Cells, Tissues and Organs

• In simple multicellular organisms cells may not show any specialisation - they all look and function similarly
 
• As organisms become more complex cell specialisation becomes apparent; the cells are said to be differentiated
 
• Where a group of cells with a similar function are grouped together within an organism the region is known as a tissue
 
• An organ is a structure within an organism with a specific function or functions. Organs are typically made up of various tissues
 

In summary:

• an organ is made up of several tissues which in turn are made up cells with a similar structure and function

The Mesophytic Leaf as an Example of an Organ

There are several tissues seen in this leaf:

• essentially protective cells around the exterior of the leaf, some cells act as guard cells regulating the exchange of gases with the air

palisade and spongy mesophyll

• mesophyll cells are specialised for photosynthesis
• palisade mesophyll is made up of densely packed, cylindrical cells with many chloroplasts
• spongy mesophyll cells are more rounded and are less densely packed leaving air spaces between them to allow gases to circulate through the leaf
 

• the transport tissues of plants.
• phloem: which transports the carbohydrates manufactured in photosynthesis
• xylem: which transports water and minerals around the plant
 

• collenchyma
• sclerenchyma

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