Photosynthesis Process in which light energy is used to drive the conversion of CO 2 to carbohydrate. 6H 2 O* + 6CO 2 C 6 H 12 O 6 + 6O 2 * light

Importance of Photosynthesis Source of atmospheric oxygen –Earth’s atm: 78% N 2, 21% O 2, 0.035% CO 2 (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 CO 2 enters leaf through open stomates; water is lost CO 2 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 Cuticle Epidermis guard cells & stomate 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 (CO 2 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 CO 2 to carbohydrates using ATP & NADPH

3 Processes in Photosynthesis (1) light O 2 CO 2 sugar starch H2OH2O (2) Light Reactions (3) Dark Reactions NADPH ATP

Light Striking Leaf Reflected back to atmosphere Passes through leaf back to atmosphere Absorbed by the leaf and used for PS

GAMMA RAYS X-RAYS UV V INFRARED RADIO WAVES ENERGY WAVELENGTH Visible Light 380 nm 430 nm 650 nm 750 nm

PAR PAR = Photosynthetically Active Radiation nm wavelengths PPF = photosynthetic photon flux PPF at full sun = 2000 µmol/m 2 /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

Chlorophyll a CHO Reaction Center Pigment PSI and PSII polar head phytol tail

beta-carotene

chlorophyll carotenoids thylakoid membrane