All living things must be able to respond, respire, take in nutrients, excrete, grow and reproduce.
The primary producers in the food chain are the photosynthetic organisms: green plants and some bacteria that are autotrophic (‘self food’) and manufacture sugars from raw materials using energy from the Sun. All other organisms in the chain are heterotrophic (‘other food’) and obtain their energy by consuming other organisms.
The lifespan of organisms is limited but the ability to perpetuate life ensures the survival of the species. Offspring have the same general characteristics as the parents. This is due to the existence of molecules, known as nucleic acids (deoxyribonucleic acid=DNA, and ribonucleic acid=RNA), that contain the coded information that passes between generations.
The smallest unit of life able to perform the life processes is the cell. Bacteria (‘small stick’) are unicellular, i.e. they consist of a single cell. Animals, plants and fungi are multicellular, i.e. they consist of many cells working together. The cell can be considered as a bag containing chemicals arranged differently and separated from those outside the bag. Without the bag the chemicals would mix freely and there would be no life. The surface of this ‘bag’ is a very thin surface membrane that controls the flow of molecules into and out of the cell.
One of the deepest divisions of life is between prokaryotic (‘before nucleus’) and eukaryotic (‘true nucleus’) cells. The nucleus is a central dense body within the cell and surrounded by its own membrane. It contains the genetic material of the cell and controls all the activity of the cell, including cell division. Eukaryotes probably evolved from prokaryotes.
As their name suggests, the prokaryotes lack a nucleus and they reproduce asexually, i.e. they self-clone; whereas eukaryotes reproduce sexually with genetic material being exchanged between organisms and this allows greatly increased genetic variation. Prokaryotes have an average diameter of 0.5-10 microns (one micron=10-6 metre); and for eukaryotes diameters of 10-100 microns are common.
Taxonomy
Organisms are classified in accordance with their shared features, and are divided into five very large groups called kingdoms: animals, plants, fungi, bacteria, and protoctists (‘first produced’). Bacteria are prokaryotes, the rest are eukaryotes. Protoctists contain all the eukaryotes that do not fit neatly into the other four kingdoms and includes organisms that resemble early plants (algae), early animals (protozoa) and early fungi (oomycota, ‘egg fungi’). The kingdoms are then broken down progressively into smaller and smaller groups: phyla, classes, orders, families, genera, tribes and species respectively.
Typical of phyla and their classes are chordates – fish, birds, mammals, reptiles, amphibians; arthropods – crabs, insects, spiders; molluscs – clams, snails, squid; annelids – leeches, worms; and the echinoderms – sea stars, sea urchins.
The orders and families of the mammal class include the carnivores – bears, cats; and the primates – apes, humans, monkeys. Organisms within a species can interbreed and produce offspring. The name given to an organism consists of two parts (binomial nomenclature): the genus (with a capital letter) and the species (with a small letter) both written in italics, e.g. Homo sapiens is the name for humans.
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