The Dark ReactionThe Dark Reaction

- light-independent

- energy stored in ATP and NADPH (from light reaction) is used to reduce CO2 to sugar

Three independent ways to reduce CO2 to make sugar:

1. the Calvin cycle (C3),

2. C4 photosynthesis,

3. crassulacean acid metabolism (CAM).

The Calvin CycleThe Calvin Cycle1.1. COCO22 is fixed by rubisco is fixed by rubisco

1.1. COCO22 + RuBP + RuBP unstable C6 unstable C6 2 2 PGAPGA

2.2. Reduction of COReduction of CO22 to make G3P to make G3P- Uses ATP and NADPHUses ATP and NADPH- G3P is exported to cytoplasm to make starch, G3P is exported to cytoplasm to make starch,

sucrose, oilssucrose, oils

3.3. Regenerating RuBPRegenerating RuBP1.1. - for enery 12 molecules of G3P - for enery 12 molecules of G3P

made in the Calvin cycle made in the Calvin cycle twotwo are “released” are “released”2.2. - the Calvin cycle needs to “turn” 6 - the Calvin cycle needs to “turn” 6

times to make one glucose!!!times to make one glucose!!!

G3PG3P

- one-third forms starch

- two-thirds are converted to sucrose and then hydrolyzed in other parts of plant into glucose and fructose

- Ultimately used as a source of C for nucleic acids, amino acids, fats…

RubiscoRubisco

● Ribulose bisphosphate carboxylase oxygenase

● fixes CO2 & O2

● Enzyme in Calvin Cycle (1st step)● Most abundant protein on Earth

– About 50% total plant protein!

Stomata - lungsStomata - lungs openings on the surface

of the leaf that allow the exchange of gases between air spaces in the leaf and the atmosphere

Guard cells – control the size of the

stoma in response to environmental conditions

● The size of the guard cell changes when water moves into or out of the cell

● K+ ions are actively pumped into the guard cell and water follows by osmosis

● Light, and CO2 concentration affect the movement of K+ ions into the cells

● Generally stomata are open during the day and closed at night

PhotorespirationPhotorespiration

● the reaction of RuBP with oxygen, reduces the efficiency of photosynthesis

● rubisco is inefficient: “fixes” O2, as well as CO2

● C3 plants lose 20% of their energy to fix one CO2

● this gets worse with heat!

- Under hot and dry conditions Under hot and dry conditions (daytime) plants will close their (daytime) plants will close their stomata to prevent water lossstomata to prevent water loss

- This causes a build up of oxygen This causes a build up of oxygen since COsince CO22 can’t enter…so MORE can’t enter…so MORE photorespirationphotorespiration

Avoiding PhotorespirationAvoiding Photorespiration● C3C3

– The majority of plantsThe majority of plants

● C4C4– COCO22 temporarily stored as 4-C organic acids resulting in temporarily stored as 4-C organic acids resulting in

more more efficient C exchange ratemore more efficient C exchange rate– Advantage in high light, high temperature, low COAdvantage in high light, high temperature, low CO22– Many grasses and crops (e.g., corn, sorghum, millet, Many grasses and crops (e.g., corn, sorghum, millet,

sugar cane)sugar cane)

● CAMCAM– Stomata open during nightStomata open during night– Advantage in arid climatesAdvantage in arid climates– Many succulents (e.g. cacti)Many succulents (e.g. cacti)

Fig. 10.21

Comparison of Photosynthesis in C3 and C4 Plants

VARIABLE C3 PLANTS C4 PLANTS

Photorespiration

Extensive Minimal

Perform Calvin cycle?

Yes Yes

Primary CO2 acceptor

RuBP PEP

CO2-fixing enzyme

Rubisco (RuBP carboxylase/oxygenase)

PEP carboxylase and rubisco

First product of CO2 fixation

3PG (3-carbon compound)

Oxaloacetate (4-carbon compound)

Affinity of carboxylase for CO2

Moderate High

Photosynthetic cells of leaf

Mesophyll Mesophyll + bundle sheath

Classes of chloroplasts

One Two

- C3 photosynthesis: about 3.5 billion C3 photosynthesis: about 3.5 billion years ago,years ago,

- C4 plants appeared about 12 million C4 plants appeared about 12 million years ago.years ago.

- A possible factor in the emergence A possible factor in the emergence of the C4 pathway is the decline in of the C4 pathway is the decline in atmospheric CO2 atmospheric CO2

CAM PLANTSCAM PLANTS- CAM is similar to C4

- CO2 is fixed to a 4-carbon compound

- Separated by time rather than space:

At night cooler and water loss is minimized stomata open and CO2 is fixed in mesophyll cells to

form the 4- carbon oxaloacetate, which is converted into malic acid.

During the day when the stomata close to reduce water loss, the

accumulated malic acid is shipped to the chloroplasts to form sugars

Fig. 10.22

CAM Plants

Global Environmental Change & Photosynthesis:

C3 vs. C4 vs. CAM

● Increasing CO2 ● Increasing chronic and acute temperatures● Changes in water

*At high CO2, C3 more efficient than C4 at all temps.(photosynthesis only, not other processes)