Living organisms can reproduce

Living organisms have certain life processes in common. There are seven things that they need to do to count as being alive. The phrase MRS GREN is one way to remember them:

• Movement - all living things move, even plants
• Respiration - getting energy from food
• Sensitivity - detecting changes in the surroundings
• Growth - all living things grow
• Reproduction - making more living things of the same type
• Excretion - getting rid of waste
• Nutrition - taking in and using food

It can be easy to tell if something is living or not. A teddy bear might look like a bear, but it cannot do any of the seven things it needs to be able to do to count as being alive.

A car can move, it gets energy from petrol (like nutrition and respiration), it might have a car alarm (sensitivity), and it gets rid of waste gases through its exhaust pipe (excretion). But it cannot grow or make baby cars. So a car is not alive.

Cells are the basic building blocks of all animals and plants. They are so small, you need to use a light microscope to see them.

Animal cells usually have an irregular shape, and plant cells usually have a regular shape. Cells are made up of different parts.

It is easier to describe these parts by using diagrams:

Animal cells and plant cells also contain tiny objects called mitochondria in their cytoplasm

A unicellular organism is a living thing that is just one cell. There are different types of unicellular organism, including:

• bacteria
• protozoa
• unicellular fungi

You might be tempted to think that these organisms are very simple, but in fact they can be very complex. They have adaptations that make them very well suited for life in their environment.

Bacteria are tiny. A typical bacterial cell is just a few micrometres across (a few thousandths of a millimetre). The structure of a bacterial cell is different to an animal or plant cell. For example, they do not have a nucleus but they may have a flagellum. This is a tail-like part of the cell that can spin, moving the cell along.

The main parts of bacteria

Multicellular organisms are organised into increasingly complex parts. In order, from least complex to most complex:

• cells
• tissues
• organs
• organ systems
• organism

Animal cells and plant cells can form tissues, such as muscle tissue in animals. A living tissue is made from a group of cells with a similar structure and function, which all work together to do a particular job. Here are some examples of tissues:

• muscle
• the lining of the intestine
• the lining of the lungs
• xylem (tubes that carry water in a plant)

An organ is made from a group of different tissues, which all work together to do a particular job. Here are some examples of organs:

• heart
• lung
• stomach
• brain
• leaf
• root

An organ system is made from a group of different organs, which all work together to do a particular job. Here are some examples of organ systems:

• circulatory system
• respiratory system
• digestive system
• nervous system
• reproductive system