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Cell Membrane Structure and Function
What is the importance of having a cell membrane?
I. Membrane Structure
a. Membranes contain proteins, lipids, and carbohydrates (which are all types of macromolecules)
b. The lipids in the cell membrane are phospholipids.
c. The current model of the cell membrane is called the fluid mosaic model.
II. Evolution of Cell Membrane Theories a. 1895 the cell membrane was thought to be made of lipids
since substances that dissolved in lipids entered the cells faster than insoluble substances.
b. 1917 first artificial membrane formed from phospholipids. c. 1925 idea that the membrane was a phospholipid bilayer. d. 1935 the Sandwich model proposed a phospholipids
bilayer between two layers of globular proteins.
e. 1972 phospholipid bilayer with membrane proteins dispersed throughout the cell membrane
(CURRENT AND ACCEPTED MODEL)
Evolution of Cell Membrane Theories Sandwich model Phospholipid Bilayer
f. Fluidity of Membranes
i. Lipids move freely in a membrane, but flip-flopping is rare.
f. Fluidity of Membranes
ii. Unsaturated hydrocarbons have kinks that keep the molecules from packing together and enhancing membrane fluidity.
f. Fluidity of Membranes
iii. Cholesterol reduces membrane fluidity (at moderate temps). At low temps it hinders solidification by disrupting the packing of the phospholipids.
g. The Mosaic Structure of the Cell Membrane
i. Mosaic – many different parts make up the cell membrane, complicated structure
Mosaic Structure of the Cell Membrane
How does the structure of the cell membrane support its function?
ii. Types of proteins in the membrane
1. Integral proteins – proteins that span the entire plasma membrane.
*What do you think the function of these proteins may be?
2. Peripheral proteins – proteins that are not embedded in the plasma membrane.
*What do you think the function of these proteins may be?
iii. Functions of Cell Proteins Transport Intercellular joining
iii. Functions of Cell Proteins Cell-Cell recognition Enzymatic activity
iii. Functions of Cell Proteins Signal Transduction Attachment to the
cytoskeleton and ECM
iv. Cell membranes have distinct inside and outside faces. Carbohydrates are found on the outer side of the membrane.
1. The carbohydrates on the surface of the cell membrane function to distinguish one cell from another.
h. THE CELL MEMBRANE IS selectively permeable!!!
So, what does that mean?
III. Traffic Across Membranes
a. Permeability of the lipid bilayer i. Hydrophobic molecules can dissolve
across the lipid bilayer 1. ex: hydrocarbons, CO2, O2, lipids
ii. Ions and polar molecules can not dissolve across the membrane 2. ex: Water, glucose, Na+
III. Traffic Across Membranes
iii. Transport proteins – helps substances across the plasma membrane, type of integral protein 1. Channel proteins – hydrophilic
channel, tunnel 2. Pump proteins – physically move
substance across membrane 3. The proteins are substrate specific
They will only allow specific molecules to pass through.
What do you see happening?
III. Traffic Across Membranes
b. Passive Transport across the membrane i. Diffusion – movement from an area of
high concentration to an area of low concentration, reaches equilibrium
III. Traffic Across Membranes
ii. Concentration gradient – change in the density of a substance
iii. Passive Transport – does not require input of ATP (energy)
III. Traffic Across Membranes
c. Osmosis – the diffusion of water across a selectively permeable membrane
*Osmosis is a special case of diffusion*
SIDEBAR: Prefixes
What do the following prefixes mean?
a. Hyper
b. Hypo
c. Iso
III. Traffic Across Membranes
i. Hypertonic – higher concentration of solutes
ii. Hypotonic – lower concentration of solutes
iii. Isotonic – equal concentration of solutes
Examples of solutions
III. Traffic Across Membranes
d. Why is water balance important? i. The control of water balance is
osmoregulation (animal cells). ii. Ex: Fish in salt water
III. Traffic Across Membranes
iii. Plant cells can be turgid (normal state), flaccid (limp), or plasmolyzed (lethal state).
III. Traffic Across Membranes
e. Facilitated Diffusion i. Facilitated
diffusion requires the help of a transport protein to move molecules or ions.
III. Traffic Across Membranes
f. Active Transport i. Active transport requires the use of ATP
Energy is required because molecules are usually going against their concentration gradient.
ii. Ex: Sodium-potassium pump
http://bio1151.nicerweb.com/med/Vid/Johnson6e/ch05/sodium_potassium_exchange_pump.swf
III. Traffic Across Membranes
III. Traffic Across Membranes
g. Some ion pumps generate voltage across membranes i. Ex: nerve cells (Sodium Potassium pump) ii. An unequal distribution of charged ions
across a membrane will cause a membrane potential.
III. Traffic Across Membranes
iii. A transport protein that generates voltage across a membrane is a Electrogenic pump.
iv. Most common example of a electrogenic pump is the proton pump.
III. Traffic Across Membranes
h. Cotransport i. Some molecules are transported across
the membrane at the same time, this is cotransport.
ii. Ex: H+ ions are transported out of a cell, and both the H+ ions and sucrose molecules are transported into the same cell.
III. Traffic Across Membranes
i. Exocytosis and Endocytosis i. When the cell secretes macromolecules
by the fusion of vesicles with the plasma membrane, this is exocytosis (think exit)
ii. When the cell takes in macromolecules and matter by forming new vesicles from the plasma membrane this is endocytosis (think enter)
III. Traffic Across Membranes **Examples of Endocytosis**
1. phagocytosis – taking in of large molecules
2. pinocytosis – taking in of water
3. receptor-mediated endocytosis – specific substances are taken in such as hormones, neurotransmitter