Some organisms do exist as single cells - for example, Amoeba, but many organisms are multicellular and consist of from hundreds to billions of cells. The functions of the organism are divided up amoungst the groups of cells, which become specialised for particular roles. Specialised cells show division of labour by being grouped into tissues.
A tissue is defined as a collection of cells, together with any extracellular secretion, that is specialised to perform one or more particular function. Tissues may contain only one type of cell, or several types.
Some examples of tissues...
Epithelial tissues are animal tissues and form sheets covering surfaces. Two tissues that you need to know about are squamous and ciliated epithelia. Both are one cell thick and so are called simple epithelia. The cells rest on a basement membrane which, is a network of collagen and glycoproteins that is secreted by cells underneath the epithelial tissue.
In this tissue, the cells are of one type and are smooth, flat and very thin. They are packed closely together like tiles on a roof and provide a low friction surface over which fluids can move. It is found lining the cheeks, inside blood vessels, lining the chambers of the heart and forms the alveoli in the lungs.
This tissue is made up of cells with cilia and so is often found in areas where it is needed to transport something - for example, lining the oviducts and bronchioles of the lungs. Sometimes the cells are shaped like cubes and the tissue is called cuboidal ciliated epithelia. If the cells are tall and narrow, it is referred to as columnar ciliated epithelia.
Xylem and phloem
These two plant tissues differ from the above examples in that they are made up of more than one cell type. Xylem has the dual function of support of the plant and transport of water and dissolved mineral salts. It is made up of vessel elements, tracheids, fibres and parenchyma cells. Phloem tissue is responsible for translocation which is the transport of soluble organic substances - for example, sugar. The substances travel along sieve elements but other types of cells are also present, the companion cells, parenchyma cells and fibres.
This tissue is found in the leaf and is made up of one type of cell. The cells are tall and thin and are tightly packed together. Their function is to harness the light energy required for photosynthesis and so each cell is packed with chloroplasts.
An organ is part of the body which, forms a structurally and functionally separate unit and is made up of more than one type of tissue. Examples of plant organs are leaves, roots and stems. Examples of animal organs are the liver, brain, heart and kidney. Organs may be organised into groups with particular functions and are then called systems - for example, the digestive system.