Light dependent reactions

What is the purpose of the light-dependent reactions in photosynthesis?

The light-dependent reactions use light energy to produce ATP and NADPH, which are energy-carrying molecules needed for the Calvin cycle.

Where do the light-dependent reactions take place in plants?

They take place in the thylakoid membranes of chloroplasts.

What are photosystems, and what role do they play in the light-dependent reactions?

Photosystems are complexes of proteins and pigments that harvest light energy. They contain chlorophyll molecules and other pigments that absorb light and pass energy to the reaction center, enabling the production of high-energy electrons.

What are the two main types of photosystems, and how are their special chlorophyll pairs named?

Photosystem II (PSII) has a special pair called P680 that absorbs light at 680 nm, and Photosystem I (PSI) has a special pair called P700 that absorbs light at 700 nm.

Describe what happens when light is absorbed by pigments in Photosystem II (PSII).

Energy from light is passed from pigment to pigment until it reaches the reaction center, P680, exciting an electron to a high-energy state (P680*). The excited electron is passed to a primary electron acceptor, pheophytin, and replaced by an electron from water, producing O₂ and H⁺ ions.

What is the chemical equation for the splitting of water in PSII?

H₂O → ½ O₂ + 2 H⁺ + 2 electrons.

How is the proton gradient formed in the thylakoid membrane?

Protons are released into the thylakoid lumen from water splitting and actively pumped from the stroma into the thylakoid interior as electrons move down the electron transport chain, creating a high H⁺ concentration inside the thylakoid.

How is ATP synthesized in the light-dependent reactions?

Protons flow down their concentration gradient through ATP synthase, driving the phosphorylation of ADP to ATP, a process called chemiosmosis. ATP is released into the stroma.

What happens to the electron after it leaves PSII?

The electron passes through an electron transport chain composed of plastoquinone (Pq), the cytochrome complex (Cyt), and plastocyanin (Pc), releasing energy used to pump H⁺ ions and eventually reaches PSI.

How does PSI (Photosystem I) re-energize the electron?

When the electron joins the P700 special pair in PSI, it is re-excited by light absorbed by the pigments, boosting it to a high-energy state for transfer to an acceptor molecule.

What is the electron pathway from PSI to NADPH?

The high-energy electron is passed from P700 to ferredoxin (Fd), then to NADP⁺ reductase, which transfers electrons to NADP⁺, forming NADPH, released into the stroma.

What is the difference between linear (non-cyclic) and cyclic photophosphorylation?

In linear photophosphorylation, electrons flow from water through PSII and PSI to NADPH, producing both ATP and NADPH. In cyclic photophosphorylation, electrons cycle from PSI back to the first electron transport chain, producing only ATP and not NADPH.

Why does cyclic electron flow occur in chloroplasts?

It occurs when the NADPH/NADP⁺ ratio is too high, when ATP demand is especially high (e.g., in C₄ photosynthetic cells), or to protect photosystem proteins from excess light damage and support repair.

What is a photosystem reaction center?

The reaction center contains a special pair of chlorophyll a molecules. When energy reaches this pair, it excites an electron, which is then transferred to the primary electron acceptor to start the electron transport chain.

How do pigments transfer energy to the reaction center?

Through resonance energy transfer, excited pigments pass energy to neighboring pigments until it reaches the reaction center. The receiving pigment may absorb light of longer wavelength (less energy) than the donor.

What is the primary electron acceptor in PSII and PSI?

In PSII, it is pheophytin. In PSI, it is a chlorophyll called A₀.

What drives the "downhill" movement of electrons in the light-dependent reactions?

Electrons move downhill because they were first excited to a high-energy state by light absorption in P680 (PSII) and P700 (PSI). The energy drop along the chain releases energy for proton pumping and ATP production.

How is oxygen produced in photosynthesis?

Oxygen is produced when PSII extracts electrons from water during the splitting of water by the manganese center, releasing O₂ and H⁺ ions.

How is the light energy ultimately converted into chemical energy?

Light excites electrons in PSII and PSI, and the energy from electrons moving through electron transport chains drives H⁺ pumping to form a gradient (used for ATP production) and reduces NADP⁺ to NADPH, storing energy in chemical form.