Variations in Photosynthesis Lecture 9 Fall 2008.

Variations in Photosynthesis Lecture 9Fall 2008

Variation in Photosynthesis

Three types of photosynthesis

• C3

• C4

• CAM

Why do plants need three different types of photosynthesis?

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Water Balance in Plants

Transpiration– Loss of water

vapor from the leaf through diffusion and evaporation

• Plants need to balance loss of water to atmosphere with uptake of water from soil

2

Water Loss from LeavesOccurs through stomata

(Path)1. Water vapor in leaf

spaces diffuses out of stomata

• Leaf air spaces are moist, air is dryer

• Driving force: Concentration gradient of water vapor inside vs. outside

2. Liquid water in leaf evaporates to replace water vapor that diffused out

Blue dots = water vapor

3

The role of stomata

Gas Exchange

• CO2 enters, O2 leaves via open stomata

Water Balance• Majority of water loss through

stomata• Stomata typically open during the

day and closed at night• Plant can control whether stomata

are open or closed– Can close stomata during the

day to prevent water loss

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Balance between photosynthesis and water loss

• Stomata need to be open for gas exchange

– No incoming CO2, then low input for Calvin cycle

• Stomata need to be closed to prevent excessive water loss– If water in the soil is limited, and– The day is hot or dry

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Balance between photosynthesis and water loss

C3

• Uses a 3-carbon sugar as substrate in Calvin cycle (3-phosphoglycerate)

• Stomata open during day, closed at night

• Not typically found in dry (arid) environments

• In hot temperatures, must close stomata– Photosynthesis reduced, so productivity

reduced

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Photorespiration

• A metabolic pathway that consumes O2 and ATP, releases CO2 and reduces photosynthetic output– ~50% in some crop plants– Produces no ATP– Produces no sugar

– Releases CO2 rather than fixing it

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Photorespiration

• Rubisco (RuBP carboxylase) adds CO2 to ribulose bisphosphate (RuBP)

• Rubisco can also bind O2

• O2 and CO2 compete for binding at active site

• As concentrations of CO2 decline, O2 gets added to Calvin cycle instead

Fig. 10.18

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Photorespiration• Results of ribulose

bisphosphate & O2– 3-phosphoglycerate (3

carbon)– Phosphoglycolate (2

carbon)

• Phosphoglycolate not used in Calvin cycle

• Carbon in phosphoglycolate must be salvaged– Needed for regeneration

of RuBPFig. 10.18

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Photorespiration• Salvage pathway involves chloroplasts,

mitochondria and peroxisomes

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Why Photorespiration?

• Rubisco evolved in bacteria before O2 in atmosphere– Enzyme had affinity for both

• Modification of active site to reduce access to oxygen may also reduce the access to CO2

• Plants compensate by increasing the concentration of rubisco. – Half of the protein in the chloroplast is rubisco

• PR may be protective in terms of preventing damage from excessive light

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Alternative Pathways: C4

C4• First carbon fixation produces a 4-carbon

sugar (oxaloacetate)• Can do photosynthesis with less CO2

coming into leaf• Can close stomata during day, without

losing much productivity• Plants found in hot/dry environments

– E.g., many grasses, sugar cane, corn

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• Two types of cells– Mesophyll cells

• Carbon fixation(C4 pathway)

– Bundle-sheath cells• Calvin cycle in chloroplasts

• Spatial separation

• Keeps high CO2 levels in bundle-sheath cells cycle

See Fig. 10.19

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In mesophyll cells• C4 Pathway• CO2 fixed to PEP

(phosphoenolpyruvate)– Requires PEP carboxylase– Forms oxaloacetate (4 carbon)

• PEP carboxylase– No affinity for O2

– Higher affinity for CO2 than rubisco has

– Able to add CO2 to PEP even in low CO2 concentrations Fig. 10.19

14


• Oxaloacetate converted to malate or aspartate

• Exported from mesophyll cell to bundle sheath cell– Plasmodesmata

In bundle-sheath cells• Malate broken down into

pyruvate and CO2

• CO2 to Calvin cycle• Pyruvate to mesophyll cells

– Regenerates to PEP– Requires ATP Fig. 10.19

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Cyclic Electron Flow

• Uses cyclic electron flow to generate extra ATP• Occurs in bundle-sheath cells

– Thylakoids only have PS1 (and cytochrome complex)

Fig. 10.15

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• PS2 primarily in well developed granum

• PS1 and ATP synthase primarily in unstacked areas of membrane

• Cytochrome complexes distributed evenly

Fig. 10.17

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Cyclic Electron Flow

• Linear electron flow produces 6 ATP & 6 NADPH– Calvin cycle requires 9 ATP

• Cyclic electron flow produces extra ATP– No water split– No O2 produced– No NADPH

Fig. 10.15

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• Besides the 4-carbon product, what else must travel between the mesophyll cells and the bundle sheath cells?

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Alternative Pathways: CAM

CAM (Crassulacean acid metabolism)• Stomata open during night• CO2 stored as 4-carbon compound to be

used the next day• Stomata can be closed during the day• Plants found in hot/dry environments

– Cactus, pineapple, many orchids– (some are members of plant family

Crassulaceae)

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• Temporal separation of carbon fixation and Calvin cycle

• Takes in CO2 at night• Uses C4 pathway to fix

CO2 into organic acids– malate

• Stored in vacuole– Malic acid– Active transport

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• Malic acid leaves vacuole• Broken down to form

pyruvate and CO2• CO2 to Calvin cycle• Pyruvate converted to

starch and stored

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Fig. 10.20

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Trade OffsWhy aren’t all plants C4 or CAM?

Type of CO2 fixation

Energy to fix one CO2

Water Transpired per CO2 fixed

C3 3 ATP 400 – 500 g

C4 5 ATP 250-300 g

CAM 6.5 ATP 50-100 g

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Importance of Photosynthesis• Provides the carbon

compounds for most organisms on the planet

• Changed Earth’s atmosphere to the current one

• Important part of the CO2

cycle – Moderates temperature on the planet

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Variations in Photosynthesis Lecture 9 Fall 2008.

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