coenzyme