Photophosphorylation: Cyclic vs. Non-Cyclic | UPSC Mains AGRICULTURE-PAPER-II 2017

What is photophosphorylation? Distinguish between cyclic and non-cyclic photophosphorylation. Give schematic structure of both the pathways.

How to Approach

This question requires a clear understanding of photophosphorylation, a crucial process in photosynthesis. The approach should begin with defining photophosphorylation and then differentiating between cyclic and non-cyclic types. A schematic representation of each pathway is essential to demonstrate understanding. The answer needs to cover the electron flow, role of photosystems, and ATP yield for each type. Finally, a concise summary emphasizing the key differences will conclude the response. Structuring the answer around the definition, differentiation, and schematic representation will ensure a comprehensive response.

Photophosphorylation is a vital process within photosynthesis, representing the light-dependent reactions that convert light energy into chemical energy in the form of ATP. This process occurs in the thylakoid membranes of chloroplasts, utilizing light energy to energize electrons, which are then used to generate ATP. The overall efficiency of photosynthesis and, consequently, plant growth and productivity, is heavily reliant on photophosphorylation. Recent research focuses on enhancing photosynthetic efficiency in crops through manipulating these light-dependent pathways, as global food security demands greater yields. Understanding the nuances of photophosphorylation, particularly the distinctions between its cyclic and non-cyclic forms, is paramount to appreciating the intricacies of plant physiology.

What is Photophosphorylation?

Photophosphorylation is the process of generating ATP (adenosine triphosphate) using light energy during photosynthesis. It's a form of chemiosmosis, where the energy from light is used to create a proton gradient across the thylakoid membrane, which then drives ATP synthase to produce ATP. The term was coined by Robin Hill in 1939.

Types of Photophosphorylation: Cyclic vs. Non-Cyclic

Photophosphorylation can be broadly categorized into two types: cyclic and non-cyclic. The key difference lies in the electron flow and the photosystems involved.

Non-Cyclic Photophosphorylation (Z-Scheme)

Non-cyclic photophosphorylation, also known as the Z-scheme, is the primary pathway for ATP and NADPH generation during photosynthesis. It involves both Photosystem I (PSI) and Photosystem II (PSII).

Electron Flow: Electrons are excited by light energy in PSII and passed down an electron transport chain. This chain includes plastoquinone (PQ), cytochrome complex, and plastocyanin (PC). The electron ultimately reaches PSI. PSI is then excited by light, and the electrons are passed down a shorter electron transport chain to NADP⁺, reducing it to NADPH. The electrons lost from PSII are replaced by the splitting of water (photolysis).
Photosystems Involved: PSII and PSI
Products: ATP, NADPH, and Oxygen (O₂)
Electron Replacements: Water molecules are split to provide electrons to PSII.

Figure 1: Schematic representation of Non-Cyclic Photophosphorylation (Z-Scheme)

Cyclic Photophosphorylation

Cyclic photophosphorylation occurs only through Photosystem I (PSI). It is an alternative pathway that occurs when the electron transport chain from PSI is blocked or when NADPH levels are high.

Electron Flow: Electrons are excited in PSI and passed down a shorter electron transport chain, ultimately returning to PSI. This creates a cyclic flow of electrons.
Photosystems Involved: Only PSI
Products: ATP only (no NADPH or O₂)
Electron Replacements: Electrons are recycled within the PSI system.

Figure 2: Schematic representation of Cyclic Photophosphorylation

Feature	Non-Cyclic Photophosphorylation	Cyclic Photophosphorylation
Photosystems	PSII and PSI	PSI only
Electron Flow	Linear, Z-scheme	Cyclic
Products	ATP, NADPH, O₂	ATP only
Water Splitting	Required	Not required
NADPH Production	Yes	No

Significance and Regulation

The ratio of ATP to NADPH produced is crucial for the Calvin cycle. Cyclic photophosphorylation helps to balance this ratio when NADPH is abundant. Environmental factors like light intensity and CO₂ concentration can influence the prevalence of each pathway.

Case Study: C4 Plants and Cyclic Photophosphorylation C4 plants, like maize and sugarcane, often exhibit higher rates of cyclic photophosphorylation, particularly under conditions of high light intensity and limited CO₂. This helps them maintain a favorable ATP/NADPH ratio for carbon fixation in the bundle sheath cells.

Additional Resources

Key Definitions

Photosystem

Photosystems are light-harvesting complexes in the thylakoid membranes of chloroplasts that contain chlorophyll and other pigment molecules that capture light energy.

Plastocyanin

Plastocyanin is a copper-containing protein that acts as an electron carrier in the electron transport chain of the thylakoid membrane during photosynthesis.

Key Statistics

Approximately 90% of the energy from sunlight is converted to chemical energy during photosynthesis. (Source: FAO, Knowledge Cutoff)

Source: FAO

The efficiency of light energy conversion in C4 plants can be up to 50% higher than in C3 plants under optimal conditions. (Source: Plant Physiology Journal, Knowledge Cutoff)

Source: Plant Physiology Journal

Examples

Example: Algae and Cyclic Photophosphorylation

Certain algae species can adapt to low CO2 conditions by increasing cyclic photophosphorylation to generate more ATP for carbon fixation, demonstrating the flexibility of this pathway.

Frequently Asked Questions

▶Why is cyclic photophosphorylation important?

Cyclic photophosphorylation is important because it helps to balance the ATP/NADPH ratio during photosynthesis, ensuring that the Calvin cycle has the necessary energy and reducing power for carbon fixation.