Photosynthesis
Process in which light energy is used to drive
the conversion of CO2 to carbohydrate.
light
6H2O* + 6CO2
C6H12O6 + 6O2*
Importance of Photosynthesis
• Source of atmospheric oxygen
– Earth’s atm: 78% N2, 21% O2, 0.035% CO2
(or 350 ppm & rising)
• Source of energy for food chain
– photoautotrophs vs. heterotrophs
• Direct/indirect involvement in all products
• Maintain stable ecosystem
Photosynthesis and Transpiration
Whole Plant Perspective
• CO2 enters leaf through open
stomates; water is lost
• CO2 is converted to carbohydrate
(photosynthate) in the leaf and
transported as sucrose in phloem
• Water is taken up by roots and
transported via xylem
• Loss of water from the leaf via
stomates is evapotranspiration
Leaf Cross-Section
guard
cells &
stomate
Cuticle
Epidermis
Palisade
parenchyma
Spongy
parenchyma
Chloroplasts
• Site of photosynthesis; several per cell
• Bounded by 2 smooth membranes
• Inner thylakoid membrane system
– grana = stacks of thylakoids
– contain PS pigments
– site of light reactions
• Stroma
– liquid matrix
– site of “dark” reactions (CO2 conversion)
3 Processes in Photosynthesis
• Absorption of light energy
– pigments in thylakoids
• Light Reactions
– pigments & proteins in thylakoids
– generates ATP and NADPH
• “Dark” Reactions
– enzymes in stroma
– reduce CO2 to carbohydrates using ATP & NADPH
3 Processes in Photosynthesis
(1) light
O2
CO2
sugar
NADPH
H2O
starch
ATP
(2) Light
Reactions
(3) Dark
Reactions
Light Striking Leaf
• Reflected back to atmosphere
• Passes through leaf back to atmosphere
• Absorbed by the leaf and used for PS
WAVELENGTH
ENERGY
GAMMA RAYS X-RAYS
UV
V
INFRARED
RADIO WAVES
Visible Light
380 nm
430 nm
650 nm
750 nm
PAR
•
•
•
•
PAR = Photosynthetically Active Radiation
400-700 nm wavelengths
PPF = photosynthetic photon flux
PPF at full sun = 2000 µmol/m2/sec
Action
Spectrum
Absorption
Spectrum
Photosynthetic Pigments
• Chlorophylls
– Chl a
– Chl b
• Carotenoids
– carotenes
– xanthophylls
• Reaction Center Pigments
– PSI [P700] and PSII [P680]
• Chl a
• Accessory Pigments
–
–
–
–
Chl a
Chl b
carotenes
xanthophylls
Chlorophyll
•
•
•
•
Chl a in all higher plants
Chl b also in most higher plants
Chl c or d replaces chl b in some algae
Chl: porphyrin-like ring (loose e-’s) and
phytol tail to anchor it in thylakoid
membrane
• Center atom of Mg surrounded by 4 N
• Fe necessary for the synthesis of chl
CHO
Chlorophyll a
polar
head
Reaction Center Pigment
PSI and PSII
phytol
tail
beta-carotene
chlorophyll
thylakoid membrane
carotenoids