The cell membrane, also called the plasma membrane, is composed of a double layer of lipids (fats) called a phospholipid bilayer. It physically separates the intracellular (inside cell) space from the extracellular (outside cell) environment. All plant and animal cells have cell membranes. The cell membrane surrounds the cytoplasm which is part of the protoplasm and is the living component of the cell.
The lipid bilayer forms spontaneously due to the properties of the phospholipids from which it is made. The majority of the lipid structure, the fatty acid chain, is non-polar (uncharged). Non-polar substances are unable to dissolve in water and are therefore known as hydrophobic compounds. Some lipids also contain polar (charged) regions. Polar substances can dissolve in water and are known as hydrophilic (water-loving) compounds. The bilayer forms because hydrophilic heads point outwards and from hydrogen bonds with water, and the hydrophobic tails point towards one another.
All the exchanges between the cell and its environment have to pass through the cell membrane. The cell membrane is selectively permeable to ions (e.g. hydrogen, sodium), small molecules (oxygen, carbon dioxide) and larger molecules (glucose and amino acids) and controls the movement of substances in and out of the cells. The cell membrane performs many important functions within the cell such as osmosis, diffusion, transport of nutrients into the cell, processes of ingestion and secretion. The cell membrane is strong enough to provide the cell with mechanical support and flexible enough to allow cells to grow and move.
Video: About the Cell Membrane
Watch the video below to learn about basic structure of the cell membrane.