net: CO₂ + 2H₂O -> (CH₂O) + O₂ + H₂O

Clues to the mechanism of the photosynthesis reaction first came from experiments by van Niel in 1920's

• used a non-oxygen-evolving photosynthetic bacterium (purple bacterium) which can grow using reduced sulfur compounds
• net reaction in these bacteria is: CO₂ + 2H₂S + light -> (CH₂O) + 2S + H₂O
• this can be compared to oxygenic photosynthesis and served as a model to predict that H₂O was the source of oxygen and also the source of electrons to reduce CO₂ (H₂O is oxidized)

This summary reaction is carried out in many steps by many different enzymes. This process can be broken down into discrete sets of reactions carried out by a particular enzyme or set of enzymes.

• light harvesting
• water oxidation and reduction of NADP⁺ to NADPH coupled to ATP synthesis
• use of ATP and NADPH for the reduction of CO₂ to sugars (CH₂O)

The first two are called the light reactions and involve three type of protein complexes:

• light harvesting complex
• photosynthetic reaction centers
• ATP synthase

The third process is called the dark reactions and occurs in a biochemical cycle called the C3 (or Calvin) cycle

Photosynthesis-Light harvesting

What is the machinery involved in the capture of sunlight used to generate sugar and oxygen?

Action spectrum:

1880's-Engelmann-filamentous green algae and aerotactic bacteria

-shown spectrum of light split by prism onto algae-saw peaks of bacterial accumulation (higher conc. of O₂) in blue and in red

Properties of light=waves (wavelength) & packets of energy (photons)

• light has properties of both a particle and a wave
• can talk about photons=a discrete packet of light energy
• also can describe light in terms of its wavelength
• E=hv (E=energy, h=Planck's const., v=frequency) so, shorter wavelength (higher v) has greater energy; longer wavelength has less energy

How do we identify the molecules which perform this reaction?

• action spectrum=a plot of the level of biological activity versus a series of wavelengths of light
• absorption spectrum=a plot of the amount of light absorbed across a series of wavelengths
• for photosynthesis, the action spectrum is very similar to the absorption spectra of chlorophylls

Light harvesting mechanism

The primary molecule responsible for capturing light energy for photosynthesis is chlorophyll

• chlorophyll makes a transition to a higher energy state when it absorbs a photon of light chl + a photon -> chl^*

-this energy can be dissipated as:

• fluorescence (at a longer wavelength=lower energy)
• heat
• by resonance transfer to another chlorophyll (very efficient), NOT a redox reaction, just energy transfer
• redox reaction transferring an electron=photochemistry

In plants and green algae - light is collected by an array of pigment molecules=the antenna system

• the light harvesting antenna array is constituted by integral membrane, protein-pigment complexes=light harvesting complexes (LHC)
• include chl a, chl b, carotenoids

The light harvesting complex acts like a funnel for energy in two ways:

• there are several hundred pigment molecules for each reaction center, so the reaction center operates at maximum efficiency
• there is a cascade of energy levels of the excited states of the different pigments
• carotenoids>chl b>chl a (progressively higher wavelength absorption maxima)
• some loss of energy as heat, but nearly every photon is funneled to the reaction center

other organisms have different light harvesting pigments

• PS bacteria have bacteriochlorophyll
• algae can have chl c or chl d

Phycobilisomes

• red algae and cyanobacteria have phycobilisome complexes made up of phycobiliproteins (which bind pigments called phycobilins)
• rod=phycoerythrin-PE  (outside, 560 nm), phycocyanin-PC (inside, 620 nm); core=allophycocyanin-AP (650 nm)
• excellent example of energy cascade, with high energy carriers on the outside of the array, cascading down in energy levels to the center of the array, and eventually to the reaction center (P680)

Plants and cyanobacteria can adapt to varying light quality (color)

• cyanobacteria-can vary the proportions of PE and PC (produce PE in green light, not in red)
• plants-LHC can associate more with PS II or more with PS I
  -this is regulated by the redox state of the cytochrome complex
  (when cyt are reduced PS II more active than PS I, LHC goes to PS I)
  (when cyt are oxid PS I more active, LHC goes back to PS II)
  association is regulated by phosphorylation of the LHC

Phenomena that confused early researchers of photosynthesis:

Red drop

• drop in quantum yield at wavelengths above 680-690 nm (quantum yield = amount of product relative to the light energy absorbed)
• this means that far-red light (longer than 690) is absorbed, but that energy is very inefficient at driving photosynthesis
• suggests two photosystems acting in series - one that works best with red light, the other with far-red light

Emerson enhancement effect

• 1957-Emerson - measured PS activity using monochromatic light
• saw a certain rate in red light, similar rate in far-red light
• PS rate with both red and far-red is greater than the sum of the individual rates
• suggests two light harvesting systems with different wavelength optima

Reduction/oxidation of cytochromes

• in 1961 - Duysens was working with red algae - measuring the oxidation state of photosynthetic cytochromes
• in far-red light alone -> cytochromes were oxidized
• turn on green light -> cytochromes become reduced
• suggests one photosystem absorbs green light (remember these bugs have phycobilisomes) to reduce cytochromes; while a second photosystem absorbs far-red light and oxidizes those cytochromes

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