Characteristics and Classification of Living Organisms

The seven characteristics of living organisms

Biology starts with a working definition of life. Cambridge lists seven characteristics, and a living organism shows all seven at some point during its life. Learn the wording exactly, because definition questions are marked against the syllabus words.

Movement is an action by an organism or part of an organism causing a change of position or place. Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Sensitivity is the ability to detect and respond to changes in the internal or external environment. Growth is a permanent increase in size and dry mass. Reproduction is the processes that make more of the same kind of organism. Excretion is the removal of the waste products of metabolism and substances in excess of requirements. Nutrition is the taking in of materials for energy, growth and development.

Two of the seven are traps. Growth must be permanent and it must be an increase in dry mass, so a plant that has merely swollen with water has not grown. Excretion removes waste that was made by metabolism inside the cells, so passing out undigested food is egestion, not excretion.

Classifying by shared features and the species concept

There are millions of species and no one can hold them all in mind, so biologists put organisms into groups by the features that they share. A good classification lets you predict: if you know an organism is a mammal, you already expect it to have hair, lungs and mammary glands.

The basic unit of classification is the species. Cambridge defines a species as a group of organisms that can reproduce to produce fertile offspring. That word fertile carries the mark. A horse and a donkey can breed, but the mule they produce is sterile, so the horse and the donkey are different species.

Every species has one scientific name, given by the binomial system. This is an internationally agreed system in which the name of an organism is made up of two parts showing the genus and the species.

• The genus comes first and takes a capital letter; the species comes second and is lower case.
• The whole name is written in italics, or underlined if you are writing by hand.
• Because the system is internationally agreed, a tiger is Panthera tigris in every country and in every language.
• Use only the two parts of the scientific name. A third name, or a common name such as 'tiger', will not score.

Dichotomous keys

A dichotomous key is a series of paired statements that you work through to identify an unknown organism. Each pair is called a couplet, and each couplet offers two choices; each choice leads either to another couplet or to a name.

Build a key by choosing features that are observable, clear-cut and present in only one of the two options. The two statements in a couplet must be opposites, so that only one of them can ever be true for the specimen in front of you.

Start with a feature that splits the group into two large halves, then keep splitting. The final couplet should separate the last two organisms, so that every specimen reaches exactly one name.

• Use features you can see and count: number of legs, presence of wings, type of skeleton, leaf shape.
• Do not use behaviour, habitat or anything you cannot observe directly from the specimen.
• Write couplets as paired opposites, for example '1a has six legs ... go to 2' and '1b has more than six legs ... go to 3'.
• Never use the same feature twice in one key, and make sure every organism in the group can be identified.

Classification reflects evolutionary relationships

A classification is not just a filing system. Modern classification aims to reflect evolutionary relationships: organisms are grouped by how recently they shared a common ancestor, not simply by how alike they happen to look.

The strongest evidence comes from DNA. The sequences of bases in DNA are used as a means of classification, because DNA is passed from generation to generation and changes only slowly over time.

Groups of organisms that share a more recent ancestor have base sequences in DNA that are more similar than those that share only a distant ancestor. So the more similar the base sequences of two organisms are, the more closely related they are.

The five kingdoms

The largest groups in classification are kingdoms. Cambridge places all organisms into one of five: animal, plant, fungus, prokaryote and protoctist.

The features used to decide are simple and countable: the type of cells (prokaryotic or eukaryotic), the number of cells (unicellular or multicellular), how the organism obtains its food (autotrophic or heterotrophic) and whether or not it has a cell wall and what that wall is made of.

Notice that fungi are not plants. A fungus has a cell wall, but it is made of chitin rather than cellulose, and a fungus feeds by absorbing nutrients from dead or living material instead of photosynthesising.

The animal kingdom: vertebrates and arthropods

Inside the animal kingdom, Cambridge asks you to place organisms into two large groups by their features: the vertebrates, which have a backbone, and the arthropods, which have an exoskeleton and jointed legs.

Vertebrates are split into five groups by their skin covering, how they breathe, how they reproduce and how they control their body temperature. Fish and amphibians must return to water to breed, while reptiles, birds and mammals can reproduce on land.

Arthropods are split into four groups by the number of legs and the number of body sections. Counting the legs is the quickest way to separate them, so do that first.

• Myriapods: many legs, one body section - for example a centipede.
• Insects: six legs, three body sections, usually with wings - for example a butterfly.
• Arachnids: eight legs, two body sections, no antennae - for example a spider.
• Crustaceans: many legs, two pairs of antennae, usually aquatic - for example a crab.

The plant kingdom and viruses

Within the plant kingdom Cambridge limits you to two groups: ferns and flowering plants. Ferns have no flowers and no seeds, and reproduce by spores. Flowering plants reproduce by seeds and are split into dicotyledons and monocotyledons.

The features that separate dicotyledons from monocotyledons are all visible in the seed, the leaf and the flower: the number of cotyledons, the pattern of veins in the leaf and the number of flower parts.

Viruses are the odd ones out. They are not placed in any kingdom, because they are not cells. A virus consists of genetic material surrounded by a protein coat. It cannot carry out the seven characteristics on its own - it can only reproduce inside a living host cell.