Water is such an everyday substance – you use water to wash, drink, make food; it is commonplace in our weather and rivers, and surrounds us in oceans. Its ubiquity is not only important for our environment: it is absolutely critical for life.
Water is simple and strong: two tiny, positive hydrogen atoms bonded to one big, fat, negative oxygen atom. This imbalanced charge sets up a strong dipole: oxygen hugs its electrons close, and the two hydrogens are pushed to be far more positive. This dipole makes water almost semi-structured, as the strongly positive side of one water molecule reaches for the strongly negative side of the next partner, and so on.
This endless dance is behind water’s entire approach to life.
Water can operate as a liquid across a far wider range of temperatures than most molecules of its size. It happily insinuates itself between other polar molecules, allowing them to dissolve.
Water accepts other molecules, from behemoth proteins to miniscule chemicals, lubricating the party so the right molecules can meet and separate with minimum fuss.
But while molecules of all sizes are at home with water, the relationships differ wildly. Small, polar molecules interact with water almost as peers. But larger molecules often drape the water around themselves, kind of like a microscopic cape, or shell.
This dynamic, fluid medium, with its random motion and friendly attitude towards other polar molecules, is the medium in which life works.
More than a medium
But water is far more than a medium for life.
It accommodates additional protons (hydrogens without their electron) easily, for example making positive H3O+ ions. This is so commonplace that is has its own vocabulary: pH, influenced by acids and alkalis. These extra protons can be donated by other molecules, or on temporary loan from neighbouring water molecules. They often play walk-on roles in enzyme catalysis.
This flow of protons is behind life’s most intensive energy-capture processes. Whether it’s capturing energy from light or releasing energy from complex chemicals, all life exploits proton gradients.
The mysterious movement of protons
Both bacterial cells and the organelles of more complex organisms have membranes that are impermeable to proton movement. By way of specialised proteins inside the cell, they use light or chemical energy to achieve a net movement of protons from one side of the membrane to another.
Protons present mainly in the form of H3O+ on each side. But exactly how one side loses a proton and the other side gains one is only partially understood – it may well involve complex, quantum chemistry. The process ultimately leads to a higher concentration of H+ ‘inside’ the cell or organelle.
At some point, ‘gatekeeper’ proteins release the protons out of the cell, often in equally mysterious ways, and harnessing the energy produced to perform actions. Intriguingly, in bacteria these gatekeepers include the flagella complex: a massive ‘tail’ that acts as a whip-like propeller to move the bacteria along. Other, less exotic but still fearsomely complex schemes make ATP, the energy currency of the cell.
The net movement and release of protons – stored as types of H+ ions on either side of a membrane – is the ultimate coupling that produces the vast majority of energy powering life on this planet. In both bacteria and organelles, proton pressure powers the creation of ATP (another ‘molecule of Christmas’).
Water in EMBL-EBI’s chemical biology resources
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