Plants do both: photosynthesis and respiration

The Photosynthetic Reaction

Chloroplast structure and function Membranes –Outer: permeable to many things Porins, large central pore –Inner: highly impermeable Specific channels for certain molecules

Membranes –Thylakoid membrane system Contained within the inner membrane system Arranged in stacks: Grana Enzymes for light capture are embedded within this membrane –Photosystem II (PSII) –Cytochrome b6f (like ETC Comp. III) (move protons) –Photosystem I (PSI) –ATP synthase Chloroplast structure and function

Enclosed spaces –Intermembrane space: between outer and inner membranes –Stroma: space enclosed by inner mem. Contains the thylakoids Contains the Calvin cycle enzymes for CO2 fixation into sugar Contains DNA, ribosomes –Lumen: Space enclosed by thylakoids Accumulates high [H+] for ATP synthesis by ATP synthase Chloroplast structure and function stroma lumen

Light dependent reactions –Capture E of light into ATP and NADPH –Produce O2 from H2O Light independent reactions –Use ATP and NADPH to capture and reduce CO2 into sugar Plants also use aerobic respiration (mitochondria) Light-dependent and independent reactions

Photon is absorbed by a molecule ‘pushes’ an electron from an inner (lower E) to an outer (higher E) orbital e - + photon --> e* (excited state) # orbitals is finite and E levels are specific Different molecules can only absorb photons of certain E (wavelength) Absorption of light by photosynthetic pigments

Chlorophyll Beta-carotene Conjugated systems –Alternating single and double bonds –Delocalized electron cloud –Can absorb more varied wavelengths –Strong absorbers of visible light Absorption of light by photosynthetic pigments

Light Harvesting Complexes –100s of chlorophyll molecules –Noncovalent link to thylakoid membrane –Group acts as an antenna for light –Photon is passed around each pass reduces E –Only one is the reaction-center P680, PSII P700, PSI Transfers e* to a carrier Organization into photosynthetic units

Photosystem II (PSII) –Boost e* halfway to NADP+ Photosystem I (PSI) –Boost e* above NADP+ H 2 O + NADP + --> 1 / 2 O 2 + NADPH + H +  E o ’ = 1.14V Cell uses 2 photons, in 2 steps Organization into photosynthetic units

Photosystem II –20 subunits, embedded in thylakoid membrane –Associated with Light Harvesting Complex II (LHCII) Antenna pigments (chlorophyll) + protein subunits –Light absorbed into D1/2 complex, e* transfer to Pheophytin P680* + Pheo --> P Pheo - (charge separation)

–P680 + = strong oxidizing agent (most powerful in biology) Will accept e- from H2O and yield O2 in process (photolysis) –Pheo - = strong reducing agent Will pass e- to Plastoquinone (PQ) --> PQH2

Cytochrome b6f (structure-function similarity to Complex III of ETC) –Accepts 2e- from PQH2 –Translocates 4H+ per pair of e- –Transfers e- to Plastocyanin protein (PC) –PC carries e- to PSI

Photosystem I (PSI) –LHCI Contains light antenna –P700 rxn center P700* + A 0 --> P A 0- –P700 + receives e- from PC –A 0- txfr e- to ferredoxin (Fd) –Fd donates e- to: NADP + + H: - --> NADPH Fd NADP + reductase (FNR)

2H 2 O + 2NADP + + 2H + + 8photons --> O 2 + 2NADPH Also, 18H + difference generated across thylakoid membrane –Acidic inside lumen ATP synthase can generate ~ 5 ATP Summary: Light-dependent reactions

Noncyclic: passage of e- from H2O to NADP+ yielding H2O and NADPH plus, the proton gradient for ATP synthase Cyclic: Fd passes e- to cytochrome b6f instead of Fd NADP+ reductase, still creates proton gradient, but no NADPH –ATP synthesis can be uncoupled from NADPH synthesis Noncyclic vs. Cyclic photophosphorylation

3D structures for the light reaction complexes

Light independent rxns: Calvin cycle Key step: Ribulose bisphosphate carboxylase (RuBisCo) –5C + CO2 --> 2x 3C (3-phosphoglycerate from glycolysis) –Plants that fix CO2 this way are called C3 plants because of the 3C intermediate 6CO2 + 18ATP + 12NADPH --> Fructose + 18ADP + 12NADP Pi –Calvin cycle enzymes are in the stroma

Cytoplasm Outer mem Inner mem

Photorespiration (distinct from respiration) RuBisCo is at the mercy of the [CO 2 ]/[O 2 ] ratio Only modest preference of enzyme for CO 2 glycolate CO 2 release

Coordination of cellular organelles in photorespiration

Hot dry climates are hard on C3 plants –Must shut stomata to prevent H2O loss during day –Also keeps CO2 out (and O2 builds up inside = photorespiration problem) –C4 plants use PEP carboxylase enzyme PEP (3C) + CO2 --> OA (4C) PEP carboxylase works at much lower [CO 2 ], open stomata less often!