Photosynthesis Part 5: C4 and CAM

Summary

This video explores alternative pathways of carbon fixation in plants, focusing on C4 and CAM plants. It begins with a review of the Calvin cycle in C3 plants, explaining how carbon dioxide is fixed into a three-carbon molecule, PGA. The challenges of photorespiration, especially in hot, dry conditions, are highlighted. The C4 pathway is introduced as a solution, utilizing a two-step process involving different cell types to effectively fix carbon while preventing oxygen interference. Finally, the video touches on CAM plants, which fix carbon at night to minimize water loss during the day. Overall, it demonstrates how plants adapt to various environmental conditions for efficient photosynthesis.

Takeaways

• 😀 Photosynthesis begins with the Calvin cycle, where carbon dioxide is fixed into a three-carbon molecule called PGA (phosphoglycerate).
• 😀 C3 plants fix carbon dioxide through rubp, but this process can lead to photorespiration when oxygen binds to rubp instead of carbon dioxide.
• 😀 The stomata in leaves allow gas exchange, but they also lead to water loss, which can be problematic in hot, dry conditions.
• 😀 To minimize water loss, plants close their stomata during the day, especially under heat stress, but this increases the risk of photorespiration.
• 😀 Photorespiration occurs when rubp binds with oxygen instead of carbon dioxide, reducing the efficiency of photosynthesis.
• 😀 C4 plants have a unique anatomical structure with mesophyll and bundle sheath cells, which helps them minimize photorespiration.
• 😀 The C4 pathway involves two stages of carbon fixation: the first occurs in mesophyll cells with PEP, and the second in bundle sheath cells with rubp.
• 😀 PEP (phosphoenolpyruvate) in C4 plants preferentially binds with carbon dioxide, avoiding oxygen and ensuring efficient carbon fixation.
• 😀 The C4 pathway enables efficient carbon fixation in environments with high oxygen levels and low carbon dioxide levels, preventing photorespiration.
• 😀 CAM (Crassulacean Acid Metabolism) plants fix carbon dioxide at night and store it as malate, allowing them to keep stomata closed during the day and minimize water loss.

Q & A

What is the primary function of the Calvin cycle in photosynthesis?

The primary function of the Calvin cycle is to fix carbon dioxide into an organic molecule, specifically PGA (phosphoglycerate), by binding it with RUBP (ribulose bisphosphate), and ultimately produce sugars used by the plant for energy.

What is photorespiration, and why is it a problem for plants in hot and dry conditions?

Photorespiration occurs when RUBP binds with oxygen instead of carbon dioxide, which reduces the efficiency of photosynthesis. In hot and dry conditions, when carbon dioxide levels drop and oxygen levels rise inside the leaf, photorespiration increases, drastically decreasing photosynthesis rates.

How do plants reduce water loss through their stomata?

Plants reduce water loss by closing their stomata during hot and dry conditions, especially at night. This helps conserve water but limits the exchange of gases like oxygen and carbon dioxide, which are essential for photosynthesis.

What is the function of the cuticle on the leaf surface?

The cuticle is a waxy coating that helps prevent water loss from the leaf, keeping moisture in and protecting the plant from excessive dehydration.

How do C4 plants differ anatomically from C3 plants?

C4 plants have a unique leaf anatomy with mesophyll cells surrounding bundle sheath cells. This structure allows them to concentrate carbon dioxide in the bundle sheath cells, minimizing photorespiration and improving carbon fixation efficiency.

What role does PEP (phosphoenolpyruvate) play in the C4 pathway?

In the C4 pathway, PEP binds with carbon dioxide to form oxaloacetate, a four-carbon molecule. This process occurs in the mesophyll cells and helps prevent oxygen from interfering with carbon fixation by RUBP in the Calvin cycle.

What happens to the oxaloacetate produced in the C4 pathway?

Oxaloacetate is converted into malate, another four-carbon molecule. Malate then moves to the bundle sheath cells, where it is broken down to release carbon dioxide, which is fixed by RUBP in the Calvin cycle.

How do CAM plants adapt to extreme heat and drought conditions?

CAM (Crassulacean Acid Metabolism) plants adapt by opening their stomata only at night to fix carbon dioxide, storing it as malate. During the day, they close their stomata and use the stored malate to carry out photosynthesis while minimizing water loss.

What is the difference between the C4 and CAM pathways?

While both C4 and CAM pathways involve fixing carbon dioxide in a separate step before the Calvin cycle to prevent photorespiration, C4 plants use a unique anatomical structure with mesophyll and bundle sheath cells, while CAM plants rely on temporal separation, fixing carbon dioxide at night and using it during the day.

What happens when stomata are closed during the day in a plant?

When the stomata are closed during the day, carbon dioxide is trapped inside the leaf, and oxygen accumulates due to photosynthesis. However, this also prevents further CO2 intake and reduces the efficiency of photosynthesis if the plant is not adapted to handle it.