Chapter 7 Photosynthesis: The light reactions

synthesis using light

is the only process of biological system that can harvest sunny energy. a deceptively simple equation 6 CO2 + 6 H2O C6H12O6 + 6 O2

stored energy can be used to power other cellular processes in the plant and provide the energy source for all forms of life. Topics: the characteristics of light, the structure of the photosynthetic apparatus, the processes of chlorophyll excitation, the synthesis of ATP and NADPH.

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Light: a wave and a particle

＊ c (the speed of wave) =λν

＊ E (the energy of photon)= hν

h : Planck’s constant (6.62 × 10-34 Js)

＊ Quantum (quanta)

each photon contains an amount of energy

discontinuous

Electromagnetic spectrum

Sunlight like a rain of photons of different frequencies.

UV-A: 320 – 400 nm

UV-B: 280 – 320 nm

UV-C : 290 nm

Light emitting diode (LED)

blue 470 nm

red 660 nm

Absorption spectrum: the amount of light energy taken up or absorbed by a molecule or

substance as a function of the light wavelength of the light.

green

Action spectrum 光譜 :the magnitude of a response of a biological system to light,

as a function of wavelength

Light-induced phenomenon

Carotenoid absorption:

400 ~500 nm

→ the pigment responsible for the particular light-induced phenomenon

O2 evolved

Late 1800s

Photosynthetic pigments:

porphyrin-like group

accessory pigments

cyanobacteria

red algae

Chl a, b, c, dxanthophyll

phytol chain

2. Chlorophyll a in nonpolar solvent

3. Chlorophyll b in nonpolar solvent

4. Phycoerythrobilin in an aqueous buffer

5. β-carotene in nonpolar solvent

Light absorption and emission by chlorophyll

1. heat loss; 2. fluorescence;

3. energy transfer; 4. photochemistry.

Chl + h → Chl* (10-9s), discontinuous

Light energy Chemical energy

Antenna天線 complex: collect light and physical resonance transfer, no chemical reactions; high efficiency of energy transfer.

Reaction center: the chemical oxidation and reduction reactions leading to long-term energy storage take place.

pigments,

e- transfer proteins

more efficiency

Chl b Chl a

Core/peripheral

The antenna funnels 注入 energy to the reaction center— fluorescene resonance energy transfer

λmax of Chl b: 650 nm

Chl a: 670 nm

red shif

A physical phenomenon

directionality and irreversibility

p. 138

1932 Emerson and Arnold

oxygen production vs. flash energy

– a suspension of Chlorella pyrenoidosa

– 10-5 s flash, 0.1 s apart

– varied light intensity 強度 ,

slope

saturation

many chlorophyll molecules

involve in energy conversion

during photosynthesis

– Quantum yield () No. of photochemical products / Total No. of quanta absorbed 0.1 for O2 production

Quantum requirement

10 for O2 evolution

– Quantum efficiency 量子效率≒ 1

engage in photochemistry / absorbed photons

– Energy efficiency 0.25≒ the stored energy of chemical compound / the energy of absorbed photons

CO2 + H2O light (CH2O) + O2 G = 467 kJ/mol

red light: 467×103/10 ×17.1×104

blue light: 467×103/10 ×23.95×104

E = hν

O2/10 photons

EE

low

Hill reaction (1937)

an artificial electron acceptors in isolated chloroplast thylakoid

No CO2 condition, still O2 production

4 Fe3+ + 2 H2O → 4 Fe2+ + O2 + 4 H+

CO2 + 2 H2S → (CH2O) + 2 S + H2O H2

18O + CO2 → (CH2O) + 18O2

(1) CO2 + 2 H2O → (CH2O) + O2 + H2O

(2) CO2 + H2O → (CH2O) + O2

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[In eukaryotic photosynthetic organisms, the reaction centers and most

of the antenna complexes are integral within the membrane of

chloroplast; in photosynthetic prokaryotes, the site of photosynthesis is

the plasma membrane or membranes derived from it. P. 132]