Topic 1: Characteristics and classification of living organisms

How do we know something is alive? A rock and a rabbit are made of similar atoms, yet only one of them grows, breathes and reproduces. In this topic you will learn the seven shared features of all living things, how scientists sort the enormous variety of life into groups, and how a simple set of questions can identify any organism. Master this and you have the foundation for the whole IGCSE Biology course.

The seven characteristics of living organisms

All living organisms share seven life processes. A useful mnemonic is MRS GREN:

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

Learn these definitions word-for-word — they are common short-answer questions.

Classification and the binomial system

Classification means putting organisms into groups based on shared features. The modern system groups species that share a recent common ancestor, often confirmed using DNA base sequences — the more similar the DNA, the more closely related the organisms.

The main groups, from largest to smallest, are: kingdom, phylum, class, order, family, genus, species. A species is a group of organisms that can reproduce to produce fertile offspring.

Each species has a two-part scientific name (the binomial system), devised by Carl Linnaeus. The first word is the genus (capital letter), the second is the species (lower case), and both are written in italics or underlined, for example Homo sapiens.

The five kingdoms and the main animal groups

Living organisms are commonly placed in five kingdoms: animal, plant, fungus, prokaryote and protoctist.

• Animals – multicellular, no cell walls or chloroplasts, feed on other organisms.
• Plants – multicellular, cellulose cell walls, contain chloroplasts and make food by photosynthesis.
• Fungi – cell walls (not cellulose), feed by saprotrophic or parasitic nutrition, e.g. Mucor and yeast.
• Prokaryotes (bacteria) – single cells with no true nucleus.
• Protoctists – mostly single-celled, e.g. Amoeba and Plasmodium.

Animals are split into vertebrates (with a backbone: fish, amphibians, reptiles, birds, mammals) and arthropods (jointed legs and exoskeleton: insects, arachnids, crustaceans, myriapods). You should know the key features of each.

Dichotomous keys

A dichotomous key is a tool used to identify organisms. It works through a series of paired statements, where each step offers two choices that lead either to the next question or to the name of the organism.

To use one, start at question 1, choose the option that matches your organism, then follow the instruction to the next numbered step until you reach a name. When constructing your own key, base each choice on a clearly visible, contrasting feature (such as “wings present / wings absent”) rather than features that vary, like colour.