Problem 3
Question
How is photosynthesis similar in \(\mathrm{C}_{4}\) plants and CAM plants? (A) In both cases, only photosystem 1 is used. (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
Verified Answer
Option (C): In both cases, rubisco is not used to fix carbon initially.
1Step 1: Understand Photosynthesis in \( \mathrm{C}_{4} \) and CAM Plants
Photosynthesis in both \( \mathrm{C}_{4} \) and CAM plants involves an additional step before the Calvin cycle for carbon fixation. This step helps to minimize photorespiration.
2Step 2: Differences in Carbon Fixation
In \( \mathrm{C}_{4} \) plants, carbon dioxide is initially fixed into a four-carbon compound using PEP carboxylase in the mesophyll cells before being transported to bundle-sheath cells for the Calvin cycle. CAM plants, on the other hand, fix carbon dioxide at night into oxaloacetate (also a four-carbon compound).
3Step 3: Identify the Similarity
In both \( \mathrm{C}_{4} \) and CAM plants, rubisco is not used for the initial fixation of carbon dioxide. Instead, PEP carboxylase is used initially. Rubisco only participates later in the Calvin cycle.
4Step 4: Determine the Correct Answer
Given the information, the correct answer to how photosynthesis is similar in \( \mathrm{C}_{4} \) and CAM plants is that rubisco is not used to fix carbon initially. This corresponds to option (C).
Key Concepts
carbon fixationCalvin cyclephotorespirationPEP carboxylase
carbon fixation
Carbon fixation is the process of converting inorganic CO₂ into an organic molecule. In photosynthesis, this is the first step for both \(\text{C}_4\) and CAM plants. In \(\text{C}_4\) plants, carbon fixation occurs in the mesophyll cells, where CO₂ is incorporated into a 4-carbon molecule called oxaloacetate by an enzyme called PEP carboxylase. The oxaloacetate then moves to the bundle-sheath cells for the Calvin cycle.
In contrast, CAM plants fix carbon at night, turning CO₂ into oxaloacetate which is later stored as malate within vacuoles. During the day, the CO₂ is released from malate and used in the Calvin cycle. This method allows CAM plants to conserve water by keeping their stomata closed during the day.
Both methods are adaptations to reduce photorespiration and efficiently fix carbon in hot and/or dry environments.
In contrast, CAM plants fix carbon at night, turning CO₂ into oxaloacetate which is later stored as malate within vacuoles. During the day, the CO₂ is released from malate and used in the Calvin cycle. This method allows CAM plants to conserve water by keeping their stomata closed during the day.
Both methods are adaptations to reduce photorespiration and efficiently fix carbon in hot and/or dry environments.
Calvin cycle
The Calvin cycle is a critical part of photosynthesis, converting CO₂ and energy (ATP and NADPH) into glucose. It takes place in the stroma of chloroplasts and involves three main stages: carbon fixation, reduction, and regeneration of the starting molecule, ribulose biphosphate (RuBP).
In \(\text{C}_4\) and CAM plants, the Calvin cycle is the same as in C3 plants but occurs after an extra step in carbon fixation. For \(\text{C}_4\) plants, the Calvin cycle happens in the bundle-sheath cells after CO₂ is transported from the mesophyll cells. For CAM plants, the cycle occurs during the day using CO₂ released from stored organic acids. This strategic separation (spatial in \(\text{C}_4\); temporal in CAM) helps these plants thrive in environments with intense sunlight and limited water.
In \(\text{C}_4\) and CAM plants, the Calvin cycle is the same as in C3 plants but occurs after an extra step in carbon fixation. For \(\text{C}_4\) plants, the Calvin cycle happens in the bundle-sheath cells after CO₂ is transported from the mesophyll cells. For CAM plants, the cycle occurs during the day using CO₂ released from stored organic acids. This strategic separation (spatial in \(\text{C}_4\); temporal in CAM) helps these plants thrive in environments with intense sunlight and limited water.
photorespiration
Photorespiration is a process where the enzyme rubisco oxygenates RuBP, leading to a loss of CO₂ and wasted energy, often occurring under high light and heat conditions. This makes it less efficient for plant growth.
\(\text{C}_4\) and CAM plants have evolved mechanisms to minimize photorespiration. \(\text{C}_4\) plants physically separate carbon fixation and the Calvin cycle into different cells, which increases the CO₂ concentration around rubisco and reduces its interaction with oxygen. CAM plants separate these processes temporally by fixing carbon at night, reducing their daytime CO₂ and O₂ competition.
By doing so, these plants reduce photorespiration and increase their efficiency in producing glucose, especially in stressed environments.
\(\text{C}_4\) and CAM plants have evolved mechanisms to minimize photorespiration. \(\text{C}_4\) plants physically separate carbon fixation and the Calvin cycle into different cells, which increases the CO₂ concentration around rubisco and reduces its interaction with oxygen. CAM plants separate these processes temporally by fixing carbon at night, reducing their daytime CO₂ and O₂ competition.
By doing so, these plants reduce photorespiration and increase their efficiency in producing glucose, especially in stressed environments.
PEP carboxylase
PEP carboxylase is an enzyme crucial in the initial carbon fixation step of both \(\text{C}_4\) and CAM photosynthesis. Unlike rubisco, PEP carboxylase has a higher affinity for CO₂ and does not interact with O₂, making it much more efficient in fixing carbon in hot and dry conditions.
In \(\text{C}_4\) plants, PEP carboxylase fixes CO₂ in the mesophyll cells to form oxaloacetate, which is later transported to bundle-sheath cells. In CAM plants, PEP carboxylase fixes CO₂ at night forming oxaloacetate, which is then converted to malate and stored for use during the day.
This enzyme's role is vital for avoiding the ineffectiveness of rubisco in oxygen-rich conditions and significantly helps in reducing photorespiration, thereby allowing plants to conserve energy and resources.
In \(\text{C}_4\) plants, PEP carboxylase fixes CO₂ in the mesophyll cells to form oxaloacetate, which is later transported to bundle-sheath cells. In CAM plants, PEP carboxylase fixes CO₂ at night forming oxaloacetate, which is then converted to malate and stored for use during the day.
This enzyme's role is vital for avoiding the ineffectiveness of rubisco in oxygen-rich conditions and significantly helps in reducing photorespiration, thereby allowing plants to conserve energy and resources.
Other exercises in this chapter
Problem 4
Which of the following statements is a correct distinction between autotrophs and heterotrophs? (A) Autotrophs, but not heterotrophs, can nourish themselves beg
View solution Problem 5
Which of the following occurs during the Calvin cycle? (A) carbon fixation (B) reduction of NADP (C) release of oxygen (D) generation of \(\mathrm{CO}_{2}\)
View solution Problem 6
In mechanism, photophosphorylation is most similar to (A) substrate-level phosphorylation in glycolysis. (B) oxidative phosphorylation in cellular respiration.
View solution