Photosynthesis
I) Introduction
a) How plants use energy from the sun, CO2 and water to make sugar (carbohydrates like starch etc.)
b) O2 is byproduct
c) 12 H2O + 6CO2 + light energy → 6CO2 + C6H12O6 + 6H2O
i) reverse reaction of aerobic respiration
(1) sustains life on earth
d) Takes place in chloroplasts
i) double membrane
(1) fluid interior = stroma
(2) interior folded membrane (thylakoid system) - form grana (um)
ii) chloroplasts found in leaves of plants- also in many protists (algae)
e) Photosynthesis occurs in 2 steps
f) light dependent rxn - harvest energy of sun, splits H2O into O2 (released into the atmosphere) and H+ and makes ATP (cannot happen without light energy; sunlight)
g) light independent rxns - uses C from CO2 to make sugar (Carbon fixation) AKA Calvin Cycle
II) Pigments - How plants catch light energy
a) pigments give color to things
i) the color seen is the light they reflect and transmit
ii) absorbs other colors
b) light travels in waves of various lengths
i) visible spectrum (380-750 nanometers (nm) in length) = ROY G BIV
(1) Red, orange, yellow, green, blue, indigo, violet
(2) red is longest wavelength
(3) violet is shortest wavelength
ii) different pigments absorb different wavelengths of light and reflect/transmit the rest
c) Chlorophyll is the primary photosynthetic pigment in plants
i) color is blue-green
ii) reflects green and some yellow light - absorbs most of the other WLs – violet and red
d) Plants also have accessory pigments
i) Do not participate directly in photosynth. But absorb other WLs
(1) Chlorophyll b
(a) Yellow-green in color
(i) Absorbs blue and orange WL’s
(b) Transfers energy to Chlorophyll A for photosynth.
ii) carotenoids- yellows and oranges
(1) fall colors seen when chlorophyll disintegrates
(2) do not transfer energy to chlor.a – absorb light that is damaging to chlorophyll
III) How energy is captured from sunlight
a) Chlorophyll molecules in thylakoid membrane form photosynthesis
i) “antenna complex” with a central rxn center
b) Molecule can only absorb light at a particular WL
c) When a photon (unit of light energy) hits a pigment molecule in antenna complex, an electron from the molecule is elevated from the ground state (low energy) to excited state (high energy)
d) Can’t stay there long (unstable) – comes back down
e) Absorbed by another chlorophyll molecule
f) Continues to happen until reaches energy state corresponding to molecule in rxn center
g) Then it is absorbed and starts the process
IV) Light Dependent rxn
a) harvests energy for next step - splits water releasing electrons, O2 and H+
b) Light dep rxn is what releases O2 to atmosphere
i) resp for our atmosphere being hospitable to aerobic life
c) Happens in thylakoid membrane compartments (grana)
d) Can be non-cyclic or cyclic
i) Non-cyclic electron flow
(1) Involves two types of photosynthesis ( I and II) *starts with what happens in II first*
(2) Sun light (photon of light energy) hits a special chlorophyll molecule = P680 (photosynthesis II)
(3) causes P680 to give up an 2e- elevated to excited state
(4) photolysis (water is split)
H2O + sunlight —> 2 H+ + 1/2O2 + 2 electrons (these replace those lost by P680)
(5) excited electrons move down the electron transport (cytochrome) system in thyl. Membrane
(a) (sim. To respiration)
(6) energy ADP → ATP ( used in the next step)
(a) called photosynthesis
(7) receptor is P700- it is also receiving photons that excite electrons
(8) electrons from the transport system replaces the excited one
(9) excited electron passes down another electron transport system
(10) NADP+ (nicotinamide adenine dinucleotide phosphate) → NADPH (reduced) - carries e-s to next step
ii) Cyclic electron flow
(1) Involves photosynthesis I only
(2) Produces ATP but no NADPH
(3) Light independent rxn (next step) requires more ATP than NADPH
(4) Cyclic provides extra ATP
iii) ATP formation in both is by chemiosmosis (similar to mitochondria during respiration)
(1) H+s from water build up in compartment
(2) also as steps of e- transport system receive e-s - H+s are drawn across membrane from stroma
(3) creates electrical and chemical gradient of H+ (conc higher inside membrane (different from resp))
(4) flow down gradient through ATP Synthase (proteins in membrane)
(a) H+s pass out into stroma
(5) synthases catalyze ADP à ATP
(a) ATP is released in stroma
(6) ATP and NADPH provide energy for next step