G Proteins

G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three different subunits) associated with

  • the inner surface of the plasma membrane and
  • transmembrane receptors of hormones, etc. These are called G protein-coupled receptors (GPCRs).

The three subunits are:

  • Gα, which carries the binding site for the nucleotide. At least 20 different kinds of Gα molecules are found in mammalian

Second Messengers

Second messengers are molecules that relay signals received at receptors on the cell surface — such as the arrival of protein hormones, growth factors, etc. — to target molecules in the cytosol and/or nucleus.

But in addition to their job as relay molecules, second messengers serve to greatly amplify the strength of the signal. Binding of a ligand to a single receptor at the cell surface may end up causing massive changes in the biochemical activities within the

Bond Energy

If we pass a direct electric current through water containing enough ions to make it a good electrical conductor, it will break down into its constituents: hydrogen and oxygen. Both hydrogen and oxygen gas exist as diatomic molecules so the equation for this chemical reaction is

2H2O -> 2H2 + O2

The prefixes tell us that it takes 2 molecules of water to produce one molecule of oxygen and two molecules of hydrogen. If we decompose 36 grams of water (2 moles), we produce 2 moles