3.5 Cell specialisation and organisation

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Cell specialisation

For multicellular organisms to stay alive, all cells perform certain basic functions. No one cell can provide the best conditions for all functions, therefore the cells of a multicellular organism are each specialised in different ways to perform a particular role. Each cell has evolved more or fewer of certain organelles and structures to suit the role it carries out.

As an embryo matures the group of cells that are initially identical, take their own characteristics that suit it to the function it will perform when it is mature. All the cells in an organism are produced by mitotic divisions from the fertilised egg, it is because of this that they all contain the same genes. So , every cell contains the genes needed for it to develop into any one of the many different cells in an organism. But, only some of these genes are expressed in any one cell at any one time, the rest are switched off.

It is not just the shape of different cells that vary but also the number of each organelle, for example muscle cells have many mitochondria whereas bone cells have very few. As each cell is adapted to its own particular function to perform it most effectively, the whole organism functions effectively.

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Tissues

To work more efficiently, a collection of similar cells that perform a specific function will work together, this is known as a tissue.

Epithelial tissues are found in animals and consist of sheets of cells. They line the surfaces of organs and often have a protective or secretory function.

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Organs

Just as cells are aggregated into tissues, tissues are aggregated into organs. An organ is a combination of tissues that are coordinated to perform a variety of functions, although they usually have one major function. For example the stomach, involved in digestion, are made up of tissues such as:

-          Muscle to churn and mix the stomach contents

-          Epithelium to protect the stomach wall and produce secretions

-          Connective tissue to hold together the other tissues

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Organ systems

Organs working together as a single unit is known as an organ system. The systems can be grouped together to perform particular functions more effectively. Examples of this are:

-          The digestive system digests and processes food. It is made up of organs that include the salivary glands, oesophagus, stomach, duodenum, ileum , pancreas and liver.

-          The circulatory system pumps and circulates blood. It is made up of organs that include the heart, arteries and veins.

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