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Lecture #4 Date ________

Chapter 10~Photosynthesis

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Photosynthesis in nature

Autotrophs:biotic producers; photoautotrophs;chemoautotrophs; obtains organic

food without eating otherorganisms Heterotrophs:

biotic consumers; obtains organicfood by eating other organisms or

their by-products (includesdecomposers)

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Photosynthesis: an overview

Redox process H2O is split, e- (along w/ H+) are

transferred to CO2, reducing it tosugar

2 major steps: light reactions (photo)

NADP+ (electronacceptor) to NADPH

Photophosphorylation:

ADP ---> ATP Calvin cycle (synthesis) Carbon fixation:

carbon into organics

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Photosystems

Light harvesting units of thethylakoid membrane

Composed mainly of protein

and pigment antenna complexes Antenna pigment molecules are

struck by photons Energy is passed to reaction

centers (redox location)

Excited e- from chlorophyll istrapped by a primary e-acceptor

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Noncyclic electron flow Photosystem II (P680):

photons excite chlorophyll e- to anacceptor e- are replaced by splitting of H2O(release of O2)

e-s travel to Photosystem I down anelectron transport chain(Pq~cytochromes~Pc) as e- fall, ADP ---> ATP (noncyclicphotophosphorylation)

Photosystem I (P700):

fallen e- replace excited e- toprimary e- acceptor 2nd ETC ( Fd~NADP+ reductase)transfers e- to NADP+ ---> NADPH(...to Calvin cycle)

These photosystems produce equal

amounts of ATP and NADPH

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The Calvin cycle

3 molecules of CO2 are fixedinto glyceraldehyde 3-phosphate (G3P)

Phases:1- Carbon fixation~each CO2 is attached to RuBP(rubisco enzyme)

2- Reduction~

electrons from NADPH reducesto G3P; ATP used up3- Regeneration~

G3P rearranged to RuBP; ATPused; cycle continues

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Calvin Cycle, net synthesis

For each G3P (and for 3 CO2).Consumption of 9 ATPs & 6 NADPH

(light reactions regenerate these molecules) G3P can then be used by the plant to make

glucose and other organic compounds

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Cyclic electron flow

Alternative cycle whenATP is deficient

Photosystem I used butnot II; produces ATP butno NADPH

Why? The Calvin cycleconsumes more ATP than

NADPH. Cyclic

photophosphorylation

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Alternative carbon fixation methods, I

Photorespiration: hot/dry days;stomata close; CO2 decrease,O2 increase in leaves; O2

added to rubisco; no ATP orfood generated Two Solutions.. 1- C4 plants: 2 photosynthetic

cells, bundle-sheath &

mesophyll; PEP carboxylase(instead of rubisco) fixes CO2in mesophyll; new 4C moleculereleases CO2 (grasses)

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Alternative carbon fixation methods, II

2- CAM plants: openstomata during night,close during day

(crassulacean acidmetabolism); cacti,

pineapples, etc.

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A review of photosynthesis