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Cardiovascular system

Transport system of the body made up of the heart, the blood vessels and the blood. Also known as the circulatory system

Cellular respiration

Breaking down glucose (food) without oxygen to provide available energy for the cells. The glucose reacts with oxygen to produce energy in the form of ATP with carbon dioxide and water as waste products

Specialised cells

Cells which are adapted to carry out a specific function in the body

Digestive system

The organ system in the body which breaks down large insoluble food molecules into small soluble molecules which can be used by the body

Epithelial cells

Cells that cover the internal and external surfaces of organs


Made up of many cells

Organ system

Several organs working together to carry out a specific function in the body


The biochemical process by which the cells in the body releases energy

Amino acid

The basic building blocks of proteins. There are twenty amino acids used, in different combinations, to make every protein required by the human body.


An endocrine gland which produces insulin


A type of sugar: a mono saccharide with 6 carbon atoms (a hexose sugar).


A group of cells in an organism that are specialised to work together to carry out a particular function.


A structure with a particular function which is made up of different tissues.

The need for a transport system

All cells need the same basic things - glucose and oxygen for respiration, amino acids and other food molecules for cell-building and growth, and the removal of poisonous waste products.

In a small, single celled organism such as an amoeba, simple diffusion is enough to supply the needs of the cell. Useful substances can diffuse in from the surroundings and waste products can diffuse out fast enough for the organism to survive successfully.

However, as organisms get bigger the surface area to volume ratio changes. It gets smaller. As a result simple diffusion is no longer enough. There is not a big enough surface area for diffusion to take place over, particularly in active organisms. Food and oxygen cannot reach the cells fast enough to supply cellular respiration. The waste products cannot be removed fast enough to prevent them damaging the cells. A transport system is needed.

The link between size and the surface area:volume ratio

The link between size and the surface area:volume ratio

Organising a transport system

In a single celled organism, all the characteristic functions of life take place in the same cell. In contrast large multicellular organisms have many layers of organisation. There are different types of specialised cells, adapted to carry out particular functions in the body eg muscle cells, epithelial cells. These cells are found in groups known as tissues, where many similar specialised cells work together to carry out a function in the body. Muscle tissue and glandular tissue are two well known examples.

Often several tissues are found together making an organ. Each organ carries out a specific function in the body eg the heart, the stomach or the pancreas. Organs may work together to form an organ system carrying out the various stages of one major function - eg the cardiovascular system which acts as the transport system of the body or the digestive system which breaks down food.

Eventually all of the organ systems come together to form the entire organism.

Organisational levels in the body

Organisational levels in the body