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Biology is the study of life. It is also a study of the systems of the natural world which make life possible.

To understand the distribution of ecosystems and the populations of species which inhabit them, it is necessary to understand the planet as a whole: its composition, its climate, and the circulations of minerals and energy which flow in and out of life systems.

In other words, to understand biology, we need to understand chemistry and physics as well. And mathematics will be useful as well!

Empirical Science

Biology, like physics and chemistry, is an empirical, or experimental, science. The empirical method really began during the period of history known as the 'Enlightenment'. The Enlightenment was driven by the scientific revolution, which began in the late 16th century and early 17th century, when scientists like Galileo Galilei proposed testing ideas, or hypotheses, by experiment.

Galileo Galilei
Galileo Galilei

The results of experiments would be measured carefully, and this data used to develop theories which tried to explain the observed phenomena. More experiments were then designed to test these new ideas, obtaining more data, which were used to develop more theories, which were then tested by experiment, which ........ and so on.

An important principle of empirical science is that the experiments be reproducible - that means anybody, anywhere in the world, could do the same experiment, and get the same results.

Sometimes, it is not possible, or difficult, to do an experiment in a laboratory to confirm a theory. It is relatively easy to run chemical tests in a laboratory, or small-scale plant experiments in a garden. but, the natural world is very complex, and there are many conditions which make controlled experiments unsuitable for replicating he complexity of the natural system.

It is therefore also necessary to make observations of nature 'in the wild'. For this, statistics and computer modelling are useful for attempting to understand the system being studied.

An example would be the effect of changing climate on species distributions and sizes. Species are counted and weather observations entered into a computer programme, which then attempts to recognise patterns. Sometimes, it takes years or decades of effort before these methods produce reliable results. It has taken several decades of careful work to model climate change, to the precision which today scientists can say for sure it is real and very damaging to the natural world and its inhabitants.


The three domains of life are: Bacteria, Archaea, Eukaryota.

Prokaryotes are single-celled bacteria and algae. Eukaryotes are multi-celled organisms, which compose animalia and flora. The word 'karyon' refers to a membrane-bound nucleus. 'Pro' therefore indicates the stage of evolution prior to this innovation.


Bacteria: prokaryote cells

Prokaryotes lack a membrane-bound nucleus in their cells. Most organisms in this category are unicellular. Exceptions are some bacteria which show multicellular forms at some stages of their life cycle, or colonies of single-cellular organisms, such as cyanobacteria and sponges.

Prokaryotes are distinguished as their type of cellular organisation, and consist of Bacteria and Archaea.


'Pro' is Greek for 'before', and 'karyon' is 'nut'. In this context, prokaryotes are the preliminary form before cooperative colonies of prokaryotes became inseparable and cells could specialise to form multi-celled organisms, which are the eukaryotes.

'Eu' is Ancient Greek for 'good' or 'genuine'.

Differences between Prokaryotes and Eukaryotes

    Eukaryote cell
  • Membrane-bound organelles
  • Eukaryotes have a nucleus and other cell parts encapsulated by membranes. Prokaryotes have all their proteins, DNA and metabolites in the one cell membrane volume.

  • Mitochondria
  • Eukaryotes have organelles which bacteria do not have. Mitochondria is a 'power pack' which makes eukaryotes more efficient and permit specialisation.


An ecosystem is a physical area with an interdependent community of species.

An ecosystem is a collective term, and attempts to create a model which is nearly self-contained. This means that most foodchains start and end within the boundaries and amongst the populations defined by the ecosystem.

Matter and energy flows are also defined within the limits of the ecosystem. There is solar energy from outside, but matter and energy exchanges between species and the physical environment to a large part are described by the ecosytem boundaries.

'Eco' is Latin for 'house'.

The hierarchy of living areas:

  • Biosphere: The entire life-supporting region of the Earth. The Biosphere is all the terrestrial surface, to a depth of about 1 km or more, the seas and waterbodies, and the troposphere.
  • Biome: a large planetary region with similar climatic and vegetation characteristics.
  • Ecosystem: a physically self-contained area and its communities, with largely intra-dependent foodchains and matter and energy flows.
  • Biota: the collective of living elements of an ecosystem.
  • Habitat: the set of conditions within a physically limited area, which are necessary for a specific species to survive.

