First 222 words of the document:
The process by which cells become specialised for different functions.
In the fertilised egg e.g. only a few genes are expressed "switched on"
The cells of multicellular organisms have therefore evolved to one function; they have lost
their ability for other functions.
A group of similar cells organised into a structural unit that serves a particular
Epithelial tissues found in animals and consist of sheets of cells. They line surfaces of organs
and protect/secrete also (e.g. alveoli)
Xylem is made of a number of cell types, they transport water and mineral ions and for
Tissues aggregated into organs.
E.g. stomach- muscle to churn, epithelium to protect and secrete, connective tissue to hold
together other tissues.
E.g. leaf- palisade mesophyll made of mesophyll cells to carry out photosynthesis, spongy
mesophyll adapted for gaseous diffusion, epidermis to protect the leaf and allow gaseous
diffusion, xylem to transport water and ions into the leaf.
Capillaries are not organs but veins/arteries are (made of up many tissues).
Organs work together in a group as a single unit.
E.g. digestive, respiratory, circulatory.