The Calvin Cycle - Lecture notes 8-9 PDF

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The Calvin Cycle The Calvin cycle is the second portion of photosynthesis, and it is sometimes referred to as the light independent reactions because it does not use light energy directly. However, it relies upon ATP and NADPH; therefore, the Calvin cycle occurs alongside the light dependent reactions. The Calvin cycle fixes inorganic carbon dioxide into organic glucose sugars. Photosynthetic leaves will obtain carbon dioxide from the atmosphere through their stomata. Then, the carbon dioxide will diffuse into mesophyll cells, and finally into their chloroplasts. The actual location where the Calvin cycle occurs is in the chloroplast stroma. In all, the Calvin cycle must accept six carbon dioxide molecules to synthesize one glucose molecule. There are four general stages the Calvin cycle goes through before it can make a single glucose molecule.

The first stage of the Calvin cycle is called carbon fixation. Here, carbon dioxide combines with ribulose-1,5-bisphosphate (RuBP), which has five-carbons. This reaction is catalyzed by an enzyme called RuBP carboxylase/oxygenase (RuBisCo). Interestingly, RuBisCo is the most common protein in the world because it is found wherever plants are photosynthesizing. The above carbon fixation reaction produces an organic molecule with six-carbons. This molecule is not stable, so it ends up splitting into phosphoglycerate (PGA) molecules, which contain three-carbon atoms each. Note: ‘normal’ photosynthesis can also be called C₃ photosynthesis because the carbon fixation step of the Calvin cycle makes three-carbon molecules.

The second stage of the Calvin cycle is where reduction takes place. ATP from the light dependent reactions will phosphorylate the PGA made above to produce another three-carbon intermediate. As soon as this intermediate is made, NADPH (again, from the light dependent reactions) reduces it to make glyceraldehyde-3-phosphate (G-3-P) sugars. Some G-3-P will be used to regenerate RuBP so the cycle can continue. This also requires ATP from the light dependent reactions. Alternatively, some G-3-P molecules will be saved for the next stage.

Carbohydrate (glucose) synthesis is the next and ‘final’ stage of the Calvin cycle. It can occur for every two G-3-P molecules saved. This is because G-3-P is a three-carbon sugar, while glucose is a six-carbon sugar. Remember, this is a cycle, so it does not end whenever a glucose is made. The G-3-P used to regenerate RuBP in the previous stage keeps the cycle going, assuming there is more carbon dioxide to fix.

Photorespiration RuBisCo is actually called RuBP carboxylase/oxygenase. This is because the enzyme has two different functions. In the Calvin cycle, RuBisCo will bind to carbon dioxide and carboxylate RuBP. This reaction fixes inorganic carbon (from carbon dioxide) into an organic molecule, leading to carbohydrate synthesis. RuBisCo can also bind oxygen, even though it has a much higher affinity for carbon dioxide. When this happens, RuBisCo will oxygenate RuBP and derail the Calvin cycle. This process is called photorespiration, and it causes the cell to undergo oxygen fixation. This process reduces the cell’s supply of fixed carbon and wastes energy. Let’s learn a little more about why this is the case. First things first, why would RuBisCo bind oxygen if its affinity for carbon dioxide is so much higher? The answer is related to the environmental conditions in which the cell finds itself. Under moderate conditions (not too hot or dry), leaves will keep their stomata open. This allows carbon dioxide to flow into the leaf as oxygen is flowing out. Therefore, the amounts of carbon dioxide in the leaf are greater than oxygen. Transpiration is when water evaporates out of the stomata, potentially desiccating the plant. When plants find themselves in hot and dry environments, they will close their stomata to minimize transpiration. In this way, gases are prevented from entering and exiting the leaf and the ratio of carbon dioxide to oxygen gets thrown off. So, when it is hot and dry, RuBisCo is encouraged to bind oxygen and photorespiration starts to occur in the chloroplast stroma. Here, RuBP is oxygenated to produce two molecules. The first is a three-carbon PGA molecule, which we saw in the normal Calvin cycle. The other molecule contains two-carbons, and it is called phosphoglycolate. DAT Pro-Tip: sometimes photorespiration will be called C₂ photosynthesis because the oxygen fixation step produces a two-carbon molecule not seen in C₃ photosynthesis. Unfortunately, phosphoglycolate can't enter the Calvin cycle, and the fixed carbon atoms it contains are essentially wasted. To battle this, mesophyll cells will spend significant energy trying to get these fixed carbon atoms incorporated back into the cycle. They seek to accomplish this by shuttling the molecule to the peroxisomes and mitochondria.

