A Level Biology : The Variety of Living Organisms

Multicellular Organisms

Multicellular Organisms


Biology – An organ is another functional unit which is comprised of more than one type of tissue. The tissues function together to perform a particular activity. For example, the heart and brain in animals, or leaves and shoots in plants.


Organs can then be further organised into those that work together to carry out a particular function, known as systems. For example, the digestive system or the respiratory system.


Multicellular Organisms

Certain organisms exist as one single cell, like Amoebas. However, a lot of organisms consist of up to billions of cells. These organisms are known as multicellular and within them groups of cells are specialised to carry out specific functions. These cells group together to form tissues which in turn group together to form organs.

Cell differentiation is the process by which cells specialise. In order for a zygote to form an organism with a complex system of cells and tissues cellular differentiation happens numerous times over. It can also happen in adults: adults contain stem cells which, during a cell turnover or to repair damage tissue, form differentiated daughter cells.



Cells, usually which are specialised in the same way, can aggregate to form tissues, which are a functional unit. There are four types of basic tissues in the human body: muscle tissue, nervous tissue, connective tissue and nerve tissue.

Humans contain two types of epithelial tissues: squamous and ciliated epithelia.

  • In squamous epithelia tissue the cells are smooth, flat and thin (only one cell thick). They’re packed tightly together and their low friction surface allows fluids to move over them easily. This tissue is found inside blood vessels and in the lining of the heart chambers and the cheek.
  • Ciliated epithelia tissue is composed of cells which have cilia. They’re found in areas where transport is necessary, for example they make up the lining of the bronchioles in the lungs. The cells can be either cube shaped (cuboidal ciliated epithelia) or narrow and tall (columnar ciliated epithelia).

Plants also contain tissues. However, they differ from these examples in that two types of cell are used as opposed to only one. Xylem tissue transports water and mineral salts as well as supporting the plant. It’s composed of vessel elements, fibres, parenchyma cells and tracheids. Phloem transports soluble organic substances (translocation) and is composed of fibres, companion cells and parenchyma cells.