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

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Presentation on theme: "Plant Ecology - Chapter 2 Photosynthesis & Light: part 3."— Presentation transcript:

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

2 Rates of Photosynthesis Basic limiting factor - amount of light energy reaching thylakoid membranes Darkness – 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

3 (PPFD = photo- synthetic photon flux density)

4 Rates of Photosynthesis Strong light - respiration plus photosynthesis - giving off and taking up CO 2, up to a point Maximum rate of photosynthesis, despite further increase in light energy

5 Ecological Significance Different plants have different photosynthetic responses to same light intensity Some do better under low light, others strong light Habitat - shade vs. sun Some can shift light compensation point to deal with changes in light availability (lots in spring, less in summer in shade)

6 Ecological Significance Spring ephemeral Constant light CP Photosynthesizes only in spring Adapted to high light before trees leaf-out Summer-green plant CP moves downward from spring to mid-summer Adapted to lower light after trees leaf out Semi-evergreen CP moves down from spring to mid-summer, but moves up again in autumn Adapted to lower light levels in summer but again makes use of higher light levels after leaf-fall

7 CO 2 Uptake Limitations CO 2 diffusion from surrounding air into leaf and into chloroplast Leaf conductance - rate at which CO 2 flows into the leaf Mostly under control of stomata


9 CO 2 Uptake Limitations 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)

10 Hornwort stomate (wet habitat) Xerophyte stomates Note countersunk guard cells and thick cuticle

11 CO 2 Uptake Limitations Controlling water loss is main reason why plants restrict their CO 2 uptake Huge amount of air required for photosynthesis - 2500 L air for each gram of glucose produced

12 CO 2 Uptake Limitations Stomata can be very dynamic, opening and closing constantly to regulate CO 2 and water loss Much variation even within same leaf Patchy closure also common in stressed plants

13 Variation in Photosynthetic Rates Increases in atmospheric CO 2 concentrations should allow C 3 plants to increase rates of photosynthesis BUT This will be mitigated by the availability of other limiting factors such as nutrients (esp. nitrogen)

14 Variation in Photosynthetic Rates: Habitats Photosynthetic rates vary within and among habitats Correlated with species composition, habitat preferences, growth rates

15 Variation in Photosynthetic Rates: Habitats Photosynthetic rates may be unrelated to species distributions, populations processes Other important components of photosynthesis: total leaf area, length of time leaves active, maintained

16 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 CO 2 and RuBP together, works only when the concentration of CO 2 is high relative to the level of O 2. When CO 2 levels drop, the enzyme, Rubisco, combines RuBP with O 2 and the Calvin cycle is disrupted. (When leaf CO 2 drops to 50 ppm Rubisco stops fixing CO 2 and starts to fix O 2 )

17 Photorespiration Get phosphoglycolic acid and PGA. Phosphoglycolic acid is hydrolyzed to glycine. 2 molecules of glycine can combine to form CO 2 (lost) and serine which can be converted into PGA using ATP. PGA stays in Calvin cycle NO ATP FROM PHOTORESPIRATION






23 Photosynthetic Pathways Carbon fixation done using 3 different pathways C3C3 C4C4 CAM (crassulacean acid metabolism)

24 Photosynthetic Pathways C 3 and C 4 named for 3- carbon and 4-carbon stable molecules first formed in these pathways CAM named after plant family Crassulaceae where it was first discovered

25 Photosynthetic Pathways Most plants use C 3 photosynthesis, and plants that use it are found everywhere C 4 and CAM are modifications of C 3, and evolved from it

26 Photosynthetic Pathways C 3 : CO 2 joined to 5- carbon molecule with assist from the enzyme RuBP carboxylase/ oxygenase - rubisco Rubisco probably most abundant protein on earth, but does its job very poorly

