Chapter 10~ Photosynthesis

Photosynthesis in nature

Autotrophs: biotic producers; photoautotrophs; chemoautotrophs; obtains organic food without eating other organisms or organic molecules.

Heterotrophs: biotic consumers; obtains organic food by eating other organisms or their by-products (includes decomposers)

The chloroplast

Sites of photosynthesis

Pigment: chlorophyll

Plant cell: mesophyll

Gas exchange: stomata

Double membrane

Thylakoids, grana, stroma

Photosynthesis: an overview

Redox process

H 2O is split, e- (along w/ H +) are transferred to CO 2, reducing it to sugar

2 major steps: • light reactions (“photo”) √ NADP + (electron acceptor) to NADPH √ Photophosphorylation: ADP ---> ATP • Calvin cycle (“synthesis”) √ Carbon fixation: carbon into organics

Photosystems

Light harvesting units of the thylakoid membrane

Pigment molecules are struck by photons

Energy is passed to reaction centers (redox location)

Excited e - from chlorophyll is trapped by a primary e - acceptor

Noncyclic electron flow

Photosystem II (P680): √ photons excite chlorophyll e- to an acceptor √ e- are replaced by splitting of H 2O (release of O 2) √ e-’s travel to Photosystem I down an electron transport chain (Pq~cytochromes~Pc) √ as e- fall, ADP à ATP (noncyclic photophosphorylation) à

Photosystem I (P700): √ ‘fallen’ e- replace excited e- to primary e- acceptor √ 2nd ETC ( Fd~NADP + reductase) transfers e- to NADP + à NADPH (...to Calvin cycle…)

These photosystems produce equal amounts of ATP and NADPH

Light reactions

The Calvin cycle

3 molecules of CO 2 are ‘fixed’ into glyceraldehyde 3-phosphate (G3P)

Phases: 1- Carbon fixation~ each CO 2 is attached to RuBP (rubisco enzyme) 2- Reduction~ electrons from NADPH are used to reduce chemicals to G3P; ATP used up 3- Regeneration~ G3P rearranged to RuBP; ATP used; cycle continues

Calvin cycle

Calvin Cycle, net synthesis

For each G3P (and for 3 CO 2)……. Consumption of 9 ATP’s & 6 NADPH (light reactions regenerate these molecules)

G3P can then be used by the plant to make glucose and other organic compounds

Cyclic electron flow

Alternative cycle when ATP is deficient

Photosystem I used but not II; produces ATP but no NADPH

Why? The Calvin cycle consumes more ATP than NADPH…….

Cyclic photophosphorylation

Alternative carbon fixation methods, I

Photorespiration : hot/dry days; stomata close; CO 2 decrease, O 2 increase in leaves; O 2 added to rubisco; no ATP or food generated

Two Solutions…..

1- C4 plants: 2 photosynthetic cells, bundle-sheath & mesophyll; PEP carboxylase (instead of rubisco) fixes CO 2 in mesophyll; new 4C molecule releases CO 2 (grasses)

Alternative carbon fixation methods, II

2- CAM plants: open stomata during night, close during day (crassulacean acid metabolism); cacti, pineapples, etc.

CO 2 fixation: CO 2 is stored as the carboxyl groups in organic acids which then release CO 2 for the Calvin Cycle. Thus stomata can remain closed during the day to retard transpiration.