Problem 27 - Biology the Easy Way (E-Z Biology)

Question

As an outcome of cyclic phosphorylation (a) \(\mathbf{P} 700\) is destroyed (c) NADPH is synthesized (b) ATP is formed (d) carbon is fixed

Step-by-Step Solution

Answer

(b) ATP is formed

Step 1: Understanding Cyclic Photophosphorylation

Cyclic photophosphorylation is part of the light reactions in photosynthesis which involves only photosystem I. It results in the generation of ATP without the production of NADPH or oxygen since it doesn't involve photosystem II.

Step 2: Analyzing the Options

For the given options: (a) P700, a chlorophyll molecule, is not destroyed; it is the reaction center for photosystem I. (c) NADPH is not synthesized since the electron does not reach NADP+ in cyclic photophosphorylation. (d) Carbon fixation occurs in the Calvin cycle, which uses the products of the light reactions and does not involve cyclic photophosphorylation directly.

Step 3: Identifying the Correct Outcome

The primary result of cyclic photophosphorylation is the formation of ATP. This occurs as electrons cycle back to photosystem I, driving the proton pump to create a proton gradient used for ATP synthesis.

Key Concepts

Cyclic Photophosphorylation

Cyclic photophosphorylation is a fascinating process in the light reactions of photosynthesis. It exclusively involves photosystem I and plays a crucial role in the production of ATP.

During this process, the electron flow is cyclic, meaning that electrons get excited by light energy and pass through a series of proteins. Eventually, these electrons return to the chlorophyll molecule, P700, where they began.

• No NADPH is produced because electrons do not reach NADP+.
• No oxygen is released because photosystem II is not involved in this cycle.
• ATP production is the primary result, essential for the energy needs of the cell.

Overall, this process helps to balance the ATP/NADPH ratio required for various cellular activities and the Calvin cycle.

Photosystem I

Photosystem I is an integral component of cyclic photophosphorylation. It contains a special chlorophyll molecule known as P700, which is the core reaction center. When light strikes P700, it excites electrons, raising them to a higher energy level.

These high-energy electrons then travel through an array of proteins in the thylakoid membrane. Unlike in other processes where electrons are passed to NADP+, in cyclic photophosphorylation, they are returned to P700.

• Photosystem I doesn't contribute to oxygen release because it does not split water molecules.
• It works with a nearby electron transport chain to produce ATP.
• The process maintains the balance of the energy currency, ATP, within the chloroplast.

By cycling the electrons back to their origin, photosystem I ensures its readiness to continue ATP production without wasting resources.

ATP Synthesis

ATP synthesis in cyclic photophosphorylation is a crucial energy conversion process. As electrons move back to photosystem I, they help power a proton pump within the thylakoid membrane. This movement of protons creates a proton gradient across the membrane.

The build-up of protons inside the thylakoid lumen provides the potential energy needed for ATP synthase to function. ATP synthase is a remarkable enzyme that facilitates the conversion of ADP and inorganic phosphate into ATP, the energy currency of the cell.

• The proton gradient is indispensable for ATP generation.
• This ATP is then used for various cellular activities, including the Calvin cycle.
• Even though NADPH is not produced, the generation of ATP through cyclic photophosphorylation is paramount for cellular energy equilibrium.

In summary, ATP synthesis through cyclic photophosphorylation supports the broader energy needs of plant cells besides the energy provided by non-cyclic pathways.