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2.3  Chemicals that Make Cells Work

or acidic depending on the concentration of hydrogen H^+​ ions when in water-​based solution.

The chemistry term pH refers to −log10 of the H^+​ ion concentration, which is a measure of the

acidity of a solution such that solutions having low values (0) are strong acids, those having

high values (14) are strong bases (i.e., with a low acidity), and neutral solutions have a pH of

exactly 7 (the average pH inside the cells of many living organism is around 7.2–​7.4, though

there can be significant localized deviations from this range).

Other broad categorizations can be done on the basis of overall electrical charge (posi­

tive, negative, neutral) at a neutral pH 7, or whether the side groups itself is electrically polar

FIGURE 2.5  Peptide and proteins. (a) Formation of peptide bond between amino acids to

form the primary structure. (b) Secondary structure formation via hydrogen bonding to form

beta sheets and alpha helices. (c) Example of a complex 3D tertiary structure, here of an enzyme

that makes ATP.