Photorespiration

Test 3

Calvin Cycle Output

G3P, which is a 3-carbon sugar used to form sucrose (transport sugar) and make starch (stored for later use)

What is the function of Rubisco?

Carbon fixation

Rubisco Enzymatic Activities

Carboxylation: adds CO2 to RuBP and is favored under normal conditions

Photorespiration: oxidizes RuBP by adding O2 and is favored when the stoma is closed in hot conditions (low CO2, high O2)

CO2 and O2 compete for active site on RuBP

Rubisco: Calvin Cycle

A dark reaction/indirectly light-dependent and occurs at low temperatures with a high CO2:O2 ratio. Light-dependent reactions initiate ATP and NADPH to reduce 3-PGA into G3P. Then, ATP and NADPH are converted into ADP and NADP+. Rubisco is regenerated to enable the system to fix more CO2.

Rubisco: Photorespiration

Occurs at high temperatures with a low CO2:O2 ratio. Generates 2 molecules of 3-phosphoglycerate when CO2 is used as a substrate. When O2 is used, Rubisco produces 1 molecule of 3-phosphoglycerate and 1 molecule of 2-phosphoglycolate.

What are the downsides of Rubisco being used in photorespiration?

2-phosphoglycolate inhibits Calvin Cycle enzymes, so it must be recycled back into 3-phosphoglycerate. It also lowers C3 plant productivity due to it being wasteful.

What prevents the formation of phosphoglycerate?

Denaturation of the Rubisco enzyme

C3 Pathway and Plants

The majority of plants produce PGA (3-phosphoglyceric acid) via the Calvin Cycle. Examples include wheat, rice, barley, rye, evergreen, etc. Found in cool, wet, temperate climates.

C4 Pathway and Plants

Fixes CO2 at low concentrations into 4 carbon compounds called oxaloacetate. Examples include sugarcane, corn, pineapple, etc. Found in hot, dry climates.

Compare C4 and CAM

C4: two pathways occur in different cells (ex: corn). Mesophyll cells fix CO2 using PEP carboxylase and transfer the fixed carbon to sheath cells where the Calvin Cycle occurs, concentrating CO2 around Rubisco.

CAM: C4 pathway at night (fixes CO2), C3 pathway during the day (releases CO2) (ex: succulents)

Both use C3 and C4 pathways