Light Dependent Reaction

What is the light energy absorbed used for?

• Making ATP from ADP and Pi (phosphorylation)

• Making NADPH from NADP

• Splitting water into protons, electrons and oxygen (photolysis)

What are the 2 types of phosphorylation?

non cyclic and cyclic

What is the electron transport chain?

A chain of proteins through which exited electrons flow

The proteins used are electron carriers

What are electron carriers?

Proteins that transfer electrons

Where is the electron transport chain found?

In the thylakoid membrane attached to photosystems

What two things happen during noncyclic photophosphorylation?

• Photolysis of water into electrons protons and oxygen

• Light energy excites electrons

Describe the process of Photolysis in noncyclic photophosphorylation

1. Light energy splits H2O into protons electrons and oxygen

H2O → (2H+) + (1/2 O2) + (2e-)

1. These electrons go on to replace the electrons lost in the photosystems when they become excited

Describe the process of non cyclic photophosphorylation

1. Photolysis occurs

2. Light (680nm) energy is absorbed by PSII

3. Light energy excites the electrons in chlorophyll

4. The electrons move to a higher energy level and are able to leave the chlorophyll and enter the electron transport chain

5. The electrons flow down the electron transport chain and reach PS1

6. As the electrons move down the ETC, they release energy

7. Energy is used to transport H+ (protons) into the thylakoids from the membrane

8. Creates an electrochemical gradient with a higher concentration of protons in the thylakoid than in the stroma

9. The protons move down their concentration gradient back into the stroma via ATP synthase

10. Energy from movement combines ADP and Pi to form ATP

11. Light (700nm) is absorbed by PSI and excites electrons to an even higher energy level

12. The electrons enter the ETC

13. The last electron carrier is NADP

14. NADP the electron and a proton from the stroma form NADPH

Describe the process of cyclic photophosphorylation

Only uses PSI

1. Light is absorbed by PSI

2. Excites electrons to a higher energy level

3. The electrons enter the ETC

4. The electrons won’t be transferred to NADP but instead back to PSI where it can be repeatedly recycled

5. The energy is used to actively transport H+ into the thylakoid creating an electrochemical gradient

6. They diffuse down their concentration gradient via ATP synthase

7. The energy from the movement combines ADP and Pi to form ATP

How is cyclic photophosphorylation different to non cyclic photophosphylation?

• cyclic only uses PSI while non cyclic uses both

• cyclic has electrons that are recycled back to the photosystem whereas noncyclic the electrons end at a final electron accepter NADP

• cyclic only produces small amounts of ATP while non cyclic produces O2, NADPH and ATP