Light Dependent Reactions (8) Flashcards
Q: What powers the light-dependent reactions, and what are their products?
A: The light-dependent reactions are powered by the radiant energy of the Sun. Their products are ATP, NADPH, and O₂ (released as waste).
Q: Why are the light-dependent reactions called “light-dependent”?
A: The light-dependent reactions are called “light-dependent” because they require the absorption of light photons to begin the process.
Q: What is the purpose of cyclic photophosphorylation, and which photosystem is used?
Cyclic photophosphorylation is a process in plants that makes ATP, which is like energy for the plant.
It uses Photosystem I (PSI), a part of the plant’s cells that contains a special type of chlorophyll called P700. This chlorophyll absorbs light best at 700 nm, a specific color in sunlight. The process is called “cyclic” because the energized electrons flow in a loop, helping to make ATP without producing any other products like oxygen.
Q: What happens to the energy that reaches the reaction center during cyclic photophosphorylation?
A: The energy is used to energize an electron, increasing its energy state. The energized electron is then passed along an electron transport chain.
Q: What is the path of the energized electron after PSI during cyclic photophosphorylation?
A: PSI passes the electron to the primary electron acceptor, which transfers it to ferredoxin (Fd). Fd carries the electron to plastoquinone (PQ), which then transfers it to a cytochrome complex (Cyt).
Q: How does the cytochrome complex (Cyt) contribute to the cyclic photophosphorylation reaction?
A: Cyt uses the energy from the electron to pump 2H+ from the stroma into the thylakoid interior (lumen). It then passes the low-energy electron to plastocyanin (PC), which returns it to PSI to be reused.
Q: How many H+ ions are needed to produce 1 ATP during cyclic photophosphorylation?
A: Every 4 H+ ions provide enough proton motive force (PMF) to synthesize 1 ATP.
Q: Can cyclic photophosphorylation be performed by organisms other than plants?
A: Yes, cyclic photophosphorylation can also be performed by some bacteria.
Q: What is the role of Photosystem II (PSII) in the light-dependent reactions?
A: Photosystem II (PSII), containing chlorophyll a p680, absorbs a photon through its antenna complex. This energy energizes an electron, which moves from chlorophyll to pheophytin (I) in the reaction center.
Q: How are the electrons lost by Photosystem II (PSII) replaced?
A: The electrons lost by PSII are replaced by the oxidation of water, where 2 H2O molecules are converted into 4 H+ ions, 4 electrons, and 1 O2 molecule.
Q: How are the electrons transported between Photosystem II (PSII) and Photosystem I (PSI)?
A: PQ picks up the electrons from pheophytin (I) and passes them to the cytochrome complex (Cyt), which uses the energy to pump 2 H+ ions. PlastoCyanin (PC) then transports the electrons from Cyt to PSI, where a second photon re-energizes the electrons at PSI.
Q: How is the electron transferred from PSI to NADP+?
A: The primary electron acceptor of PSI passes the electron to ferredoxin (Fd). Ferredoxin, carrying the electron, is oxidized by the NADP+ reductase enzyme, transferring the electron to NADP+, reducing it to NADPH.
Q: How are two electrons used to fully reduce NADP+ to NADPH?
A: The first electron reduces NADP+ to NADP. The second electron, along with a proton (H+), fully reduces NADP+ to NADPH.
Q: What is produced during noncyclic photophosphorylation for every 4 photons absorbed?
A: 1 ATP and 1 NADPH are produced for every 4 photons absorbed (2 by PSII and 2 by PSI).
