Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Energy-Yielding Processes
Capture of Light Energy by Biomolecules

It is safe to say that life on our planet owes its existence to sunlight. Light energy is absorbed by photosynthetic organisms and stored within the chemical bonds of Organic compounds, whereas all other inhabitants of Earth (perhaps with the sole exception of chemosynthetic Bacteria) utilize the energy of these bonds to drive oxidative or substrate-level phosphorylation within their Cells.

Visible light represents a form of electromagnetic radiation with a wavelength of 400–700 nm, and its origin stems from complex processes occurring within the Sun. One of the outcomes of these processes is the emission of energy in the form of quanta of visible light (photons) that reach the Earth's surface. Photon energy is inversely proportional to its wavelength, meaning that photons with the shortest wavelengths—corresponding to the violet end of the visible spectrum—possess the highest energy.

The ability of a substance to absorb light depends on its atomic Structure. When a photon collides with an atom or molecule capable of absorbing light of a given wavelength, the photon's energy is absorbed by one of the electrons, elevating the atom or molecule to a higher-energy excited state. This excitation lasts for 10^-9–10^-8 s, after which the molecule returns to its initial ground state, which possesses lower energy than the excited state. Upon returning to the ground state, the excited molecule can dissipate its energy in several ways: 1) the energy can be dissipated as heat; 2) a portion of the absorbed energy can be immediately re-emitted as light (fluorescence) or after a certain delay (phosphorescence). The light emitted during Fluorescence and Phosphorescence typically features a longer wavelength and lower energy than the light that initially excited the molecule. Additionally (3), Light absorption can trigger photochemical reactions in which excited molecules are capable of participating.

Photochemical reactions involve dissociation into ions or radicals, proton addition (which may be accompanied by bond Cleavage), photoaddition and photoelimination reactions, as well as isomerization. Excited molecules are capable of turning into strong oxidizing and reducing agents, thereby inducing corresponding processes in other molecules. All of these reactions involving light-excited molecules become possible because, upon gaining energy, the molecules acquire chemical properties uncharacteristic of their unexcited forms.

Photochemical reactions play a particularly critical role when they affect vital cellular structures. Of paramount importance in this regard are Nucleic Acids, whose nitrogenous bases are known to undergo the photodynamic action of short-wavelength light. As a result of various photochemical transformations (primarily The formation of pyrimidine dimers), a multitude of alterations arise within the Introduction/20.html">DNA Structure which, if left unrepaired, become fixed as Mutations (Chapter 2).

On the other hand, photochemical reactions are critically important for phenomena such as Photosynthesis and photoreception. Among biological molecules, specialized ones exist that are capable of driving specific cellular processes in response to light absorption. These molecules are primarily Photosynthetic Pigments, which participate in capturing light energy during photosynthesis. In their excited state, chlorophyll molecules can initiate the photooxidation of Water molecules, accompanied by The transport of excited electrons through the components of Photosystems. Another Class of light-sensitive Biomolecules is represented by visual photoreceptors, which generate a Nerve Impulse in response to light absorption. The photochemical reaction triggering this process consists in the isomerization of a visual pigment molecule.