II. The Cell Membrane

A. Cell Membrane Function1. Transport Proteins are selectively permeable-

regulate what materials enter and exit the cell 2. Enzymes are often attached to membranes with

the active sites exposed3. Receptor Proteins (signal transduction)-

receive information from a chemical messenger4. Cell-cell Recognition- glycoproteins sever as ID

tags that are recognized by other cells 5. Intercellular Joining- membrane proteins join

to cells together6. Attachment Proteins- attach to the cytoskeleton

that maintains the cell’s shape

Cholesterol

Proteins

Oligosaccharide Chains

Enzyme

B. Membrane Structure

Phospholipid

Cell Membrane Structure

1. The cell membrane consists of a phospholipid bilayer in which globular proteins are embedded.

2. The phospholipids are amphipathic - they have both hydrophilic and hydrophobic ends. Membrane is stabilized by hydrogen bond which causes membranes to be self sealing

3. Phospolipids are Fluid -The lipids and protein are free to move within the plane of the membrane.

4. Two types of Membrane proteins:

a) Integral Proteins - extend through one or both lipid layers.

b) Peripheral Proteins- on membrane surface

5. Cholesterol molecules are found in the hydrophobic lipid layer of the membrane. Cholesterol helps to maintain membrane structure:

a) Cholesterol acts like a speed bump to slow down phospholipids when they are moving to fast due to increased heat

b) Cholesterol helps spread out phospholipids when they are cold and prevents the hydrophobic tails from sticking together

6. Membrane Carbohydrates Allow Cell Recognition

a) Cells recognize each other by the pattern of oligosaccharides attached to the cell membrane

1) Glycolipids – carbohydrates attached to lipids

2) Glycoproteins - carbohydrates attached to proteins

b) Oligosaccharides- contain 15 or fewer sugars

C. Passive Transport Mechanisms- move molecules across the membrane using no ATP

1. Diffusion- movement of molecules from areas of high concentration to areas of low concentration. Diffusion is due to Brownian Movement -random motion of small particles. Factors that affect diffusion are:

a) Concentration - the higher the concentration of solute, the greater the rate of diffusion

b) Temperature - increased temperature increases the kinetic energy of the particles causing them to move faster and collide more thus increasing diffusion

c) Pressure - the more particles in a fixed volume, the greater chance for molecular collision, therefore increasing diffusion

2. Facilitated Diffusion- the use of protein channels to move specific molecules across the membrane with their concentration gradient. Transport may be:

a) Uniport- one molecule or ion one way.

b) Symport- two different solutes one way

c) Antiport- two different types of molecules are transported opposite ways.

3. Osmosis-diffusion of water from an area of high water concentration to an area of low water concentration.

a) The concentration of dissolved solutes and water is

inversely proportional. Therefore, diffusion and osmosis are opposites.

OSMOSIS

High Solute concentrationHigh H2O concentration

Low H2O concentrationLow solute concentration

DIFFUSION

b) Cells may be in One of Three Conditions:

1) Isotonic Environment- equal amount of solute and water on both sides of a membrane

Cell5% solute95% H2O

Beaker with 5% solute and 95% water

Water will move equally in and out of the cell

2) Hypertonic Environment- more solute outside the

cell and more water in the cell

Cell5% solute95% H2O

Beaker with 10% solute and 90% water

Water will move out of the cell

Shriveled

3) Hypotonic Environment- less solute and more

water outside the cell.

Cell5% solute95% H2O

Beaker with 0% solute and 100% water

Water will move into the cell

Cells with no cell wall

Burst!

D. Water balance in cells: 1. Cells lacking cell walls must be isotonic with

their environment or have a mechanism to remove water hypotonic situations. Ex. Contractile vacuoles

2. Plants, prokaryotes, fungus, and some protists have cell walls

a) In hypotonic solution they are turgid which is ideal and gives them support

b) In isotonic solutions they are flaccid or wiltyc) In hypertonic solutions plant cells plasmolyze. This

is lethal. The cell membrane is pulled from the cell wall.

E. Mediated Transport Systems - transport systems that require energy.

1. Active Transport- using transport proteins to bind with specific substances to move them across the cell membrane AGAINST the concentration gradient. Example: Sodium- potassium pump

2. Ion Transport- diffusion of charged solutes depend on a chemical and electrical gradient called the electrochemical gradient

a) Electrochemical gradient determines the net direction of ionic diffusion

b) Membrane Potential- the voltage across a membrane is determined by ion distribution. The sodium-potassium pump and proton pump generate voltage across a membrane.

3. Transport of Large Molecules

a) Endocytosis- movement of large particle into the cell

1) Phagocytosis - engulfing solids

2) Pinocytosis- engulfing liquids for the desired solutes

3) Receptor Mediated Endocytosis- receptor sites on membrane proteins bind with specific substrates in an area called a coated pit

1) when an appropriate number of substantes have been captured, the coated pit is engulfed

2) The cell can aquire bulk quatities of specific solutes this way

b) Exocytosis - moves large materials out of the cell