Cell Membranes
Chapt 4
www.cellsalive.com/
The Cell
Membrane
Fluid Mosaic Membrane
Text pg 80
Cell Membrane: Fluid Mosaic Model
At Very High
Magnification
& in color
Membrane Structure
Flash movie too..
CLICK ON THE PICTURE TO SEE AN
ANIMATION OF THE CELL MEMBRANE
Cell Membrane
1. Every cell is encircled by a membrane
2. Membranes fence off the cell's interior from its
surroundings. Membranes let in water, certain ions
and substrates and they excrete waste substances.
They act to protect the cell.
3. Without a membrane the cell contents would pass
into the surroundings
4. Allows the cell to maintain
HOMEOSTASIS = a constant internal environment
kind of like the cell’s happy place 
Cell Membranes
Fluid-like structure…like soap bubbles
Structure
1. Lipids in a bilayer
2. Protein-coated pores extend through membrane (MOSAIC)
– Pores are very small and give the membrane polar
properties
– H2O penetrates membrane easily
– Large polar molecules do not
3. Proteins embedded in lipid layer
4. Some proteins floating within the lipid sea (called integral
proteins)
5. Some proteins associated outside the lipid bilayer (peripheral)
Membrane Lipids
• Composed largely of phospholipids
• Phospholipids composed of….glycerol and
two fatty acids + PO4 group
• P-Lipids are polar molecules…
P-Lipids are represented like this
Membrane Lipids
form a Bilayer
Outside layer
Inside Layer
Quiz
• If Phospholipids are polar, which end seeks
out water and which avoids water?
Phospholipid Molecule
Model
phosphate (hydrophilic)
glycerol
fatty acids (hydrophobic)
Fluid Mosaic Membrane
Text pg 80
Functional Parts of the Cell Membrane
• Cell Recognition Proteins unique markers that identify cells
• Channel Proteins allows molecules and ions to move across
• Carrier Proteins combine with substrate to move across membrane
• Receptor Proteins bind to specific molecules (ex. hormone receptors)
• Enzymatic Proteins proteins that run specific metabolic reactions
• Cell membrane contains carbohydrate (CHO’s) attached to proteins or
lipids ( makes Glycoproteins and Glycolipids)
• Found on outside of cell functioning as markers to identify cells to the
immune system
Auto-Immune Disease
• Inflamed body cells in the joints
or colon respectively being
recognized by white blood cells
as “non-self”.
• Result is an immune response
that causes injury to the body’s
cells – in this case, the cartilege
at the joints
Membrane Proteins
• Integral: embedded within bilayer
• Peripheral: reside outside hydrophobic
region of lipids
Membrane Proteins
Integral membrane proteins
Peripheral membrane proteins
Integral
Membrane Models
Fluid Mosaic Model - lipids arranged in
bilayer with proteins embedded or
associated with the lipids.
Evidence for the Fluid Mosaic
Model (Cell Fusion)
More Evidence for the Fluid Mosaic
Model
Membrane Permeability
• Biological membranes are physical
barriers..but which allow small uncharged
molecules to pass…
• And, lipid soluble molecules pass through
• Big molecules and charged ones do NOT
pass through
How to get other molecules
across membranes??
There are three ways that the molecules typically move through the
membrane:
1. Facilitated transport
2. Passive transport
3. Active transport
•
Active transport requires that the cell use energy that it has obtained
from food to move the molecules (or larger particles) through the cell
membrane.
•
Facilitated and Passive transport does not require such an energy
expenditure, and occur spontaneously.
Membrane Transport Mechanisms
I. Passive Transport
• Diffusion- simple movement from regions of high
concentration to low concentration
• Osmosis- diffusion of water across a semi-permeable
membrane
• Facilitated diffusion- protein transporters which assist in
diffusion
Membrane Transport Mechanisms
II. Active Transport
• Active transport- proteins which transport against
concentration gradient.
• Requires energy input
Diffusion
• Movement generated by random motion of particles.
• Applies to any molecule/requires NO ENERGY!!
• Movement always from region of high concentration to low concentration
Diffusion continued
• Lipid soluble molecules (alcohols) and gases (O2
and CO2) pass easily thru the membrane
• H2O passes through protein channels
• Large molecules and charged ions have difficulty
passing thru (charge on membrane = polarity)
Osmosis
• Movement of water across
a semi-permeable barrier.
