BIOL10008 · Foundations Of Biology: Life's Machinery
The Chemistry of Life
Biology runs on a handful of light elements joined by covalent bonds, and on the much weaker forces between molecules that those bonds create. This chapter sets up the chemistry the whole subject reuses: which six elements dominate life, how electronegativity — an atom's pull on shared electrons — predicts whether a bond is ionic, polar or non-polar, and how the weakest attractions of all (intermolecular forces, led by the hydrogen bond) give water its life-supporting properties. It ends with the single most reused reaction in the course, condensation (build, release water) and hydrolysis (break, add water), and with the nested hierarchy — atoms → molecules → macromolecules → organelles → cell — that maps the rest of the bible. The test move here is to predict a bond or force from electronegativity and to explain a water property from H-bonding.
What this chapter covers
- 01What counts as alive — the living checklist, and where viruses sit
- 02The elements & bonds of life: C, H, O, N, P, S and why carbon is the scaffold
- 03Electronegativity → ionic vs polar vs non-polar covalent bonds
- 04Intermolecular forces, weakest to strongest (LDF → H-bond)
- 05Water explained by hydrogen bonding (heat capacity, cohesion, solvent)
- 06The master reaction: condensation vs hydrolysis
- 07Life's hierarchy of organisation and emergent function
Worked example: predict the bond and the force from electronegativity
- +1(a) Classify the bond: oxygen is much more electronegative than hydrogen, so the shared electrons are pulled unevenly — a polar covalent bond, with a partial negative O (δ−) and partial positive H (δ+).
- +1(a) Justify with the gap: a large electronegativity gap transfers an electron (ionic); a moderate gap shares unevenly (polar covalent); a tiny gap shares evenly (non-polar). O–H sits in the moderate band.
- +1(b) Name the force: the strongest intermolecular force is the hydrogen bond. It needs an exposed H bonded to F, O or N on one molecule, drawn to a lone pair on another.
- +1(b) Check water qualifies: water's H is bonded to O and its O carries lone pairs, so each water both donates and accepts H-bonds with its neighbours.
- +1(c) Explain heat capacity: heating water must first break many H-bonds before the molecules can speed up, so a lot of energy is absorbed for a small temperature rise — a high heat capacity that keeps body and ocean temperatures stable.
Key terms
- Electronegativity
- An atom's pull on a shared (bonding) electron pair. A bigger nuclear charge or fewer electron shells means a stronger pull. The difference in electronegativity between two bonded atoms predicts the bond type.
- Covalent bond vs intermolecular force
- A covalent bond holds atoms together within a molecule; an intermolecular force (IMF) attracts whole molecules to each other. A hydrogen bond is a force between molecules, not a covalent bond — a classic exam mix-up.
- Hydrogen bond
- The strongest intermolecular force: an exposed H (bonded to F, O or N) is attracted to a lone pair on another molecule. Repeated billions of times, it gives water its life-supporting properties.
- Condensation reaction
- Joins two monomers into a polymer, forming a covalent bond and releasing one water molecule. It is anabolic (build, energy in). The bond is glycosidic in sugars, peptide in proteins, ester in lipids.
- Hydrolysis
- The reverse of condensation: a water molecule is consumed to split a bond, breaking a polymer into monomers. It is catabolic (break, energy out).
The Chemistry of Life FAQ
How do I tell an ionic bond from a polar covalent one?
Look at the electronegativity gap between the two atoms. A large gap means an electron is effectively transferred — an ionic bond (e.g. Na⁺Cl⁻). A moderate gap means electrons are shared but unevenly — a polar covalent bond (e.g. O–H in water). A small or zero gap means even sharing — a non-polar covalent bond (e.g. C–H). You are not asked for exact numbers; you are asked to rank the pull and map the gap to a bond type.
Why is water such a good solvent and so resistant to temperature change?
Both come from one feature — hydrogen bonding. Water is polar, so it surrounds and separates other polar solutes and ions (dissolving them), and its many H-bonds must be broken before the molecules can speed up, so a lot of heat is absorbed for a small temperature rise (high heat capacity). Cohesion and adhesion, which let water climb plant xylem, are the same H-bonds acting between water molecules and surfaces.
Is condensation really the same reaction in carbohydrates, proteins and lipids?
Yes — that is the point. Condensation always joins two units and releases one water; only the bond's name changes: glycosidic in carbohydrates, peptide in proteins, ester in lipids, phosphodiester in nucleic acids. Hydrolysis always reverses it by adding water. Learning one reaction unlocks the chemistry of the whole biomolecules chapter.
What does 'emergent' mean in the hierarchy of life?
An emergent property appears at one level of organisation and not the level below. A single phospholipid is not a barrier, but billions arranged as a bilayer are; a single brick is not a house, but bricks arranged as walls are. The test rewards explaining how the arrangement of the lower level creates the new function of the higher one.
Exam move
Build a reflex: given any two atoms, rank their electronegativity (more protons / fewer shells = stronger pull) and map the gap to ionic / polar / non-polar. Then practise explaining a water property from H-bonding in one sentence each — high heat capacity, cohesion/adhesion, solvent power. For the master reaction, train the two-word test: if two molecules become one and water appears, it is condensation (anabolic); if one becomes two and water disappears into the products, it is hydrolysis (catabolic). Finally, be able to order the hierarchy and place an example (glucose = molecule, mitochondrion = organelle) — and to say why arrangement creates the new function.
