Membrane Structure
& Function Notes
About Cell Membranes
1.All cells have a cell
membrane
2.Functions:
a.Controls what enters
and exits the cell
b.Maintain an internal
balance (homeostasis)
c.Provides protection and
support for the cell
TEM picture of a
real cell membrane.
About Cell Membranes (continued)
3.Structure of cell membrane
Lipid Bilayer -2 layers of
phospholipids
a.Phosphate head is polar,
hydrophilic
Phospholipid
b.Fatty acid tails non-polar
hydrophobic
c.Proteins embedded in
membrane
d. Cholesterol give flexibility/
Lipid Bilayer
fluidity
Fluid Mosaic
Polar heads
Model of the
love water
& dissolve. cell membrane
Non-polar
tails hide
from water.
Carbohydrate cell
markers
Proteins
(pieces that
move)
Membrane
movement
animation
http://www.youtube.com/watch?NR=1&v=moPJkCbKjBs&feature=endscreen
Membrane
Proteins
1.“Framework”
proteins =
Integerins; span
membrane, attach to
cytoskeleton on
inside & E.C.M on
outside
2. Cell Recognition
Glycoproteins(carb
surface markers =
ID cells by
species/cell
type/etc.)
4. Receptors – messenger molecules
(hormones) bind to; relay “message” & trigger
rxns inside cell (signal transduction)
3. Enzymes – carry
out rxns
6. Junctions – connect cells
5. Transport Proteins – channels for large,
/polar molecules/ions
Functions of Membrane Proteins
Junctions
Framework
Selectively Permeable
Pores (holes) in membrane allow some
molecules in & keeps other out.
Structure helps it be selective (form/fxn)
Pores
Structure of the Cell Membrane
Outside of cell
Proteins
Lipid
Bilayer
Transport
Protein
Go to
Section:
Carbohydrate
chains
Animations
of membrane
structure
Phospholipids
Inside of cell
(cytoplasm)
Cholesterol in
membrane
keeps
membrane
fluid.)
Membrane Transport
Pass freely
Small, nonpolar molecules (O2, CO2) – DIFFUSION
Need help of TRANSPORT PROTEINS
Polar molecules (glucose)
Larger molecules (starches)
Charged ions (Na+, K+
Aquaporins: transport proteins for water (can
diffuse, but slow).
Types of Cellular Transport
•
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis (water)
•
Weeee!!
!
Active Transport
cell uses energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
high
low
•Animations of Active
Transport & Passive
Transport
high
low
This is
gonna
be hard
work!!
Passive Transport
•
•
•
cell uses no energy
molecules move randomly
Molecules spread out from an area of
high concentration to an area of low
concentration.
• (HighLow)
•
Three types:
3 Types of Passive Transport
1. Diffusion
2. Facilitative Diffusion – diffusion with
the help of transport proteins
3. Osmosis – diffusion of water
Passive Transport:
1. Diffusion
Simple Diffusion
Animation
1. Diffusion: random movement
of particles from an area of
high concentration to an
area of low concentration.
(High to Low)
•
Diffusion continues until all
molecules are evenly spaced
(equilibrium is reached)-Note:
molecules will still move around
but no NET movement.
http://bio.winona.edu/berg/Free.htm
Passive Transport:
2. Facilitated Diffusion A
2. Facilitated diffusion:
diffusion of specific particles
through transport
proteins found in the
membrane
a.Transport Proteins are
specific – they “select”
only certain molecules
to cross the membrane
b.Transports larger or
charged molecules
Facilitated
diffusion
(Channel
Protein)
B
Diffusion
(Lipid
Bilayer)
Carrier Protein
Passive Transport:
2. Facilitated Diffusion
Cellular Transport From aHigh Concentration
Glucose
molecules
High
• Channel Proteins
animations
Cell Membrane
Low Concentration
Through a 
Go to
Section:
Transport
Protein
Protein
channel
Low
Passive Transport:
3. Osmosis
Osmosis
animation
• 3.Osmosis: diffusion of
water through a
selectively permeable
membrane
• Water moves from high
to low concentrations
•Water moves freely
through pore space or
through aquaporins.
•Solute (green) to large
to move across.
Active Transport
•cell uses energy
•actively moves molecules to where they are
needed
•Movement from an area of low
concentration to an area of high
concentration
•(Low  High)
•Three Types:
Types of Active Transport
1. Protein Pumps transport proteins that
require energy to do
work
•Example: Sodium /
Potassium Pumps
are important in nerve
responses.
Sodium
Potassium Pumps
(Active Transport
using proteins)
Protein changes
shape to move
molecules: this
requires energy!
