Although there are many possible biochemical reactions, they fall into only a few types to consider:
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Oxidation and reduction: For example, the interconversion of an alcohol and an aldehyde.
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Movement of functional groups within or between molecules For example, the transfer of phosphate groups from one oxygen to another.
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Addition and removal of water: For example, hydrolysis of an amide linkage to an amine and a carboxyl group.
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Bond-breaking reactions: For example, carbon-carbon bond breakage.
The complexity of life results, not from many different types of reactions, but rather from these simple reactions occurring in many different situations. Thus, for example, water can be added to a carbon-carbon double bond as a step in the breakdown of many different compounds, including sugars, lipids, and amino acids.
Regulating biochemical reactions
Mixing gasoline and oxygen can run your car engine, or cause an explosion. The difference in the two cases depends on restricting the flow of gasoline. In the case of the car engine, you control the amount of gasoline entering the combustion chamber with your foot on the accelerator. Like that process, it's important that biochemical reactions not go too fast or too slowly, and that the right reactions occur when they are needed to keep the cell functioning.
Large molecules provide cell information
The ultimate basis for controlling biochemical reactions is the genetic information stored in the cell's DNA. This information is expressed in a regulated fashion, so that the enzymes responsible for carrying out the cell's chemical reactions are released in response to the needs of the cell for energy production, replication, and so forth. The information is composed of long sequences of subunits, where each subunit is one of the four nucleotides that make up the nucleic acid.
Weak interactions and structural stability
Heat often destroys a biochemical system. Cooking a slice of liver at temperatures only slightly over 100°F. destroys the enzymatic activity. This isn't enough heat to break a covalent bond, so why aren't these enzymes more robust? The answer is that enzymatic activity and structure depend on weak interactions whose individual energy is much less than that of a covalent bond. The stability of biological structures depends on the sum of all these weak interactions.
Biochemical reactions occur in a downhill fashion
Life on earth ultimately depends on nonliving energy sources. The most obvious of these is the sun, whose energy is captured here on Earth by photosynthesis (the use of the light energy to carry out the synthesis of biochemicals especially sugars). Another source of energy is the makeup of the Earth itself. Microorganisms living in deep water, the soil, and other environments without sunlight can derive their energy from chemosynthesis, the oxidation and reduction of inorganic molecules to yield biological energy.
The goal of these energy-storing processes is the production of carbon-containing organic compounds, whose carbon is reduced (more electron-rich) than carbon in CO2. Energy-yielding metabolic processes oxidize the reduced carbon, yielding energy in the process. The organic compounds from these processes are synthesized into complex structures, again using energy. The sum total of these processes is the use of the original energy source, that is, light from the sun, for the maintenance and replication of living organisms, for example, humans.
The energy available from these reactions is always less than the amount of energy put into them. This is another way of saying that living systems obey the Second Law of Thermodynamics, which states that spontaneous reactions run “downhill,” with an increase in entropy, or disorder, of the system. (For example, glucose, which contains six carbons joined together, is more ordered than are six molecules of CO2, the product of its metabolic breakdown.)
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