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 CO 2. 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 CO 2, the product of its metabolic breakdown.