Problem 8 - Campbell Biology Concepts & Connections

Question

How is photosynthesis similar in \(\mathrm{C}_{4}\) plants and CAM plants? a. In both cases, the light reactions and the Calvin cycle are sepa. rated in both time and location. b. Both types of plants make sugar without the Calvin cycle. c. In both cases, rubisco is not used to fix carbon initially. d. Both types of plants make most of their sugar in the dark.

Step-by-Step Solution

Answer

c. In both cases, rubisco is not used to fix carbon initially.

Step 1 - Understanding Photosynthesis Types

Understand the basic differences and similarities between C4 and CAM photosynthesis. Both types are adaptations to arid conditions and differ from the typical C3 pathway.

Step 2 - Separation of Light Reactions and Calvin Cycle

The statement in option a says, 'In both cases, the light reactions and the Calvin cycle are separated in both time and location.' This is partially true as the reactions are separated spatially in C4 plants and temporally in CAM plants, but not both.

Step 3 - Sugar Production without the Calvin Cycle

Option b claims, 'Both types of plants make sugar without the Calvin cycle.' This is incorrect because both C4 and CAM plants utilize the Calvin cycle in the final steps of sugar production.

Step 4 - Initial Carbon Fixation

Option c states, 'In both cases, rubisco is not used to fix carbon initially.' This is correct. Both plant types initially fix CO2 into a four-carbon compound using PEP carboxylase, not rubisco.

Step 5 - Sugar Production in the Dark

Option d claims, 'Both types of plants make most of their sugar in the dark.' This is incorrect as photosynthesis requires light to generate the energy needed to produce sugar.

Step 6 - Conclusion

Combining the information, the correct answer is option c: 'In both cases, rubisco is not used to fix carbon initially.'

Key Concepts

C4 Plants

C4 plants have evolved a unique way to fix carbon dioxide, which helps them survive in hot and dry environments. They first capture CO2 into a four-carbon compound (hence the name C4). This process occurs in mesophyll cells and uses an enzyme called PEP carboxylase, which is more efficient than rubisco in fixing CO2. The four-carbon compound then moves to bundle-sheath cells, where the CO2 is released and enters the Calvin cycle to produce sugars. This spatial separation between initial CO2 fixation and the Calvin cycle helps minimize photorespiration, making C4 plants more efficient in hot climates.
Examples of C4 plants include maize, sugarcane, and sorghum.

CAM Plants

CAM (Crassulacean Acid Metabolism) plants take a different approach to carbon fixation to adapt to arid conditions. They open their stomata at night to capture CO2, which is fixed into a four-carbon compound and stored in vacuoles. During the day, when the stomata are closed to reduce water loss, the CO2 is released from these compounds and enters the Calvin cycle to produce sugars. This temporal separation between carbon fixation and the Calvin cycle allows CAM plants to conserve water while still carrying out photosynthesis.
Examples of CAM plants include cacti, succulents, and pineapple.

Carbon Fixation

Carbon fixation is a crucial step in photosynthesis, where inorganic carbon dioxide is converted into organic compounds. In C3 plants, this is done directly by the enzyme rubisco in the Calvin cycle. However, in C4 and CAM plants, carbon fixation initially uses the enzyme PEP carboxylase to form a four-carbon compound. This initial step helps these plants efficiently capture CO2 even in conditions where CO2 is scarce or the risk of water loss is high.
This efficiency is particularly beneficial in hot and arid environments, allowing C4 and CAM plants to thrive where C3 plants might struggle.

Calvin Cycle

The Calvin cycle, also known as the dark reactions or light-independent reactions, is where the actual production of sugars occurs. Regardless of whether a plant is C3, C4, or CAM, the Calvin cycle takes place in the stroma of chloroplasts. CO2 is incorporated into organic molecules through a series of reactions powered by ATP and NADPH produced in the light reactions. This cycle results in the production of glucose, which plants use for energy and building materials.
Though the initial steps of carbon fixation differ in C4 and CAM plants, the Calvin cycle is a shared pathway vital to all plants for synthesizing sugars.