PHOTOSYNTHESIS
Autotrophs
Photoautotrophs
chemoautotrophs
Heterotrophs
Leaf Anatomy
Upper, lower epidermis
Stomata
Palisade Parenchyma
Lower Mesophyll
Veins, xylem, phloem
Chloroplasts, in eukaryotes
Anything green on a plant probably has chlorophyll but bulk of chloroplasts in mesophyll
Double membrane bound organelle
Stroma = matrix
Grana = stacks of thylakoids
Thylakoids = membrane bound sacs containing chlorophyll
Photosynthetic prokaryotes lack chloroplasts but have photosynthetic membranes
6 CO2 + 6 H20 + solar E à C6H12O6 + 6 O2
(compare to respiration equation, with same caveat regarding glucose)
Splitting of Water
O2 given off by plants comes from H2O, not CO2. CO2 is reduced to CHO.
Van Niel (1930s) hypothesized, following studies on sulfur bacteria (use H2S) that O in oxygen came from H20, not CO2 as generally thought. Later confirmed with stable isotope studies using 18O.
1: CO2 + 2 H2O à CH2O +H2O + O2
2: CO2 + 2 H2O à CH2O + H2O + O2
Respiration vs. Photosynthesis
Respiration: series of redox rxns in which E released from oxidation of CHO (glucose, in our example), H20 byproduct, electronegative 1/2O2 pulls E rich e- down e- transport chain, proton gradient.
Photosynthesis: series of redox rxns in which water is split generating O2, e-, and protons. e- and protons used to reduce CO2 to CHO (glucose, in our example).
In both rxns, e- travel “downhill” along e- transport chain.
How do e- travel “downhill” in both directions? E provided by sun energizes PS e-.
Photosynthesis
Light Reactions and Calvin Cycle (Dark Reactions)
Light Reactions, occur in thylakoid membranes:
Solar E captured,
water split,
e- pair and H+ reduce NADP+ to NADPH (chemical (stored, potential) energy)
photophosphorylation to generate ATP (chemical energy)
O2 released
NO CHO produced
Calvin Cycle (Melvin Calvin, 1940s), occurs in chloroplast stroma:
Carbon Fixation: incorporation of atmospheric C, in CO2, into organic cmpds.
CO2 reduced by NADPH from Light Rxns
Cycle powered by ATP from Light Rxns
Not called dark rxns because take place in dark, but because don’t use light
Details
Sun generates immense amounts of electromagnetic radiation in all wavelengths
8 minutes later, some reaches earth’s atmosphere which filters out most non-visible radiation
Pigments absorb some light and reflect other light, the color we see
Photosynthetic chlorophyll absorbs red and blue light and reflects green
Action spectrum determined by Thomas Engelmann (1883) using aerobic bacteria and a filament of algae receiving light passed through prism.
Action spectrum is not complete reflection of chlorophyll a absorption
ONLY chl a participates in Light Rxns, but other pigments can transfer E to chl a.
Solar E strikes the pigment complex (many molecules), exciting an e- at Rxn Center
Rxn Center: pair of chl a molecules
Typically the excited e- would drop back to a lower E state, releasing heat
In PS, the excited e- is captured by an e- acceptor, initiating Light Rxns
Light Reactions Two possible paths
Cyclic Electron Flow
PS I high E e- returns to PSI reaction center via e- transport, chemiosmosis, ATP
Cyclic photophosphorylation (remember oxidative phosphorylation?)
No NADPH or CHO
One Photosystem
Non-Cyclic Electron Flow
Excited e- from PSI do not return to PSI, leaving powerful oxidant
Involves PSII and PSI
PSII focuses Solar E on chl a pair at Reaction Center
Excited e- goes down SAME e- Transport Chain as in Cyclic Electron Flow
Generate H+ gradient, enter oxidizing PSI
e- enter from Split Water, PSII, e- transport, ATP, PSI, NADP+
Noncyclic makes equal ATP and NADPH
Calvin Cycle requires more ATP than NADPH
Deficit filled by Cyclic
Mitochondrial and Photosynthetic ATP Generation
Both use H+ gradients generated by e- transport of High E e-
Both use similar ATP synthase
Some e- carriers are the same
Respiration uses Chemical Energy, Photosynthesis uses Solar Energy
Mitochondria pump H+ from matrix to intermembrane space, generate ATP in matrix
Chloroplasts pump H+ from stroma to thylakoid, generate ATP in stroma
Krebs (Citric Acid) Cycle in Matrix, Calvin in Stroma
Calvin Cycle
5C RuBP accepts incoming (fixed) CO2
Rubisco catalyzes this (perhaps most abundant protein on earth)
Unstable 6C intermediate immediately splits into to 3C cmpds
Phosphorylated by ATP
Reduced by NADPH
Forming same sugar formed in Glycolysis by splitting glucose, energy investment
Whereas Glycolysis takes two cycles to complete, Calvin takes three
Finally five 3C cmpds form three 5C cmpds, RuBP
PRODUCT is 3C cmpd, glyceraldehyde phosphate
Photorespiration
Rubisco accepts O2, rather than CO2, in CO2 deficit
Product is Calvin’s 3C cmpd and a released 2C cmpd
Digested in perioxisome and mitochondria, releasing CO2
No ATP generated
May be a relic from a time when atmospheric CO2 equaled O2
C4
Plants adapted in hot, arid climates to minimize Photorespiration
C3 plants close stomata in these conditions, CO2 depleted, O2 accumulates
Corn, sugarcane, crabgrass – all in the Grass family
Greater metabolic cost in ATP, but maintains CO2 supply with closed stomata
CAM
Open stomata at night, closed during day, opposite
Manufacture organic acids at night using CO2
Release CO2 from organic acids during day, when stomata closed
Common in succulents
Photosynthetic Products
Used for E production
Used to build other macromolecules
About ½ and ½
Much of CHO produced used to make cellulose
Perhaps most abundant molecule on earth