C 3 Photosynthesis Chapter 10

What you need to know! How photosystems convert solar energy to chemical energy. How linear electron flow in the light reactions results in the formation of ATP, NADPH, and O 2. How chemiosmosis generates ATP in the light reactions. How the Calvin cycle uses the energy molecules of the light reactions to produce G3P

Photosynthesis Radiation energy is transformed into chemical bond energy in two distinct stages: 1.Light reactions Occur in the thylakoid membrane Water donates electrons to NADP+ to make NADPH Water is split, O 2 is released Photophosphorylation turns ADP into ATP 2.Calvin cycle Occurs in the stroma CO 2 transformed into sugar Net Rx: 6 CO H 2 O + Light  C 6 H 12 O 6 + 6O 2

Big Picture

Light Reactions Location: thylakoid membrane Needs: Light, H 2 O, NADP+, ADP, P Makes: NADPH, O 2, ATP Includes: Linear (non-cyclical), cyclical, & chemiosmosis

Linear (Non-cyclical) Light Rxs Photosystem II (P680) pigments absorb light (photons) A photon excites chlorophyll which kicks an electron e- out of the reaction center The excited e- is captured by the Electron Transport Chain (ETC) between P680 and Photosystem I (P700) The missing e- is replaced by splitting water (photolysis of water): H 2 O  O + 2e- + 2H+

Linear Light Rxs The excited e- moves down the ETC The e-’s energy (excited) is used to pump H+ into the thylakoid space (creating a concentration gradient) e- is deposited into P700 P700 pigments absorb light (photons) A photon excites chlorophyll which kicks an electron e- out of the reaction center The e- is captured by another shorter ETC At the end of the 2 nd ETC the e- binds to NADP+ 2 e- and NADP+ are combined with H+ to form NADPH

Linear Light Reactions

Cyclical Light Reactions Some e-’s, when kicked out of P700 do not go down the 2 nd shorter ETC Instead they fall back on the first ETC between P680 and P700 This produces less NADPH and more H+ gradient

Cyclical Light Reactions

Chemiosmosis This process makes ATP by using the H+ concentration gradient H+ concentration gradient across the thylakoid membrane means: H+ inside the thylakoid is high, while H+ in the stroma is low –On a sunny day it is 1000x’s more acidic in the thylakoid space (pH 5 in thylakoid, pH 8 in stroma) ATP Synthase in the membrane functions like a turbine: when H+s rush through ATP Synthase (down the electrochemical gradient) ATP Synthase turns and uses kinetic energy to phosphorylize ADP ADP + P  ATP aka: Photophophorylation

Calvin Cycle aka: light independent reactions Location: stroma Needs: CO 2, ATP, NADPH Makes: G3P, ADP, P, NADP+

Calvin Cycle Multiple enzyme pathways that uses ATP and NADPH to reduce CO 2 into C 6 H 12 O 6 (glucose) One turn of the cycle reduces one CO 2 3 distinct steps: 1.Carbon fixation 2.Reduction 3.Regeneration

Carbon Fixation First enzyme of the cycle is Rubisco (Ribulose Bisphosphate Carboxylase) which binds 3 CO 2 to an acceptor molecule RuBP Rubisco is the most famous and abundant enzyme on earth: no other organic molecule can chemically binding CO 2

Reduction Several enzymes later the 3 CO 2 have been reduced to a C3 sugar called G3P (glyceraldehyde phosphate) –powered by 6 ATP and 6 NADPH G3P leaves the cycle –2 G3P can combine to form glucose

Regeneration RuBP needs to be regenerated –powered by 3 ATP

3. Regeneration 1. Carbon Fixation 2. Reduction

Water Balance If water is running low, plants will close their stomata to avoid transpiration When stomata are closed CO 2 is not replenished ADP and NADP+ are not replenished by the Calvin Cycle Light Reactions run out of ADP and NADP+ Energized e-’s fall back to the reaction center of chlorophyll –This can emit light (plant fluorescence)

Review Biology Crash Course n0s-Mhttp:// n0s-M Mr. Anderson (Bozeman) QrJ4http:// QrJ4