First 416 words of the document:
What is it?
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
We all start our lives as a single fertilised egg, a zygote, which then divides by mitosis. In
early embryos the cells are unspecialised, but have the potential to differentiate into any of
200 or so specialised cell types that make up the human body. These embryonic cells are the
ultimate stem cells. They are known as totipotent because they have the power to turn
into any other cell type.
Each body cell has a full set of genes, so the key to differentiation is the selective
activation of genes different genes are active in different cells. The tissue and organs of
the body develop because cells differentiate in the right way at the right time.
For working efficiency, cells are normally aggregated together. Such a collection of similar
cells that perform a specific function is known as a tissue.
Epithelial tissues, which are found in animals and consist of sheets of cells. They
line the surfaces of organs and often have a protective or secretory function. There
are many types, including those made up of thin, flat cells that line organs where
diffusion takes place e.g. alveoli of the lungs and ciliated epithelium that lines duct
such as the trachea. The cilia are used to move mucus over the epithelial surface.
Xylem, which occurs in plants and is made up of a number of cell types. It is used to
transport water and mineral ions throughout the plant and also gives mechanical
Organs are aggregation of tissues performing specific physiological functions. In animals, for
example, the stomach is an organ that carries out the digestion of certain types of food. It
is made up of tissues such as:
Muscles to churn and mix the stomach contents
Epithelium to protect the stomach wall and produce secretions
Connective tissue to hold together all the tissues
In plants, a leaf is an organ made up of the following tissues:
Palisade mesophyll made up of leaf palisade cells that carry out photosynthesis
Spongy mesophyll adapted for gaseous diffusion
Phloem to transport organic materials away from the leaf
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Xylem to transport water and ions into the leaf
NB: it is not always easy to determine which structures are organs. Blood capillaries are not
organs but both arteries and veins are.
Organ systems are groups of organs that work together to achieve a major physiological
function some examples 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