Example:
• Salt in water, cell
membrane is barrier
• Salt will NOT move across
membrane, water will.
Click Picture Above for
an Osmosis Demo
Tonicity of Solutions
Tonicity = the strength of a solution that a cell is placed in
3 Tonicity scenarios you must know!!
1.
Isotonic
• equal conc. of particles inside / out of the cell
• Cells placed in isotonic solutions do not gain or lose H2O
2.
Hypertonic
• outside of cell has greater particle conc.
• Cells placed in hypertonic solutions lose H2O and SHRINK
3.
Hypotonic
• inside of cell has greater conc.
• Cells placed in hypotonic solutioins gain H2O and SWELL
Osmosis in Hypertonic Medium
cell
Solute outside cell
is greater water is
drawn from inside
the cell to outside
solute
10% NaCl = hypertonic to blood cells (accustomed to 0.9%)
CELL SHRINKS OR “CRENATES”
Hypertonic solutions- shrink cells
Osmosis in Hypotonic medium
Solute inside cell
is less and
attracts water
inside, causing
swelling
> 0.9 % NaCl =
Hypotonic to blood cells
CELL SWELLS
Hypotonic solutions- swell cells
Pressure on cell due to the flow of water is called
OSMOTIC PRESSURE
Check out the animation ---> click me
Importance of Osmosis
• Allows for absorption of H2O by the large intestine
• Retention or shedding of H2O by kidneys
• Uptake of H2O by the blood affects our blood pressure
• Increased blood pressure creates a greater risk of heart attack and
stroke
• Watch these Flash Movies --->
Endocytosis
• Transports macromolecules and large particles into
the cell.
• Part of the membrane engulfs the particle and
folds inward to “bud off.”
Endocytosis
Putting Out the Garbage
• Vesicles (lysosomes, other secretory
vesicles) can fuse with the membrane and
open up the the outside…
Exocytosis
(Cellular Secretion)
Movies!
A wee flash tour of endocytosis and exocytosis – just click
the image below
Membrane Permeability
1) lipid soluble solutes go through faster
2) smaller molecules go faster
3) uncharged & weakly charged go faster
1
4) Channels or pores may also exist in
membrane to allow transport
2
3 Types of Endocytosis
•
Click on active transport, then next a few times until the menu
shows “endocytosis”
1. Phagocytosis = cell engulfs large amounts
2. Pinocytosis = the cell takes in (drinks) a small amount
3. Receptor-guided = when receptors must first be filled before
endocytosis is allowed. This often happens with hormones.
Three Types of Transport
1. Active Transport
– requires energy ATP
– Uses a transport protein in the membrane
2. Facilitated Transport = Passive
– NO ATP necessary
– Like active transport, uses a transport protein in the
membrane
3. Diffusion and Osmosis = Passive
– no ATP needed
Transport Proteins
Facilitated Diffusion & Active Transport
• move solutes faster across membrane
• highly specific to specific solutes
ACTIVE
• can be inhibited by drugs
FACILITATED
Active Transport
• Movement of particles low concentration into high
concentration (against a concentration gradient)
• Requires energy input from ATP
• Cells use active transport to build up their stores of
important particles, such as vitamins, minerals, salts,
etc.
Active Transport
Sodium-Potassium Pump
•Balance of the two ions is done at same time
•Helps to create a dipole inside and out of the cell
•This is necessary for nerve cells to pass an electric impulse
Na+
high
Na+
low
K+
low
K+
high
Click here to see
a cool Flash
Animation
ATP required for maintenance of the pump
Sodium-Potassium Pump
Click image for an animation
Types of Protein Transporters:
Active Transport
• carrier proteins
• go against the concentration gradients Low
to High
• require Energy to function (ATP)
Facilitated Diffusion
•
Proteins in the membrane
assists in diffusion process
•
Solutes go from High conc to
Low conc.
Flash Animation
Example: Glucose transporters
http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm
Glucose Transporter:
How it works..
• glucose binds to outside of
transporter (exterior side with
higher glucose conc.)
• glucose binding causes a shape
change in protein
• glucose drops off inside cell
• protein reverts to original shape
Facilitated Diffusion without a Shape Change
Ion Channels
• Works fast: No protein shape changes needed
• Not simple pores in membrane:
– proteins are specific to different ions (Na, K, Ca...)
– gates control opening
– Toxins, drugs may affect channels
Toxins…how they work
They Plug the Ion Channels