Alternative Methods of Carbon Fixation Photorespiration C 3

Alternative Methods of Carbon Fixation Photorespiration & C 3 Plants n C 4 Photosynthesis & Plants n CAM & CAM Plants n (pages 168 -172) n

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

Under Hot and Dry Conditions…. n 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 n This leads to PHOTORESPIRATION.

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

n n Rubisco (the enzyme that binds Ru. BP to CO 2 in the Calvin Cycle) can also bind Ru. BP to O 2. When Ru. BP 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 will be returned to the cycle as G 3 P to regenerate Ru. BP

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

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

Photorespiration & C 3 Plants n n n 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? n n 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.

n 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)

C 4 Plants n n 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 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.

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

C 4 Photosynthesis n n 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.

n n n Malate diffuses from the mesophyll cells into bundle-sheath cells through plasmodesmata. 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.

n n n 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. Because there is CO 2 available, rubisco won’t bind to O 2 and photorespiration is minimized.

n n Photorespiration is minimized 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. The process is called C 4 photosynthesis, because the first product of CO 2 fixation is a 4 -carbon molecule (OAA)

CAM Plants n n 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.

n n n 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 is converted into C 4 organic acids (such as malate) using PEP carboxylase.

n n n 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. 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 n n 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.