PHOTOSYNTHESIS: OVERALL REACTION
6CO2 + 6H20
Light,
Chlorophyll
C6H12O6 + 6O2
PHOTOSYNTHESIS: LIGHT REACTIONS VS. DARK
REACTIONS
• THE FUNCTION OF THE LIGHT REACTIONS IS TO
CONVERT SUNLIGHT INTO CHEMICAL ENERGY (ATP AND
NADPH).
• THE DARK REACTIONS FIX CO2 INTO SUGARS, USING
THE ATP AND NADPH GENERATED BY THE DARK
REACTIONS.
LIGHT HAS CHARACTERISTICS OF BOTH A
PARTICLE AND A WAVE
A WAVE IS CHARACTERIZED BY A
WAVELENGTH AND A FREQUENCY
c = 
LIGHT IS ALSO A PARTICLE WHICH WE
CALL A PHOTON
 EACH PHOTON CONTAINS AN AMOUNT OF ENERGY
CALLED A QUANTUM.
 THE ENERGY CONTENT OF LIGHT IS NOT
CONTINUOUS, BUT IS DELIVERED IN DISCREET
PACKETS OR QUANTA.
 E = h,
where h is Planck’s constant
Chlorophyll b
Chlorophyll a
Phycoerythrobilin
-carotene
Bacteriochlorophyll a
ENGELMANN EXPERIMENT (1881)
PHOTOSYNTHESIS TAKES PLACE IN
COMPLEXES CONTAINING LIGHTHARVESTING ANTENNAS AND
PHOTOCHEMICAL REACTION CENTERS
EMERSON AND ARNOLD’S FLASHING LIGHT
EXPERIMENTS WITH CHLORELLA (1932)
1. Gave brief flashes (10-5 s) of light to Chlorella
suspension and measured the amount of O2 produced.
(The flashes were spaced 0.1 s apart for maximum
efficiency of dark reaction.)
2. Varied intensity (light energy) of flash and
plotted the O2 produced per flash versus the
flash energy (number of photons).
QUANTUM YIELD = 0.1
Carotenoid absorption: carotenoids have
photoprotective effect
RED DROP
EFFECT
EMERSON
ENHANCEMENT
EFFECT
Stroma lamellae
Double
membrane
Grana stack
(thylakoids)
Stroma
PSI
PSII
PSI
Antenna complex as energy funnel
LHCII PROTEIN
~15
LHCII Antenna Complex
PSI
CHEMIOSMOTIC
MECHANISM