Membrane Structure & Function

1.         Membrane structure

            a.         Our current model of the cell membrane is called the fluid mosaic model. It is called this because the membrane is fluid and contains several different components embedded in it.

            b.         It is a lipid bilayer, meaning it is made of two layers of fatty molecules (called phospholipids) sandwiched together.

                        i.         Phospholipids have a hydrophilic (or polar) head and two long, hydrophobic (or nonpolar) tails. This means the heads like to be near (or interact with) water while the tails prefer to be away from (or not interact with) water. Molecules like this are called amphipathic.

                        ii.        In general, nonpolar molecules do not interact with polar molecules. This can be seen when oil (nonpolar) is mixed with water (polar). Polar molecules interact with other polar molecules and ions while nonpolar molecules interact with other nonpolar molecules. E.g., table salt (ionic) dissolves in water (polar).

                        iii.       The easiest way to satisfy both heads and tails is for the phospholipids to from two layers with the tails between them and the heads facing water on both sides. Phospholipids do this spontaneously.

            c.         The fluid nature of the membrane means that

                        i.         components of the membrane can move sideways like buoys floating in water.

                        ii.        small tears in a membrane will repair themselves spontaneously so that the tails avoid contact with water.

                        iii.       two membranes can fuse together very easily.

            d.         The membrane contains proteins that have a variety of functions:

                        i.         channels for allowing substances to move into and out of the cell

                        ii.        pumps for transporting substances across the cell membrane

                        iii.       receptors for hormones

                        iv.       binding adjacent cells together

                        v.         markers used for cell recognition

                                    (1)       Glycoproteins are proteins with sugars attached. They function in cell recognition, allowing cells to identify other cells. This is particularly important in the immune system where cells need to be able to identify and destroy foreign invaders such as bacteria or viruses.

2.         Selective permeability

            a.         The cell membrane forms a barrier between the cell and the external environment. It separates the contents of the cell from its environment and regulates the passage of molecules into and out of the cell. The membrane allows the cell to

                        i.         retain the molecules it requires.

                        ii.        exclude unwanted molecules.

                        iii.       recognize one another by molecules attached to their membranes.

            b.         Membranes are selectively permeable barriers. This means that some molecules are able to pass through while others cannot.

                        i.         Substances able to pass through a membrane

                                    (1)       Nonpolar molecules like fats lipids.

                                    (2)       Small polar molecules like water.

                        ii.        Substances that are unable to pass through a membrane

                                    (1)       Ions and charged molecules like salts dissolved in water.

                                    (2)       Polar molecules like glucose.

                                    (3)       Macromolecules