Problem 14 - Biology Concepts and Applications

Question

In \(C 3\) plants, ___ makes sugar production inefficient when stomata close on hot, dry days. a. photosynthesis b. photolysis c. photorespiration d. carbon fixation

Step-by-Step Solution

Answer

Photorespiration makes sugar production inefficient.

Step 1: Understanding C3 Plants

C3 plants are so-named because they produce a 3-carbon compound as the first product of carbon fixation. These plants tend to be inefficient in hot and dry conditions.

Step 2: Recognizing Stomata Function

Stomata are small openings on plant leaves that allow gas exchange. When stomata close, typically on hot, dry days, carbon dioxide intake is reduced, impacting photosynthesis.

Step 3: Analyzing the Process of Photosynthesis

Photosynthesis in C3 plants utilizes carbon dioxide to produce glucose. Reduced carbon dioxide intake due to closed stomata leads to an alternative process instead of normal photosynthesis.

Step 4: Identifying Photorespiration

Photorespiration occurs when the enzyme RuBisCO fixes oxygen instead of carbon dioxide, often when stomata close. This process is energy-expensive and produces no sugars, making it inefficient for sugar production.

Key Concepts

C3 Plants

C3 plants are the most common type of plants on Earth. They are called "C3" because the first stable product of the carbon fixation process in these plants is a three-carbon compound known as 3-phosphoglycerate. This happens because the enzyme RuBisCO reacts with carbon dioxide during photosynthesis.

• C3 plants thrive in cool, wet environments.
• They can include crops like rice, wheat, and soybeans.
• In these plants, photosynthesis occurs only during the day when light is available.

However, C3 plants become inefficient under hot or dry conditions. When the stomata in C3 plants close to prevent water loss, the intake of carbon dioxide is reduced. This inefficiency often leads to the process of photorespiration, which is less effective in sugar production.

Photosynthesis

Photosynthesis is a vital process through which plants, algae, and some bacteria convert light energy into chemical energy. This chemical energy is stored in the form of glucose, which is a type of sugar. The overall photosynthesis process can be summarized with the equation: \[ 6CO_2 + 6H_2O + light ightarrow C_6H_{12}O_6 + 6O_2 \] In simpler terms, plants take in carbon dioxide and water, absorb sunlight, and produce glucose and oxygen.

• The process occurs mainly in the chloroplasts of plant cells.
• Photosynthesis includes two main stages: the light-dependent reactions and the Calvin cycle (or light-independent reactions).

It's important to note that the efficiency of photosynthesis can be severely impacted by environmental factors. For C3 plants, the presence of more oxygen and less carbon dioxide—often due to closed stomata on dry days—leads to photorespiration, which is an alternative, less efficient pathway.

Carbon Fixation

Carbon fixation is a critical component of photosynthesis where inorganic carbon dioxide is converted into organic compounds within a plant. This process essentially "fixes" carbon from its gaseous form to a solid form that can be used by the plant.

• In C3 plants, carbon fixation begins in the Calvin cycle, with the enzyme RuBisCO catalyzing the reaction between carbon dioxide and ribulose bisphosphate (RuBP).
• This reaction forms a three-carbon molecule, 3-phosphoglycerate, the hallmark of C3 photosynthesis.

However, when stomata close, carbon dioxide levels inside the leaf fall, oxygen levels rise, and RuBisCO may fix oxygen instead. This starts the photorespiration process, which consumes energy but does not produce sugars. Thus, efficient carbon fixation is central to maintaining high levels of photosynthesis and sugar production in plants.