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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