Photosynthesis: The Light Reactions

Outline
History and intro
Properties of light and pigments
Light-dependent reactions
photosystem II and I
ATP synthesis
Light-independent reactions
Calvin cycle
Rubisco and photorespiration
CAM and C4 plants
Physiological and ecological considerations
light
plant anatomy
plant responses
excess light and photoinhibition
greenhouse effect and consequences

History
1600’s van Helmont - soil alone does not nourish plant
1700’s Priestley - plants restore air from burning candles
1700’s Ingenhousz - only green parts of plants restore air, suggests CO2 split to release O2
1931 van Niel - Ps in purple sulfur bacteria produced S2 instead of O2 during Ps thus proposed O2 released from Ps comes from H2O, not CO2
1937 Hill - isolated chloroplasts produced O2 w/o CO2 confirming O2 released from Ps comes from H2O, not CO2
1905 Blackman - Ps composed of light-dep. + light-indep. rxns, enzymes involved

Intro
Light Σ
H2O
O2
Light-dependent reactions
Chemical Σ
(ATP, NADPH)
Chemical Σ
(ATP, NADPH)
CO2
Light-independent reactions
Chemical Σ
(C H2O)
Photosynthesis (Ps) = process by which plants convert sunlight into chemical Σ, how Σ enters biosphere
chemical Σ used to convert water and CO2 into sugars, O2 is produced as byproduct
6 CO2 + 6 H2O → C6H2O6 + 6 O2
Consists of light-dependent + light-independent rxns

Intro
Light-dependent rxns (a.k.a) light rxns, thylkoid rxns, light-transduction rxns
require light, occur on thylakoid membranes
split water (oxidize water to O2)
produce NADPH, ATP (via PMF)
uses 2 photosystems
Light-independent rxns (a.k.a) dark rxns, carbon fixation rxns, stroma rxns, Calvin cycle
don’t require light, occur in chloroplast stroma
use ATP, NADPH
produce reduced carbon cmpds (i.e. glucose) from CO2
Ps primarily occurs in leaf mesophyll cells
mesophyll contains lots of chloroplasts

Properties of Light
Light travels in waves
wavelength (λ) = distance btwn 2 crests
frequency (ν) = # of wave crests that pass a pt in a given time
c = λ ν where: c = wave speed, clight = 3.0 x 108 m s-1
Light composed of particles of Σ (photons)
Σ contained in discrete packets (quantum)
Σ of photon inversely proportional to frequency
E = hν where: ν = frequency of light
h = Planck’s constant = 6.626 x 10-34 J s

Properties of Light
Electromagnetic spectrum = entire range of radiation
visible spectrum = what we can see
each wavelength has particular amnt Σ
shorter wavelength = ↑ Σ (violet)
longer wavelength = ↓ Σ (red)
UV has ↑ Σ, infared has ↓ Σ

Absorption
Chlorophyll a
most effective
400
600
500
700
Chlorophyll b
Carotenoids
Wavelength (nm)
Properties of Light
Absorption spectrum = amount of light Σ absorbed by a substance as a func. of wavelength
chlorophyll a absorbs blue (430 nm) and red (660 nm) portion of spectrum
other pigments extend Ps useful portion of spectrum

Quantum Efficiency vs Energy Efficiency
Quantum efficiency = fraction of absorbed photons that engage in photochemistry = 100%
energy efficiency = fraction of absorbed Σ that is stored as chemical products = 27%
other 73% converted to heat
of the 27%, most is used for Rm

Pigments
Pigment = substance that absorbs photons of light
photon hits pigment it can be absorbed, transmitted or reflected
we see transmitted or reflected
pigments absorb specific wavelengths of light = absorption spectrum
pigment absorbs all wavelengths in visible spectrum = black
pigment absorbs green and blue wavelengths but transmits or reflects red wavelengths = red (750 nm)
chlorophyll = reflects green, absorbs violet, blue and red
Action spectrum = effectiveness of wavelengths

Pigments
When photon hits pigment, e- bumped to higher orbital (↑ potential Σ b/c further from nucleus) (excited state)
once e- in higher orbital (unstable) it has 4 fates
re-emit photon and fall back to original position (florescence and heat)
convert Σ to heat
transfer Σ to another chlorophyll until reaches reaction center (a.k.a. resonance energy transfer)
transfer Σ to other chemical rxns in ETS

Pigments
All Ps pigments found in chloroplast
primary photosynthetic pigment = chlorophyll a
must have chl. a
ring structure w/ Mg
similar to hemoglobin
hydrophobic tail embedded in thylakoid membrane
chl. b, carotenoids and phycobilins are accessory pigments
accessory pigments = not directly involved in Σ transduction, pass Σ to chl. a which transforms it to chemical Σ, extend useful spectrum, antioxidant func.
carotenoids = red/orange/yellow, embedded in thylakoid, fall color
2 types carotenoids - carotene and xanthophyll

Anatomy
Ps occurs in chloroplast
double-membrane, DNA, RNA, ribosomes
extensive 3rd membrane = thylakoid membrane
chlorophyll embedded (light-dep. rxns)
stroma (light-indep. rxns)
grana lamellae (PS II)
stroma lamellae (PS I)
granum
lumen

Overview of Photosynthesis
2 main events of Ps (+50 rxn steps in Ps discovered)
light-dependent rxns
light Σ transferred to chemical bond in ADP and reduction of NADP+, forming ATP and NADPH
thylakoid membranes
light-independent rxns
ATP used to link CO2 to organic molecule
NADPH used to reduce C to simple sugar
carbon or CO2 fixation = conversion of CO2 into organic cmpds
stroma

