Principles of Biochemistry Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Photosynthesis
Light absorption excites molecules to a higher energy state

Visible light is electromagnetic radiation with wavelengths ranging from 400 to 700 nm. Solar radiation originates from the fusion of hydrogen nuclei to form helium atoms and electrons. This process is made possible by the extremely high temperatures deep within the Sun. In general terms, the reaction can be written as follows:

Class="center">4Н → ⁴Не + 2е^- + hv,

where hv is a quantum of light energy, also referred to as a photon. Recall that light exhibits both wave and particle (corpuscular) properties. The energy possessed by photons is inversely proportional to the wavelength of light (Table 23-1). Photons in the short-wavelength (violet) region of the visible spectrum possess the highest energy.

Table 23-1. Photon Energy

¹⁾ One Einstein (1 "mole" of photons) contains 6.023∙10²³ photons.

The ability of a chemical compound to absorb light depends on the distribution pattern of electrons around the atomic nuclei within its molecule. When a molecule absorbs a photon, one of its electrons is promoted to a higher energy level. This occurs in an "all-or-none" fashion: to boost an electron to a higher energy level, the photon must possess a specific minimum amount of energy (Latin quantum, meaning "how much"; hence the alternative name for a photon, "quantum"). A molecule that has absorbed a photon is in a high-energy excited state, which is generally unstable. When the light source is turned off, the "high-energy" electrons typically drop back down to their low-energy orbitals almost immediately, returning the molecule to its initial stable ground state while releasing the excitation energy (in the form of light or heat). The light emitted by an excited molecule as it returns to its ground state is called fluorescence (Fig. 23-7). The transition of a molecule to an excited state upon Light absorption and the emission of energy via fluorescence are extremely rapid processes. Exciting a chlorophyll molecule in vitro takes only a few picoseconds (1 ps = 10^-12 s). The lifetime of the molecule in the excited state is also remarkably short: calculations show that during the time a chlorophyll molecule remains in the excited state, a Concorde supersonic airliner flying at maximum speed would travel a mere 6 µm.

Fig. 23-7 Transition of an atom to an excited state resulting from the absorption of light energy. When the atom returns to its initial ground state, it releases the absorbed light energy (as fluorescence or heat). However, in photosynthetic Cells, the absorbed energy is not dissipated as fluorescence, but is instead conserved through The formation of NADPH and ATP.

We now arrive at a critical point. If a solution of chlorophyll in vitro is illuminated, the absorbed energy is rapidly re-emitted as fluorescence or dissipated as heat. However, when chlorophyll in intact spinach leaves is excited by visible light, no fluorescence is observed. Instead, high-energy electrons leave the excited chlorophyll molecules and "leap" to the first electron carrier in the Electron Transport Chain. Light thus induces a flow of electrons along this chain. Coupled to this electron flow are the processes that generate ATP and NADPH.