Cell Transport Notes: Cell Membrane

I. Introduction: A basic principle in biology is that all living things are made of cells. While they all come in a wide variety of sizes and shapes, all cells have a fluid filled inside (called cytoplasm) surrounded by a protective layer.

A. The cell membrane, also known as the plasma membrane, is the boundary that separates all living cells from their nonliving surroundings.

This membrane: Protects the cells from the outside environment Controls chemical traffic into and out of the cell. It is described as being selectively permeable which means it allows some substances to cross more easily than others.

B. In 1972 S. J. Singer and G. L Nicolson proposed the Fluid Mosaic Model, which describes the structure of the cell membrane.

1. The cell membrane is made up of three types of organic compounds: -Lipids (phospholipids) -Carbohydrates -Proteins

2. In this model, the membrane is a flexible layer made of lipid molecules. It is mixed together with large protein molecules that act as channels through which other molecules enter and leave the cell.

3. Cell Membrane Diagram (Fluid Mosaic Model): *If cell membrane were solid, molecules such as oxygen and water would not be able to move into/out of the cell very easily.

II. Components of the Fluid Mosaic Model C. Lipids 1. Phospholipids- makes up the majority of the cell membrane. (has two parts which are seen below) The heads are water-soluble (hydrophilic- water loving) The tails are not water soluble (hydrophobic- water fearing) -By forming a bilayer the hydrophobic tails are sheltered from the water that surrounds the cell inside and out. Head (hydrophilic) Tail (hydrophobic)

Phospholipid Bilayer

2. Steroids (or sterols)- a second, less abundant type of lipid that is found in the cell membrane. a. The function is to regulate the fluidity of the cell membrane. -At high temperatures steroids helps hold the cell membrane together -At cold temperatures it keeps the bilayer from packing too tightly together

b. Cholesterol is the main membrane steroid found in animal cells -Cholesterol is made by all animals and is found in varying quantities in the food you eat. -HDL = good cholesterol (Healthy) -LDL = bad cholesterol (want Less of this kind)

B. Proteins 1. Membrane Proteins are used to transport material into the cell (things that cannot easily pass through the cell membrane).

2. There are two types of membrane proteins: (a) Peripheral proteins- which are located on the surface of the cell membrane (inside and outside) (b) Integral proteins- pass through the lipid bilayer. Just like the phospholipids, the protein is made up of parts that are hydrophobic and hydrophilic.

C. Carbohydrates 1) Carbohydrates in the membrane are usually attached to the outer surface of the cell membrane. 2) There ‘antenna’ are used by cells to help identify one another. 3) They are divided into two types, depending on what it is attached to: (a) Glycolipids- a carbohydrate attached to a lipid. (b) Glycoproteins- a carbohydrate attached to a protein.