One universal feature of all cells is an outer limiting membrane called the plasma membrane.
In addition, all eukaryotic cells contain elaborate systems of internal membranes, which set up various membrane-enclosed compartments within the cell.
Cell membranes are built from lipids and proteins.







A thin membrane surrounds every living cell, delimiting the cell from the environment around it. 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, some 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 that 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 and passed in a different form into the cell. For example, a hormone binding to a receptor could open an ion channel in the receptor and allow calcium ions to flow into the cell. Other proteins on the surface of the cell membrane serve as "markers" that 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.



A historical perspective


In the early 1930's-40's, Danielli and Davson studied triglyceride lipid bilayers over a water surface. They found that they arranged themselves with the polar heads facing outward. However, they always formed droplets (oil in water) and the surface tension was much higher than that of cells














Fluid Mosaic Model

A model conceived by S.J. Singer and Garth Nicolson in 1972 to describe the structural features of biological membranes.

The plasma membrane is described to be fluid because of its hydrophobicintegralcomponents such as lipids and membrane proteins that move laterally or sideways throughout the membrane. That means the membrane is not solid, but more like a 'fluid'.
The membrane is depicted as mosaic because like a mosaic that is made up of many different parts the plasma membrane is composed of different kinds of macromolecules, such as integral proteins, peripheral proteins, glycoproteins, phospholipids, glycolipids, and in some cases cholesterol, lipoproteins.
According to the model, the plasma membrane is a lipid bilayer (interspersed with proteins). It is so because of its phospholipidcomponent that can fold in itself creating a double layer - or bilayer - when placed in a polar surrounding, like water. This structural feature of the membrane is essential to its functions, such as cellular transport and cell recognition.







Phospholipids


The main constituent of the membrane. The hydrophilic heads of the molecule face the outside and inside of the cell while the hydrophobic tails face each other in the middle of the membrane.
Cholesterol
This lipid with the four fused rings structure acts to stiffen the membrane and control its fluidity.
Proteins
Membrane proteins have many different functions. Protein molecules in their tertiary three dimensional form can act as channels, carriers and pumps, cell recognition proteins, receptors for hormones or other signal molecules, and enzymes. The function of membrane proteins will be the topic of other lessons in this unit.
Carbohydrates
Carbohydrate chains are found attached to membrane lipids and proteins on the cells outer surface. These molecules called, glycolipids or glycoproteins, are responsible for giving each cell a unique identity. Our bodies recognize these cells as our own. Transplanted organs and incompatible blood types have different sugar chains than our own and are attacked by our immune systems causing rejection.