Photosynthesis

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• Lecture 10
• Photosynthesis
• Light absorption mechanisms

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Motivation
Plants provide the primary organic food for all
living organisms. Plants provide oxygen in the
atmosphere, fossil fuel, fiber.
How do plants convert CO2 into organic food?
Carbohydrates, proteins, lipids, secondary
products (medicine)
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General Concepts

• - mesophyll of leaves is most active
  photosynthetic tissue
• mesophyll cells have many chloroplasts
  containing
• chlorophyll
• light reactions take place in thylakoid
  membranes,
• producing high energy compounds ATP and NADPH
• (light energy is converted into chemical
  energy)
• dark reactions (carbon fixation reactions) take
  place in
• chloroplast stroma
• - understand
• nature of light
• properties of pigments
• various roles of pigments

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Light has characteristics of both a particle and
a wave
- Light is a transverse electromagnetic wave,
consisting of oscillating electric and magnetic
fields that are perpendicular to each other -
Speed of light c ?? 3108 m s-1 - Light is
a particle, a photon - Quantum energy of a photon
E h? (Plancks law)
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The Electromagnetic Spectrum
- Sunlight is like a rain of photons of different
frequencies - Eyes are sensitive to only the
visible region of the spectrum
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When molecules absorb or emit light, they change
their electronic state
(loss of energy by emission of light of longer ?)
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When molecules absorb or emit light, they change
their electronic state
Fates of sunlight absorbed in the
light-harvesting chlorophyll complexes. Chl,
chlorophyll 1Chl, excited singlet Chl 3Chl,
excited triplet Chl P, photochemistry D,
dissipation of excess excitation energy as
heat F, fluorescence 3T, triplet pathway,
leading to the formation of singlet oxygen (1O2)
and photo-oxidative damage. Taken from
Demmig-Adams B and Demmig-Adams W III 2000 Nature
403, 371.
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Photosynthetic pigments absorb the light that
powers photosynthesis
complex ring structure porphyrins involved in
electron transition (conjugated system)
Hydrocarbon tail, anchors Chl to thylakoids
Is chlorophyll soluble in water or ethanol?
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Photosynthetic pigments absorb the light that
powers photosynthesis
Functions of carotenoids 1. Accessory
pigments 2. Absorb blue light (400-500 nm)
for photosynthesis 3. Antioxidants
Open-chain tetrapyrrols in antenna structures of
cyano-bacteria and red algae
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Absorption spectra of photosynthetic pigments
1 bacteriochlorophyll 2 chlorophyll a 3
chlorophyll b 4 phycoerythrobilin 5 ?-carotene
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Action spectra relate light absorption to
photosynthetic activity
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Concept of Action Spectrum
Engelmann late 1800s What wavelength of light is
effective in O2 evolution?
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Photosynthesis takes place in complexes
containing light-harvesting antennae and
photochemical reaction centers
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Quantum yield of photosynthesis
F Number of photochemical products
Total number of quanta absorbed
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Enhancement Effect
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Oxygen-evolving organisms have two
photosystems operating in series
P680 and P700 - Reaction Center pigments (Chl a)
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The chloroplast is site of photosynthesis
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Organization of the Photosynthetic Apparatus
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Organization of light-absorbing antenna systems
Antenna funnels energy to the reaction center
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Organization of light-absorbing antenna systems
2-dimensional view of the structure of the LHCII
antenna complex from higher plants Transmembrane
pigment protein with three helical regions 14
Chl a and b molecules are associated with the
complex