The cell membrane is the outside layer (“skin”) of the cell. 10 I. STructure Proteins are found floating around in the membrane. Some of them only go half-way through the membrane, others stick the whole way through. This layer can then act like a ‘tough’ fluid much like that of a nearly unpoppable soap bubble. The membrane will “flow” around the cell. An important part of membranes is the lipid Cholesterol. It is keeps the “balls & tails” from being tangled together. These proteins have many very important functions: Section 3: The Cell Membrane The cell membrane is the outside layer (“skin”) of the cell. One type of proteins act as “Sign Holders” that tell other parts of your body exactly what kind of cell it is. This is important for cells like your White Blood Cells, which are supposed to go around and kill things that don’t belong. One great example is your Red blood cells. It is made from a combination of Lipids & Proteins. The lipids form two “back-to-back” layers known as a Lipid Bilayer.

What do you think an AB type blood cell would look like? 11 Type O blood is blood that doesn’t have any signs on it. It looks like: A doctor can give Type O blood to anyone, since it does not have any signs, there is nothing to make the WBCs mad, so they won’t attack! This is why hospitals want people who are Type O to donate blood as often as they can. So, if your blood is Type A your white blood cells would recognize the big green protein sticking out of it and leave it alone. However, if for some reason, a doctor gave you Type B blood, you would be in trouble. Your WBC’s would not recognize the purple protein and it would attack and destroy the blood cell. This would end up clogging up your blood stream with dead cells and you would die. There are four different types of blood: A, B, AB, & O If you were Type AB, though, you could receive any type of blood because your WBC’s would recognize either the A-signs or the B-signs, so they would not attack anything! There are two different “signs” a protein in a red blood cell membrane can hold: What do you think an AB type blood cell would look like? AB O A B

3 Analogy: This is like the offensive line on a football team. The line wants to keep the defensive players from getting through at all costs, but they WANT the running back to get through so he can run forward to get yards. Other proteins can act as “doorways” to get materials into the cell. A cell membrane is Semi-Permeable. In Latin, “Permeable” means “to pass through”. If something is semi-permeable, it allows some objects through, but keeps others out. Oxygen (O2), Carbon Dioxide (CO2), & Water (H20) can pass through a membrane any time they wish. II. On the Move

Diffusion Osmosis Selective Transport 1 There are three ways that materials can cross a membrane: Diffusion Osmosis Selective Transport

of water affect the rate Diffusion: 6 2. You drop food coloring into a glass of water. The dye is VERY concentrated at first at the top where the drop landed. The dye will spread to areas of LOWER concentration (clear). Eventually, the dye will spread evenly throughout the glass and the water will be one color. Will the temperature of water affect the rate of diffusion? 1. One person farts in a classroom. A HIGH concentration of smelly molecules is found directly behind the person that farted. These smelly molecules will spread to areas of the room that don’t have fart molecules (LOW concentration). Eventually, the molecules will evenly fill the entire room. Usually, by then the molecules spread so far apart that you can no longer detect them. So, even though those people across the room cannot smell it, they will eventually still be breathing pieces of your fart. Material will move from areas of HIGHER concentration to LOWER concentration. How it works: Molecules are always moving. (remember temperature is the measure of their speed) They are also bumping into one another as they move. This causes them to bounce off each other and spread out. Eventually, they will spread evenly everywhere. This works automatically without the use of energy. Examples:

Which way will the Water Osmosis: Which way will the Water move through Osmosis? 5 Osmosis is a special type of Diffusion. Osmosis is the diffusion of Water. Water This state of balance is called Equilibrium. Water that has 20% Salt Water that has 30% Salt Water that has 10% Salt Water that has 20% Salt In osmosis, water will move from places with more water to places with less water. Salt Which way will the Salt move through Diffusion?

10 Another way a cell can get objects inside that would not ordinarily diffuse is by Endocytosis. “Endo” means inside and “cytosis” refers to the cytoplasm. Therefore, Endocytosis brings objects into the cytoplasm. (it is also called Engulfing) Sometimes there is a higher concentration of a material (like SUGAR) inside a cell than there is in the bloodstream. If diffusion were to happen, then the cell’s sugar would leave the cell. This would be BAD because then the cell would not be able to create as much energy. The cell is actually able to absorb MORE sugar, even though it is against the concentration gradient. But, to do so requires active transport & ENERGY. A cell can do this using TRANSPORT PROTEINS found in the membrane. Active Transport These proteins can capture molecules that the cell needs inside and ‘push’ them inside. Diffusion & Osmosis is like riding a bicycle downhill. They do not take any energy to occur. Active Transport, on the other hand, is like riding uphill. It takes energy to move. The opposite, or Exocytosis, is when a cell removes large amounts of material from its cytoplasm, since “Exo” means “out”. This is how single-celled creatures usually EAT. If you put together the different types of proteins and lipids in a plasma membrane, you get something that looks like this: