There are four traditional levels of organisation in living things, starting from the microscopic cells and increasing in complexity of structure and function to tissues, organs and systems.

Cells
Cells are the most basic unit of structural organisation within living things. It can be said that cells are the characteristic feature that distinguish between living and non-living material.

Cells are made up of many parts, called organelles, which are specialised to do a specific job. The cell membrane allows essential materials to move in and out of the cell. The nucleus contains genetic material that determines an organism's chemical and physical characteristics. These are common to both animal and plant cells, but the plant cell also has a rigid cell wall that maintains the cell's shape, and some have chloroplasts to capture the Sun's energy in photosynthesis to produce sugars.

Cells vary in their shape and function. Humans have over twenty different forms such as skin, muscle, and fat-storing cells. They all differ in structure to allow them to perform their specific functions.

Some living things like bacteria consist only of one cell. Their organisational structure is very simple. Due to their small size, they do not have tissues, organs and systems like more complex animals and plants. Others in this category are protists such as paramecium and euglena (See The five kingdoms of living things).

Tissues
Similar cells working together are collectively called a tissue. An example of a tissue is muscle, such as cardiac muscle in the heart. Other animal tissues are nerve, bone and skin (epithelial tissue). Examples of plant tissues are xylem which transports water around a plant, and phloem which transports sugars made in the leaves to all parts of the plant.

Organs
In most plants and animals, tissues are grouped together to form organs. An organ is a complicated structure that has a particular function. An example in animals is the heart that pumps blood around the body. It consists of tissues called cardiac muscle, and connective tissue (e.g. ligaments and valves). In plants, the root is an organ which takes up nutrients and water from the soil. It is made up of xylem, phloem and other simple tissues such as parenchyma tissue. The pistil of the flower is the female reproductive organ.

Systems
Systems are made up of a set of organs that work together with a common overall function. In humans there are several systems each with its own specific role. These include the skeletal, muscular, endocrine, reproductive, digestive, respiratory, circulatory, excretory and nervous systems. Complex systems allow an organism to be very much larger and to survive in a wider range of habitats which pose greater environmental problems (see also Systems, transport and exchange).

The circulatory system consists of the heart, the blood vessels, lymph vessels and blood. Together they ensure transport of nutrients and wastes throughout the organism.