scrutinizing photosynthesis

[wanabe]

Photosynthesis, the process by which plants get their energy from sun light. What part(s) of sun light(ex: visible infrared uv... ) do they actually get energy from? How is this done?

[CharonY]

Check out light-dependent reaction and/or photophosphorylation. In short: light harvesting protein complexes (that also define the optimum wavelength) use light to initiate an electron flow to NADH+. During this transfer and due to the associated split of water a proton gradient is formed that powers the ATP synthase.

[wanabe]

Ah yes, thank you. I remember this from bio 100 now! Quite enlightening (sorry, it had to be said).

 

The light-harvesting system

 

A common misconception is that photosynthesis relies only on chlorophyll pigments. The truth is that photosynthesis would be rather inefficient using only chlorophyll molecules. Chlorophyll molecules absorb light only at specific wavelengths (see image). A large gap is present in the middle of the visible regions between approximately 450 and 650 nm. This gap corresponds to the peak of the solar spectrum' date=' so failure to collect this light would constitute a considerable lost opportunity. That's why photosynthesis organisms have developed a light-harvesting system, which bundles different pigments to create a much wider absorption spectrum.

 

The light harvesting-system is composed of numerous light-harvesting complexes that completely surround the reaction center where the photoinduced charge separation takes place. Chlorophyll, carotenes and xanthophylls are arranged in such light-harvesting complexes or LHC-proteins. These pigments are referred to as accessory pigments and funnel the energy to a special pigment in the reaction center of PSI or PSII.

 

If a pigment molecule absorbs a photon, an electron in the molecule becomes excited. For most compounds that absorb light, the electron simply returns to the ground state and the absorbed energy is converted into heat and/or fluorescence. But in a LHC-protein the pigments are so arranged that the excitation energy can be transferred from one molecule to a nearby molecule. The rate of this process, called resonance energy transfer, depends strongly on the distance between the energy donor and energy acceptor molecules. For reasons of conservation of energy, energy transfer must be from a donor in the excited state to an acceptor of equal or lower energy. If the energy of a photon becomes lower, the wavelength also becomes longer. The pigments in an LHC-protein are so arranged that pigments are very close to each other and a pigment is near another pigment that absorbs photons with a longer wavelength. As a consequence, the pigment in the reaction center has to absorb photons with the longest wavelength and can not transfer this energy to another pigment. The function of LHC-proteins is to create a constant supply of excitation-energy to the reaction center pigment. Every reaction center has a couple of LHC-proteins.[/quote']

 

This quote from wiki(Light-dependent reactions) some what specifies wave lengths.

Edited September 5, 2010 by wanabe

[Greippi]

Further to what's been said,

The wavelength of light that can be used for photosynthesis depends on the types of pigments used, using many different types of pigments will increase the spectral width of the antenna. For example, purple photosynthetic bacteria have carotenoids (which absorb green light), as well as various chlorophylls. Proteins are used to fine-tune the absorbance characteristics of the pigments.

 

Many organisms use the pigments best suited for their environment and what wavelengths of light are most abundant- for example red seaweeds can survive in low light conditions whereas green ones cannot.