Through a series of reactions involving two phosphoglycolate molecules, the cells make a three-carbon molecule that can be converted back to PGA at the chloroplast. Unfortunately, one fixed carbon molecule is lost at the mitochondria as carbon dioxide. If RuBisCo were to bind six carbon dioxide molecules to carboxylate RuBP, there would be a net gain of six fixed carbon atoms as one glucose molecule. If this same RuBisCo enzyme were to bind six oxygen molecules to oxygenate RuBP, there would be a net loss of three fixed carbon atoms and no glucose would be made. This would be a problem.

Alternative Pathways The vast majority of photosynthesis is C₃ photosynthesis. Remember, it is called C₃ photosynthesis because the initial products of carbon fixation in the Calvin cycle are three-carbon molecules (PGA). Unfortunately, these plants are handicapped by photorespiration (C₂ photosynthesis) when RuBisCo fixes oxygen into RuBP. Plants have evolved to utilize different mechanisms of photosynthesis to reduce photorespiration. The two most important alternative pathways for the DAT are called C₄ photosynthesis and crassulacean acid metabolism (CAM) photosynthesis. C₄ photosynthesis uses an enzyme called PEP carboxylase, which has virtually no affinity for oxygen. PEP carboxylase fixes inorganic carbon dioxide into a three-carbon molecule called PEP. This carboxylation reaction produces oxaloacetate, which quickly turns into malate at the mesophyll cell. DAT Pro-Tip: both oxaloacetate and malate are four carbon compounds, hence the name C₄ photosynthesis. The malate will then be transferred from the mesophyll cells (where the PEP carboxylase reaction occurred) to the bundle sheath cells. Bundle sheath cells surround the vascular bundles of plants, where oxygen concentration is much lower. Here, malate can be decarboxylated to release inorganic carbon dioxide and a three-carbon pyruvate molecule. The carbon dioxide can now undergo the conventional Calvin cycle (with RuBisCo) in an environment where oxygen is not as prevalent. Therefore, RuBisCo will have a lower risk of photorespiration. Unfortunately, C₄ plants will need to spend energy to shuttle pyruvate back to the mesophyll cells. This energy comes from the hydrolysis of ATP to AMP, essentially equal to spending two ATPs. Once back at a mesophyll cell, pyruvate can be converted to PEP for more C₄ cycles.

C₄ photosynthesis occurs in a small percentage of plants living in hot environments. It allows them to prevent photorespiration through the spatial isolation of carbon dioxide. Spatial isolation means that inorganic carbon is transported to a different location within the cell to prevent photorespiration. Thus, carbon dioxide is sent to a location where oxygen will not compete with it for RuBisCo.

A small percentage of plants can undergo CAM photosynthesis. CAM plants use temporal isolation, which prevents photorespiration through timing. Here, processes occur in the same part of the leaf, but the plant does different processes at different times. During the day, CAM plants close their stomata to prevent transpiration. Remember, transpiration is when water evaporates out of the stomata, potentially desiccating the plant. This prevents gases from entering the leaf during the day, which sounds like a recipe for photorespiration. Despite this, CAM plants are still able to photosynthesize. At night, CAM plants have their stomata open, allowing carbon dioxide to enter the leaf. After diffusing into the mesophyll cells, PEP carboxylase will fix inorganic carbon dioxide into PEP. This is a carboxylation reaction that produces oxaloacetate, which converts to malate just like in C4 photosynthesis....