Plant Ecology - Chapter 2 Photosynthesis & Light: part 3.

Plant Ecology - Chapter 2

Photosynthesis & Light: part 3

Rates of Photosynthesis

Basic limiting factor - amount of light energy reaching thylakoid membranesDarkness – net loss of carbon and energy due to respiration, no photo-synthesis Compensation point – amount of usable light energy at which fixation and respiration are equal

Carbon dioxide gain = carbon dioxide loss

(PPFD = photo-synthetic photon flux density)

Rates of Photosynthesis

Strong light - respiration plus photosynthesis - giving off and taking up CO2, up to a pointMaximum rate of photosynthesis, despite further increase in light energy

Ecological Significance

Different plants have different photosynthetic responses to same light intensitySome do better under low light, others strong light

Habitat - shade vs. sunSome can shift light compensation point to deal with changes in light availability (lots in spring, less in summer in shade)

http://www.pbase.com/naomismith/garden_flowers&page=2

Ecological SignificanceSpring ephemeral

Constant light CPPhotosynthesizes only in springAdapted to high light before trees leaf-out

Summer-green plantCP moves downward from spring to mid-summerAdapted to lower light after trees leaf out

Semi-evergreenCP moves down from spring to mid-summer, but moves up again in autumnAdapted to lower light levels in summer but again makes use of higher light levels after leaf-fall

CO2 Uptake Limitations

CO2 diffusion from surrounding air into leaf and into chloroplastLeaf conductance - rate at which CO2 flows into the leafMostly under control of stomata


Stomata open, close to maintain water balance (seconds, minutes)Stomata change as leaf morphology, chemistry change (days, months)Natural selection modifies (100s, 1000s of years)

Hornwort stomate (wet habitat)

Xerophyte stomates

Note countersunk guard cells and thick cuticle


Controlling water loss is main reason why plants restrict their CO2 uptakeHuge amount of air required for photosynthesis - 2500 L air for each gram of glucose produced


Stomata can be very dynamic, opening and closing constantly to regulate CO2 and water lossMuch variation even within same leafPatchy closure also common in stressed plants

Variation in Photosynthetic Rates

Increases in atmospheric CO2 concentrations should allow C3 plants to increase rates of photosynthesisBUT This will be mitigated by the availability of other limiting factors such as nutrients (esp. nitrogen)

Variation in Photosynthetic Rates: Habitats

Photosynthetic rates vary within and among habitatsCorrelated with species composition, habitat preferences, growth rates

Variation in Photosynthetic Rates: Habitats

Photosynthetic rates may be unrelated to species distributions, populations processesOther important components of photosynthesis: total leaf area, length of time leaves active, maintained

Photorespiration

Under very hot and dry conditions, many plants must close their stomata to minimize water loss. During these times the ratio of oxygen to carbon dioxide in the leaf increases, and this favors a process called photorespiration. Rubisco, the enzyme that brings CO2 and RuBP together, works only when the concentration of CO2 is high relative to the level of O2.

When CO2 levels drop, the enzyme, Rubisco, combines RuBP with O2 and the Calvin cycle is disrupted. (When leaf CO2 drops to 50 ppm Rubisco stops fixing CO2 and starts to fix O2)

Photorespiration

Get phosphoglycolic acid and PGA.Phosphoglycolic acid is hydrolyzed to glycine. 2 molecules of glycine can combine to form CO2 (lost) and serine which can be converted into PGA using ATP.PGA stays in Calvin cycleNO ATP FROM PHOTORESPIRATION

Photosynthetic Pathways

Carbon fixation done using 3 different pathwaysC3

C4

CAM (crassulacean acid metabolism)


C3 and C4 named for 3-carbon and 4-carbon stable molecules first formed in these pathwaysCAM named after plant family Crassulaceae where it was first discovered


Most plants use C3 photosynthesis, and plants that use it are found everywhereC4 and CAM are modifications of C3, and evolved from it


C3: CO2 joined to 5-carbon molecule with assist from the enzyme RuBP carboxylase/ oxygenase - rubiscoRubisco probably most abundant protein on earth, but does its job very poorly

