Cell Biology

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photosynthesis

Photosynthesis
Oxygenic photosynthesis apparently developed several billion years ago in an ancestor of present Cyanobacteria. Oxygenic photosynthetic machinery is located within the specialized internal thylakoid membrane system. The ability to construct and modify this thylakoid membrane system appears to be an important feature of oxygenic photosynthesis. Light-dependent photosynthetic reactions employ the thylakoid membrane-embedded antenna system to harness energy delivered by a photon. The light-dependent, photosystem reactions ultimately transduce the energy of light to generate molecules of ATP and NADPH, which act as energy-transfer molecules in the “dark” reactions of the Calvin cycle.

6 CO2 + 12 H2O - > C6H12O6 + 6 O2 + 6 H2O
or,
CO2 + 2 H2O = CH2O + H2O + O2

Several photosynthetic bacteria, such as the sulfur bacteria, perform non-oxygenic photosynthesis and do not generate oxygen. Nonoxygenic photosynthesis, or cyclic photophosphorylation differs from oxygenic photosynthesis in several ways other than lack of oxygen production.

Hydrogen sulfide (H2S) is utilized by the purple and green sulfur bacteria:

CO2 + 2H2S = CH2O + H2O + 2S

The chloroplast is the site of (oxygenic) photosynthesis in eukaryotic cells. The current consensus is that chloroplasts originated from Cyanobacteria that have become endosymbionts. This is an origin analogous to the endosymbiotic origin of mitochondria, which are believed derived from the "purple bacteria".

The thylakoid membrane, with its embedded photosystems, is the structural unit of photosynthesis. Both photosynthetic prokaryotes and photosynthetic eukaryotes possess membranes with embedded photosynthetic pigments. Only eukaryotes, which have a nuclear membrane and membrane-bound organelles, have chloroplasts with an encapsulating membrane.

Pigments are molecules with their own characteristic absorption spectra in response to light. The perceived color of the pigment depends upon the wavelengths of light not absorbed. Chlorophyll absorbs all wavelengths of visible light except green, which it reflects, producing the green color of leaves. Various chlorophylls and accessory pigments have characteristic absorption spectra. The action spectrum of photosynthesis relates to the relative electron-exciting effectiveness of different wavelengths of light.

Absorption Spectrum of pigments: results from the ability of pigments to absorb incident electromagnetic radiation. Like the action spectrum, the parameter of interest (light absorbed) is plotted as a function of the wavelength of the radiation. This is the opposite of an emission spectrum where wavelengths emitted by a substance are measured.

Action Spectrum of pigments: the efficiency of photochemical response to incident electromagnetic radiation. Like the absorption spectrum, the parameter of interest (here photochemical reaction) is plotted as a function of the wavelength of the radiation.

The action spectrum of photosynthesis within an organism resembles the absorption spectra of chlorophyll though it is not identical. This difference arises because accessory pigments play a part in photosynthetic efficiency.

Photosynthesis: Cyanobacterial cell

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