Alternative Methods of Carbon Fixation Photorespiration & C 3 Plants Photorespiration & C 3 Plants C 4 Photosynthesis & Plants C 4 Photosynthesis & Plants CAM & CAM Plants CAM & CAM Plants (pages ) (pages )

Photorespiration & C 3 Plants Remember the STOMA? Remember the STOMA? Stomata allow for plants to take in CO 2, release O 2 and H 2 O. Stomata allow for plants to take in CO 2, release O 2 and H 2 O. O2O2O2O2

Under Hot and Dry Conditions…. Guard cells close the stomata (or decrease its size) to conserve water. Guard cells close the stomata (or decrease its size) to conserve water. → H 2 O can’t get out. → O 2 can’t get out. → CO 2 can’t get in. ↓↓↓ CO 2 and ↑↑↑ O 2 This leads to PHOTORESPIRATION. This leads to PHOTORESPIRATION.

Photorespiration C 3 plants (i.e. soybeans, and sunflowers) use the Calvin Cycle to fix carbon. C 3 plants (i.e. soybeans, and sunflowers) use the Calvin Cycle to fix carbon. CO 2 is required for the Calvin Cycle. CO 2 is required for the Calvin Cycle. Photorespiration is a process in which O 2 is used to produce CO 2. Photorespiration is a process in which O 2 is used to produce CO 2. HOW??? HOW???

Rubisco (the enzyme that binds RuBP to CO 2 in the Calvin Cycle) can also bind RuBP to O 2. Rubisco (the enzyme that binds RuBP to CO 2 in the Calvin Cycle) can also bind RuBP to O 2. When RuBP binds to O 2 it produces a 3- carbon PGA molecule and a 2-carbon GLYCOLATE molecule. When RuBP binds to O 2 it produces a 3- carbon PGA molecule and a 2-carbon GLYCOLATE molecule. Some glycolate will leave the chloroplast and go to the mitochondria to yield CO 2. Some glycolate will leave the chloroplast and go to the mitochondria to yield CO 2. Some will be returned to the cycle as G3P to regenerate RuBP Some will be returned to the cycle as G3P to regenerate RuBP

Normal Conditions Sugars are produced Sugars are produced Cycle regenerated Cycle regeneratedPhotorespiration CO 2 is produced CO 2 is produced No (net) sugar produced No (net) sugar produced

Photorespiration & C 3 Plants The CO 2 produced can be used for photosynthesis BUT overall, photorespiration decreases photosynthetic output. The CO 2 produced can be used for photosynthesis BUT overall, photorespiration decreases photosynthetic output.

Photorespiration & C 3 Plants Under normal conditions, 20% of fixed carbon is lost to photorespiration. Under normal conditions, 20% of fixed carbon is lost to photorespiration. The optimum temperature for photorespiration is 30ºC – 40ºC. The optimum temperature for photorespiration is 30ºC – 40ºC. The optimum temperature for photosynthesis is 15ºC – 25ºC. The optimum temperature for photosynthesis is 15ºC – 25ºC.

Why do Plants Undergo Photorespiration? Hypothesis: Rubisco evolved when the Earth’s atmosphere was rich in CO 2 and poor in O 2, so it did not matter that rubisco also had oxigenase activity. Hypothesis: Rubisco evolved when the Earth’s atmosphere was rich in CO 2 and poor in O 2, so it did not matter that rubisco also had oxigenase activity. Over evolutionary time, as O 2 levels increased, plants did NOT evolve a modified enzyme that would only bind to CO 2 and not O 2. Over evolutionary time, as O 2 levels increased, plants did NOT evolve a modified enzyme that would only bind to CO 2 and not O 2.

However, some plant species have evolved alternative mechanisms of carbon fixation that effectively suppress the rate of photorespiration. However, some plant species have evolved alternative mechanisms of carbon fixation that effectively suppress the rate of photorespiration. 1. C 4 photosynthesis 2. CAM (Crassulacean Acid Metabolism) Metabolism)

C 4 Plants C 4 plants including sugar cane, corn, and many grasses, undergo C 4 photosynthesis. C 4 plants including sugar cane, corn, and many grasses, undergo C 4 photosynthesis. C 4 plants have a unique leaf anatomy that facilitates this form of photosynthesis. C 4 plants have a unique leaf anatomy that facilitates this form of photosynthesis. C 4 plant leaves contain two types of photosynthetic cells: bundle sheath cells and mesophyll cells. C 4 plant leaves contain two types of photosynthetic cells: bundle sheath cells and mesophyll cells. Chloroplasts in C 4 plants are concentrated in the bundle sheath cells. Chloroplasts in C 4 plants are concentrated in the bundle sheath cells.

