In the cell, single amino acids, sugars, and nucleotides can be joined together into polymers. Polymers are large molecules composed of small subunits arranged in a “head to tail” fashion. Living systems are based on polymers. There are several reasons why this is true:
- Economy of synthesis: Chemical reactions occur much more quickly and specifically in living cells than they do in an organic chemical reaction. The speed and specificity of biochemical reactions are due to the enzymes that catalyze the reactions in a cell. How does the cell get the many catalysts needed to support life? They can be made one by one or mass‐produced. Mass production is much more efficient, as can be seen by the following exercise.
Suppose that a living system needs 100 catalysts. These catalysts could be synthesized one by one. Where would the catalysts to make the catalysts come from? Making the set of 100 catalysts would require at least 100 more catalysts to synthesize them, which would require 100 more catalysts, and so on. A living cell would need a huge number of catalysts, greater than the number of known organic molecules (or even the number of atoms in the universe). Suppose, on the other hand, that the catalysts were mass-produced. Joining the amino acids to each other by a common mechanism allows a single catalyst to join 20 different amino acids by the same chemical reactions. If two amino acids join together, they can make 20 × 20 = 400 possible dimers (molecules composed of two similar subunits); joining three together makes 20 × 20 × 20 = 8,000 trimers (molecules made of three similar subunits), and so on. Because a single protein may contain 1,000 or more amino acids joined end to end, a huge number of different catalysts can be made from the relatively few monomer compounds.
- Economy of reactions: Joining monomers to make macromolecules is economical if the monomers can be joined by the same chemistry. If the monomers contained different functional groups, synthesis of each polymer would require a different kind of catalyst for each monomer added to the chain. Clearly, it is more economical to use a generic catalyst to put together each of the many monomers required for synthesis.
- Stability of cells: This argument is based on the properties of water. If red blood cells are placed in distilled water, they burst. Water moves across the membrane from the outside to the inside. In general, water moves across a membrane from the side with a lower solute concentration to the side with higher solute concentration; the side with higher solute con centration has a higher osmotic pressure. The cell has to expend energy to maintain its osmotic pressure. The osmotic pressure of a system is based on the number of atoms or molecules dissolved in water, not on their size. Thus, 100 molecules of a carbohydrate monomer (a sugar) have the same osmotic pressure as 100 polysaccharide molecules, each containing 100 monomers; however, the latter macromolecule can store 100 times more energy.