The Role of Climate

Why things live where they live is a complex question. A simple answer is: 'because they can'.

Life is found all over the planet. It is very determined, and adapts to the conditions in which it finds itself. The most obvious factor in determining these conditions is the climate. The latitude (how far north or south) is the most important of all climatic factors, since the intensity of the Sun's radiation varies most directly with latitude.

The Earth is a sphere, so the part of the Earth receiving the most vertical sunlight (the middle, or equatorial regions) will receive the most sunlight, and be hotter. As we move further north or south, the same amont of sunlight is spread over a larger area, and therefore the temperature and irradiation (sunlight available for photosynthesis) decreases (for those who like maths: by the sine of the angle of incidence).

Seasons are caused by the angle of tilt of the Earth, making the sun's heat inconsistent at any one point on the surface

The Earth is tilted to the plane of its orbit around the Sun. This means the line between the north and south poles (axis of rotation) is not at right angles to the line Earth-Sun (in the orbital plane). The angle of tilt is 23.44°. As the Earth goes around the Sun, the tilt causes the angle the sunlight strikes the Earth's surface at (angle of incidence) to change, and therefore the amount of sunlight that hits any place on Earth will vary continuously throughout the year. This causes seasons.

'Eco' is Latin for 'house'. An ecosystem is a physical area with an interdependent community of species.

Why are there are so many species? What are the living and non-living factors that make every spot on the Earth unique in some way, and result in unique solutions from nature, whose aim is survival of the species.

An ecosystem is a collective term, and attempts to create a model which is nearly self-contained. This means that most foodchains start and end within the boundaries and amongst the populations defined by the ecosystem.

Matter and energy flows are also defined within the limits of the ecosystem. There is solar energy from outside, but matter and energy exchanges between species and the physical environment to a large part are described by the ecosystem boundaries.

Foodchains and foodwebs

A foodweb is a model of the feeding interactions between populations in a community

A foodweb is a network of at least two foodchains. It shows the complexities of interactions between populations in a community.

A foodchain is a series of prey-predator relationships demonstrating the trophic level interactions with single relationships.


Bioaccumulation is the process by which substances not readily broken down or excrete can build up and be stores in living tissue (usually in fatty tissue).

Some chemicals can bind to specific sites in the body, such as fatty tissue, causing them to stay in the body and not be excreted. The degree of bioaccumulation is proportional to the length of exposure to the chemical, which is in turn dependent on the size, metabolic rate and life expectancy of species.

An example of a human chemical which bioaccumulates in the environment is DDT. This is a pesticide which was widely used, until it was banned in the 1970s. DDT accumulates in living tissue, particularly fat tissue. When high concentrations occur in bird species, it can cause eggshell thinning, which prevents reproduction success. In humans it can cause nerve diseases.


Biomagnification is the process by which substances become more concentrated in the bodies of consumers as they move up the food chain (trophic levels).

An example is mercury. In the form methyl mercury, it can be absorbed by fish easily, and only slowly can be eliminated by the body. It then passes up the food chain, by birds and humans, who biomagnify the concentration. Methyl mercury was responsible for the Japanese Minamata disaster in the 1950s. It causes nervous system deterioration, involuntary muscle movements, impairment to speech and walk, corrosion to the skin and mucous membranes, and causes problems for basic bodily actions like swallowing.

The multiplication factor in bioaccumulation is called the Bioaccumulation Factor (BAF). The BAF for some persistent pollutants can be millions. This means that the concentration in a higher trophic level animal may be millions of times greater than the concentration in the water, where the chain of accumulation started.


A group of pollutants which bioaccumulate are PCBs, polychlorinated biphenyls. Since their invention in 1929, they have been widely used in industry, for such things as electrical transformers, cosmetics, varnishes, inks, carbonless copy paper, pesticides and for wood and plastic coatings (for weatherproofing and fire resistance).

When it was revealed that PCBs could affect the immune system, fertility, child development and cause cancer, the US government banned the production of PCBs.

Scientists: Edward Wilson

Content © Andrew Bone. All rights reserved. Created : December 23, 2013 Last updated :March 20, 2016

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