Organisms capture and store free energy for use in biological processes

Calvin CycleOrganisms capture and store free energy for use in biological processesWhere does the Calvin Cycle take place?Stroma of the chloroplast the fluid filled area outside of the thylakoid membraneHow does CO2 enter the Calvin Cycle?CO2 enters through the stomata microscopic pores in leaves

Once in the leaf the CO2 diffuses into mesophyll cells where it can enter the chloroplast

Within the chloroplast carbon fixation takes place

Fig. 10-3a5 mMesophyll cellStomataCO2O2ChloroplastMesophyllVeinLeaf cross section4Figure 10.3 Zooming in on the location of photosynthesis in a plantWhat occurs during carbon fixation?Carbon dioxide joins a five-carbon molecule called ribulose bisphophate (RuBP)This reactions is catalyzed by RuBP carboxylase, aka RibiscoRibisco the most abundant enzyme in natureThis enzyme often takes up 50% of the total chloroplast protein contentRibisco is a slow only catalyzing 3 molecules of substrate per second (compared to 1,000 per second)Unstable 6 carbon compound is formed which splits to form 2 three carbon molecules of PGA (phosphoglycerate)How is PGA turned into sugar?Each molecule of PGA is systematically reduced by enzyme action.NADPH provides the hydrogen atoms and ATP provides the energy for these reactions to occur. (NADPH and ATP from Light Reactions)PGAL (phosphoglyceraldehyde), also called G3P (glyceraldehyde-3-phosphate) is the final product of the Calvin CycleG3P can be exported to the cytoplasm and combined to form fructose-6-phosphate and glucose 1-phosphate.Fructose and glucose can join to form sucroseHow does the Calvin Cycle get back to 5-C RuBP?For every 3 molecules of carbon dioxide fixed, 6 molecules of G3P are formed

Only 1 of the G3P exits the cycle

The other five G3P (3C) molecules are used to regenerate 3 molecules of RuPB (5C) using ATP from the Light Reactions

Fig. 10-18-3Ribulose bisphosphate(RuBP)3-Phosphoglycerate Short-livedintermediatePhase 1: Carbon fixation(Entering oneat a time)RubiscoInputCO2P3633PPPPATP66 ADPPP61,3-Bisphosphoglycerate6PP666 NADP+NADPHiPhase 2:ReductionGlyceraldehyde-3-phosphate(G3P)1POutputG3P(a sugar)Glucose andother organiccompoundsCalvinCycle33 ADPATP5PPhase 3:Regeneration ofthe CO2 acceptor(RuBP)G3P8Figure 10.18 The Calvin cycleAlternative Carbon Fixation MechanismsOrganisms use feedback mechanisms to maintain their internal environments and respond to external environmental changesWhy do plants need alternative mechanisms for carbon fixation?Dehydration is a problem for plants, sometimes requiring trade-offs with other metabolic processes, especially photosynthesisOn hot, dry days, plants close stomata, which conserves H2O but also limits photosynthesisThe closing of stomata reduces access to CO2 and causes O2 to build upThese conditions favor a seemingly wasteful process called photorespiration

What is photorespiration?In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound

In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle

Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

How do C4 plants avoid photorespiration?C4 plants minimize the cost of photorespiration by incorporating CO2 into four-carbon compounds in mesophyll cells

This step requires the enzyme PEP carboxylase

PEP carboxylase has a higher affinity for CO2 than rubisco does; it can fix CO2 even when CO2 concentrations are low

These four-carbon compounds are exported to bundle-sheath cells, where they release CO2 that is then used in the Calvin cycle

Fig. 10-19C4 leaf anatomyMesophyll cellPhotosyntheticcells of C4plant leafBundle-sheathcellVein(vascular tissue)StomaThe C4 pathwayMesophyllcellCO2PEP carboxylaseOxaloacetate (4C)Malate (4C)PEP (3C)ADPATPPyruvate (3C)CO2Bundle-sheathcellCalvinCycleSugarVasculartissue13Figure 10.19 C4 leaf anatomy and the C4 pathwayHow do CAM plants avoid photorespiration?Some plants, including succulents, use crassulacean acid metabolism (CAM) to fix carbon

CAM plants open their stomata at night, incorporating CO2 into organic acids

Stomata close during the day, and CO2 is released from organic acids and used in the Calvin cycle

Fig. 10-20CO2SugarcaneMesophyllcellCO2C4Bundle-sheathcellOrganic acidsrelease CO2 to Calvin cycleCO2 incorporatedinto four-carbonorganic acids(carbon fixation)PineappleNightDayCAMSugarSugarCalvinCycleCalvinCycleOrganic acidOrganic acid(a) Spatial separation of steps(b) Temporal separation of stepsCO2CO21215Figure 10.20 C4 and CAM photosynthesis comparedReviewThe energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds

Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells

Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits

In addition to food production, photosynthesis produces the O2 in our atmosphere

Fig. 10-21LightReactions: Photosystem II Electron transport chain Photosystem I Electron transport chainCO2NADP+ADPPi+RuBP3-PhosphoglycerateCalvinCycleG3PATPNADPHStarch(storage)Sucrose (export)ChloroplastLightH2OO217Figure 10.21 A review of photosynthesis