concept 10.5: alternative mechanisms of carbon fixation have evolved in hot arid climates

what is the biggest issue for all terrestrial plants

dehydration

what is the trade off most plants do to conserve water

they must balance betweeb photosynthesis and water conservation

stomata

control thing coming in and out of the leaf

what happens when a stomata remains closed

it can conserve H2O but limit photosynthesis and reduce CO2 levels while causing a build up of O2 levels

why is a build up of O2 bad for plants

oxygen is oxidizing

C3 plants

plants that initially fixate CO2 via rubisco and create a three-carbon compound called 3-phosphoglycerate

example of C3 plants

rice, wheat, soybeans

what happens in photorespiration

rubisco binds with O2 instead of CO2 in the calvin cycle and produces a two carbon compound

why is photorespiration “costly”

because it consumes O2 and organic fuel without outputting any ATP or sugar

what is strange about rubisco

it is an enzyme that evolved when the atmosphere had more CO2 than O2 and it has still not evolved to exclude O2 from binding to it

what does photorespiration take in and put out

takes in O2 and organic fuel and puts out CO2

what is a benefit of photorespiration

it can provide some protection from the damaging products of the light reactions that build up when the Calvin cycle slows due to low CO2

why is photorespiration bad in most plants

on a hot dry day it can drain up to 50% of the carbon fixed by the calvin cycle

what have some plants done in response to photorespiration

they had evolved to have alternate modes of carbon fixation that minimize photorespiration

C4 plants

minimize the cost of photorespiration by incorporating CO2 into 4-carbon compound (C4) as the first product of the calvin cycle

example of C4 plants

sugar cane and corn

what are the two distinct types of cells in the leaves of C4 plants

bundle sheath cells and mesophyll cells

bundle sheath cells

arranged in tightly packed sheaths around the veins of the leaf

mesophyll cells

loosely packed between the bundle sheath and the leaf surface

what occurs to C4 plants in hot dry weather

they partially close their stomata which conserves water but reduces CO2

where does photosynthesis occur in C4 plants

photosynthesis begins in mesophyll cells, but it completed in bundle sheath cells

first step in C4 plants sugar production

first the production of the four carbon precurors is catalyzed by the enzyme PEP carboxylase in the mesophyll cells

second step of C4 plants sugar production

the four carbon compounds are exported to bundle sheath cells through plasmodesmata

third step of C4 plants sugar production

within the bundle sheath cells, CO2 is released from the four-carbon compound and then used in the calvin cycle

PEP carboxylase

has a higher affinity for CO2 than rubsico and can fix CO2 even when CO2 concentrations are low

what occurs in C4 plants when CO2 is released

pyruvate is formed and transported back to the mesophyll cell to be converted back to the PEP with energy from ATO

what photosystem do bundle sheaths have

they only have PS I and use cyclic electron flow

CAM plants

plants that use crassulacean acid metabolism to fix carbon

example of CAM plants

succulents and pinneapples

what do CAM plants do

open their stomata at night and incorporate CO2 into organic acids that are stored in the vacuoles of mesophyll cells and close their stomata during the day

what is CAM plants most similar to and why

they are most like C4 plants because they incorporate CO2 into organic intermediates before it enters into the calvin cycle