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Biology Unit 1: Chemistry of life

Living things are built from a few elements arranged into four big families of molecules. Water's polarity makes life's chemistry possible.

Covers: Water and hydrogen bonding, carbon, carbohydrates, lipids, proteins, nucleic acids, monomers and polymers

Study guide

Life is built mostly from a few elements (carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur), and its chemistry depends on water. A water molecule is polar: oxygen pulls electrons more strongly, so it's slightly negative and the hydrogens are slightly positive. That polarity lets water molecules form hydrogen bonds with each other, which explains cohesion (water sticking to water, as in surface tension), adhesion (sticking to other surfaces, as in capillary action), its high specific heat that stabilizes temperatures, ice floating because it's less dense than liquid water, and water's power as a solvent for polar and ionic substances.

Carbon forms four covalent bonds, so it builds chains, branches and rings, the backbones of organic molecules. Functional groups attached to these skeletons give molecules their properties.

Large biological molecules are usually polymers made of repeating monomers. Dehydration synthesis joins monomers by removing a water molecule, and hydrolysis breaks them apart by adding water. Carbohydrates (monomers: monosaccharides such as glucose) store energy as starch and glycogen and build structures like cellulose. Proteins (monomers: amino acids linked by peptide bonds) do most of the cell's work as enzymes, transporters and structural parts; their function depends on their folded shape, built up through primary, secondary, tertiary and quaternary structure. Nucleic acids (monomers: nucleotides of sugar, phosphate and base) store and transmit genetic information as DNA and RNA. Lipids, including fats, phospholipids and steroids, aren't true polymers; they store energy, form membranes and act as signals.

Carbohydrates and most lipids contain only carbon, hydrogen and oxygen. Proteins also contain nitrogen, and some contain sulfur. Nucleic acids contain nitrogen and phosphorus, and phosphorus also appears in phospholipids and ATP. Every amino acid has a central carbon bonded to an amino group, a carboxyl group, a hydrogen and a variable R group (side chain). R groups can be nonpolar (hydrophobic), polar or charged, and those properties control how a protein folds and where it sits. Nonpolar sections, for example, tend to fold inward or sit inside membranes.

Saturated fatty acids have only single carbon-carbon bonds and pack tightly, so they are solid at room temperature. Unsaturated fatty acids have double bonds that kink the chain, so they stay liquid. A phospholipid has a hydrophilic phosphate head and two hydrophobic tails, so in water phospholipids arrange themselves into a bilayer.

Key ideas

• Water is polar, so it forms hydrogen bonds: that gives cohesion, high specific heat, and its power as a solvent.
• Carbon forms four bonds, so it builds long chains and rings.
• Monomers join by dehydration synthesis (water released) and split by hydrolysis (water added).
• Carbohydrates store energy; lipids store energy and build membranes; proteins do most cell work; nucleic acids store information.
• A protein's shape (primary → quaternary structure) determines its function.

Formulas and rules

Common mistakes

• Calling lipids polymers. They aren't built from repeating monomers.
• Mixing up dehydration synthesis (builds) with hydrolysis (breaks).

Words to know

Amino acid

The monomer of proteins.

Nucleotide

The monomer of nucleic acids: sugar, phosphate and base.

Cohesion

Water sticking to water, from hydrogen bonds between polar molecules.

Dehydration synthesis

Joining monomers by removing a water molecule.

Phospholipid

A lipid with a polar head and nonpolar tails; forms cell membranes.

Carbohydrate

Sugars and starches; polymers built from monosaccharides.

Protein

A polymer of amino acids (polypeptide) that does most cell work.

Nucleic acid

A polymer of nucleotides: DNA or RNA.

Lipid

Fats, phospholipids and steroids; not true polymers.

Polar molecule

A molecule with uneven charge; water's O is slightly negative, its H's slightly positive.

Hydrogen bond

A weak attraction between a slightly positive H and a slightly negative atom.

Adhesion

Water sticking to other surfaces, as in capillary action.

Specific heat (water)

Water's high specific heat stabilizes temperatures.

Hydrolysis

Breaking polymers apart by adding water.

Monosaccharide

A simple sugar, like glucose; the monomer of carbohydrates.

Peptide bond

The covalent bond joining amino acids.

Protein structure levels

Primary (sequence), secondary (helices/sheets), tertiary (3D fold), quaternary (multiple chains).

Functional group

A cluster of atoms that gives an organic molecule its properties.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.