6.6: Simple Phototrophs

Phototrophs are organisms that capture photons (particles of light) and transform their electromagnetic energy into energy stored in unstable molecules, such as ATP and carbohydrates. Phototrophs “eat” light. Light can be considered as both a wave and a particle (that is quantum physics for you) and the wavelength of a photon reflects its "color" as perceived by a brain and the amount of energy it contains. Due to quantum mechanical considerations, a particular molecule can only absorb or emit photons of specific wavelengths (energies). This property makes possible spectroscopic methods, and enables us to identify molecules even when located at interstellar/intergalactic distances based on the photons they absorb or emit. Our atmosphere allows mainly visible light from the sun to reach the earth's surface, but most biological molecules do not absorb visible light. To capture this energy, evolution has led organisms to synthesize molecules, known as pigments, that can absorb visible light. The colors we see for a typical pigment are the colors of light that are not absorbed but reflected. For example chlorophyl appears green because light in the red and blue regions of the spectrum is absorbed while green light is reflected. So, how do organisms' use absorbed electromagnetic energy?

One of the simplest examples of a phototrophic system, a system that can directly capture the energy of light and transform it into the energy stored in a molecule, is provided by the archaea Halobacterium halobium.^[243] Halobacteriais an extreme salt-loving organism, a halophile. It lives in waters that contain up to 5M NaCl. H. halobium uses the membrane protein bacteriorhodopsin to capture light. Bacteriorhodopsin consists of two components, a polypeptide, known generically as an opsin, and a non-polypeptide prosthetic group, the pigment retinal, a molecule related tovitamin A.^[244] Together the two, opsin + retinal, form the functional bacteriorhodopsin protein.

When a molecule of retinal absorbs a photon, an electron moves from a lower to a higher energy molecular orbital. Extended molecular orbitals (← highlighted here) are associated with molecular regions that are drawn as alternating single and double bonds between carbon atoms, known as conjugated π orbital systems. Conjugated π systems are responsible for the absorption of light by pigments such as chlorophyll and heme, the pigment that makes blood red. Heme (not shown) includes an iron ion while chlorophyll includes a magnesium ion. When a photon of light is absorbed by the retinal group, it undergoes a reaction that leads to a change in molecular shape (↑) and composition, which in turn leads to a change in the structure of the polypeptide to which it is attached. This is called a photo-isomerization reaction. Isomers are similar molecules with different configurations.

Bacteriorhodopsin proteins are embedded within the plasma membrane where they associate with other bacteriorhodopsin molecules to form patches. These patches of bacteriorhodopsin give the organisms their purple color and are known as purple membranes. When a bacteriorhodopsin molecule (→) absorbs light, the change in its retinal group produces a light-induced change in protein structure that results in the movement of an H⁺ ion from the inside to the outside of the cell. The protein and its associated pigment molecule then returns to its original low energy (ground) state; its state before it absorbed a photon. The return of bacteriorhodopsin to its ground state is NOT associated with the movement of an H⁺ ion across the membrane. Because all of the bacteriorhodopsin molecules have the same orientation, as light is absorbed H⁺ ions move in the same direction across the membrane, leading to the formation of an H⁺ concentration gradient with [H⁺]_outside > [H⁺]_inside. This H⁺ gradient is associated with an electrical gradient because the movement of H⁺ leads to more positive charge outside the cell. As light is absorbed the concentration of H⁺ outside the cell increases and the concentration of H⁺ inside the cell decreases. One question is, where are the moving H⁺’s coming from? As you (perhaps) learned in chemistry, water can undergo a dissociation reaction (although this reaction is quite unfavorable): \[\mathrm{H}_2 \mathrm{O} \rightleftharpoons \mathrm{H}^{+}+\mathrm{OH}^{-}\nonumber\]

At pH 7.0 water contains 10^-7 moles of H⁺ and it is these H⁺ s that move.

As the absorption of light drives the movement of H⁺s across the membrane, they leave behind OH^– ions in the cytoplasm. The result is an electrical field with excess positive charges outside and excess negative charges inside the cell. As you may remember from physics, positive and negative charges attract, but the intervening membrane stops them from reuniting. The result is that when exposed to light there is an accumulation of positive charge on the outer surface of the membrane and negative charge on the inner surface. This charge separation produces an electric field across the membrane. An H⁺ ion outside of the cell will experience a force associated with the electric field together with the effect of a H⁺ concentration gradient. If there is a way back across the membrane, both the electrical field and the [H⁺] gradient will act to drive the movement of H⁺ ions back into the cell. The [H⁺] gradient acts as a battery, a source of energy that the cell can use.

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