Active Transport in Neurons
Sodium-Potassium Pump
Sodium
Potassium Pumps
(Active Transport
using proteins)
http://www.sumanasinc.com/webcontent/animations/content/electricalsignaling.htm
l
Sodium-Potassium Pump
Sodium
Potassium Pumps
(Active Transport
using proteins)
Cells use energy in the form of ATP to pump
Na+ and K+ ions against gradient.
1. 3 Na+ ions bind to protein pump (based
on shape).
2. ATP phosphorylates protein pump
(Breaks bond b/w phosphate groups. Pi
binds to protein pump and causes the
pump to change its shape – this costs
energy)
3. Pump changes shape & releases 3 Na+
outside of cell
4. 2 K+ bind to activated pump
(phosphorylated)
5. Phosphate is released; pump returns to
original shape; 2K+ ions released in cell
End Result of Na+/K+ pump
1. Maintain concentration gradients of Na+ and
K+
3 Na+ out; 2 K+ in
High Na+ outside cell (wants to reenter; but cell is
not permeable to it). Na+ pumped from [low]
[high]
High K+ inside cell (wants to leave and will leak
out if concentration gradient becomes imbalanced.
K+ pumped from [low] [high]
2. Inside of cell is (-) relative to outside of cell (+).
Resting potential inside of cell is -70 mV
3. When neurons receive “stimulation” special
channels will open that allow Na+ in  this change
in charge is the basis for electrical impulse
transmission along neurons!
Sodium-Potassium Pump
1. When neurons receive “stimulation” (heat,
pressure, binding of neurotransmitters);
2. Na+ rushes into the cell (cell membrane is
DEPOLARIZED and ACTION POTENTIAL is
triggered. This is an electrical impulse!
3. K+ leaks out (membrane is more permeable
to K+) to restore membrane to resting potential
(Polarized).
http://biomhs.blogspot.com/2012/07/nerveimpulse.html
Types of Active Transport
2. Endocytosis: taking bulky
material into a cell
•
•
–
–
•
•
Uses energy
Cell membrane in-folds
around food particle
“Cell eating”(phagocytosis);
“Cell Drinking” (pinocytosis=
dissolved substances)
forms food vacuole & digests
food
This is how white blood cells
eat bacteria!
Types of Active Transport
2.
Endocytosis:
receptor –
mediated
Take in specific
molecules
(complimentary
shape to surface
receptors)
Ex: take in
cholesterol
needed for
membrane
Types of Active Transport
3. Exocytosis: Forces
material out of cell in bulk
• membrane surrounding the
material fuses with cell
membrane
• Cell changes shape –
requires energy
• EX: Hormones or
wastes released from
cell
Endocytosis &
Exocytosis
animations
Effects of Osmosis on Life
• Osmosis- diffusion of water through a
selectively permeable membrane
• Water is so small and there is so much
of it the cell can’t control it’s movement
through the cell membrane.
Hypotonic Solution
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than
inside the cell. (Low solute; High water)
•
Osmosis Animations for isotonic,
hypertonic, and hypotonic
solutions
Result: Water moves from the solution to inside the
cell): Cell Swells and bursts open (cytolysis)!
Hypertonic Solution
Hypertonic: The solution has a higher concentration
of solutes and a lower concentration of water than
inside the cell. (High solute; Low water)
•
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Result: Water moves from inside the cell into the
solution: Cell shrinks (Plasmolysis)!
Isotonic Solution
Isotonic: The concentration of solutes in the solution
is equal to the concentration of solutes inside the cell.
•
Osmosis
Animations for
isotonic,
hypertonic, and
hypotonic solutions
Result: Water moves equally in both directions and
the cell remains same size! (Dynamic Equilibrium)
What type of solution are these cells in?
A
B
C
Hypertonic
Isotonic
Hypotonic
How Organisms Deal
with Osmotic Pressure?
•
Paramecium
(protist) removing
excess water
video
•Bacteria and plants have cell walls that prevent them from overexpanding. In plants the pressure exerted on the cell wall is called
tugor pressure.
•A protist like paramecium has contractile vacuoles that collect
water flowing in and pump it out to prevent them from overexpanding.
•Salt water fish pump salt out of their specialized gills so they do
not dehydrate.
•Animal cells are bathed in blood. Kidneys keep the blood isotonic
by remove excess salt and water.
Video Clips
• http://www.youtube.com/watch?v=JaCCKPyE6I4 - cytoplasmic
streaming of chloroplasts and plasmolysis of Elodea cells
•
• http://www.youtube.com/watch?v=lzDlGl3b4is – plasmolysis in onion
cells
http://www.youtube.com/watch?v=gYbt7hhIxPo – plasmolysis in red
onion cells (goes with the one below showing water reentering the
cells)
•
• http://www.youtube.com/watch?v=5uGroik-6QE&feature=related –
cytolysis of plasmolyzed red onion cells