Water and Hydrogen Bonding in Living Systems
Hydrogen bonds between water molecules give water its cohesion, its high heat capacity and its behaviour as a solvent. Every one of those properties traces back to the same charge separation across the molecule.
What a learner can do afterwards
- Explains cohesion and surface tension from bonds between neighbouring molecules rather than from water being sticky.
- Links the high specific heat capacity to what hydrogen bonds do when heat is added.
- Predicts which solutes dissolve readily in water and says what about the molecule decides it.
1 · Read
You saw water enter red blood cells so they can swell and may burst, and enter plant cells until walls hold them firm. A water molecule is polar. Oxygen pulls shared electrons toward itself, leaving the oxygen end slightly negative and the hydrogen ends slightly positive. The hydrogen side of one molecule is drawn to the oxygen side of its neighbour, and that link is called a hydrogen bond. Each bond is weak, but together they act strong.
Those links explain everyday sights. Drops bead up and insects dent the surface without sinking because neighbours pull on each other, which is called cohesion. Water climbs thin plant stems by gripping the walls and dragging the next molecule along, which uses adhesion with cohesion.
Heating water disrupts its shifting network of bonds while temperature rises slowly. Much added energy goes into that disruption, so high heat capacity steadies the temperature inside living things.
Let polarity decide what dissolves. The charged ends of water surround ions and polar molecules and tug them apart, which is why salt vanishes into water. Molecules without charged regions get no such grip, so fats and oils bead up instead of mixing.
Surface denting needs many neighbours sharing the load. Pressing down stretches their links without snapping them. The shared pull holds the insect up.
Glue sticking to other things is adhesion. Drops rounding from water pulling on itself is cohesion. Climbing stems uses both: grip on the walls plus drag to the next molecule.
One polar molecule builds a network of bonds, and that network explains clinging, slow warming and dissolving.
The rewritten reading was written for this dataset.
2 · Watch
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Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.