Plants do both: photosynthesis and respiration
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Transcript of Plants do both: photosynthesis and respiration
Plants do both: photosynthesis and respiration
The Photosynthetic Reaction H2O + CO2 --> CH2O + O2
– For a long time, the O2 released was thought to come from CO2 (wrong)– Studies on sulfur bacteria showed:
H2S + CO2 --> CH2O + 2S
– So, van Niel postulated a generic scheme:
H2X + CO2 --> CH2O + 2X
– And it was later shown that indeed the O2 comes from H2O
6H2O + 6CO2 --> C6H12O6 + 6O2
weak reducer + weak oxidizer --> strong reducer + strong oxidizer
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+
H2O + NADP+ --> 1/2O2 + NADPH + H+
Eo’ = 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 --> P680+ + Pheo- (charge separation)
P680* + Pheo --> P680+ + 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* + A0 --> P700+ + A0-
– P700+ receives e- from PC
– A0- txfr e- to ferredoxin (Fd)
– Fd donates e- to:
NADP+ + H:- --> NADPH
Fd NADP+ reductase (FNR)
2H2O + 2NADP+ + 2H+ + 8photons -->
O2 + 2NADPH
Also, 18H+ difference generated acrossthylakoid 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+ + 18Pi
– Calvin cycle enzymes are in the stroma
Cytoplasm
Outer mem
Inner mem
Photorespiration (distinct from respiration)• RuBisCo is at the mercy of the [CO2]/[O2] ratio• Only modest preference of enzyme for CO2
glycolate
CO2 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 [CO2],open stomata less often!