A eucaryotic cell is an elaborately structured chemical system. We have taken note throughout this chapter of where different chemical reactions occur, and these are summarized in the table on the following page. The individual chemical reactions and reaction networks all are controlled by enzymes, and are regulated by several different factors:

1. the concentrations of reactants and products
2. the availability of enzymes that can speed up one reaction over another, and hence decide which pathways will make the most use of a given starting material
3. the availability of a supply of ATP to make an energetically unfavorable reaction possible
4. direct inhibition of an enzyme by its immediate products
5. indirect or feedback control, positive or negative, of an allosteric enzyme by a molecule produced later in the reaction network
6. physical separation of the enzymes for a given process in one part of the cell or another
7. control by selectively permeable membranes over the circulation of metabolites and ions between various parts of the cell.

The structure of the cell thus has a strong influence on the chemistry that goes on within it.