C 4 Plant Leaf Substances can move from mesophyll to bundle- sheath cells via plasmodesmata: cell-cell connections

C 4 Photosynthesis In the cytoplasm (NOT the chloroplast) of mesophyll cells, the enzyme PEP carboxylase catalyzes the reaction of CO 2 and PEP to form the 4-carbon molecule oxaloacetate (OAA). In the cytoplasm (NOT the chloroplast) of mesophyll cells, the enzyme PEP carboxylase catalyzes the reaction of CO 2 and PEP to form the 4-carbon molecule oxaloacetate (OAA). OAA is converted into the 4-carbon acid malate. OAA is converted into the 4-carbon acid malate.

Malate diffuses from the mesophyll cells into bundle-sheath cells through plasmodesmata. Malate diffuses from the mesophyll cells into bundle-sheath cells through plasmodesmata. Malate converts into CO 2 and 3-carbon pyruvate. Malate converts into CO 2 and 3-carbon pyruvate. Pyruvate diffuses back into the mesophyll to regenerate PEP, and CO 2 enters the Calvin cycle to be catalyzed by rubisco and produce sugar. Pyruvate diffuses back into the mesophyll to regenerate PEP, and CO 2 enters the Calvin cycle to be catalyzed by rubisco and produce sugar.

Since the Calvin Cycle is localized to the bundle-sheath cells, CO 2 is continuously pumped into the bundle-sheath chloroplasts from surrounding mesophyll cells via malate and the C 4 pathway. Since the Calvin Cycle is localized to the bundle-sheath cells, CO 2 is continuously pumped into the bundle-sheath chloroplasts from surrounding mesophyll cells via malate and the C 4 pathway. The concentration of CO 2 is increased and rubisco is saturated with CO 2. The concentration of CO 2 is increased and rubisco is saturated with CO 2. Because there is CO 2 available, rubisco won’t bind to O 2 and photorespiration is minimized. Because there is CO 2 available, rubisco won’t bind to O 2 and photorespiration is minimized.

Photorespiration is minimized Photorespiration is minimized Sugar production is maximized. Sugar production is maximized. C 4 photosynthesis uses almost TWICE the amount of ATP (compared to C 3 photosynthesis) BUT without it, photorespiration would stress the plant. C 4 photosynthesis uses almost TWICE the amount of ATP (compared to C 3 photosynthesis) BUT without it, photorespiration would stress the plant. The process is called C 4 photosynthesis, because the first product of CO 2 fixation is a 4-carbon molecule (OAA) The process is called C 4 photosynthesis, because the first product of CO 2 fixation is a 4-carbon molecule (OAA)

CAM Plants CAM plants are water-storing plants (succulents) such cacti and pineapples. CAM plants are water-storing plants (succulents) such cacti and pineapples. To conserve water, they open their stomata at night and close them during the day – the REVERSE of other plants. To conserve water, they open their stomata at night and close them during the day – the REVERSE of other plants.

Closing the stomata during the day prevents water loss, but also prevents CO 2 from entering the leaves. Closing the stomata during the day prevents water loss, but also prevents CO 2 from entering the leaves. At night, the stomata open to allow the intake of CO 2. At night, the stomata open to allow the intake of CO 2. CO 2 is converted into C 4 organic acids (such as malate) using PEP carboxylase. CO 2 is converted into C 4 organic acids (such as malate) using PEP carboxylase.

The 4-carbon organic acids are stored in the vacuole until the morning. The 4-carbon organic acids are stored in the vacuole until the morning. When the stomata close in the morning, the organic acids release CO 2 molecules to enter the Calvin cycle. When the stomata close in the morning, the organic acids release CO 2 molecules to enter the Calvin cycle. This process is called CAM – Crassulacean Acid Metabolism because it was first discovered in the crassulacean family of plants. This process is called CAM – Crassulacean Acid Metabolism because it was first discovered in the crassulacean family of plants.

In C 4 plants, 1 st part of carbon fixation and the Calvin cycle occur in different compartments. In CAM plants, the steps occur in same compartment, but at different times of the day.

C 4 and CAM The C 4 and CAM pathways present evolutionary solutions to the problem of maintaining photosynthesis when stomata close on sunny, hot, dry, days. The C 4 and CAM pathways present evolutionary solutions to the problem of maintaining photosynthesis when stomata close on sunny, hot, dry, days. Both methods produce organic acids that eventually transfer CO 2 to the Calvin Cycle. Both methods produce organic acids that eventually transfer CO 2 to the Calvin Cycle.