C4 photosynthesis

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akimhaneul

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I'm confused about C4 photosynthesis. It says in the cliffs book that they have a higher rate of photosynthesis. They also reduce the time that the stomata are open to reduce the water loss. Apparently, reducing water loss is one of their advantages. But if they reduce the time that stomata are open, how can they have a higher rate of photosynthesis? Don't you need to keep the stomata open to let CO2 in for photosynthesis?

Thanks!

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I'm confused about C4 photosynthesis. It says in the cliffs book that they have a higher rate of photosynthesis. They also reduce the time that the stomata are open to reduce the water loss. Apparently, reducing water loss is one of their advantages. But if they reduce the time that stomata are open, how can they have a higher rate of photosynthesis? Don't you need to keep the stomata open to let CO2 in for photosynthesis?

Thanks!
C4 plants have evolved to thrive in hot climates. They must keep their stomata half closed in order to reduce water loss (transpiration). You are correct in thinking that will reduce the amount of CO2 taken in. However, you must understand that C4 use different mechanism.
They have an enzyme called PEP carboxylase. It has a higher affinity for CO2 than RUBISCO. When this enzyme combines Phosphoenolpyruvate with CO2 it forms Oxaloacetate (4 carbon compound). This mechanism makes CO2 available for Rubisco, but at the cost of ATP. Thus, C4 plants can maintain relatively high CO2 concentration needed for their Calvin cycle without fully opening up their stomata.

Hope this helps.
 
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Hi, just wanted to add that Rubisco also catalyzes a reaction between oxygen and 1,5-bisphosphate which leads to a non-usable product for the plant.. With that in mind, open stomata means that not only CO2 but also O2 can enter and compete to bind at the active site of Rubisco. You can see how that can lead to inefficiency, and with the process mentioned above C4 plants actually have higher rate of photosynthesis.
 
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