Cellular orgnaisation

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

How are cells grouped??

similar cells

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tissues

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organs

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systems

Each function requires 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 multicellular 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 the function that it will perform when it is mature.

In other words, each cell becomes specialised in structure to suit the role that it will carry out. this is known as cell differentiation.

So what is cell differentiation??

Cellular differentiation is the process by which a less specialised cell becomes a more specialised cell type

Differentiation dramatically changes a cell's size, shape, membrane potential, metabolic activity, and responsiveness to signals.

With a few exceptions, cellular differentiation almost never involves a change in the DNA sequence itself. Thus, different cells can have very different physical characteristics despite having the same genome.

All the cells in an organism, such as a human, are derived by mitotic divisions of the fertilised egg. 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. only a few of these genes, however, are switched on (expressed) in any one cell.

Differenct genes are switched on in each type of differentiated cell. the rest of the genes are switched off.

So how does the cell differenciate if they all start will the same genes??

Every cell contains the genes needed for it to develop into any one of the different cells in an organism.

However, only some of these genes are expressed.

The rest are switched 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 or sperm cell will have many mitochondria, while a bone cell has very few. white blood cells have many lysosomes while a muscle cell has very few.

So how may a specialised cell be adapted in ways other than its shape??

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 capability to carry out other functions. They therefore are dependent on other cells to perform these activites for them. these other cells perform their own particular function more effectively. As a result the whole organism functions efficitently.

A specialised cell will be adapted in that its numbers of organelles will vary.

For working efficiently, cells are normally aggregated together. Such a collection of similar cells that perform a specific function is known as a tissue.

For Example:

• Epithelial tissues - 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. alvioli 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 transport water and mineral ions throughout the plant and also gives mechanical support.

1) So what is a tissue??
2) And what are some examples of a tissue??

1) A tissue is a group of similar cells organised into a structural unit that serves a particular function.

2) Epithelial tissues - found in animals
                                            - consist of sheets of cells
                                             - line the surface of organs
                                                           - often have a protective or secretory function
For example - those that line the surfaces of organs where diffision takes place like the alveoli of the lungs.

Xylem - found in plants
                                       - made up of a number of cell types
                                                 - used to transport water and mineral ions
                         - gives mechanical support

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. for example, in animals the stomach is an organ that carries out digestion or certain types of food. it is made up of tissues such as:

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

1) So what is an organ??

2) What different tissues is the stomach made up of??

1) An organ is a combination of tissues that are coordinated to perform a variety of functions

2) Muscle - to churn and mix
                           Epithelium - to protect and produce secretions 
        Connective tissue - to hold together other tissues

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

• Palisade mesophyll, made up of leaf paliside cells that carry out photosynthesis
• spongy mesophyll adapted for gaseous diffusion
• epidermis to protect the lead 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 tell which structures are organs. Blood capillaries for example, are not organs where as arteries and veins are both organs. all these structures have the same major function however capillaries are made up of just one tissue whereas the others are made up of many.

So what tissues is a leaf made up of??

Paliside mesophyll - carry out photosynthesis

  Spongy mesophyll - adapted for gaseous diffusion

   Epidermis - to protect the leaf and allow gaseous diffusion 

Phloem - transport organic materials away from the leaf

Xylem - transport water and ions into the leaf

Organs work together as a single unit know as an organ system. thses systems may be grouped together to perform particular functions more efficiently. there are a number or organ systems in humans:

• the digestive system - digests and processes food. it is made up of organs that include the salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver.
• the repiratory system - used for breathing and gas exchange. it is made up of organd that include the trachea, bronchi and lungs.
• the cirulatory system - pumps and circulates blood. it is made up of organs that inclue the heart, arteries and veins.

1) So what is an organ system??

2) And what are the organ systems in a human??

1) An organ system is a group of organs that work together as a single unit to perform a particular function

2)

• The Digestive System - digests and processes food
                                     - includes the salivary glands, oesophagus, stomach, duodenum, ileum, pancreas and liver
• The Respiratory System - breathing and gas exchange
                                         - includes the trachea, bronchi and lungs
• The Circulatory system - pumps and circulates blood
                                        - includes the heart, arteries and veins