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Cell Organisation

Anatomy Of Cells by Jane Thurnell-Read

The cell is the basic structural and functional unit of the body. All cells share the same basic structure. The average adult human body consists of nearly 100 trillion cells. Looked at simply a cell consists of a series of compartments which are separated from each other and from the outside environment by cell membranes. Cell membranes are often viewed as simple dividing walls, but, in fact, they are active structures determining what enters and leaves the cell or an area within the cell.

The basic cell design is modified depending on the function of the cell. Cells are grouped together in tissues: all the cells in a particular tissue are the same.

There are 4 basic types of cells; each cell is specialised to perform a particular function:

  • Muscle cells: generation of mechanical force and movement
  • Nerve cells: rapid communication
  • Connecting and supporting tissue cells: including blood and lymph
  • Epithelial cells: protection, secretion and absorption
cells

 

All cells contain the same genetic information, but the process of specialisation comes about because different parts of the genetic code are switched on and are active. This then determines which proteins are made and so determines the cell function(s).

Some of the main components of all cells are:


OUTER CELL MEMBRANES (PLASMA MEMBRANE)

a continuous thin sheet of fatty molecules (rather like cooking oil) with protein embedded in it; some of the proteins regulate the flow of materials into and out of the cells by providing pathways for transport; others serve as receptors for chemical signals coming from other cells. The membrane allows some things in, while stopping other things from entering the cell. The fluid mosaic model describes its structure: The molecular arrangement of the plasma membrane resembles an ever-moving sea of lipids that contain a mosaic of many different proteinsThe proteins may float freely like icebergs, be moored at specific locations, or be moved through the lipid sea. (Johnjoe McFadden) Some of the proteins go right through the membrane, projecting at both sides some have a pore or channel through which specific substances can enter the cell. Some proteins bind with a substance on one side of the membrane and then, by changing their shape, move the substance to the other side of the membrane.
CYTOPLASM

the space between the nucleus and the cell membrane, enclosing many specialist structures (organelles) in a fluid. (CYTOSOL), which is 75-90% water. The cytoplasm contains the raw materials for the cells activities.


ORGANELLES

(= little organs): the collective name for the structures within the cytosol, (e.g. cytoskeleton, ribosomes, lysosomes, Golgi apparatus, etc.). The many different chemical reactions that need to occur within a cell at the same time are possible because many of the reactions are isolated inside.


CYTOSKELETON

this gives the cell its shape and allows for movement both within the cell and of the cell itself the cells version of bones and muscles. It is made up of protein filaments that extend throughout the cytosol, giving the cell its shape and organising the placement of the cell contents. The cytoskeleton is also responsible for movement within the cell (e.g. the movement of chromosomes in cell division, the movement of some chemicals within the cell, etc.) and the movement of some entire cells (e.g. phagocytes).


MITOCHONDRIA

these are where chemical energy contained within nutrients is trapped and stored through the formation of ATP molecules. ATP is the energy used to carry out cellular work. There are between a hundred and several thousand mitochondria in each cell, depending on the amount of work done by the cell. The inner membranes of mitochondria are folded to increase the surface area for the chemical reactions: enzymes are attached to these shelves.


CENTROSOME

two components, (the pericentriolar area and the centrioles), involved in cell division.


RIBOSOMES

are sites where proteins are made. Some ribosomes are also located within the mitochondria, where they make mitochondrial proteins.


ENDOPLASMIC RETICULUM (ER)

an elaborate and extensive network of tubes distributed throughout the cytoplasm; involved in the synthesis of proteins and detoxification of drugs and deactivation of sex hormones. Ribosomes become attached to the ER when they become involved in some forms of protein manufacture.


GOLGI APPARATUS (also known as GOLGI COMPLEX)

fluid-filled sacs that are stacked. The Golgi apparatus modify, sort and package proteins for delivery to other structures within the cells or for secretion out of the cells.


ENDO- AND EXOCYTOTIC VESICLES

travel from and to the plasma membrane carrying proteins for delivery into and out of the cells.


LYSOSOMES

they are produced in the Golgi apparatus and contain up to 40 enzymes that can digest damaged, toxic and spent material within the cells. They then make this material available for re-use within the cells and also export some to other sites.


NUCLEUS

contains genetic material (genes, DNA and chromosomes); expresses information stored in genes and directs activity within the cell; most cells have a single nucleus, but mature red blood cells do not have any and skeletal muscle cells have several; the nucleus has a double membrane surrounding it containing many pores that allow materials (such as mRNA) to leave the nucleus and enter the rest of the cell; it also allows substances (e.g. some hormones, nutrients etc.) to enter the nucleus.

"The nuclear pores are the most complex structures inside a cell, consisting of hundreds of molecules. They are presumably a bureaucrats dream of a customs and immigration department, with each molecule that goes in or out of the nucleus being checked, and probed in its intimate orifices, and pushed about." Mark Ridley Mendels Demon page 18.

CILIA AND FLAGELLA

on the outside of the cell. The cilia move fluids across the surface of the cell. Flagella move the whole cell. (In humans the only example of flagella is the sperm).

Copyright Jane Thurnell-Read 2008

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