AQA AS Biology Unit 2: Cell Differentiation and Organisation

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Cell Differentiation and Organisation
In multicellular organisms, cells are specialised to perform specific functions.
Similar cells are then grouped together into tissues, tissues into organs and organs into
systems for increased efficiency.
Cell Differentiation
Multicellular organisms are made up from many different cell types.
All cells in an organism are initially identical.
As it matures, each cell takes on its own individual characteristics that suit it to the function
that it will perform when it is mature.
Each cell becomes specialised to suit its particular job.
The process of becoming specialised is known as differentiation.
All the cells of an organism are derived by mitotic divisions of the fertilised egg.
It follows that they all contain the same genes.
Every cell contains the genes needed for it to develop into any one of the many different
cells in an organism.
But only a few of these genes are switched on, or expressed, in any one cell.
Different genes are switched on in each type of differentiated cell and the rest are switched
off.
The cells of multicellular organisms have evolved to become more and more suited to one
specialised function.
In doing so, they have lost the ability to carry out other functions.
They are therefore dependent on other cells to perform these activities for them.
These other cells are specially adapted to their own particular function and perform is more
effectively.

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Tissues
For working efficiency, cells are normally aggregated together.
Such a collection of similar cells that perform a specific function is known as a tissue.
Tissue A collection of similar cells that perform a specific function
Examples of tissues:
Epithelial tissues
Epithelial tissues are found in most animals and consist
of sheets of cells.
They line the surface of organs and often have a
protective or secretory function.…read more

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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 often have one predominant major function.
Organ A combination of tissues that are coordinated to
perform a variety of functions
In animals, the stomach is an organ that carries out the digestion of certain types of food.…read more

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Organ Systems
Organs work together as a single unit known as an organ system.
These systems may be grouped together to perform particular functions more efficiently.…read more

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