NOTES

The Light Harvesting Complex (LHC) and the Photosynthetic Electron Transport Chain are key components of the photosynthetic process in plants, algae, and some bacteria. They work together to convert light energy into chemical energy, which is ultimately used to synthesize sugars from carbon dioxide and water. Here’s a detailed look at each:

Light Harvesting Complex (LHC)

1. Function: The primary role of the LHC is to capture light energy and transfer it to the reaction centers of photosystems (Photosystem I and II). The LHC is made up of proteins and pigments (like chlorophyll and carotenoids) that absorb various wavelengths of light.

2. Structure: The LHC consists of several antenna proteins that bind pigments. These pigments are arranged in such a way that they can efficiently absorb light and transfer the energy between each other.

3. Process: When light is absorbed by a pigment molecule in the LHC, the energy is transferred from one pigment to another until it reaches the reaction center of a photosystem. This process is known as resonance energy transfer.

4. Variability: Different pigments absorb different wavelengths of light, allowing plants to utilize a broader spectrum of sunlight.

Photosynthetic Electron Transport Chain

1. Components: The photosynthetic electron transport chain includes Photosystem II, the cytochrome b6f complex, Photosystem I, and several other electron carriers.

2. Function: Its main function is to transfer electrons from water to NADP+, creating a flow of protons across the thylakoid membrane, which drives the synthesis of ATP.

3. Process:

• Photosystem II (PSII): It absorbs light, which excites electrons to a higher energy state. These electrons are then passed to the electron transport chain. PSII also splits water molecules to replace the lost electrons, releasing oxygen as a by-product.

• Cytochrome b6f complex: This complex facilitates the transfer of electrons between PSII and PSI. The energy released during electron transfer is used to pump protons into the thylakoid lumen, creating a proton gradient.

• Photosystem I (PSI): It receives electrons from the cytochrome b6f complex. PSI also absorbs light, further boosting the energy level of electrons. These high-energy electrons are then used to reduce NADP+ to NADPH.

• ATP Synthesis: The proton gradient across the thylakoid membrane drives ATP synthase to produce ATP from ADP and inorganic phosphate.

PHOTOPHOSPHORYLATION:

Photophosphorylation is a process in photosynthesis where light energy is converted into ATP (adenosine triphosphate), a form of chemical energy that is usable by the cell. There are two main types of photophosphorylation: cyclic and non-cyclic.

1. Cyclic Photophosphorylation:

• In cyclic photophosphorylation, the electrons are cycled back to the original chlorophyll molecule.
• This process only involves Photosystem I.
• It produces ATP but does not produce NADPH or oxygen.
• The process starts with a photon hitting the pigment in the photosystem, which results in the transfer of an electron to a higher energy state.
• The high-energy electron is passed along a series of proteins in the thylakoid membrane (electron transport chain).
• As the electron passes through the electron transport chain, its energy is used to pump protons across the thylakoid membrane, creating a proton gradient.
• The electron eventually cycles back to the photosystem.
• The proton gradient is used by ATP synthase to generate ATP.

2. Non-Cyclic Photophosphorylation:

• Non-cyclic photophosphorylation involves both Photosystem I and II.
• It results in the production of ATP, NADPH, and oxygen.
• The process starts with Photosystem II absorbing light and losing an electron, which is then passed to Photosystem I.
• The missing electron in Photosystem II is replaced by splitting a water molecule, releasing oxygen as a by-product.
• The high-energy electron from Photosystem II is passed through an electron transport chain to Photosystem I, producing ATP in the process.
• Photosystem I also absorbs light and transfers its high-energy electron to NADP+, reducing it to NADPH.