Living Organisms: A Brief Overview

Living organisms are categorised according to their characteristics. During this part of the Biology course, your biology tutors will inform you out about the different kingdoms that exist on our planet and how species are classified within them.

Vertebrates, which are animals with backbones, include amphibians, birds, bony fish, mammals and reptiles. Invertebrates, on the other hand, do not have backbones and include annelids, arthropods, molluscs and nematodes.

Flowering plants make up two groups: monocotyledons and eudicotyledons. The first includes plants such as grasses, orchids and palms whilst the latter consists of buttercups, dandelions and oak trees.

Keys are used to identify and classify the different species, asking questions that can only have one of two possible answers. These dichotomous keys are usually displayed in the form of a branching diagram, similar to those you see in magazine quizzes (i.e. "Are you X or are you Y?" style).

The binomial system assigns two-word names in Latin.
All living things are given a Latin name which describes the genus and species. Photo on Visual Hunt

As your course develops, you may begin to learn about the variety of living organisms and the levels at which they are organised.

To help you to follow the biological parts, processes and functions described below, we have provided a glossary of terms, put together by the experts at BBC Bitesize, which can be referred to whilst you read this text, or can be saved for future reference. Go to their website for tonnes more guides, tests and other useful references during your course.

Note: Having a list of key terminology can be very useful when you come to revise for your exam, as this topic is likely to come up in at least one question on your paper.

GCSE Biology Glossary of  Terms

(Extracted From BBC Bitesize)

"Arthropods: Arthropods are an important group of invertebrates.

Binomial: Having two parts to a name. In the binomial system of classification, each organism is named for its genus then its species.

Cladistic: A method of classification that groups organisms according to characteristics of a common (shared) ancestor.

DNA: Deoxyribonucleic acid. The material inside the nucleus of cells, carrying the genetic information of a living being.

Genus: A rank in classification below family and above species.

Gills: The respiratory organ found in fish and other aquatic animals. Gills have a large surface area, and a good blood supply, for efficient gas exchange to happen in water.

Invertebrate: An animal without a backbone.

Parasite: An organism that lives in or on another organism (the host). The parasite receives nutrients from the host, harming the host as it does so.

Phylum: A taxonomic rank between kingdom and class (the plural is phyla). The arthropods, for example, are a phylum.

RNA: Ribonucleic acid, a type of genetic material.

Species: A type of organism that is the basic unit of classification. Individuals of different species are not able to interbreed successfully."

Classification Of Species

Classification refers to the groups of species on our planet, which are put into groups based on their similarities.

There are five kingdoms:  animals, plants, fungi, prokaryotes and protoctists. These are divided up according to their cell structures. Beyond these kingdoms, living organisms can then be placed in groups according to class, family, genus, order, phylum and species.

You may have come across strange names for species that have alternative, more common names designated to them, particularly in the world of plants. This system is called the binomial system and assigns Latin words in two-parts, which describe the genus and the species respectively. For example, a daisy is also a Bellis (genus) Perennis (species). This also applies to other organisms;  for instance, humans are Homo (genus) Sapiens (species).

The reason why these names are so important in Science is that they give scientists accurate information on the individual species which then allows them to classify them appropriately. This is particularly useful when there are different types of a similar species (like the European and American robins which are distinctively different in size).

There are yet more ways to classify living organisms, and one is described as Cladistics. This process relies on DNA rather than just the appearance of the species.


Vertebrates can be described as animals that have backbones like amphibians, birds, bony fish, mammals and reptiles. They are classified based on their physical features.

Vertebrates include bony fish, birds and mammals.
Vertebrates are species with a backbone. Photo by James St. John on


Invertebrates, or arthropods, are, as you might expect, animals without backbones. The species that fall into this category are annelids, arthropods, molluscs and nematodes.

Invertebrates are species like molluscs.
Invertebrates do not have a backbone. Photo on Visual hunt

Arthropods can be divided into different groups depending on how many legs they have.

Flowering Plants

Flowering plants are set into groups according to their leaves, seeds and seedlings. There are various groups but the two largest known to man are monocotyledons and eudicotyledons (sometimes referred to as dicotyledons).

The first consists of plants with one embryonic leaf and leaves that have parallel veins. The parts of their flowers come in threes. These include, as an example, grasses, orchids and palms.

The second group is made up of plants with two embryonic leaves and leaves that have branching veins. The parts of their flowers come in fours or fives. Buttercups, dandelions and oak trees make up just part of this group.

Keys And Identification

In order to identify the different species, like those mentioned above, one must use keys to ask relevant questions. These dichotomous keys are questions that can have only one of two possible answers, like "Does the animal have feathers?" (YES/NO).

These questions are usually presented in either a table of questions or are set out like a branching tree diagram leading to a final result. You may be familiar with this style of question if you read magazines and answer quiz questions about yourself to find out your classification in respect to the nature of the quiz.

Useful Links

Other topics you might be interested in are:

GCSE Biology Syllabus: Topics At A Glance
GCSE Biology Revision: Reproduction In Humans
GCSE Biology Evolution
GCSE Biology: Digestive System
GCSE Biology Revision: Inheritance and Genetics

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Laura is a Francophile with a passion for literature and linguistics. She also loves skiing, cooking and painting.