Life and Energy
Living versus non-living is one of the most fundamental distinctions we can make. For the most part, this distinction is an easy one to make: a person, shark, or tree is clearly alive, while a rock, cloud, or stream is clearly not. Yet defining life itself is not so easy – it is, in fact, one of our all-time great puzzles. Most biology texts list a series of characteristics that all living things seem to possess: metabolism (the exchange of energy and materials with the environment, including energy/food acquisition and waste excretion), growth, responsiveness to stimuli, self-maintenance (including homeostatic [physiological balance], immunological and anti-predatory mechanisms), movement (on at least a cellular level), and reproduction. But the characteristics of life is not the same thing as what life is.
Fundamentally, life is an energy-handling process. What most clearly separates the living from the non-living is the fact that life uses energy to maintain itself. Non-life cannot do this, and – as a direct consequence – passively erodes, corrodes, and otherwise breaks-down. The multitudinous ways in which living things handle energy are subtle and complex. But perhaps the most astonishing and revealing consequence of perceiving life as an energy-handling process is the realization that the very same principles of chemistry and physics governing non-living things can be seen in living things in easy-to-understand ways.
The study of energy flow is called thermodynamics (a holdover from one of the earliest-studied examples: heat flow in steam engines). Animals – such as human beings and White Sharks – cannot manufacture their own energy, and thus must steal it from plants or other animals. Virtually all multicellular animals obtain the energy that fuels their life-sustaining processes by combining the food they eat with oxygen in a kind of slow, controlled burn. Breaking apart chemical bonds in food releases energy. This energy, in turn, is packaged in a convenient, easily handled form: the molecule adenosine triphosphate (or ATP, for short). ATP is a kind of common energy currency used by all living cells. As in the non-living world, energy can change form from one kind to another – for example, chemical energy of ATP to mechanical energy in muscles, and from elastic energy in tendons to thermal energy of metabolic heat – but this never occurs with 100% efficiency. As an inevitable consequence, living things need to continually acquire new energy and non-living things eventually run-down.
Heat and molecules flow inevitably from areas of high concentration to areas of low. The former process is known as heat dissipation, the latter as diffusion (the diffusion of water is a special case, known as osmosis). The rate of this flow is dependent on the thermal or concentration gradient: the greater the difference in temperature or concentration, the more quickly heat or molecules will flow. These processes continue at an ever-decreasing rate until an equilibrium is reached, then stop. The same processes explain why even the hottest cup of coffee eventually cools and why milk and sugar introduced into a cappuccino at any specific location ultimately spread evenly throughout. Such passive flows of heat and molecules hold true for living things, too. Yet, unlike non-living things, living things are able to move heat and molecules against the thermal or concentration gradient. This process requires an input of energy, which is usually achieved by cashing in some ATP in what is termed active transport.
As in non-living things, the rate of chemical reactions in living things varies with concentration and temperature of reactants. A greater concentration of reactants increases the likelihood that some of them will combine and interact. A greater proportion of molecules are likely to have sufficient energy to combine and react at higher temperatures than at lower. Compounds known as catalysts change (usually decrease) the amount of energy required for a reaction to occur. Biological catalysts are referred to as enzymes. Like many biological mechanisms, enzymes are composed of protein.
Proteins are the work force of living things. The precise shape of a protein dictates its characteristics, including how it reacts with other proteins. After two or more proteins react, another fundamental energy law causes the participants to shift to the lowest possible energy state. For proteins, this is usually achieved by way of an alteration in shape, known as a conformational change. Although the rate of protein interactions generally increases with temperature, for a given protein this trend continues only to a certain point. Beyond this point, the protein's shape is permanently altered. This is why it is virtually impossible to un-hardboil an egg: the proteins that compose its albumin and yolk have been 'denatured'.
Metabolism is accomplished essentially through two antagonistic processes: building up (anabolism) and breaking down (catabolism). The minimum rate of anabolism and catabolism needed to maintain an organism is termed its basal metabolic rate (or BMR, for short). For aerobic animals such as humans, race horses, and White Sharks, BMR is usually measured as a function of oxygen use. Metabolic rate varies with such factors as temperature, activity level, age, and body size. Within limits, increasing temperature and activity level elicit increases in metabolic rate, while increasing age and body size usually induce reductions in metabolic rate. Thus, the metabolic rate of virtually all animals is fastest when they are youngest and smallest, slowing gradually over time. When metabolic rate equals zero, the organism is no longer alive – in other words: dead.
Therefore, thermodynamically speaking, life is an energy juggling act which ultimately fails.* What a given living thing accomplishes between birth and death is a measure of how successfully it has lived.