What distinguishes living organisms from non-living things? Non-living things are not aware of their surroundings, and do not react to them. Living organisms, on the other hand, actively use their environment to help ensure their survival: they are ‘aware’ of it and react to it; they grow; and they reproduce to make more, similar organisms. Our concept of ‘life’ is based on a set of features called vital functions, considered here in some detail.
Taken in its broadest sense, metabolism encompasses all the chemical processes whereby food is converted into energy and body-building materials. All ingested food is broken down into the basic units of glucose and other sugars, fats and amino acids. Glucose and fats are ‘burned’ in the presence of oxygen, taken in from the environment by the process of respiration, to yield energy in a form useful for driving the body’s chemical processes. All these chemical reactions produce waste products which may poison the system if allowed to accumulate, and so need to be broken down and/or removed from the body by the process of excretion.
In multicellular organisms the conversion of food into energy takes place inside individual, just as it does in single-celled animals and plants. A complex series of chemical processes which include a specialized set of reactions called the Krebs cycle yields molecules of ATP (adenosine triphosphate), the ‘energy currency’ of living .
Most living things grow bigger as they get older. Some organisms, such as trees, continue growing almost until they die; while others, such as human beings, grow to a set point in their life cycle and then stop. Growth usually involves an increase in the number of cells of the original organism. We each start off as one cell, the fertilized egg, which multiplies again and again, increasing the number of cells and therefore our size.
Even though we stop growing in terms of overall body size when we become adults, the organs of our body continue to replace worn or damaged tissue by new growths ofy tissue. This occurs either through an increase in the number of cells, hyperplasia, or in the size of individual cells, hypertrophy.
Movement takes place both inside an organism and as a result of the organism changing position within its environment. Inside the cells of simple organisms, a fluid material known as cytoplasm ensures the adequate distribution of nutrients and other substances. In more complex organisms such as ourselves the internal distribution of nutrients and various chemicals is carried out by thestream. In a similar fashion, plants move nutrients around their body by diffusion or as sap flowing along small tubes. The simplest form of movement is termed amoeboid, from the single-celled amoeba in which it is clearly seen. The amoeba extends part of its cell membrane as a projection called a pseudopodium (’false foot’) and its cell cytoplasm flows into this. This apparently simple process occurs through the continual construction and dismantling of submicroscopic tubules and filaments within the cytoplasm, called cell organelles. Other single-celled organisms have small fur-like projections called cilia on their cell membranes which wave to and fro to propel the organism along. Others have tail-like structures called flagellae which, when lashed about, also cause the organism to move. In a human being a large proportion of the total body is concerned with movement: the bones, joints, muscles and the nerves that control them.
The Frenchman Claude Bernard (1813-1878) was the first to develop the concept of the internal environment, which he termed the milieu interieur. He realized that the body does not exist in a continuing state of flux in response to changing external conditions, but instead adjusts internal physical and chemical conditions so that they remain at a constant level. This process, known as homeostasis, is the primary function of most of the body’s organs. There are optimum environmental conditions in which any living cell functions most efficiently. Important factors are temperature, the amount of moisture available, osmotic pressure (the concentrations of dissolved substances) of the surroundings, the presence of nutrients and the absence of poisons. Single-celled and other small organisms are very dependent on their immediate environment, which they have little chance of altering except by moving to somewhere else. Larger organisms such as human beings have developed a barrier, the skin, that to a certain extent keeps the external environment at bay. The internal environment around body cells is controlled and kept roughly constant by various homeo-static mechanisms, so that, despite changes in the surroundings, body cells are always working in optimum conditions.
For example, the kidneys help to control the osmotic pressure of the, chiefly by adjusting the excretion of sodium; they also get rid of waste products. Body temperature is also continously monitored and kept at or near the ideal figure of 37°C. Below this temperature body functions would slow down and endanger life; so if we become cold our muscles contract involuntarily (shivering) thereby releasing heat and warming us up. Above 37°C damage is done to the enzymes that speed up many metabolic reactions; so if our temperature rises we sweat and evaporation of the sweat causes heat loss to cool us down.
The environment provides many stimuli, which we perceive as sensations because highly evolved organisms such as ourselves have special sensory organs and nerves to detect and interpret these stimuli. When we sense something we may also react to it, and the speed of this reaction is measured in terms of nervous response. The most fundamental reaction, seen in almost all animals and some plants (especially growing ones) is the withdrawal reflex: when a harmful stimulus is perceived the organism pulls away. This basic response protects it from further injury.
The genes inside a cell carry all the information needed to build and maintain that cell (indeed, to build and maintain the whole organism). Duplicating the genes is therefore the basis of reproduction. In fact some biologists have gone so far to suggest that the guiding force behind life is merely the urge of genes to reproduce. In this view, the organism is only a carrier of the genes and has the simple function of gathering enough raw material and staying out of harm’s way long enough to make more genes. The simplest form of reproduction is that of a single-celled organism diving into two, with the genes being duplicated and one set passing into each of the two new cells. More complex organisms reproduce sexually: two organisms each contribute a sex cell (egg or sperm) that contains half the normal complement of genes. The sex cells combine to form a new individual resembling, but not identical to, its parents. The function of reproduction is of course not vital for the individual, but it is vital for the continuation of the species. The mixing up of genetic material in sexual reproduction has the advantage that the species can better adapt to a changing environment.