Biological life would not be possible without chemical interactions. In turn, chemical interactions would not be possible without physical interactions, and no physical interaction occurs that does not involve an energy exchange; consequently, all biological processes depend upon energy exchanges. Clearly it is important to understand a little about this phenomenon if we are to understand how life began and how it proceeds.
We learned in school that almost all life on our planet depends, either directly or indirectly, upon the sun’s energy, converted to usable form through photosynthesis. During photosynthesis, chlorophyll converts simple molecules of water and carbon dioxide into more complex molecules (particularly sugars). Photons of sunlight provide the energy required to join the simple molecules together. This energy doesn’t disappear, it becomes locked within the larger molecules (held within the electromagnetic forces that bind the chemical elements together). Plants utilize these larger molecules as nutrients fuelling other biochemical processes (breaking the molecules apart releases the binding energy). When consumed, these plants in turn fuel micro-organisms, other plants, insects and animals. In this way, the sun provides most of the energy needed by life on this planet.
Before delving a little deeper into what happens during energy exchanges within living entities, it is helpful to review a few features of non-living energy exchanges. The latter have been occurring since the universe began, and they hold the key to understanding life’s creation—that instant when energy-processing chemical molecules first became energy-processing living molecules.
All chemical processes, living or non-living, involve energy transfers. Energy is either added to (or taken from) the involved atoms and molecules (by rearranging their electronic configurations). This energy is either taken from (or added to) the external environment. For instance, the energy required to form an iron compound when iron dissolves in an acid solution, is obtained from the energy released as the relatively complex configurations of electron orbitals in the acid are rearranged to form somewhat simpler ones. Forming iron sulphate in this manner needs no additional energy from outside the interacting molecules. (Quite the opposite; this process is exothermic—it releases energy in the form of heat as it proceeds.) Burning wood is another exothermic reaction; once started, the process sustains itself. When ignited, complex organic wood molecules break apart and release energy, only some of which is needed to join carbon from the wood and oxygen from the atmosphere to form carbon dioxide and other molecules. The rest of the energy is radiated away as heat and warms the universe.
Many chemical interactions do not release energy. These processes, termed endothermic, will not proceed, even after being started, without the continuous addition of energy. Producing plastics from oil, or forming sugar molecules by photosynthesis, are examples of endothermic reactions. In such cases, the final molecular compounds contain more energy than was originally held within the atomic structures of the forming components. This energy must be added before the bonds that hold the more highly structured molecules together will form.
Although every chemical process involves energy transfers, there is a significant difference between non-living (abiotic) chemical processes and living or biological (biotic) chemical processes. Abiotic chemical processes destroy or permanently rearrange the molecular structure of the constituents taking part in the process—the end products are different from those present at the start. However, healthy living cells do not permanently destroy, rearrange (other than when growing, learning or reproducing), nor deplete their own internal molecular configurations to obtain energy. They take what energy they need from their environment, eventually giving all of it back (in degraded form). While molecular configurations change continuously during life’s many and varied processes, they are re-established before these processes end. A living entity, at the end of a long day of processing food, is much the same as it was when it started. (Indeed, unless growing, learning or reproducing, any difference between start and end configurations would be due to disease or damage.)
This energy transformation process, whereby molecules gather energy from their environment, utilize it in various ways, yet retain their unique identity unchanged after the energy utilization, distinguishes biotic from abiotic matter. Thus, the first molecular complex able to sustain an energy-transfer process unchanged, using energy extracted exclusively from the external environment, became the first living entity.
- It is precisely because life’s processes are basically chemical processes that we can treat illnesses with chemically synthesized drugs, and can chemically manipulate emotions and genes. (Indeed, genes themselves are simply chemical molecules—and not even very complex ones.)
- Not all life on Earth depends directly upon sunlight, but all life requires an energy supply of some kind. As earlier noted, many simple life forms living near deep ocean hydrothermal vents or in subterranean rock crevices obtain their energy by chemosynthesis.