SynthesisCells contain some very large molecules. Proteins, polysaccharides, polynucleotides and hydrocarbons (part of lipids) are all huge molecules made up of hundreds of thousands of atoms all joined together by covalent bonds.
All of these very large molecules are made within the cytoplasm (and nucleus) of the cell in a complex series of chemical reactions in which smaller molecules (called 'monomers') are joined together in long chains (called 'polymers'). All the macromolecules are made the same way; smaller subunits are united in long chains to form the final structures.
Although the details may vary, most of these 'joining reactions', which unite the subunits into larger structures, are basically similar. One of the subunits (or monomers) has a hydroxyl group (-OH) sticking out from it. The second subunit (another monomer) has a simple hydrogen (-H) sticking out somewhere. Thus:
The chemical reaction that joins these two monomer subunits together also removes the -OH and the -H, and joins them together to form a molecule of water H-OH. Thus:
This type of reaction is very common inside cells and is called a dehydration synthesis, because a new molecule is made or 'synthesized', and a molecule of water is removed (de-hydrated, or 'water removed').
There is one problem however. A dehydration synthesis is an endergonic (or 'energy in') type of reaction that cannot take place without the input of energy from somewhere else. It is non-spontaneous, and by the second law of thermodynamics will not take place on its own. It needs help.
Cells, therefore, must somehow provide this extra energy from other exergonic reactions in order to be able to synthesize the very large, very complicated, very ordered macromolecules that are so important to life.