Gb1- Chapter 10

plants

photorespiration

hot, dry days cause the closing of stomata to conserve H2O but also decreases photosyntehsis.

• decreased access to CO2 and builds up O2 concentration = wasteful

rubisco binds with O2 instead of CO2

• costly because it consumes O2 and organic fuel without producing ATP or sugar

a key trade-off in balancing the needs

C3 plants

rubisco fixes carbon to RuBP and forms 3 phosphoglycerate

C4 plants

minimize the cost of photorespiration by incorporating CO2 into a four carbon compound as the first product of the Calvin Cycle.

ex) corn and sugarcane

in hot, dry weather, they partially close their somata to conserve water. this also decreases CO2

photosynthesis begins in mesophyll cells but is completed in bundle sheath cells that are arranged in tightly packed sheaths around the leaf veins

sugar production in C4 plants

1. In mesophyll cells, PEP carboxylase makes oxaloacetate using CO2

• has higher CO2 affinity than Rubisco so it can fix Co2 even when the concentrations are low

2. Oxaloacetate is converted to malate and is exported to bundle sheath cells through plasmodesmata.

3. CO2 is released from four carbon compounds and used in the Calvin cycle. Pyruvate is transported back to the mesophyll cell where one ATP is used to convert it back to PEP. ATP comes from cyclic electron flow

C4 photosynthesis uses less water and resources than C3 photosynthesis

structural separation of Co2 from Calvin cycle

crassulacean acid metabolism (CAM)

succulents conserve water by opening their somata at night to take in CO2 and close during the day. CO2 is for the calvin cycle used during the day

Temporal separation of CO2 from the calvin cycle