Plasma Membrane Or Cell Membrane.

The outer covering of a cell is called plasma membrane. It is made of lipids and proteins. It defines the shape and size of a cell, encloses cytoplasm and protect it from external environment. 

Any substance enters or leaves the cell through this membrane only. This exchange of substances through this membrane occurs very selectively hence it is known as 'selectively permeable membrane'. 

 Experiment to observe plasma membrane. 

Take a rheo leaf ,tear it in single stroke 

and observe it against the light. Take a small piece of light colored transparent portion keep it on a slide, put one drop of water and observe it under a microscope. 

Now put some dilute salt solution over the specimen and observe it under a microscope. 

The salt solution removes the water present inside the cell and hence resulting in shrinking of cytoplasm. The outer boundary of cytoplasm is plasma membrane. 

The separated layer is called the cell wall. It is made up of cellulose. It is a tough and flexible layer that gives protection to cell. It exerts an inward pressure (turgor pressure) to resist the outward pressure exerted by cell sap. Hence the plant cells can withstand greater changes in surrounding medium than the animal cells.