The strong bonds (ionic, covalent, metallic) build molecules. But the interesting behaviour — how proteins fold, how DNA pairs, how cell membranes form, how the protomolecule could in principle reorganise biological matter — happens through weaker forces between molecules. This module covers the three you need to know.
Hydrogen bonds
Established Remember water's polarity from the previous module? The slightly positive hydrogen on one water molecule is attracted to the slightly negative oxygen on another water molecule. This attraction is a hydrogen bond.
Hydrogen bonds are much weaker than covalent bonds — about 5–10% as strong. They constantly form and break at room temperature. But they are crucial to almost every biological structure.
Hydrogen bonds are why water has surface tension. Why water has an absurdly high boiling point for such a small molecule. Why ice floats — hydrogen bonds force water into a less-dense crystal structure when it freezes. Why DNA's two strands pair up (A-T and G-C base pairs are held together by hydrogen bonds). Why proteins fold into specific shapes.
Individually weak, collectively enormous. A single hydrogen bond breaks at room temperature. But a DNA molecule with millions of hydrogen bonds holding it together is extremely stable. This is the general principle: weak forces become powerful when you have many of them acting in concert.
Van der Waals forces
Established Even non-polar molecules attract each other weakly. Because electrons are in constant motion, at any given instant an atom might have its electrons slightly more on one side, giving a momentary partial charge. This induces a momentary opposite charge on a neighbouring atom, and the two briefly attract.
These are called van der Waals forces (or London dispersion forces, after Fritz London who explained them quantum-mechanically). They are extremely weak individually but accumulate over large surfaces. Gecko feet stick to walls because of cumulative van der Waals forces between the gecko's tiny foot hairs and the wall surface — millions of nearly-nothing interactions adding up to enough force to hold the gecko's weight.
Hydrophobic interactions
Established This is technically not a force but a consequence of water's behaviour. Non-polar molecules (like oils) don't form hydrogen bonds with water. When oil is in water, water molecules huddle together to keep their hydrogen bonds with each other, effectively excluding the oil. The oil molecules clump together not because they attract each other strongly, but because water actively excludes them.
This is the hydrophobic effect, and it is how cell membranes form (a bilayer of lipids with water-loving heads pointing outward and oily tails pointing inward), how proteins fold (oily amino acids tuck inside, water-loving ones face outward), and how soap works (one end binds oil, the other end binds water, letting the two mix).
Why does oil and water separate but salt dissolves in water?
Show answer
Water is polar. Salt is made of ions (charged particles), and water's partial charges can surround and stabilise them — the positive end of water faces Cl⁻, the negative end faces Na⁺, and the ions get pulled apart. Oil molecules are non-polar — they have no charges for water to grip onto. Water prefers to stay with its own hydrogen-bonded community and excludes the oil.
Why this module matters for everything that follows
Once you have these three weak forces in your toolkit, you can explain protein folding (a complex dance of hydrogen bonds, van der Waals attractions, and hydrophobic exclusion), DNA stability (massive hydrogen bond networks), cell membrane integrity (hydrophobic exclusion organising lipids), and the entire principle behind prion propagation (M-Bio-01) — which is the closest real biological analog to how the protomolecule converts matter on contact.