Membrane Structure & Function

General

· The cell membrane forms a barrier between the cell and the external environment

· A membrane performs several jobs for the cell:

1.The cell must be able to retain the molecules it requires.

2.The cell must be able to exclude unwanted molecules.

3.Cells recognize one another by molecules attached to their membranes.

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

1. Substances able to pass through a membrane

a) Nonpolar molecules like fats lipids.

b) Small polar molecules like water.

2 Substances that are unable to pass through a membrane

a) Ions and charged molecules like salts dissolved in water.

b) Small polar molecules like glucose.

c) Macromolecules

Membrane structure

· 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.

· The membrane contains proteins that have a variety of functions. For example, some proteins are receptors which can detect the presence of certain kinds of molecules in the surrounding environment

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

· Because the membrane a fluid, components of the membrane can move sideways like buoys floating in water.

· 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.

· 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).

· 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. The nature of phospholipids also means that any small tear in a membrane will repair itself spontaneously so that the tails avoid contact with water. It also means that two membranes can fuse together very easily.

Membrane function

· The primary function is to control the passage of materials into and out of the cell

· Membranes contain other molecules as well

1. Proteins in the membrane do a variety of jobs like:

a) channels for moving materials into and out of the cell

b) markers used for cell recognition

c) receptors for hormones

d) pumps for transporting materials across the membrane

2. 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.

3. Proteins associated with the cell membranes of animal cells may bind to proteins of adjacent cells.