AQA AS Level Biology Unit 2 - Cellular Organisation

• Single celled organisms perform all essential life functions inside the boundaries of a single cell
• Although they perform all functions adequately they cannot be totally efficient at all of them because each function requres a different type of cellular structure.
• One activity may be best carried out by a long thin cell while another might suit a spherically shaped cell
• No one cell can provide the best conditions for all functions
• For this reason the cells of mulicellular organisms are each adapted in different ways to perform a particular role All cells in an organism are initially identical
• As it matures, each cell takes on its own individual characteristics that suit it to its function that it will perform when it is mature. so each cell becomes specialised in structure to suit the role that it will carry out. This is cell differentiation
• All the cells in an organism are derived by mitotic divisions of the fertilised egg. It follows that they contain exactly the same genes. Each cell contains 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 in any one cell. Different genes are switched on in each typr of differentiated cell. The rest are swtched off.

• It is not just the shape of the different cells that varies, but also the numbers of each of their organelles. e.g a muscle of sperm cell will have many mitochondria, while bone cells will have very few.
• White blood cells have many lysosomes while a muscle cell will have few
• The cells of a multicellular organism have therefore 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 dependant on other cells to perform these other activities for them.
• These other cells are specially adapted to their own particular function and perform it more effectively.
•  As a result the whole organism functions efficiently

For working efficency, cells are normally aggregated togethe. Such a collection of similar cells that perform a specific function is known as a tissue. E.g:

• Epithelial tissues - which are found in animals and consist of sheets of cells. They line the surface 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 a 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 transpost water and mineral ions throughout the plant and also gives mechanical support

Tissues are aggregated together 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. In animals , e.g the stomach is an organs that carries out digestion of certain foods. It is made uo of tissue succh as:

• muscles to churn and mix the stomach contents
• epithelium to protect the stomach wall and produce secretions
• connective tissue to hold together the other tissues

In plants a leaf is an organ made up of the following tissues:

• palisade mesophyll made up of leaf palisade cells for photosynthesis
• spongy mesophyll adapted for gaseous exchange
• epidermis to protect the leaf and allow gaseous diffusion
• phloem to transport organic materials away from the leaf
• xylem to transport water and ions into the leaf

It is not always easy to determine which structures are organs. Capillaries arent whereas ateries and veins are because capillaries are only made of one tissue.

Organs work together as a single unit known as an organ system. These systems may be grouped together to perform particular functions more efficiently. There are a number of organ systems in humans.

• The digestive system - digests process food. It is made up of organs that include the salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver
• The respiratory system - is used for breathing and gas exchange. It is made up of organs that include the trachea, bronchi and lungs
• The circulatory system - pumps and circulates blood. It is made up of organs that include the heart arteries and veins,