Function: to control the movement of substances into and out of the cell. 


·    Maintains homeostasis.


Membranes are able to maintain homeostasis because they are selectively permeable (allows some materials to pass through while keeping other out).





Phosphate Head (hydrophilic): Water loving; Contains phosphate


Fatty Acid Tail (hydrophobic): Water fearing;

Contains fatty acids: saturated are rigid and unsaturated are flexible




A. phospholipids

     1.phosphate group – polar

     2.two fatty acid chains – nonpolar

·    One fatty acid chain is saturated making it rigid and the other is unsaturated which is more flexible. The unsaturated chain will also be bent.

·    form a bilayer with the phosphate heads lined up on the outside and the fatty acids tails on the inside of the bilayer (not the outside or inside of the cell).

·    Small nonpolar molecules are able to pass right through the plasma membrane. Ex: O2, CO2

·    Polar molecules, ions or large molecules are not able to pass through the membrane. Ex: Na+, Cl-, H+, Ca+, H2O, C6H12O6

·    Polar molecules, ions and large molecules can pass through the membrane with the help of proteins.

B. cholesterol

·    helps to give a little rigidity (to strengthen) to the plasma membrane

C. proteins

·    can be embedded in the membrane or located outside the membrane

·    carry out many different functions

1. protein channels -used in diffusion.

2. protein pumps-used in active transport.

3. enzymes-speeds up reactions.

4. receptor proteins-accepts certain molecules.

5. marker proteins-where the carbohydrates are attached.

D. carbohydrates

·    attached to marker proteins

·    help in cell recognition so the body can distinguish between foreign substances

·    act like a “name tag”

·    called the glycocalyx.




·    Describes the plasma membrane.

·    The phospholipids are able to move sideways around each other (fluid) but still stay within the membrane.

·    The membrane is made up of so many different molecules it is like a mosaic.

·    The dual-natured polarity of the phospholipids maintains the structure of the cell membrane.

·    The phospholipid bilayer is an example of the fact that oil (fats) and water don’t mix!!!


The lipid bilayer is held together by the fact that the polar heads (hydrophilic) are attracted to water both inside and outside the cell.


The fatty acid tails (hydrophobic) are repelled by the water so they “hide” inside the bilayer.





A.Fatty acid tail          F. Channel protein (integral)


B.Phospholipid          G. Phosphate head


C.Cholesterol              H. Protein pump (integral)


D.Carbohydrate          I. Inside


E.Outside                  J. Protein (peripheral)