Problem 14 - Biology: The Unity and Diversity of Life

Question

In \(\mathrm{C} 3\) plants,_____________ makes sugar production inefficient when stomata close during the day. a. photosynthesis c. photorespiration b. photolysis d. carbon fixation

Step-by-Step Solution

Answer

Photorespiration makes sugar production inefficient when stomata close in C3 plants.

Step 1: Understand the Context

In C3 plants, the process of photosynthesis is directly affected by environmental factors. One key challenge occurs when the stomata, small openings on the leaves, close to prevent water loss during hot days. This closure limits the intake of carbon dioxide, affecting the efficiency of sugar production.

Step 2: Identify Effect of Stomatal Closure

When stomata close, CO2 levels inside the leaf decrease, while O2 levels increase due to ongoing respiration. This change in concentration affects the enzyme Rubisco's role, which usually fixes CO2 during the Calvin cycle.

Step 3: Recognize the Role of Rubisco

Rubisco, the enzyme responsible for fixing carbon, has a dual affinity for both CO2 and O2. When CO2 levels are low and O2 levels are high, Rubisco tends to fix O2 instead, initiating a process called photorespiration.

Step 4: Explain Photorespiration's Effect on Sugar Production

Photorespiration is inefficient for sugar production as it consumes energy and releases fixed carbon as CO2 rather than synthesizing glucose. This decreases the overall efficiency of photosynthesis and sugar production in C3 plants when stomata are closed.

Step 5: Choose the Correct Answer

Given the inefficiency caused by stomatal closure through the process induced, the correct term for this phenomenon is 'photorespiration.' Thus, the best answer is option c: photorespiration.

Key Concepts

Photosynthesis

Photosynthesis is a vital process that allows plants to convert light energy into chemical energy in the form of glucose. C3 plants, which are a major group of plants using this process, derive their name from producing a three-carbon compound, 3-phosphoglycerate, as the first product of carbon fixation. For photosynthesis to occur efficiently, plants need a steady supply of carbon dioxide, water, and sunlight.
During photosynthesis, chlorophyll in the plant's chloroplasts captures sunlight. This energy drives the conversion of carbon dioxide and water into glucose and oxygen. The overall reaction can be represented by the equation:

• \(6 \text{CO}_2 + 6 \text{H}_2\text{O} + light \text{ energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6 \text{O}_2\)

This process is crucial for the survival of most life forms on Earth as it is the foundation of the food chain. In C3 plants, the challenge arises when external conditions like high temperatures cause the stomata to close, disrupting photosynthesis.

Photorespiration

Photorespiration is an alternative pathway that occurs in plants when the stomata are closed. This pathway is not as productive as photosynthesis because it uses more energy and releases carbon dioxide without producing sugars. When stomata are closed, carbon dioxide levels inside the leaf fall while oxygen levels rise due to continued respiration.
Rubisco, the enzyme involved in fixing carbon dioxide during photosynthesis, inadvertently captures oxygen instead. This activity leads to the process known as photorespiration. During photorespiration, rather than synthesizing more glucose, the plant actually breaks down and consumes some of the intermediates:

• Energy is used without producing sugars, resulting in a loss of carbon as it's converted back to carbon dioxide.
• The efficiency of C3 photosynthesis decreases, impacting the plant's growth and productivity.

Understanding photorespiration is key to optimizing plant growth, especially in warming climates where stomatal closure is more frequent.

Stomatal Closure

Stomata are small openings on the surfaces of leaves that allow gas exchange—carbon dioxide in and oxygen out, essential for photosynthesis. However, these openings also permit water vapor to escape. During hot and dry conditions, C3 plants may close their stomata to retain moisture. This closure, while necessary for conserving water, poses a significant challenge to photosynthesis.
When stomata are closed:

• The intake of carbon dioxide is restricted, limiting the resources available for photosynthesis.
• Increased oxygen levels inside the leaf favor photorespiration, as Rubisco is more likely to interact with oxygen.

Stomatal closure thus presents a dilemma where the plant needs to balance water retention with efficient sugar production. This trade-off is a critical area of study, aiding in the development of plant varieties that can better withstand stressful environments.

Rubisco Enzyme

Rubisco, short for ribulose-1,5-bisphosphate carboxylase/oxygenase, is the enzyme at the heart of the Calvin cycle in photosynthesis. Rubisco has a unique ability to fix carbon dioxide, turning it into organic compounds that the plant can use for energy and growth. Yet, Rubisco also reacts with oxygen, the foundation of photorespiration.
Rubisco's dual function:

• When carbon dioxide is plentiful, Rubisco catalyzes the attachment of CO2 to ribulose bisphosphate (RuBP), leading to sugar production through the Calvin cycle.
• Conversely, when carbon dioxide is low and oxygen is high, Rubisco mistakenly fixes oxygen instead, initiating photorespiration.

This inherent flexibility is crucial for survival but also results in inefficiencies under certain conditions. Enhancing Rubisco's specificity for carbon dioxide, or developing crops with alternative pathways, are exciting frontiers in agricultural science.