Alternative Methods of Carbon Fixation Photorespiration & C3 Plants C4 Photosynthesis & Plants CAM & CAM Plants

Photorespiration & C3 Plants Remember the STOMA? Stomata allow for plants to take in CO2, release O2 and H2O. O2

Transpiration Transpiration: is the loss of H2O from the plant. It has a cooling effect that prevents plant leaves from overheating and denaturing enzymes required for photosynthesis. When a molecule of water is lost from a stoma, it creates a transpiration pull that moves water, minerals and other substances from the roots to leaves where they are used.

Under Hot and Dry Conditions…. Guard cells close the stomata (or decrease its size) to conserve water. → H2O can’t get out. → O2 can’t get out. → CO2 can’t get in. ↓↓↓ CO2 and ↑↑↑ O2 This leads to PHOTORESPIRATION.

Photorespiration C3 plants (i.e. soybeans, and sunflowers) use the Calvin Cycle to fix carbon. CO2 is required for the Calvin Cycle. Photorespiration is a process in which O2 is used to produce CO2. HOW???

Rubisco (the enzyme that binds RuBP to CO2 in the Calvin Cycle) can also bind RuBP to O2. When RuBP binds to O2 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 CO2. Some will be returned to the cycle as G3P to regenerate RuBP

No (net) sugar produced RuBP is used up ( a little regenerated) Normal Conditions Sugars are produced Cycle regenerated Photorespiration CO2 is produced No (net) sugar produced RuBP is used up ( a little regenerated) No ATP produced

Photorespiration & C3 Plants The CO2 produced can be used for photosynthesis BUT overall, photorespiration decreases photosynthetic output. It siphons materials from the Calvin Cycle and produces little CO2.

***Note, photorespiration will regenerate some RuBP but it will also use RuBP reserves (because the RuBP is providing the carbon to make sugar in photorespiration) So, the plant will eventually use up all its RuBP. If CO2 becomes available, there will be no RuBP for the Calvin Cycle.

Photorespiration & C3 Plants Under normal conditions, 20% of fixed carbon is lost to photorespiration. The optimum temperature for photorespiration is 30ºC – 40ºC. The optimum temperature for photosynthesis is 15ºC – 25ºC.

Why do Plants Undergo Photorespiration? Hypothesis: Rubisco evolved when the Earth’s atmosphere was rich in CO2 and poor in O2, so it did not matter that rubisco also had oxigenase activity. Over evolutionary time, as O2 levels increased, plants did NOT evolve a modified enzyme that would only bind to CO2 and not O2.

However, some plant species have evolved alternative mechanisms of carbon fixation that effectively suppress the rate of photorespiration. 1. C4 photosynthesis 2. CAM (Crassulacean Acid Metabolism)

C4 Plants C4 plants including sugar cane, corn, and many grasses, undergo C4 photosynthesis. C4 plants have a unique leaf anatomy that facilitates this form of photosynthesis. C4 plant leaves contain two types of photosynthetic cells: bundle sheath cells and mesophyll cells. Chloroplasts in C4 plants are concentrated in the bundle sheath cells.

C4 Plants

C4 Plant Leaf Substances can move from mesophyll to bundle-sheath cells via plasmodesmata: cell-cell connections

C4 Photosynthesis In the cytoplasm (NOT the chloroplast) of mesophyll cells, the enzyme PEP carboxylase catalyzes the reaction of CO2 and PEP to form the 4-carbon molecule oxaloacetate (OAA). OAA is converted into the 4-carbon acid malate.

Malate diffuses from the mesophyll cells into bundle-sheath cells through plasmodesmata. Malate converts into CO2 and 3-carbon pyruvate. Pyruvate diffuses back into the mesophyll to regenerate PEP, and CO2 enters the Calvin cycle to be catalyzed by rubisco and produce sugar.

Since the Calvin Cycle is localized to the bundle-sheath cells, CO2 is continuously pumped into the bundle-sheath chloroplasts from surrounding mesophyll cells via malate and the C4 pathway. The concentration of CO2 is increased and rubisco is saturated with CO2. Because there is CO2 available, rubisco won’t bind to O2 and photorespiration is minimized.

Photorespiration is minimized Sugar production is maximized. C4 photosynthesis uses almost TWICE the amount of ATP (compared to C3 photosynthesis) BUT without it, photorespiration would stress the plant. The process is called C4 photosynthesis, because the first product of CO2 fixation is a 4-carbon molecule (OAA)

CAM Plants 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.

CAM Plants

Closing the stomata during the day prevents water loss, but also prevents CO2 from entering the leaves. At night, the stomata open to allow the intake of CO2. CO2 is converted into C4 organic acids (such as malate) using PEP carboxylase.

The 4-carbon organic acids are stored in the vacuole until the morning. When the stomata close in the morning, the organic acids release CO2 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.

In C4 plants, 1st 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.

C4 and CAM The C4 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 CO2 to the Calvin Cycle.