Photosynthesis

Locations and Functions of Pigments

The location of pigments in photosynthetic organisms depends on whether the organism is prokary-otic (does not have a cell nucleus or organelles) or eukaryotic (has cell nucleus and organelles). The prokaryote Halobacterium halobium and other photosynthetic bacteria have pigments embedded in their cell membranes. Prokaryotic blue-green alga has pigment proteins inserted in a more complicated system of stacked membranes interior to the cell wall. Higher plants, such as needle-leaved plants and flowering plants, have a specialized organelle for photosynthesis within the plant cell, the chloroplast. The double-membraned organelle contains photosynthetic membranes that are embedded most commonly with the pigments, chlorophyll-a and chlorophyll-b.

Pigments are essential to photosynthesis, because they can absorb energy from photons, the units of light energy. Each pigment absorbs a characteristic wavelength, which is a stream of photons. For example, chlorophyll-a absorbs wavelengths in the range between 550 and 700 nanometers (nm, 1 × 10∼9meter), and bacteriochlorophyll-a in bacteria absorbs wavelengths between 470 and 750 nanometers.

Pigments efficiently absorb energy because they contain chemical bonds that accommodate fluctuating levels of energy. The characteristic carbon rings in pigments include many double bonds. Carbon atoms joined by double bonds share their electrons; thus the electrons are not strongly attracted to a particular carbon nucleus and move in a loose cloud around the entire molecule. When photons strike a pigment, their energy is accepted by the pigment's electrons, which can easily move from a lower energy level to a higher one in the cloud of electrons. Chlorophyll-a has five carbon rings with a total of 10 double bonds, making it an excellent acceptor of energy from light. The pigment can either donate the energized electrons to other molecules or release the energy from the electrons as longer wavelengths than those the pigment absorbed.

Structures in Photosynthesis

Organisms have structures in their photosynthetic membranes called reaction centers and antennae, respectively, both of which are necessary for photosynthesis to occur. The reaction center is composed of the unique pigments capable of initiating the chemical reactions of photosynthesis by donating electrons to molecules within cells; the pigments are bacteriochlorophyll-α in bacteria and chlorophyll-a in algae and plants. Scientists have identified special forms of these chlorophylls that are responsible for the actual work of changing light energy into chemical energy in the reaction centers. The chlorophylls are P870 in bacteria and P700 and P680 in algae and plants, where P stands for pigment and the number refers to an absorption wavelength. However, the specialized chlorophylls cannot absorb enough light energy on their own to drive photosynthesis; they are fed energy by the antennae.

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