Light Reactions and Electron Transport in Photosynthesis

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Questions and Answers

What role does photosystem II (PS II) play in the process of water splitting during photosynthesis?

Photosystem II is responsible for the splitting of water, which provides electrons to replace those lost during the light reactions.

Describe the difference between cyclic and non-cyclic photophosphorylation in photosynthesis.

Cyclic photophosphorylation involves only PS I and produces ATP without generating NADPH, while non-cyclic photophosphorylation involves both PS I and PS II, producing ATP and NADPH.

Why is the location of the water splitting complex significant in photosynthesis?

The water splitting complex is located on the inner side of the thylakoid membrane, allowing direct access to the electron transport chain.

How do electrons from water splitting enhance the function of photosystem I (PS I)?

Electrons produced from water splitting are vital for replacing those lost from PS I, maintaining its function in the light reactions. Signup and view all the answers

What are the primary roles of light reactions in photosynthesis?

The light reactions primarily involve light absorption, water splitting, oxygen release, and the formation of high-energy intermediates like ATP and NADPH. Signup and view all the answers

Explain the relationship between PS II and the formation of oxygen during photosynthesis.

The splitting of water associated with PS II produces oxygen as a byproduct, along with electrons and protons. Signup and view all the answers

Describe the function of the light-harvesting complexes (LHC) within the photosystems.

The light-harvesting complexes (LHC) contain pigment molecules that absorb various wavelengths of light, enhancing the efficiency of photosynthesis. Signup and view all the answers

Explain the significance of the chlorophyll a absorption peaks in Photosystem I and II.

Chlorophyll a in Photosystem I absorbs light at 700 nm (P700), while in Photosystem II it absorbs at 680 nm (P680), allowing for efficient light harvesting and energy transfer. Signup and view all the answers

What is the Z scheme in the context of photosynthesis?

The Z scheme refers to the electron transfer process in photosynthesis, depicting the movement of electrons from water through photosystem II and I before reducing NADP+ to NADPH. Signup and view all the answers

How do electrons move from Photosystem II to Photosystem I?

Electrons are excited in Photosystem II, jump to an electron acceptor, and then pass through a series of cytochromes to reach Photosystem I. Signup and view all the answers

What occurs when chlorophyll a in Photosystem I is excited by light?

When chlorophyll a in Photosystem I absorbs light at 700 nm, it excites electrons that are transferred to an acceptor molecule with a greater redox potential. Signup and view all the answers

What is the role of NADP+ in the light reactions of photosynthesis?

NADP+ serves as the final electron acceptor in the electron transport chain, getting reduced to NADPH + H+. Signup and view all the answers

Differentiate between the absorption maxima of chlorophyll a in Photosystem I and II.

Chlorophyll a in Photosystem I has an absorption maximum at 700 nm, while in Photosystem II it is at 680 nm. Signup and view all the answers

Study Notes

Light Reactions (Photochemical Phase)

Involves light absorption, water splitting, oxygen release, and synthesis of ATP and NADPH.
Key protein complexes are involved, notably Photosystem I (PS I) and Photosystem II (PS II).
Two light harvesting complexes (LHC) are formed by hundreds of pigment molecules attached to proteins; these complexes enhance the efficiency of photosynthesis.
Reaction centers in each photosystem have distinct absorption peaks:
- PS I's chlorophyll a: absorbs at 700 nm (P700).
- PS II's chlorophyll a: absorbs at 680 nm (P680).

Electron Transport

Electrons in PS II become excited upon absorbing 680 nm light and move to a higher orbital.
Excited electrons are transferred to an electron acceptor, entering an electron transport chain involving cytochromes.
Electrons are recycled, moving from PS II through the chain to PS I.
In PS I, electrons are excited through 700 nm light and passed to another acceptor with higher redox potential.
This electron transfer reduces NADP+ to NADPH + H+, completing the process called the Z scheme, which has a characteristic shape on a redox potential scale.

Water Splitting Mechanism

Photosystem II continuously replenishes lost electrons by splitting water molecules.
Water is split into 2H+, oxygen (O), and electrons, producing oxygen as a byproduct.
The water-splitting complex is integral to PS II, found on the thylakoid membrane's inner side, influencing where protons and O2 are released.

Cyclic and Non-cyclic Photophosphorylation

Organisms extract energy from oxidizable substances, storing it as bond energy in ATP.
ATP serves as the primary energy carrier in biochemical reactions, allowing for energy transfer and use within the cell.

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Description

This quiz covers the light reactions of photosynthesis, focusing on the photochemical phase, water splitting, and the roles of Photosystem I and II. Explore the intricate details of electron transport and the specific absorption peaks in chlorophyll. Test your understanding of how these processes contribute to ATP and NADPH synthesis.