Cells are specialised to perform a specific function at an optimum efficiency. These cells are usually found in groups of other similar cells to them which is called a tissue. In animal cells there are four main tissue categories, these are epithelial tissues- tissues used in lining of the uterus, next are connective tissues-hold structures in the body together, then there is muscle tissue-used in the actuation of movement of limbs and body parts, finally there is the nervous tissue-communication via electrical impulses in the body. These tissues can be further organised into organ systems that carry out a further functions, e.g. the nervous system or the digestive system. In humans, early embryos consist of Stem Cells that can produce any type of cell. These cells are described as totipotent. Stem Cells are also found in a few places in adults, but these can only differentiate into a limited number of types of cell and are called multipotent.

Here are some examples of cells that are specialised to perform their function properly:

-Erythrocytes or red blood cells, carry oxygen from the lungs to respiring tissues. They contain large amounts of Haemoglobin, that holds oxygen, and many of the organelles (including the nucleus) present in normal cells are not present in erythrocytes, to make space for more Haemoglobin. Their shape is biconcave, which maximises surface area, and so the speed of movement of Oxygen. Red blood cells are part of the respiration system

-Neutrophils are a type of White Blood Cell that ingests and destroys invading microorganisms. They have large amounts of lysosomes, which contain enzymes that kill ingested cells. They have a Lobed Nucleus, which allows for greater flexibility. -Squamous epithelial cells make up squamous epithelial tissue. They are flat cells that form a single thin, smooth layer that lines tubes where diffusion occurs. They occur, for example, in alveoli and some blood vessels. They are held in place by a basement Membrane. This is composed of Collagen and Glycoproteins, secreted by epithelial cells that binds them to connective tissue.

-Ciliated epithelial cells are cells that cover many surfaces. They have tiny projections on their exposed surface, called cilia, which beat in a synchronised pattern to move mucus, along the surface. They can be found, for example, in the trachea, the uterus and the bronchi - Spermatozoon are Sperm Cells. They are the male sex cell involved in sexual reproduction. They travel toward the female's Egg and attempt to fertilize it to produce a Totipotent Zygote. Sperm Cells are specialised in a number of ways. They have an Undulipodium (tail) which moves by energy (ATP) generated