1. Cell membranes are gatekeepers.
Learning objectives
Structure of cell membrane?
Function of cell membrane?
Section 3-2 Opener
Cell membranes control the movement of material into and out of the cell.
What is a cell membrane made up of? (structure)
What is its importance to living cells? (Function)
Cell membranes are gatekeepers. 1

2. Faulty membranes can cause diseases
Cystic Fibrosis
Mutations in the CFTR gene cause cystic fibrosis.
The CFTR gene provides instructions for making a channel that transports negatively charged particles called chloride ions into and out of cells. The flow of chloride ions helps control the movement of water in tissues, which is necessary for the production of thin, freely flowing mucus.
Mutations in the CFTR gene disrupt the function of the chloride channels, preventing them from regulating the flow of chloride ions and water across cell membranes. As a result, cells that line the passageways of the lungs, pancreas, and other organs produce mucus that is unusually thick and sticky. This mucus clogs the airways and glands, causing the characteristic signs and symptoms of cystic fibrosis
Hypercholesterolemia

3. Membrane Structure Membrane structure:
The foundation of all plasma membranes is a layer of lipid molecules all packed together. These are a special type of lipid, called phospholipids, which, as you may recall from Chapter 2, have what appears to be a head and two long legs.
The head molecule consists of a molecule of glycerol linked to a molecule containing phosphorous (Figure Good membrane material).  This head region is said to be “polar,” because it has an electrical charge. As you may also recall from Chapter 2, water is also a polar molecule and for this reason, other polar molecules mix easily with water.
Molecules that can mix with water are called hydrophilic (“water loving”) molecules. The two legs of the phospholipid are long chains of carbon and hydrogen atoms. Because they have no electrical charge, the carbon-hydrogen chains are non-polar molecules. And because they are non-polar, these molecules do not mix with water and are said to be hydrophobic (“water fearing”). The chemical structure of phospholipids gives them a sort of split-personality: their hydrophilic head region mixes easily with water, whereas their hydrophobic tail region does not mix with water.
The split personality of phospholipids makes them good membrane material. Once a large number of phospholipids are packed together with all of their heads facing one way and their legs the other, we have a sheet with one side that is hydrophilic and one that is hydrophobic. In a cell’s plasma membrane, two of these sheets of phospholipids are arranged so that the hydrophobic tails are all in contact with each other and the hydrophilic heads are in contact with the watery solution outside and inside the cell (Figure 3-9). This arrangement gives us another way to describe the structure of the plasma membrane as a phospholipid bilayer.
Membrane structure:
Main component is Phospho bilipid layer
In addition it has cholesterol, surface proteins and transmembrane proteins 3

4. Low temp: tightly packed
High temp: more fluid

5. Sources of Cholesterol

6. Lipid bilayer with mostly phospholipids, but also cholesterol, and proteins embedded in the bilayer.
Fluid Mosaic

7. Fluid Mosaic Membrane http://www.youtube.com/watch?v=Qqsf_UJcfBc
What determines whether a protein resides on the surface or extends through the bilayer? Its tertiary structure.
Remember from Chapter 2 that all the amino acids that make up each protein have side chains that differ from one another chemically.
Some of these side chains are hydrophilic, others are hydrophobic, and as a protein is assembled into its final shape, these side chains can cause parts of the protein to be attracted to hydrophobic or hydrophilic regions.
Because a transmembrane protein has both hydrophobic and hydrophilic regions, part of the protein can exist in the hydrophobic region in the center of the membrane while the other resides in hydrophilic regions.
Peripheral membrane proteins, on the other hand, have an entirely hydrophilic structure, and so can bind only to the head regions of the phospholipids.
As a consequence, they can be positioned on either the outer or inner side of the membrane.
Once membrane proteins are in place, the hydrophobic and hydrophilic forces keep them properly oriented. Because all of the components of the plasma membrane are held in the membrane in this manner, they can float around without ever popping out.
Fluid mosaic model 7

8. What is structure F? A. Receptor protein B. Cholesterol
C. Transmembrane protein
D. Phospholipids
E. Extracellular matrix
Ans:C

9. Main function: Transport of material
______ do most of the membrane functions
Proteins do most of the membrane functions
“Membranes are Selectively Permeable”

10. 2. Facilitated diffusion 3. Active transport
Transport Processes
1. Diffusion :
2. Facilitated diffusion
3. Active transport
Carrier (protein) Mediated Transport
Carrier protein will bind to solute and then transport it by conformation change in the protein
Channel protein will make a channel or pore and transport solutes (water channel is called aquaporin)

11. Simple Diffusion and Facilitated diffusion
No energy is required in both
The molecules move from a area of high concentration to low concentration.
Molecules such as oxygen and carbon dioxide, that are small and carry no charge, can pass directly through the lipid bilayer of the membrane without the assistance of any other molecules in a process called simple diffusion.
Each time you take a breath, for example, there is a high concentration of oxygen molecules in the air you pull into your lungs. That oxygen diffuses across membranes of the lung cells and into your bloodstream where red blood cells pick it up and deliver it to parts of your body where it is needed. Similarly, because carbon dioxide in your bloodstream is at a higher concentration than in the air in your lungs, it diffuses from your blood into your lungs and is released to the atmosphere when you exhale (Figure Simple and facilitated diffusion). 
What is the difference between the two types of transport? 11

12. In active transport, cells use energy to move small molecules into and out of the cell.
Molecules can’t always move spontaneously and effortlessly in and out of cells.
Molecules can’t always move spontaneously and effortlessly in and out of cells. Sometimes their transport takes energy, in which case the process is called active transport.
Such energy expenditures may be necessary if the molecules to be moved are very large or if they are being moved against their concentration gradient. 12

13. Which of these cannot pass directly through the phospholipids of the plasma membrane?
A. Only A
B. Only B
C. Only C
D. Only D
E. B, C, and D
Ans: E (glucose, water and hydrogen ions cannot pass through the phospholipids)
Any ion because of their charge cannot cross cell membrane
Glucose and water are hydrophilic molecules and so cannot cross the phospholipid membrane

14. Faulty membranes can cause diseases
Cystic fibrosis: Faulty ______
Hypercholesterolemia: Faulty ______
Cystic fibrosis: faulty transmembrane protein
Hypercholesterolemia: faulty receptor protein

15. Why is it extremely unlikely that a person will catch HIV from casual contact—such as shaking hands—with an infected individual?
The AIDS-causing HIV virus uses the molecular markers on a cell’s plasma membranes to infect an individual’s cells. These same molecular markers are also the reason why it is extremely unlikely that a person can catch AIDS from casual contact—such as shaking hands—with an infected individual.
The specific molecular markers involved in infection by the HIV virus belong to a group of identifying markers called “clusters of differentiation.” Abbreviated as “CD markers” and having names such as CD1, CD2, and CD3, these marker molecules are proteins embedded within the plasma membrane that enable a cell to bind to outside molecules and, sometimes, transport them into the cell.
One CD marker is called the CD4 marker. It is found only on cells deep within the body and in the bloodstream, such as immune system cells and some nerve cells.
It is the CD4 marker, in conjunction with another receptor, that is targeted by the HIV virus. If the HIV virus can find a cell with a CD4 marker, it can infect you, and because the CD4 markers never occur on the surface of your skin cells, casual contact such as touching is very unlikely to transmit the virus (Figure HIV is not transmitted through casual contact).  
Even if millions of HIV particles are present on one person’s hands, they just can’t gain access to any of the other person’s surface cells. 15