Chemical Composition of Living Tissue

Elemental composition

  • No chemical element is unique to living matter. Living tissue and the earth's crust contain broadly the same elements but in very different relative proportions.
  • Carbon, hydrogen and oxygen are relatively more abundant in living systems. Carbon is about 0.03% of the earth's crust but roughly 18.5% of the human body; silicon shows the opposite trend, forming over 27% of the crust but only a negligible fraction of the human body.
  • A carbon compound isolated from living tissue is called a biomolecule. Living organisms also contain inorganic elements and compounds.

Wet weight, dry weight and ash

  1. Weigh fresh tissue to obtain its wet weight.
  2. Dry it: the lost mass is mainly water, while the remaining material gives the dry weight.
  3. Burn the dry tissue completely: organic carbon compounds are oxidised to gaseous products. The non-combustible residue is ash.

Ash reveals inorganic constituents such as calcium and magnesium and salts containing phosphate or sulphate. This experiment is different from acid extraction.

Acid-soluble and acid-insoluble fractions

Grind living tissue in trichloroacetic acid (Cl3CCOOH) and filter the slurry:

  • Acid-soluble pool (filtrate): small molecules, mostly about 18-800 Da and conventionally below 1000 Da. Examples include amino acids, monosaccharides, nucleosides, nucleotides, nitrogen bases, fatty acids, glycerol and organic acids. It roughly represents the cytoplasmic composition.
  • Acid-insoluble fraction (retentate): proteins, nucleic acids, polysaccharides and lipids. The first three are true biomacromolecules, generally around 10,000 Da or above.
  • Lipid exception: individual lipid molecules generally do not exceed about 800 Da, but membrane lipids form water-insoluble vesicles after grinding and are therefore recovered with the acid-insoluble fraction. The separation reflects acid solubility, precipitation and cellular association, not a simple molecular-size sieve.

Average cellular composition by mass

  • Water: 70-90%
  • Proteins: 10-15%
  • Nucleic acids: 5-7%
  • Carbohydrates: about 3%
  • Lipids: about 2%
  • Ions: about 1%

Water is the most abundant chemical in living organisms. Proteins form the largest organic fraction in the average composition table.

Amino Acids - Building Blocks of Proteins

  • Proteins are heteropolymers assembled from 20 standard amino acids.
  • A general amino acid is a substituted methane: the alpha-carbon carries an amino group, a carboxyl group, a hydrogen and a variable R group. In glycine the R group is also hydrogen, so the four attachments are not four different groups.
  • Named R groups: hydrogen in glycine, methyl in alanine and hydroxymethyl in serine.

Classification by side-chain character

  • Acidic: an extra carboxyl group, for example glutamic acid.
  • Basic: an extra amino/basic group, for example lysine.
  • Neutral: for example valine.
  • Aromatic: tyrosine, phenylalanine and tryptophan.

The alpha-amino and alpha-carboxyl groups are shared by the standard amino acids; differences arise from the R group.

pH, charge and the zwitterion

  • Amino and carboxyl groups are ionisable, so charge changes with pH and, where relevant, with ionisable R groups.
  • At sufficiently low pH an amino acid tends to have a net positive charge; at sufficiently high pH it tends to have a net negative charge.
  • At its isoelectric pH, positive and negative charges balance. The molecule can be a zwitterion with charged groups but zero net charge.

Essential and non-essential amino acids

  • Essential amino acids cannot be synthesised adequately by the human body and must be supplied through food. Examples include lysine, valine, methionine, leucine and phenylalanine.
  • Non-essential amino acids can be synthesised by the body. Examples include glycine, alanine and glutamic acid.
  • These are examples, not complete lists. "Essential" describes dietary need, not greater biological importance.

Visual - General Amino-Acid Structure

General amino acid and its zwitterionic form

The alpha-carbon carries an amino group, carboxyl group, hydrogen and R group. Net charge depends on pH and the R group; it is zero at the isoelectric pH.