27 Photosynthetic Pathways Rubisco inefficient at capturing CO 2 Also takes up O 2 during photorespiration O 2 uptake favored over CO 2 uptake as temperatures increase Limits photosynthesis Plants must have HUGE amounts of rubisco, especially those in warm, sunny habitats, to compensate for poor performance

28 How to Beat the Heat Crassulacean acid metabolism (CAM) light and dark reactions of photosynthesis are uncoupled stomates are closed during the day Temporal separation of light and dark reactions C4 Photosynthesis couple CO 2 with PEP (phosphoenolpyruvic acid) get C4 intermediates split to get CO 2 back store CO 2 in special cells, keeps CO 2 level high Spatial separation of light and dark reactions


30 Photosynthetic Pathways C 4 : Mesophyll cells for carbon fixation, bundle sheath cells for Calvin cycle - keeps O 2 away from Calvin cycle C 3 : Mesophyll cells for carbon fixation and Calvin cycle - allows O 2 access to Calvin cycle


32 C 4 photosynthesis C 4 photosynthesis contains additional step used for initial CO 2 capture 3-carbon PEP (phosphoenol-pyruvate) + CO2 = 4-carbon OAA (oxaloacetate) Catalyzed by PEP carboxylase


34 Plasmodesmata Materials are transferred from one cell to another across plasmadesmata

35 C 4 photosynthesis PEP carboxylase only captures CO 2 Higher affinity for CO 2 than rubisco Not affected by warmer temperatures Decarboxylation (CO 2 removal) process allows standard Calvin cycle (including rubisco)

36 C 4 requires special leaf anatomy Spatial separation of C 4 and C 3 reactions C4 plants fix CO 2 in mesophyll cells as 4- carbon compounds, and later release CO 2 in bundle sheath cells. Calvin-Benson cycle occurs in bundle sheath cells in C4 plants. Rubisco exposed only to CO 2, not O 2 in atmosphere like in C 3 plant In C3 plants, photo- synthesis occurs in both types of mesophyll cells; not in bundle sheath cells




40 Carbon fixation in a C4 plant. CO 2 is fixed in mesophyll cells as oxaloacetate which quickly converts to malate. Malate is transported across plasmodesmata to bundle sheath cells where a CO 2 is released to the Calvin cycle. The remaining pyruvate is sent back to the mesophyll cell where it is phosphorylated to PEP.

41 Carbon fixation in a C4 plant.

42 Plasmodesmata

43 Photosynthetic Pathways Requires additional energy to run C 4 pathway, but easily compensated for by photosynthetic gains at high light levels Very successful in warm, full-light habitats, e.g., deserts

44 Photosynthetic Pathways C 4 plants generally have higher maximum rates of photosynthesis, and have higher temperature optima

45 Photosynthetic Pathways C 4 plants generally do not become light- saturated, even in full sunlight Also have better nitrogen use and water use efficiencies because of reduced needs for rubisco

46 C4 Plants Many grasses such as corn, sugar cane, sorghum C4 photosynthesis CO 2 fixed by mesophyll cells as a C4 compound C4 cpd is transported to adjacent bundle sheath cells C4 cpd is split, and CO 2 is refixed by C3 pathway Keeps CO 2 level high in bundle sheath cells CO 2 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 40 o C

47 Photosynthetic Pathways

48 Crassulacean acid metabolism (CAM) – temporal separation Night Stomates open Take up CO 2 Produce crassulacean acid stores CO 2 as a C4 acid Day Stomates closed Use stored CO 2 for standard C3 photosynthesis





53 Photosynthetic Pathways CAM photosynthesis - Crassulacean acid metabolism Uses basically same biochemistry as C 4, but in very different way Rubisco found in all photosynthetic cells, not just bundle sheath cells


55 CAM Photosynthesis CAM uses temporal separation of light capture, carbon fixation rather than spatial separation as in C 4 CO 2 captured at night, converted into organic acids

56 Photosynthetic Pathways During daylight, organic acids broken down to release carbon, used normally in Calvin cycle Stomata remain closed during day


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