The Chemical Composition of Living Things: Quick Revision

A brief note on The Chemical Composition of Living Things. Read it before a test. Try each question at the end before you open its answer.

Analysing a tissue

• Dry a sample to find the water. Burn the dry matter to find the ash, the inorganic elements.
• Grind in trichloroacetic acid and strain. The filtrate is the acid-soluble pool of small molecules: amino acids, sugars, nucleotides and ions.
• The retentate is the acid-insoluble fraction: proteins, nucleic acids, polysaccharides and lipids.
• Lipids are small, but their membrane vesicles come down with the large molecules. So they are not true macromolecules.
• A cell is 70 to 90 per cent water. Proteins, 10 to 15 per cent, are the most abundant organic compounds.
• Primary metabolites have known roles. Secondary metabolites include alkaloids, rubber, drugs, pigments and gums.

Amino acids and proteins

• An amino acid has a hydrogen atom, an amino group, a carboxyl group and an R group on the alpha carbon. Twenty kinds build proteins.
• In water an amino acid is a zwitterion.
• A peptide bond joins carboxyl to amino and releases a water. A chain of $n$ amino acids has $n - 1$ bonds.
• Levels: primary (sequence), secondary (helix), tertiary (fold), quaternary (subunits, as in haemoglobin).
• Collagen is the most abundant animal protein. RuBisCO is the most abundant protein of the biosphere.

Sugars and lipids

• Sucrose is glucose and fructose, lactose is glucose and galactose, maltose is two glucose units.
• Starch, glycogen and cellulose are chains of glucose. Inulin is fructose. Chitin is an amino sugar.
• Starch holds iodine in its helix and turns blue. Cellulose does not.
• A saturated fatty acid with $c$ carbons has $2c$ hydrogens. Fats and oils are glycerides; oils melt at a lower temperature.
• Lecithin is a phospholipid of membranes.

A worked example

A tissue extract holds glucose, alanine, starch and haemoglobin. Which of them is in the acid-soluble pool?

1. Glucose and alanine are small molecules, far below $1000\ \text{Da}$.
2. Starch and haemoglobin are far above $10\,000\ \text{Da}$, so they are macromolecules.
3. Small molecules pass through the cloth.
4. So glucose and alanine are in the acid-soluble pool. Starch and haemoglobin stay on the cloth.

Practice with new numbers

A program to try

The program lists the four classes of large molecule with their units, their links and an example.

Mistakes to avoid

• Calling lipids macromolecules. They are small molecules that come down in membrane vesicles.
• Counting one bond for each amino acid. A chain has one bond fewer.
• Mixing up starch and cellulose. Both are glucose chains, but only starch turns blue with iodine.
• Treating secondary metabolites as waste. Many are drugs, spices and pigments.

Check yourself

1. Which pool holds the amino acids and sugars of a tissue?

2. How many peptide bonds are in a chain of twelve amino acids?

3. Which polysaccharide is the energy store of animals?

4. What makes a fatty acid unsaturated?