Problem 7 - Campbell Biology Concepts & Connections

Question

Why is it difficult for \(\mathrm{C}_{3}\) plants to carry out photosynthesis in very hot, dry environments such as deserts? a. The light is too intense and destroys the pigment molecules. b. The closing of stomata keeps \(\mathrm{CO}_{2}\) from entering and \(\mathrm{O}_{2}\) from leaving the plant. c. They must rely on photorespiration to make ATP. d. \(\mathrm{CO}_{2}\) builds up in the leaves, blocking carbon fixation.

Step-by-Step Solution

Answer

b. The closing of stomata keeps \(\text{CO}_2\) from entering and \(\text{O}_2\) from leaving the plant.

Step 1: Understand the characteristics of \(\text{C}_3\) plants

\(\text{C}_3\) plants are the most common type of plants that use the Calvin cycle for carbon fixation. They tend to have issues in hot, dry climates.

Step 2: Analyze the environmental conditions

In desert-like conditions, where it is hot and dry, plants close their stomata to reduce water loss. This action, however, also affects gas exchange.

Step 3: Consider the impact on gas exchange

When stomata are closed, \(\text{CO}_2\) cannot enter the leaves, and \(\text{O}_2\) cannot leave. This disrupts the normal process of photosynthesis, as carbon dioxide is required for the Calvin cycle.

Step 4: Evaluate each option

a. The intensity of light does generally not destroy pigment molecules since plants have mechanisms to protect themselves.b. The closing of stomata indeed prevents \(\text{CO}_2\) from entering and \(\text{O}_2\) from leaving. This makes option (b) a very strong candidate.c. Photorespiration does use energy, but it is not the primary reason photosynthesis is impaired.d. \(\text{CO}_2\) does not build up in the leaves under these conditions; rather, it is diminished.

Step 5: Conclude with the correct answer

The correct answer is (b). In very hot, dry environments, \(\text{C}_3\) plants have difficulty carrying out effective photosynthesis primarily because the closing of stomata keeps \(\text{CO}_2\) from entering and \(\text{O}_2\) from leaving the plant.

Key Concepts

Photosynthesis

Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize foods from carbon dioxide and water. It is a fundamental biological process that sustains life on Earth. The process involves the chlorophyll in the plant cells capturing light energy to produce glucose and oxygen. This transformation takes place in two main stages: the light-dependent reactions and the Calvin cycle.
The light-dependent reactions convert solar energy into chemical energy in the form of ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). These energy molecules are then used in the Calvin cycle to convert carbon dioxide into glucose.
For C3 plants, the Calvin cycle takes place in the chloroplasts and is the primary pathway for carbon fixation. This process is efficient under normal conditions but can be problematic in specific environments such as hot and dry climates.

Carbon Fixation

Carbon fixation is the process of converting inorganic carbon dioxide into organic compounds within the plant cells. This is a crucial step in the photosynthesis process. In C3 plants, carbon fixation happens through the Calvin cycle, where the enzyme RuBisCO incorporates carbon dioxide into a five-carbon sugar called ribulose biphosphate (RuBP).
However, RuBisCO is not efficient in high oxygen and low carbon dioxide conditions. In hot and dry environments, plants close their stomata to minimize water loss, reducing the carbon dioxide uptake while increasing oxygen concentration. This can lead to a process called photorespiration, where RuBisCO starts binding oxygen instead of carbon dioxide, causing a loss of energy and decrease in photosynthesis efficiency.
This challenge of carbon fixation in unfavorable conditions makes it difficult for C3 plants to thrive in hot and dry climates.

Stomata Function

Stomata are tiny openings on the surface of leaves that regulate gas exchange. They allow carbon dioxide to enter the leaf for photosynthesis and oxygen to exit as a by-product. Stomata also facilitate the release of water vapor from plant tissues to the atmosphere—a process known as transpiration.
In hot and dry environments, plants must balance the need for carbon dioxide with the risk of losing too much water. Plants close their stomata to conserve water, but this also means that carbon dioxide cannot enter the leaves and oxygen cannot leave. This closure disrupts the plant’s ability to carry out photosynthesis efficiently, as the Calvin cycle cannot proceed without sufficient carbon dioxide.
Thus, the function of stomata is vital but also a limiting factor for photosynthesis under extreme environmental conditions.

Hot Dry Environments

Hot and dry environments present significant challenges for C3 plants. The main issue is water conservation, as these plants need to minimize water loss while still maintaining essential physiological processes. When temperatures are high and the air is dry, the risk of dehydration increases. Plants respond by closing their stomata to limit water loss through transpiration.
However, the closing of stomata restricts the influx of carbon dioxide and the release of oxygen. This leads to a reduction in the photosynthetic rate since carbon dioxide is a key substrate for the Calvin cycle. Additionally, high temperatures can increase the rate of photorespiration, where oxygen is used instead of carbon dioxide, wasting energy and reducing glucose production.
These combined factors illustrate why C3 plants struggle to carry out photosynthesis efficiently in desert-like conditions. Adapting to such environments requires specialized mechanisms that are typically seen in other types of plants, such as C4 and CAM plants, which evolved different strategies to minimize the adverse effects of hot, dry climates.