Question

Assertion: Photorespiration is a wasteful process. Reason: It results in \(\mathrm{CO}_{2}\) release with the utilization of ATP and there is no synthesis of ATP or NADPH

Short answer

The claim is correct. Photorespiration indeed results in wastage as it leads to the release of \( \mathrm{CO}_{2} \) and uses ATP without the synthesis of more ATP or NADPH, leading to a net energy loss with no productive output.

Step-by-step solution

Understanding Photorespiration

Photorespiration is a metabolic pathway in plants, involving oxygenation of RuBP, a molecule involved in the Calvin cycle, in the process of photosynthesis. When the concentration of carbon dioxide (CO2) is low, and oxygen (O2) is high, oxygen binds with RuBP leading to photorespiration.

Understanding the outcomes of Photorespiration

With photorespiration, the plant not only fails to produce sugars (as it does with normal photosynthesis via the Calvin cycle), but it also results in a net loss of previously fixed carbon (as \( \mathrm{CO}_{2} \)) and nitrogen (as ammonia) which can be detrimental to plant productivity.

Understanding ATP and NADPH role

ATP and NADPH are vital molecules produced in the light-dependent reactions of photosynthesis and are used in the light-independent reactions to fix CO2 into organic molecules in the Calvin cycle. In photorespiration, however, ATP energy is expended, and neither ATP nor NADPH are produced, reinforcing the claim that it is a wasteful process.

Summarizing the Assertion and Reason

Based on the information, it's clear that the original assertion that photorespiration is a wasteful process is correct as it leads to the release of CO2 and utilizes ATP without synthesizing new ATP or NADPH. The reason accurately describes why photorespiration is considered wasteful.

Key concepts

RuBP

Ribulose bisphosphate, or RuBP, is a key molecule in the process of photosynthesis. It acts as a carbon acceptor in the Calvin cycle, where it plays a critical role in the fixation of carbon dioxide (\(\mathrm{CO}_{2}\)) from the atmosphere into organic molecules.
RuBP is a five-carbon compound that readily interacts with carbon dioxide in the presence of the enzyme RuBisCO. This reaction is essential for the continuation of the Calvin cycle, leading to the synthesis of glucose and other sugars in plants.
However, RuBP can also react with oxygen instead of carbon dioxide, particularly when \(\mathrm{CO}_{2}\) levels are low and \(\mathrm{O}_2\) levels are high, leading to photorespiration. This alternate pathway is less efficient as it does not lead to sugar production and can actually result in the loss of already-fixed carbon. Understanding RuBP and its interactions is crucial in grasping how photorespiration affects plant efficiency and metabolism.

Calvin cycle

The Calvin cycle, also known as the light-independent reactions or dark reactions, is a fundamental process of photosynthesis where carbon fixation occurs. It uses the energy and reducing power of ATP and NADPH generated in the light-dependent reactions to convert \(\mathrm{CO}_{2}\) into organic compounds.
The cycle involves three main stages: carboxylation, reduction, and regeneration.

• Carboxylation: RuBP combines with \(\mathrm{CO}_{2}\) to form a 6-carbon intermediate, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA).
• Reduction: ATP and NADPH are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P), a sugar molecule.
• Regeneration: Some G3P molecules are recycled to regenerate RuBP, enabling the cycle to continue.

The Calvin cycle is pivotal in forming the building blocks of life, contributing directly to the plant's ability to grow and reproduce. Disruption in this cycle, such as by photorespiration, can lead to reduced biomass and productivity.

ATP

Adenosine triphosphate (ATP) is often referred to as the energy currency of the cell. In photosynthesis, ATP is produced during the light-dependent reactions when photophosphorylation converts light energy into chemical energy.
This energy is then used in the Calvin cycle to drive the reactions that fix carbon dioxide into sugars.
During the normal photosynthetic process, ATP is utilized efficiently to build glucose molecules, vital for plant growth and energy storage.
The role of ATP in photorespiration is less favorable as it does not result in the synthesis of sugars. Instead, ATP is consumed without an equivalent energy payoff, contributing to the characterization of photorespiration as a wasteful process. Understanding ATP's involvement in both photosynthesis and photorespiration highlights its critical role in plant metabolism and stresses the inefficiencies within alternative pathways like photorespiration.

NADPH

Nicotinamide adenine dinucleotide phosphate (NADPH) is another crucial molecule generated during the light-dependent reactions of photosynthesis. It serves as a reducing agent that provides the hydrogen and electrons necessary to convert carbon dioxide into glucose within the Calvin cycle.
Together with ATP, NADPH helps drive the series of reactions necessary to sustain plant life and promote growth.
Unlike in the regular Calvin cycle, photorespiration does not produce or utilize NADPH. Instead, the energy-rich electrons carried by NADPH are somewhat wasted as the cycle diverts from its usual productive route. This aspect of photorespiration underscores its inefficiency; the energy and reducing power from NADPH are not harnessed to form new organic matter. This lack of NADPH usage accentuates why photorespiration is often seen as detrimental compared to the traditional Calvin cycle.

plant metabolism

Plant metabolism encompasses all the chemical processes that allow plants to grow, reproduce, and respond to their environment. Key elements of this include photosynthesis, respiration, and nutrient absorption.
Photosynthesis, notably, is central to plant metabolism, enabling them to convert light energy into chemical energy in the form of glucose, which serves as both a fuel and a building block for growth.
Within plant metabolism, photorespiration plays a controversial role. While it was traditionally viewed as a purely wasteful process, recent studies suggest it might have protective roles under specific environmental conditions, such as avoiding excess photodamage during intense light exposure.
However, the inefficiencies such as the unnecessary use of ATP and release of previously fixed carbon make its impact on overall plant metabolism complex. Understanding these processes is critical as it influences how plants are cultivated, how they adapt to different environmental conditions, and how efficient they are in converting resources into growth.