Photosynthesis Chapter 10

n Objectives F Compare the overall reaction of photosynthesis with the overall reaction for respiration F Describe where the processes of photosynthesis occur F Describe the basic processes involved in photosynthesis: water splitting to obtain electrons, redox reactions of the electron transport chains, electron and energy shuttling by means of ATP and NADPH, and the coupling of the light-dependent reactions and the Calvin cycle

F Explain how pigments capture light and excite electrons F Describe the structural and functional differences between photosystem I and photosystem II, and cyclic and noncyclic photophosphorylation F Outline the steps in the cyclic fixation of carbon in the Calvin cycle and where these occur F Contrast the C 4 and CAM photosynthetic systems with the simpler C 3 system, and how they are adaptions to hot, dry climates

Introduction n Overall equation is reversal of cellular respiration u 6CO 2 +12H 2 O+energy--->C 6 H 12 O 6 +6O 2 +6H 2 O n Increasingly probing studies provided knowledge about how photosynthesis works u van Helmont-developed early ideas about where plants obtain materials for growth F showed that soil not sufficient F concluded that water important

u Priestly-showed that plants restore “bad” air u Ingenhousz-plants only restore air when exposed to light

Autotrophs are Producers n Autotroph-means self-feeding u applies to any organism that makes own food without eating, decomposing or absorbing other organisms or organic molecules n Photosynthetic autotrophs include plants, algae and photosynthetic bacteria

Site of Photosynthesis n Photosynthesis occurs in chloroplasts in all photosynthetic organisms except monerans n Leaves (specifically, mesophyll cells) are primary site of photosynthesis n Light-absorbing pigment is chlorophyll u located in protein complexes in internal membranes of chloroplasts n Sugars assembled in stroma

Underlying Processes n Oxygen produced by splitting water u demonstrated using 18 O-labeled reactants F plant given C 18 O 2 does not release 18 O 2 F plant given H 2 18 O does give off 18 O 2

n Photosynthesis is redox process u H 2 O oxidized---> 1 / 2 O 2 +2H + +2e - u CO 2 reduced to glucose by addition of e - ’s and H + ’s u compare with respiration where glucose oxidized and O 2 reduced

n In photosynthesis, electrons travel “uphill” from water to glucose, adding light energy captured by chlorophyll n In respiration, electrons travel “downhill” from glucose to water, releasing energy to ATP

Overview n Photosynthesis is a two-stage process u light-dependant reactions F convert light energy to chemical energy, releases O 2 as waste product F occurs in thylakoid membranes and produces energy shuttles ATP and NADPH u Calvin cycle F cyclic series of steps that assemble organic molecules from CO 2

F occur in stroma and use energy and electrons from ATP and NADPH in carbon fixation F light not required but usually run during day as require shuttles from light-dependant reactions

The Light Reactions n Driven by visible light u light is electromagnetic radiation u only small fraction of em radiation perceived by organisms F different wavelengths=different colors

u leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green)

u in plants light absorbed by chlorophyll a, chlorophyll b and carotenoids

n only chlorophyll a directly involved in light reactions; other pigments act as “antenna” molecules to broaden range of energy absorbed

In the photossynthetic light reaction where does oxygen come from???? A. Carbon dioxide B. Surrounding air C. water

The Photosystems n Light behaves like particles-photons n When pigment absorbs photon, energy level of one electron is raised to excited, unstable state u if pigment is isolated from molecular environment, excited electron loses energy as heat or light and returns to normal level F chlorophyll fluoresces red

n In chloroplasts, chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll a molecules-reaction center chlorophylls u excited electrons passed from antenna chlorophylls to reaction center chlorophylls then to primary electron acceptor F series of redox reactions final is oxidation of reaction center chlorophyll and reduction of primary electron acceptor

n Two photosystems (antenna assembly+primary electron acceptor) identified u absorb at different wavelengths F photosystem I-absorbs maximally at 700nm (P700) F photosystem II-absorbs maximally at 680nm (P680) u function together to carryout non-cyclic electron transport F also known as non-cyclic photophosphorylation

u photosystem I can also carryout cyclic electron transport (cyclic photophosphorylation) F thought to be the earliest form of photosynthesis present in many primitive photosynthetic bacteria F synthesizes only ATP

Chemical Energy Generation n Electron transport chains generate ATP, NADPH and O 2 u kinetic energy of light absorbed and excites electrons u excited electrons passed along electron transport chain-series of redox reactions u released energy used to generate ATP, NADPH and O 2

u production of NADPH requires 2 electrons F supplied to PS I by PS II F replaced in PS II by splitting water F H 2 O ---> 1 / 2 O 2 + 2H + + 2e -

Chemiosmosis n Powers ATP synthesis u H + ions from splitting water and those pumped across thylakoid membrane by electron transport chain form gradient across thylakoid membrane (inside to outside) u ATP synthase provides port for H + to diffuse back into stroma F releases energy and phosphorylates ADP to ATP F similar process to ATP generation in mitochondria F known as photophosphorylation

Carbon Fixation n ATP and NADPH from light-dependant reactions power Calvin cycle u net result of Calvin cycle is 3C molecules from CO 2 using energy and electrons in ATP and NADPH from light-dependant reactions u CO 2 added to 5C intermediate ribulose-1,5- bisphosphate (RuBP) F catalyzed by RuBP carboxylase/oxygenase (rubisco)

u Number of rearrangements occur in many steps, using energy in ATP and oxidation of NADPH F last step in cycle regenerates RuBP F all steps occur simultaneously but ultimately regenerate starting reactants, hence cycle

n Three RuBP enter cycle for each 3C molecule released from chloroplast n Calvin cycle occurs in chloroplast stroma n 3C molecules exported to cytoplasm u used to synthesize glucose and other organic molecules

n Plants that use only Calvin cycle to fix carbon called C 3 plants u first identifiable product of carbon fixation is 3C molecule

Carbon-fixing Variations n C 3 plants conserve water by closing stomata u allows buildup of O 2 in leaves u Rubisco fixes O 2 rather than CO 2 u called photorespiration F uses ATP and NADPH but makes no sugars

u C 4 plants adapted to conserve water and prevent photorespiration F CO 2 incorporated into 4C molecule in mesophyll cells F diffuses into bundle sheath cells and released F enters Calvin cycle in bundle sheath chloroplasts

n CAM (crassulacean acid metabolism) plants incorporate carbon during night u stomata open at night, closed during day u CO 2 incorporated in 4C molecule and stored in vacuole at night u during day, 4C molecules exported into cytoplasm and CO 2 released u CO 2 enters Calvin cycle

n C 4 separate carbon incorporation and fixation spatially n CAM plants separate carbon incorporation and carbon fixation temporally