WHAT IS LIFE?


INTRODUCTION

Many definitions of life exist although there is no general agreement on any of them as they all fall short for some aspects of life. Viruses do not get included in many classifications of life as they only ‘become alive’ within the cells of other life forms.

This lack of a comprehensive definition of life is even more surprising as humans generally have an immediate recognition of what things are alive, even organisms observed through a microscope. There are some exceptions.

No definition is very useful in experiments to determine the presence of life, particularly robotic experiments which are often fooled by non-organic processes. This caused a big controversy in the Viking life experiments on Mars. NASA, who always likes to avoid controversy has shied away from life detection payloads ever since this incident.


LIFE AS WE KNOW IT

One of the defining properties of life is that it consists of cells. The cell is the fundamental biological unit and is a common denominator of life. Al living things are composed of one or more cells, which may vary in size from one micron (or micrometre) to almost 30 millimetres. The other defining characterisitic of life as we know it is that it based upon the chemistry of carbon. Carbon can form up to four bonds with other atoms and can thus form more compounds than any other single element. This leads to a wide diversity of life.

Cells have many features in common across all forms of life. A cell consists of a nucleus which directs cell activities, a membrane which serves as a cell boundary and which selectively allows only certain moelcules to enter and leave the cell. Inside the cell is cytoplasm, vacuoles, chloroplasts and mitochondria, all of which have specialised tasks in the activities of the cell. The mitochondira supplies energy to the cell.

There are two main types on cells in macroscopic life - plant cells and animal cells. However, they all share the same basic components in their cells.

In addition to structural unity, there is a major degree of chemical unity in living organisms on Earth. The main elements in cells are carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Together with many other minor elements these atoms form molecules of three types: proteins, lipids(fats) and carbohyrates.

Cell size is one of most differentiating features between plant and animal cells. Plant cells are larger with sizes around 100 microns wheras animal cells are centred around 10 microns.

Other differentiating features of life are:

Not all these characterisitics are unique.

Three other definitions of life:

The second and third definitions are closely related.


LIFE AS WE DON'T KNOW IT

It is difficult to imagine life forms of a type that we have never experienced.

One suggestion is life based on the silicon atom rather than carbon. Although silicon can form four bonds as with carbon, it is a much heavier atom and the range of compounds it does form are far inferior to carbon. It is difficult to see much diversity coming out of a silicon biology, even though silicon is the basis of much of our information technology which is now heading in the direction of 'artificial intelligence'. This however has nothing to do with a diversity of silicon compounds.

It might be more useful to look at some of the life forms conjured up by true science fiction writers like Arthur C Clarke.


REFERENCES


ASAAustralian Space Academy