# GCSE Biology/Classification of Organisms
## Living or Non-Living

Biologists have to decide whether organisms are alive or dead. This provides the foundation for biology itself, so the categories must be clearly defined. A living organism must complete certain life processes to be classed as "alive". These life process form an acrostic known as MRS GREN.
* M - Movement - All living things must be able to move.
* R - Respiration - All living things respire, this is to say release energy through a chemical reaction within their cells.
* S - Sensitivity - All living things can sense their environment and are able to interact with it.
* G - Growth - All living things grow and develop during their lifespan.
* R - Reproduction - All living things are able to make more of themselves.
* E - Excretion - All living things remove waste matter from themselves.
* N - Nutrition - All living things use nutrients from the environment.
Therefore we can say that humans are living as they complete all of these processes, but organisms such as viruses are non-living as they do not complete some of these processes. Viruses are not traditionally included within the classification of life because of this reason.

## The Five Kingdoms of Life

There are five kingdoms in which all living things are divided into.

### Plants

Plants are autotrophs. This means that an organism can make its own food, using photosynthesis in the particular case of plants. They contain chlorophyll which acts as a catalyst for the photosynthesis reaction. They are multicellular, the largest plants being trees. Plant cells have cell walls which provide rigid structure to the plant.

### Animals

Animals are heterotrophs. This means that an organism must eat other organisms to survive, for example herbivores eat plants and carnivores eat other animals. Animals do not contain chlorophyll or cell walls, but they are multicellular and have a nucleus.

### Fungi

Fungi are saprophytes. This means that an organism eats dead or decaying organic matter. Fungi have cell walls, but do not contain chlorophyll. Like animals and plants they are multicellular.

### Protoctists

Protoctists are single-celled (unicellular). They have nuclei in their cells, for example algaes.

### Prokaryotes

Prokaryotes are single-celled (unicellular). They do not have nuclei in their cells, for example bacteria.

## Subdivision of Kingdoms

The 5 kingdoms are further divided into smaller and smaller groups. These follow the order: Kingdom, Phylum, Class, Order, Family, Genus and Species. Thus a genus contains a few species that share common features.

### The Binomial System

The biological name of any organism is its genus and species, for example a lion belongs to the genus "panthera", and the species "leo", so its name in biological terms is "panthera leo". This is called the binomial name of the organism and is useful for four main reasons:
* Easy identification of unknown organisms, avoiding confusion
* Studying species which look alike but are in fact separate
* Conserving species which look alike, preventing the extinction of one
* Protecting large areas with a diverse range of species, for example tropical rainforests

### Chordata

The phylum "chordata" is found in the animal kingdom. This contains all of the animals that have a rod-like structure used to give them support. In most cases this is the spine or backbone. The common name for members of "chordata" is vertebrates, and those that do not belong to the phylum but are within the animal kingdom are known as invertebrates. Invertebrates can however have an external skeleton, for example crabs.
Within "chordata" there are five classes of animals. These are fish, amphibians, reptiles, birds and mammals. We can tell the difference through three dividing factors:
* Regulation of body temperature - animals are either homeothermic (can regulate their internal temperature so that it is kept at an optimum level) or poikilothermic (cannot regulate their internal temperature, the environment affects how hot or cold they are)
* Oxygen Absorption - the way in which oxygen is taken in from the air, which can be through gills, the skin (amphibians) or lungs
* Reproduction - this factor is particularly varied. Animals can be oviparous (lay eggs) or viviparous (birth live young). Fertilisation can occur externally or internally. In mammals, the mother produces milk for the young.

## Accurate Classification

Often organisms are hard to classify. For example, the duck-billed platypus is homeothermic, has lungs and provides milk for the young. These are the classic features of mammals, yet the platypus also is oviparous. This is not a feature of mammals, as mammals are the only viviparous class of animal. Where does it go? We must consider many factors before placing newly discovered species into a division in the classification system.

### The Species Definition

Two organisms are in the same species if they can reproduce to create fertile offspring. This means that a lion and lioness are members of the same species because they produce fertile cubs, however horses and donkeys are not as they reproduce to create infertile mules.

### Asexual Reproduction

Some organisms do not require two individuals to reproduce. This is known as asexual reproduction, for example in bacteria. We cannot easily classify these organisms as they do not follow the species definition. However, they are still members of the same species.

### Ring Species

Some organisms live in neighbouring populations. Often populations near each other can breed successfully, but populations further away cannot. This results in difficulty classifying the organisms, as we can't tell if the different populations are different species or not.

### Hybridisation

Hybrids (when two species breed, e.g. the mule) can sometimes be fertile. This often occurs in many duck species. This goes against the species definition, because the organisms are different species but have still produced fertile offspring.