What is life? In a nutshell, scientists would define it as a self-replicating process that requires at least one ‘thermodynamic cycle’. To put that another way, something that is alive is able to make a copy of itself, and it does this by harnessing a source of energy, using it to transform chemical resources in some way. The supply of energy must be continuous; if the energy source were to become unavailable, or the life form became unable to tap it, then the result would be death. That is something else unique that life can do: it can die.
According to this definition, the simplest life form is a strand of nucleic acid, something like RNA (see here). This chemical is able to use its own molecule as a template for a copy of itself. However, such a life is incredibly precarious, and over billions of years of evolution, a multitude of life forms have developed abilities that ensure survival. These abilities are set out in genes, and they govern the success or failure of a life. To understand life, one must begin with genetics.
The number of different types, or species, of organism on Earth is estimated to be anywhere between 3 and 30 million, with most biologists erring towards about 9 million.
The simplest and oldest life forms are the bacteria, which have a body made from a single tiny ‘prokaryotic’ cell (see here). They are joined by the archaea, which to the uninitiated look more or less the same but have some important distinctions. Other single-celled organisms, including things like amoebae and protozoa, have much larger and more complex cells, and this ‘eukaryotic’ cell type (see here) is the one used by multicellular organisms such as plants, animals and fungi.
Every species of organism has a unique way of life, but members of any biological group share more characteristics with each other than with the members of other groups. However, all life forms share a set of abilities: they sense the surroundings, excrete, reproduce, grow, respire and require nutrition.
The broad term ‘metabolism’ encapsulates the link between chemical activity and life – the myriad chemical processes that are occurring inside every organism are described as its metabolism. In very general terms, these include the way the organism handles its energy supply, and how it uses this to grow and repair its body, making use of simple chemical building blocks.
Metabolic processes fall into two general types: anabolism and catabolism. The former involve building larger, more complex and more ordered structures out of smaller units. (That is why a sports cheat might use an ‘anabolic steroid’, a chemical that builds muscle.) Catabolism, in contrast, involves splitting large structures into smaller ones (this includes processing unwanted waste materials to generate energy). Anabolic and catabolic processes are constantly working together to release manageable packets of energy and then put them to work in keeping the organism alive.
Every living thing must feed, or putting it more precisely, they must access a source of nutrition. Plants get this in the form of sugars from photosynthesis and mineral nutrients absorbed from their surroundings (soil is a good place to start). Animals and fungi get their nutrition from the bodies of other organisms. Some single-celled organisms can get nutrition using both techniques!
Nutrition has two main purposes. First, it is a source of chemical energy that can be extracted and put to work in the body (the best examples of this are glucose and other sugars). The second purpose is as a stockpile of the raw ingredients required to build a body. The requirements of different organisms vary wildly: plants are able to build everything they need from water, carbon dioxide, and a menu of minerals such as nitrates and phosphates, while animals need more complex nutrition, such as fats, starches, proteins and a range of crucial helper chemicals, known collectively as ‘vitamins’.
When most people hear the term ‘respiration’, they tend to assume it relates to breathing. But while this is indeed the word’s common medical context, biology gives it a wider meaning: in fact, all organisms respire, whether or not they breathe in and out in the way that vertebrate animals do.
Biologically, respiration is defined as the metabolic process that releases energy from sugar or other chemical fuels. Typically, this involves the fuel molecules being oxidized – exactly the same chemical reaction involved when materials combust in air. The respiration of glucose, one of the most common sugars, for example, can be written in the form of a chemical equation as shown opposite. This demonstrates that glucose reacts with oxygen to produce carbon dioxide and water, plus some energy. If raw glucose is burnt in air, the reaction produces flames and heat, but within a living cell it can be heavily regulated, allowing small packets of energy to be released in several steps.
As the word suggests, ‘photosynthesis’ is the process of ‘making with light’, and the end product in question is glucose sugar. Photosynthesis takes place in the leaves and the other green parts of plants and other photosynthetic organisms. The colour is important because the energy from sunlight is absorbed by a pigment chemical called chlorophyll in the plant’s cells – chlorophyll itself appears green because it traps the blue and red wavelengths of sunlight while reflecting other colours.
Chemically, photosynthesis is the reverse of respiration, with carbon dioxide and water molecules being combined to make glucose molecules and oxygen, all powered by the energy channelled from the chlorophyll molecules. While carbon dioxide is the waste product of respiration, photosynthetic organisms produce waste oxygen, which is released into the air. Nearly all of the oxygen in Earth’s atmosphere (about 20 per cent of all the air) originated as the by-product of photosynthesis.
To qualify as living, an organism needs to be able to grow – at least at some point in its life. For most complex organisms this is a simple thing to verify. The majority of multicellular life forms – those with bodies of more than one cell – grow from a single cell into an embryo and on to a fully developed adult. This growth is achieved by the division of cells (see here and here), with every cell in the body being descended by some route from that first single cell, known as the zygote.
The growth of single-celled organisms – things like bacteria and amoebae – is less clear cut. They too can divide their cells, but instead of creating a larger body, they produce a new and independent individual. In these cases, growth and reproduction are two sides of the same coin. Therefore, the best definition of growth is the ability to produce new cells from older cells. This is the concept that lies at the heart of cell theory (see here), a central tenet of life science.
It could be said that the primary goal of an organism is to survive. However, that survival is really a means to an end – all organisms are striving to make a copy of themselves or something close to it. In other words the true purpose of biological life is reproduction. There are many modes of reproduction, ranging from organisms simply dividing in two to a complex process of courtship, mate selection and parental care. However, broadly speaking there are two types of reproduction: sexual and asexual. The former involves two parents and the latter requires only one (see here and here).
The struggle to survive and reproduce is the driving force behind evolution by natural selection (see here), the process that shapes the millions of species that live on Earth. However, this evolution is a by-product of reproduction. The genetic purpose of reproduction is to make new copies, and many of them, of the DNA molecules in all bodies, reproducing the information that we call genes.
Just as an organism takes in nutrients and other raw materials from its surroundings, it must also remove the waste products of metabolism – a process known as excretion. Despite common usage, the voiding of the bowel, passing faecal matter, out of the body is not actually excretion in biological terms: instead, it is defecation or egestion. The crucial difference is that the unused food has not really entered the body – it has only passed through the gut, a hollow tube that runs through the body. True excretion is the process of taking waste products – which may be harmful if left to accrue – from the body’s tissues and expelling them.
In human biology the chief mode of excretion is urination, whereby excess water and nitrogen-rich waste in the form of urea are released. Excretion can also occur directly through the skin as sweating. In addition, the release of carbon dioxide generated by respiration processes is also a form of excretion.
All life forms are able to detect changes in their surroundings and respond to them. For single-celled organisms this may be simply a matter of detecting a chemical change, such as the salinity of water or the presence of nutrients or toxins. Plants, meanwhile, are sensitive to light, gravity and sometimes pressure – they grow towards light and away from the pull of gravity, and some adjust their growth patterns to wrap themselves around other objects they contact.
Animal senses are much more advanced, befitting their active lifestyles. The five used by humans are somewhat ubiquitous: hearing, smell, taste, vision and touch. The last of these is a complex mix of detectors on the body surface, sensitive to heat, cold, vibrations and pressure. Other animals can sense things beyond a human’s abilities. Many insects and other arthropods can detect ultraviolet light; sharks and their cousins can detect electrical activity in another body, while many other animals appear to sense Earth’s magnetic field.
