Light dependent stage

Light dependent stage

Photosynthesis is a two stage reaction: the light dependent reaction (LDR) and the light independent reaction (LIR)

These reactions both occur within the chloroplast but in a different location

The LDR occurs on the thylakoid membranes or grana and the LIR occurs in the stroma

LDR is the first stage of photosynthesis and it requires light

Light energy and water are used to create ATP and reduced NADP which are needed for the light independent stage

Occurs in three stages: photolysis of water, photoionsiaton of chlorophyll and chemiosmosis

Production of ATP and reduced NADP

Photolysis of water

Photo(light) and lysis (splitting)
Protons- used to reduce NADP and help maintain the proton gradient

Electrons- replace those lost from chlorophyll

Oxygen- released as a by product

Light energy is absorbed by chlorophyll and splits water into oxygen H+ and e-

H2O →1/2 O2 + 2e- + 2H+

Photoionsiation of chlorophyll

Light energy is absorbed by the chlorophyll and the energy results in electrons becoming excited and raising up and energy level to leave chlorophyll

Therefore the chlorophyll has been ionised by light

Some of the energy from the released electrons is used to make ATP and reduced NADP in chemiosmosis

Chemiosmosis

1. Electrons pass down an electron transfer chain, a series of carrier proteins embedded in the thylakoid membrane

2. As the electrons move along the chain, they lose energy which is used to pump protons (H+ ions) from the stroma into the thylakoid membrane

3. Protons move down their electrochemical gradient back into the stroma via ATP synthase, an enzyme embedded in the membrane. This movement provides energy to combine ADP and inorganic phosphate (Pi) to form ATP. This is described as the chemiosmotic theory

4. The electrons from the electron transfer chain along with protons from photolysis are used to reduce NADP (NADPH)