Theory:

Structure and function of cell:
The living organisms comprise of many organs that make up the various systems such as the digestive, respiratory, circulatory, excretory and reproductive systems of the body. The organs of these systems perform various specific functions. Similarly, the different organs of a plant perform specific functions.
 
cell-48542_1280.png
Example:
The roots help in the absorption of water and minerals.
Each organ is made up of specific smaller structures known as the tissues. The tissues are a group of cells that perform a similar function. Thus an organ is made up of tissues which are made up of cells that are the basic structural unit of a living organism. The cell comprises of various parts such as the cell membrane, cell wall and the cell organelles involving the nucleus.
Cell membrane:
The membrane that separates the cell's components from the surrounding medium and from each other is known as the cell membrane or plasma membrane.
The cell membrane is a thin and elastic material that gives shape to the cell and protects its contents. The plasma membrane is a porous structure and hence allows the movement of substances or materials both inward and outward. As it allows only certain substances such as water, minerals and a few other vital substances, it is known as a selectively permeable membrane.
 
Anatomy_and_physiology_of_animals_structure_plasma_membrane.jpg
Plasma membrane
Reference:
https://commons.wikimedia.org/wiki/File:Anatomy_and_physiology_of_animals_structure_plasma_membrane.jpg