The cell contains highly organized physical structures called intracellular organelles.The physical nature of each organelle is as important as the cell's chemical constitutes for cell function.

1.Cell membrane:

The cell membrane also called plasma membrane which envelops the cell,is a thin,pliable,elastic structure only 7.2-10 nanometers thick.It is also composed almost entirely of proteins and lipids. The basic lipid layerof the cell is composed of phospholipid molecules.One end of each phospholipid molecule is soluble in water that is hydrophilic.

The other end is solublein water only in fats,that is hydrophobic.The cholesterol molecules in the membrane are also lipid in nature because their steroid nucleus is highly fat soluble.. Cell membrane protein,most of which are glycoproteins

.Two types of proteins occur :integral protein that protrude all the way through the membrane and the peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.Integral proteins provide structural channels through which water molecules and water soluble substances,especially ions can diffuse between extracellular and intracellular proteins.Integral membrane proteins can also serve as receptor for water soluble chemicals such as peptide for hormones,that do not easily penetrate the cell membrane.

2.Cytoplasm and its organelles:

Cytoplasm is filled with both minute and large dispersed particles and organelles.Five special important organelles are :Endoplasmic reticulum,the Golgi apparatus,mitochondria,lysosomes and peroxisomes..Others are glycogen granules,ribosomes,secretary vesicles . *Endoplasmic reticulum:In the figure shows a network of tubular and flat vesicular structures in the cytoplasm,this is endoplasmic reticulum.

The tubules and vesicles interconnect with one another.The space inside the tubules and vesicles is filled with endoplasmic matrix is a watery medium.Their walls are constructed of lipid bilyer membranes that contains large amount of proteins. *Golgi apparatus:Golgi apparatus are closely related to the endoplasmic reticulum.This apparatus is prominent in secretary cells ,where it is located on the site of the cell from which the secretary substances are extruded.Its functions are associated with endoplasmic reticulum.Substances entrapped in the endoplasmic reticulum are transported from the endoplasmic reticulum to the Golgi apparatus.

The transported substances are then processed in the Golgi apparatus to form lysosomes,secretary vesicles and other cytoplasmic components. Lysosomes:Lysosomes are vesicular organelles that formed by breacking off from the Golgi apparatus and then dispersing throught the cytoplasm.

The lysosome provide an intracellular digestive system : •

damaged cellular structure

•food particles that have been ingested by the cells.

•unwanted matter such as bacteria. Lysosome usally 250-750 nanometer in diameter. It is surrounded by lipid bilayer membrane and is filled with large amount of small granules.In lysosome,protein is hydrolyzed to form amino acid,glycogen are hydrolyzed to form glucose,lipids are hydrolyzed to form fatty acid and glycerol.

3.Peroxysome:They are form by self replication and contains oxidases rather than hydrolases.Oxidase combine with hydrogen and to form hydrogen peroxide.It is higly oxidizing substances.

4.Secretory vesicles:These vecsicles store protein proenzymes.Secretory vesicles are formed by endoplasmic reticulum-Golgi apparatus system.

5.Mitochondria:Mitochondria which is also called " powerhouse"of the cell.The basic structures of the mitochondria composed mainly of two lipid bilayer protein membrane :an outer and an inner membrane.Many unfoldings of the inner membrane form shelves onto which oxydative enzymes are attached.Inner cavity of the mitochondrium is matrix is filled with dissolved enzymes.

This enzyme association with oxidative enzyme to form energy.The liberated energy is used to synthesized a high energy substances called adenosin triphosphate.ATP is then transported out of the mitochondria and it diffuses throughout the cell to release its own energy wherever it is needed for performing cellular functions.

6.Nucleus:The nuclus is the control center of the cell.It contains large quantities of DNA which are the genes.These genes determine the characteristics of the cell's proteins,also control and promote reproduction of the cell itself.Nucleus contains many other structures: Nuclear membrane Neucleoplasm, Nucleolus Endoplasmic reticulum, Cromatin network