Light dependent stage

What is the light dependent stage?

Energy from sunlight is absorbed and used to form ATP.

Hydrogen from water is used to reduce coenzyme NADP to reduced NADP.

Where does the light dependent stage occur?

Thylakoid membrane

PSII

Water is absorbed from the roots, travels up to the leaf and goes into the thylakoid membrane.

Water is first at PSII

What is PSII also known as?

P680

Because chlorophyll a in this system has a max absorption at 680nm

What about PSI?

P700

Chlorophyll a in this system has a max absorption of light at 700nm

Photolysis

Light from the sun hits the leaf, and the light energy is used to break this water molecule down.

1/2 O₂ has been released

2 H⁺ are also made

Electrons are released from the water molecule, go into PSII, and they get excited because of the light energy.

Why is this process called photolysis?

Light energy break down

4)

The 2e⁻ from the water molecule are transported in structures called electron carriers. They electrons will go through the electron transport chain

Electron transport chain

ETCs play a major role in ATP synthesis. It's the same one in respiration (oxidative phosphorylation)

6)

These electrons eventually lose their energy as they go along a series of electron carriers, and reach PSI. In the process, they will make ATP, releasing energy

7)

PSI absorbs light energy from the sun, and again, the electrons get excited and travel via an electron carrier again

Cyclic Photophosphorylation

The electrons can either go back down to PSI to release more energy, or continue with the cycle. This depends on what the plant needs - if the plant is lacking in ATP, it will travel back down to PSI, releasing ATP again

ADP + Pi → ATP

8)

The electrons can continue with the cycle, arriving at an enzyme called NADP reductase

9)

NADP will receive the 2 electrons and 2H⁺ from the water molecule at the beginning

The 2e⁻ and 2H⁺ will combine with the NADP to produce reduced NADP

NAD

Coenzyme in respiraton

NADP

Coenzyme in photosynthesis

12)

When the 2H⁺ is travelling to NADP reductase, it also passes an enzyme called ATP synthase.

ATP synthase releases energy in the form of ATP (ADP+Pi → ATP)

What's the process called when the electrons continue with the cycle from PSI?

Non-cyclic photophosphorylation

Proton gradient

There's a build up of protons in the intermembrane space. This establishes a proton gradient, so H⁺ moves through ATP synthase, down its electrochemical gradient, in a process called chemiosmosis to make ATP

15)

The hydrogen ion that's come through the ATP synthase will be added on to NADP along with the e⁻ to form NADPH