Alternative Methods of Carbon Fixation in Plants

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  • 0:00 Carbon Fixation in Plants
  • 1:15 Photorespiration
  • 2:20 C4 Photosynthesis
  • 4:44 CAM Plants
  • 6:16 Lesson Summary
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Lesson Transcript
Instructor: Christopher Muscato

Chris has a master's degree in history and teaches at the University of Northern Colorado.

What happens to plants when it gets too hot or dry for the normal cycles of creating energy to keep functioning? In this lesson, we are going to check out a couple alternative solutions adapted by plants to maintain energy production when the need arises.

Carbon Fixation in Plants

Outside of science fiction, plants don't generally eat people. That's Biology 101 for you right there. So if they don't eat, how do plants produce energy? Well, like most autotrophs, or living things that create their own energy, plants rely on a form of carbon fixation; basically this is the process of converting, referred to as 'fixing,' inorganic carbon into organic compounds. In plants, carbon fixation is the first step of a larger three-step process called the Calvin Cycle, which adds carbon dioxide to the carbohydrate glucose in order to make energy. But really, is any of this new information? I mean, we all know that in order to make energy, plants need sunlight, carbon dioxide and water. Wait, where is the water when you are in the desert? This is a rather harsh environment, but there are still plants. How is this possible? Well, for something without a brain, plants are actually pretty clever, and when things get tough, they find alternative methods of carbon fixation to get things going.

Photorespiration

So let's start by looking at some of the challenges plants face in a dry, hot environment, like this. Now under normal conditions, plants take in carbon dioxide through stomata, which are small pores that allow for gas to enter the plant. However, when it gets especially hot and dry, the plants close up their stomata to prevent the water inside from evaporating. This means that there is less CO2 entering the plant. When the concentration of carbon dioxide gets too low, the enzyme responsible for carbon fixation, RuBisCO, stops fixing carbon dioxide and starts fixing oxygen molecules instead. This results in any leftover CO2 actually being released from the plant. We call the process when plants replace CO2 with O2 in carbon fixation, photorespiration. Now the plant can technically still survive this way and still create energy, but it's drastically less. Only about 25% of the normal rate in fact. So it's a problem.

C4 Photosynthesis

If dry, hot, climates lead to photorespiration, how is it that plants are able to keep growing? Well, some plants adapted techniques to minimize photorespiration and optimize the Calvin Cycle in hot climates. One such technique is C4 photosynthesis, in which plants fix CO2 onto extra carbons that are stored and used for energy. Here's how it works; the leaves of C4 plants have an extra system of cells surrounding the veins holding the water, so the stomata can stay open. CO2 enters the stomata, and is then taken in by a mesophyll cell, which fixes CO2 onto a 3-carbon molecule, storing it as a molecule with four carbons. This is where the C4 name comes from. This 4-carbon molecule then must be actively pumped across a thick membrane into a bundle sheath cell, surrounding the vein, where the 4-carbon molecule is separated back into a 3-carbon molecule and a separate CO2 molecule. Which then enters the Calvin Cycle and becomes energy. So why is this two stage process so effective? Basically the thick membrane between the mesophyll and bundle sheath means that oxygen can't just float in. Only four carbon molecules can be actively transported, and this creates an artificial shortage of oxygen inside the bundle sheath cell, where the Calvin Cycle actually occurs. Since there is so much more CO2 than oxygen, the RuBisCo never stops fixing the CO2, and the Calvin Cycle stays very efficient.

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