Antenna Complex and Reaction Center
Most pigments serve as antenna
antenna collect light and transfer its Σ to reaction center
antenna complex = group of antenna molecules
means of increasing efficiency
integral proteins
reaction center complex = Σ is stored by transferring e- from chlorophyll to e- acceptor (REDOX rxns)
e- boosted to higher orbital
integral proteins

Photosystems I and II
Enhancement effect = Ps rate greater w/ red and far-red light together than w/ each separate
due to 2 photochemical complexes (photosystem I and II)
work in conjuction, independent antenna and rxn centers
linked by electron transport chain
e- flow = H2O → PS II → PS I → NADP+ (Z scheme)
PSII chl. a = P680 (red) vs PSI chl. a = P700 (far-red)

Photosystems I and II
PS I and II spatially separated on thylakoid membrane
PS II on grana lamellae
PS I on stroma lamellae
ETC that connects PSII to PSI found throughout
PSII produces
4 photons + 2H2O → 4 H+ + 4e– + O2 (photolysis)
H+ released in lumen (H+ gradient)
increases efficiency b/c pool of reducers vs having to associate w/ single PS

ETC

Photosystem II
Photon absorbed by PS II
e- in P680 (chlorophyll) gets excited (P680*)
e- passed from P680* to Pheo (pheophytin)
e- acceptor similar to chlorophyll but lacks a Mg, instead 2 H
P680 is oxidized as looses e- to pheophytin
NOTE: P680 re-reduced by Yz who got e- from splitting (oxidation) water this is where O2 comes from!!!
4 photons + 2H2O → 4 H+ + 4e– + O2 (photolysis)
occurs in lumen (contributes to H+ gradient (PMF) across thylakoid membrane)
Higher
Lower
Σ of electron
P680
Photon
e–
Pheo
PQ
Cytochrome
complex
P680*

Photosystem II
e- passed from Pheo to QA and QB (plastoquinones/PQ)
as e- passed by QA and QB, H+ pumped into thylakoid lumen thereby creating H+ gradient (PMF) across thylakoid membrane
e- passed from PQ to cytochrome b6f complex
large multisubunit protein w/ heme groups
e- passed from b6f complex to plastocyanin (PC)
protein w/ copper
e- passed from PC to P700 (PS II)
PQ
PQ

Photosystem I
e- passed to P700 (reduced) from PC in PS II
Photon absorbed by reduced P700
e- in P700 gets excited (P700*)
e- passed from P700* to A0 (chlorophyll?) to A1 (phylloquinone, a.k.a. vitamin A)
e- passed from A1 to FeSx to FeSA to FeSB (Fe-S proteins)
e- passed from FeSB to Fd (ferredoxin) (Fe-S protein)
Ferredoxin/NADP+ reductase (FNR) txf e- and H+ to NADP+ to form NADPH
NADPH highly reduced, used to reduce CO2 in Calvin Cycle
occurs in stroma
P700*

Noncyclic Photophosphorylation
Z-scheme used to describe how PS I and II interact
Noncyclic photophosphorylation (uses light to produce ATP)
what we’ve covered so far
2 photons from each photosystem = 1 NADPH and 1O
products go to Calvin Cycle
H2O + NADP+ → NADPH + H+ + ½ O2
6 e- = 6 ATP + 6 NADPH

Cyclic Photophosphorylation
Cyclic photophosphorylation when extra ATP needed
PS I donates e- to back to PS II resulting in production of additional ATP (no NADPH)
e- txf via PQ
PS II generates ATP only
PS I generates NADPH

ATP Synthesis
ATP produced via chemiosmosis = ion conc. differences and electric potential differences across membrane are source of free Σ that can be harnessed to do work
2nd law of thermodynamics = any nonuniform distribution of matter or Σ represents a source of Σ
ATP synthase (a.k.a. ATPase, CF0-CF1) uses PMF to generate ATP
H+ in thylakoid lumen = electrochemical gradient
due to splitting of H2O, cytochrome b6f complex
pH in stroma = alkaline, pH in lumen = acidic
gradient drives ATP synthesis via ATP synthase complex

ATP Synthesis
ATP synthase consists of hydrophobic portion CF0 (in membrane) and CF1 (sticks out in stroma)
found on stroma lamellae and edge of grana of thylakoid membrane
CF0 contains channel which H+ pass,
rotates along w/ internal stalk
CF1 where ATP synthesized
when H+ pass potential energy converted to kinetic
kinetic Σ converted to chemical bond
4 H+ translocated per ATP

Summary
4 major protein complexes
PS II oxidizes H2O, releases H+ into lumen
b6f complex pumps additional H+ into lumen
PS I reduces NADP+ to NADPH
ATP synthase produces ATP
initial acceptor?
final donor??

Summary II

Repair and Regulation
Regulatory and repair mechanisms needed to safely dissipate excess Σ or repair if damaged
carotenoids dissipate excited state of chlorophyll
excited state can react w/ O2 to produce singlet oxygen (extremely reactive, damaging to lipids)
xanthophylls (type of carotenoid) also help dissipate Σ and heat
prolonged photoinhibition (inhibition of Ps by excess light) = damage to PS II rxn center, esp. D1 protein
D1 protein removed and replaced

Chloroplast Genetics
Chloroplast have their own DNA, mRNA, ribosomes
import some genes from nucleus
circular DNA
Reproduce via division
chloroplasts divided btwn daughter cells
chloroplasts come from female plant
non-Mendelian genetics, maternal inheritance