Question

In cyclic photophosphorylation which one of the following is formed? (a) ATP (b) NADP and ATP (c) \(\mathrm{NADH}\), and \(\mathrm{O}_{2}\) (d) NADPH, ATP and \(\mathrm{O}\).

Short answer

The correct answer is option (a)- ATP

Step-by-step solution

Recalling the concept

Understand that cyclic photophosphorylation generates ATP via a 'cyclic' process where electrons from Photosystem I are transported back to it via an electron transport chain, rather than being passed on to Photosystem II. Also, remember that this process only involves Photosystem I and does not produce NADPH nor O₂.

Understanding the options

Look at the options given. Three out of four include NADPH, NADP or O₂, which as we just recalled are not products of cyclic photophosphorylation. Thus, these options are incorrect.

Matching the concept with the options

We're only left with the first option - (a) ATP which is indeed the product of cyclic photophosphorylation. It is therefore the correct answer.

Key concepts

Photophosphorylation

Photophosphorylation is the process by which light energy is converted into chemical energy during photosynthesis. This chemical energy is stored as adenosine triphosphate (ATP), a molecule that serves as the energy currency for the cell. There are two types of photophosphorylation: cyclic and non-cyclic.

As the term 'cyclic' suggests, cyclic photophosphorylation involves the movement of electrons in a circular path. In this process, electrons are excited by light in photosystem I and then travel down an electron transport chain before returning to the same photosystem. This movement of electrons allows the cell to generate ATP through a protein complex called ATP synthase. However, unlike in non-cyclic photophosphorylation, no oxygen or nicotinamide adenine dinucleotide phosphate (NADPH) is produced in this cycle.

In the exercise, students were reminded that the cyclic process generates solely ATP without the production of NADPH or oxygen, guiding them towards identifying the correct answer.

Photosystem I

Photosystem I (PSI) is one of two photosystems, the other being Photosystem II (PSII), involved in the light-dependent reactions of photosynthesis. Located in the thylakoid membranes of chloroplasts, PSI plays a pivotal role in both cyclic and non-cyclic photophosphorylation. Its key function is to capture light energy, which is essential for energizing electrons.

During cyclic photophosphorylation, PSI is the starting and ending point for electrons. It absorbs light and uses the energy to promote electrons to a higher energy state. These high-energy electrons are then passed along the electron transport chain, driving the production of ATP. PSI is critical for the process, as it allows plants to adjust to the needs for ATP and NADPH. The students can improve their understanding by visualizing PSI as the main power station that harnesses light energy and sets the electrons on their cyclic journey.

Electron Transport Chain

The electron transport chain (ETC) in photosynthesis is a series of protein complexes and other molecules that transfer electrons from one to another via redox (reduction-oxidation) reactions. As electrons travel down the ETC, they lose energy, which is then used to pump protons across the thylakoid membrane, creating a proton gradient.

In cyclic photophosphorylation, the ETC provides the pathway for electrons to return to PSI. It's important for students to realize that the flow of electrons is not just a random journey; it has a purpose. The energy released from these moving electrons is harvested to build up a proton gradient, upon which ATP production is dependent. By understanding the ETC's role, students can better relate the cyclic flow of electrons to the broader context of energy conversion in photosynthesis.

ATP Production

ATP production is the end goal of photophosphorylation, providing energy for the cell's various activities. ATP is produced by a process known as chemiosmosis, where a proton gradient generated by the electron transport chain is used by ATP synthase to add a phosphate group to adenosine diphosphate (ADP), creating ATP.

In the context of cyclic photophosphorylation specifically, the electrons cycling back to PSI enable the continuous pumping of protons, which maintains the proton gradient necessary for chemiosmosis. Therefore, the ATP formed in this process is the direct result of the photosynthetic light reactions harnessed by cells to perform work. The role of ATP production is central to understanding cellular energy transfer and highlights the crucial nature of cyclic photophosphorylation as a means to ensure plants have a sufficient energy supply, particularly when the demand for NADPH is not as high.