C3 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 O2.

• 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, O2 is released

• Photophosphorylation turns ADP into ATP

2. Calvin cycle• Occurs in the stroma

• CO2 transformed into sugar

Net Rx: 6 CO2 + 6 H2O + Light C6H12O6 + 6O2

Big Picture

Light Reactions• Location: thylakoid membrane

• Needs: Light, H2O, NADP+, ADP, P

• Makes: NADPH, O2, 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):

H2O 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 2nd 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 2nd 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 ATPaka: Photophophorylation

Calvin Cycle

aka: light independent reactions• Location: stroma

• Needs: CO2, ATP, NADPH

• Makes: G3P, ADP, P, NADP+

Calvin Cycle

• Multiple enzyme pathways that uses ATP and NADPH to reduce CO2 into C6H12O6 (glucose)

• One turn of the cycle reduces one CO2

• 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 CO2 to an acceptor molecule RuBP

• Rubisco is the most famous and abundant enzyme on earth: no other organic molecule can chemically binding CO2

Reduction

• Several enzymes later the 3 CO2 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• CO2 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• http://www.youtube.com/watch?v=wEPUfJ

n0s-M• Mr. Anderson (Bozeman)• http://www.youtube.com/watch?v=g78utcL

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