Question

In cyclic photophosphorylation in photosystem I, ATP is produced, even though water is not split. Explain how the process takes place.

Short answer

In cyclic photophosphorylation in photosystem I, light energy excites electrons, which generate a proton gradient used by ATP synthase to produce ATP while electrons return to PSI.

Step-by-step solution

- Understand Photophosphorylation

Photophosphorylation is the process of converting ADP to ATP using the energy of sunlight, occurring in the chloroplasts of plant cells during photosynthesis.

- Introduction to Cyclic Photophosphorylation

In cyclic photophosphorylation, photosystem I (PSI) is used, and this process produces ATP but does not produce NADPH or oxygen. The electron transport is cyclic, meaning that electrons expelled by PSI return to the system.

- Role of Photosystem I

Photosystem I absorbs sunlight, which excites the electrons, raising them to a higher energy level. These high-energy electrons are transferred to a primary electron acceptor.

- Electron Transport Chain

The electrons then move through a series of proteins in the electron transport chain, including ferredoxin and cytochrome, driving the transport of protons (H+) across the thylakoid membrane.

- ATP Production

The transport of protons creates a proton gradient across the thylakoid membrane. The energy from this proton gradient is used by ATP synthase to convert ADP to ATP.

- Return of Electrons

After passing through the electron transport chain and contributing to ATP production, the electrons return to PSI, completing the cycle.

- Significance of Cyclic Pathway

This cyclic pathway ensures continuous production of ATP without the need for water splitting (photolysis) and generation of oxygen.

Key concepts

photosystem I

Photosystem I (PSI) is a crucial complex in the photosynthetic process of plants. It's located in the thylakoid membrane of chloroplasts. PSI primarily absorbs light at a wavelength of 700 nm, which is why it's often referred to as P700. When PSI absorbs sunlight, it excites electrons, boosting them to a higher energy state. These high-energy electrons are immediately captured by a primary electron acceptor. This initial step is vital for the subsequent processes in cyclic photophosphorylation. Understanding PSI's role helps elucidate how plants convert light energy into a usable chemical form without splitting water.

electron transport chain

The Electron Transport Chain (ETC) is a series of protein complexes within the thylakoid membrane. Once the high-energy electrons are captured by the primary electron acceptor in photosystem I, they move through the ETC. The chain includes key proteins such as ferredoxin and cytochrome. As the electrons travel through these proteins, they release energy. This energy is used to pump protons (H+) from the stroma into the thylakoid lumen. The movement of protons across the membrane is crucial in creating a proton gradient, which is essential for ATP production. The ETC ensures that the excited electrons from PSI are utilized efficiently, facilitating the continuous flow of electrons in cyclic photophosphorylation.

ATP production

ATP production is the end goal of cyclic photophosphorylation. The energy released by electrons moving through the electron transport chain is used to transport protons (H+) across the thylakoid membrane, creating a proton gradient. This gradient represents potential energy, which is harnessed by ATP synthase, an enzyme embedded in the membrane. As protons flow back into the stroma through ATP synthase, the enzyme converts ADP and inorganic phosphate (Pi) into ATP. This process of converting light energy into the chemical energy of ATP makes it possible for the plant cells to perform various functions that require energy.

proton gradient

The proton gradient is a key feature of cyclic photophosphorylation. As electrons pass through the electron transport chain, protons (H+) are pumped from the stroma into the thylakoid lumen. This creates a higher concentration of protons inside the lumen compared to the stroma, forming a proton gradient. The buildup of protons creates potential energy because they naturally want to flow back to the area of lower concentration in the stroma. ATP synthase, a membrane-bound enzyme, provides a channel for protons to flow back into the stroma. The energy from this flow is used to convert ADP and inorganic phosphate (Pi) into ATP, effectively using the proton gradient to produce the energy currency of the cell.