What is a Plasma Membrane?

The plasma membrane is the outer most wall surrounding cells in the body. it is how the cells are able to keep its boundaries of inside and outside the cell.

The phospholipids are what hold the membrane together. They are like the parts of the floating sea per se that makes up both sides of the membrane. If you look at the pictures at any of the below links, it will make sense on how it all ties together.

http://en.wikipedia.org/wiki/Cell_membrane

http://scientia.org/cadonline/biology/eukaryotic/plasmembrane.ASP

http://www.okc.cc.ok.us/biologylabs/Images/Cells_Membranes/plasmamembr ane2.jpg

The cell membrane surrounds the cytoplasm of a cell and, in animal cells, physically separates the intracellular components from the extracellular environment, thereby serving a function similar to that of skin. In fungi, bacteria, and plants an additional cell wall forms the outermost boundary, however, the cell wall plays mostly a mechanical support role rather than a role as a selective boundary. The cell membrane also plays a role in anchoring the cytoskeleton to provide shape to the cell, and in attaching to the extracellular matrix to help group cells together in the formation of tissues.

The barrier is selectively permeable and able to regulate what enters and exits the cell, thus facilitating the transport of materials needed for survival. The movement of substances across the membrane can be either passive, occurring without the input of cellular energy, or active, requiring the cell to expend energy in moving it. The membrane also maintains the cell potential.

Specific proteins embedded in the cell membrane can act as molecular signals which allow cells to communicate with each other. Protein receptors are found ubiquitously and function to receive signals from both the environment and other cells. These signals are transduced into a form which the cell can use to directly effect a response. Other proteins on the surface of the cell membrane serve as "markers" which identify a cell to other cells. The interaction of these markers with their respective receptors forms the basis of cell-cell interaction in the immune system.

http://en.wikipedia.org/wiki/Cell_membrane or you can try

http://micro.magnet.fsu.edu/cells/plasmamembrane/plasmamembrane.html

http://scientia.org/cadonline/biology/eukaryotic/plasmembrane.ASP

The cells of all living organisms are bounded by a thin and delicate structure known as Plasma membrane or cell membrane. It is selectively permeable in nature. In certain absorptive cells, it projects in the form of a number of microvilli to increase the area of absorption. Plasma membrane maintains the shape and form of the cell. It acts as a barrier between the extracellular and intracellular fluids. It plays a vital role in the movement of molecules into and out of the cell by diffusion, osmosis and active transport. It is basically composed of lipids, proteins and carbohydrates.

The lipids present in the cell membrane are mostly in the form of phospholipids. They are small molecules with a phosphate head group which is polar and hydrophilic in nature; a glycerol back bone, and two fatty acid tails which are nonpolar and hydrophobic.

The fluid mosaic model of plasma membrane by S.J Singer and G.L. Nicolson is helpful to explain the working of phospholipids in the plasma membrane. The phospholipids form a bilayer with mosaics of proteins embedded within the bilayer. The word 'fluid' refers to the fact that the lipid and protein molecules in the membrane are not rigid, but have mobility. Both the lipids and proteins are arranged in a mosaic pattern. The phospholipid bilayers are polarized so that the polar hydrophilic heads are directed outwards. One layer faces the outside and the other faces the interior of the cell or the cytoplasm. The non-polar hydrophobic fatty acid tails project inwards so as to face each other. Both lipids and proteins exhibit semisolid and semifluid properties. The transport proteins help to carry various substances in and out of the cell. The binding proteins or receptors translocate signals to genetic and metabolic machinery in the cytoplasm. Cholesterol also forms a part of the membrane in animal cells to maintain fluidity. Sugars are attached to some phospholipids and proteins. They protect the cell and are involved in cell to cell recognition.

Reference:
Biology Class XI .Aasha Pillai and C.S.Mishra.Foundation books.