Photosynthesis

Alternative methods of carbon fixation in hot, arid climates

Your goals◊ Explain how C4 and CAM plants

have adapted to live in hot, windy climates

You may want to set up your notes like this

C3 Plants C4 Plants CAM Plants

Plant evolutionary historyLife began in water

The first plants were aquatic

Since plants evolved to living out of water, they have had a hard time dealing with dehydration

Plant adaptationsMost adaptations involve some sort of

trade offFor plants, the trade off with

photosynthesis is that they lose water to the environment through transpiration

A little plant anatomyThe openings on leaves are called stomataThe openings on leaves are called stomataThis is where CO2 and O2 can enter and exit

the plantWater is also lost through the stomataPlants often close the stomata on very hot Plants often close the stomata on very hot

days, but COdays, but CO22 intake is cut off intake is cut off

C3 plants

Most plantsFix carbon by Fix carbon by

attaching COattaching CO22 to RuBP to RuBP

“3” comes from the 3-carbon molecule produced after CO2 is fixed

C3 PlantsOn hot days, they close their stomata

part wayProduce little sugar then

C3 plantsAs the COAs the CO22 that the plant does have that the plant does have

gets used up, rubisco will fix Ogets used up, rubisco will fix O22 instead instead and send it into the Calvin cycle instead of CO2

Peroxisomes & mitochondria split the new compound and release CO2

This is called photorespiration

Occurs in the light

Consumes oxygen and releases CO2

Photorespiration uses ATP but makes no ATP

Photorespiration makes no sugars

Photorespiration Photorespiration decreases decreases photosynthesis photosynthesis outputoutput

So why would any plant do

this…Photorespiration may be Photorespiration may be an evolutionary leftoveran evolutionary leftover

In early atmosphere, with little O2, it didn’t matter if Rubisco had an affinity for O2

Today, with so much O2 in the atmosphere, it is inevitable that some O2 will be fixed instead of CO2

C3 Crop plantsRice, wheat, soybeansRice, wheat, soybeansMay lose as much as 50% of the

Carbon fixed in the Calvin Cycle to photorespiration

3 ThingsWhat are 3 things you know for sure

about C3 plants?

C4 plantsC4 plants have

adaptations that allow them to minimize the effects of photorespiration

Many are tropical grasses

C4 plants

Have an alternate means of fixing carbon

C4 plants fix carbon out in the cytoplasm before it enters the Calvin Cycle

“4” comes from the 4-carbon compound that is formed when CO2 is fixed in C4 plants

C4 plant adaptations1st step is binding CO2 to PEP

(phosphoenolpyruvate) by the enzyme PEP carboxylase to make a 4-C compound: oxaloacetate

PEP has a high affinity for CO2 and none for O2

2. The 4-carbon compound enters photosynthetic cellsphotosynthetic cells

CO2 is released from PEP and the Calvin cycle continues as normal

This adaptation allows C4 plants to keep a high concentration of CO2 in the photosynthetic cells, preventing Rubisco from binding to O2 instead of CO2

C4 plants thrive in hot climates where stomata will be closed often

Some important C4 plants are sugarcane, corn, tropical grasses

3 ThingsWhat are 3 things you know for sure

about C4 plants?

CAM plantsAnother alternate method of carbon

fixationCommon in succulents like cacti

CAM plantsCAM plants keep stomata closed during

the day to minimize water lossOnly open stomata at night

Closed stomata during the day means no CO2

entering the plant during the day

CAM plantsWhen stomata are

open at night, CAM plants take in CO2 and fix it to PEP

This is called crassulacean acid metabolism

Crassulaceae is the name of the family of plants that include succulents

CAM plantsCells of CAM plants store

the organic molecules that CO2 was fixed to in their vacuoles until morning

Light reactions then produce ATP and NADPH to drive the Calvin Cycle

The CO2 in the organic molecules is released for use in the Calvin Cycle

CAM plantsImportant CAM plants include pineapple

and agave

3 ThingsWhat are 3 things you know for sure

about CAM plants?

Comparison of C3, C4, and CAM plantsC3 plants C4 plants CAM plants

Most plantsTropical grasses like corn, sugarcane

Succulents, pineapple, agave

Fix carbon in Calvin cycle - attach CO2 to RuBP

Fix carbon in cytoplasm - attach CO2 to PEP

Fix carbon at night only, fix it to organic molecules

Enzyme - Rubisco Enzyme – PEP-ase Enzyme – PEP-ase

Most energy efficient method

1/2 way between these two

Best water conservation

Loses water through photorespiration

Loses less water Loses least water