Photosynthetic PathwaysRubisco inefficient at capturing CO2

Also takes up O2 during photorespirationO2 uptake favored over CO2 uptake as temperatures increaseLimits photosynthesisPlants must have HUGE amounts of rubisco, especially those in warm, sunny habitats, to compensate for poor performance

How to Beat the Heat

Crassulacean acid metabolism (CAM)light and dark reactions of photosynthesis are uncoupledstomates are closed during the dayTemporal separation of light and dark reactions

C4 Photosynthesiscouple CO2 with PEP (phosphoenolpyruvic acid)

get C4 intermediatessplit to get CO2 back

store CO2 in special cells, keeps CO2 level high

Spatial separation of light and dark reactions


C4: Mesophyll cells for carbon fixation, bundle sheath cells for Calvin cycle - keeps O2 away from Calvin cycleC3: Mesophyll cells for carbon fixation and Calvin cycle - allows O2 access to Calvin cycle

C4 photosynthesis

C4 photosynthesis contains additional step used for initial CO2 capture3-carbon PEP (phosphoenol-pyruvate) + CO2 = 4-carbon OAA (oxaloacetate)Catalyzed by PEP carboxylase

Plasmodesmata

Materials are transferred from one cell to another across plasmadesmata

C4 photosynthesis

PEP carboxylase only captures CO2

Higher affinity for CO2 than rubiscoNot affected by warmer temperaturesDecarboxylation (CO2 removal) process allows standard Calvin cycle (including rubisco)

C4 requires special leaf anatomySpatial separation of C4 and C3 reactionsC4 plants fix CO2 in mesophyll cells as 4-carbon compounds, and later release CO2 in bundle sheath cells. Calvin-Benson cycle occurs in bundle sheath cells in C4 plants. Rubisco exposed only to CO2, not O2 in atmosphere like in C3 plantIn C3 plants, photo-synthesis occurs in both types of mesophyll cells; not in bundle sheath cells

Carbon fixation in a C4

plant. CO2 is fixed in

mesophyll cells as oxaloacetate which quickly converts to malate. Malate is transported across plasmodesmata to bundle sheath cells

where a CO2 is released

to the Calvin cycle. The remaining pyruvate is sent back to the mesophyll cell where it is phosphorylated to PEP.

Carbon fixation in a C4 plant.

Plasmodesmata


Requires additional energy to run C4

pathway, but easily compensated for by photosynthetic gains at high light levelsVery successful in warm, full-light habitats, e.g., deserts


C4 plants generally have higher maximum rates of photosynthesis, and have higher temperature optima


C4 plants generally do not become light-saturated, even in full sunlightAlso have better nitrogen use and water use efficiencies because of reduced needs for rubisco

C4 PlantsMany grasses such as corn, sugar cane, sorghumC4 photosynthesis

CO2 fixed by mesophyll cells as a C4 compound

C4 cpd is transported to adjacent bundle sheath cellsC4 cpd is split, and CO2 is refixed by C3 pathway

Keeps CO2 level high in bundle sheath cells

CO2 doesn’t leak out through stomates

Since stomates don’t have to open so much don’t lose so much water

Very efficient; C4 plants do better at high temps but not when temps are below about 40oC

Crassulacean acid metabolism (CAM) – temporal separation

NightStomates openTake up CO2

Produce crassulacean acid

stores CO2 as a C4 acid

DayStomates closedUse stored CO2 for standard C3 photosynthesis


CAM photosynthesis - Crassulacean acid metabolismUses basically same biochemistry as C4, but in very different wayRubisco found in all photosynthetic cells, not just bundle sheath cells

CAM Photosynthesis

CAM uses temporal separation of light capture, carbon fixation rather than spatial separation as in C4

CO2 captured at night, converted into organic acids


During daylight, organic acids broken down to release carbon, used normally in Calvin cycleStomata remain closed during day

Plant Ecology - Chapter 2 Photosynthesis & Light: part 3.

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Transcript of Plant Ecology - Chapter 2 Photosynthesis & Light: part 3.