Lecture on Photosynthesis: The Z Scheme (Light Dependent Stage)

Overview

• Focus on the first stage of photosynthesis known as the Z scheme or the light-dependent stage.
• Requires light to occur, taking place in the thylakoid membrane.
• Key structures involved are the photosystems which absorb light energy.

Process Steps

Photosystem II

1. Water Absorption
   • Water is absorbed by plant roots and travels to the leaf's thylakoid membrane.
   • Water first interacts with Photosystem II.
2. Light Absorption
   • Light from the sun hits Photosystem II.
   • Light energy breaks down water molecules (Photolysis).
     • Releases oxygen and hydrogen ions (protons).
     • Electrons are released and enter Photosystem II.
3. Electron Excitation
   • Electrons get excited by light energy.

Electron Transport Chain (ETC)

1. Electron Carriers

   • Excited electrons are transferred to electron carriers.
   • Carriers transport electrons through the ETC.

2. ATP Production

   • As electrons move through the ETC, they lose energy.
   • Energy is used to synthesize ATP.

Photosystem I

1. Arrival at Photosystem I

   • Electrons reach Photosystem I, absorbing additional light energy.
   • Electrons get excited again.

2. Cyclic vs. Non-Cyclic Photophosphorylation

   • Cyclic Photophosphorylation
     • Electrons return to Photosystem I to release more energy and produce ATP again.
   • Non-Cyclic Photophosphorylation
     • Electrons proceed to NADP reductase, reducing NADP to NADPH.
     • Combines with protons to form reduced NADP.

Enzymes Involved

1. NADP Reductase

   • Receives electrons and protons to form NADPH.

2. ATP Synthase

   • Produces ATP by utilizing the energy from protons moving across the membrane.

Products of the Z Scheme

• ATP
• Reduced NADP (NADPH)

Next Steps

• ATP and NADPH are used in the Calvin Cycle to synthesize glucose.
• Further details on the Calvin Cycle to be discussed in the follow-up lecture.