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The Cell Membrane
What do you notice about the picture below?
What parts can you name?
Do you know their functions?
Cell Membrane
8,000 membranes to equal
one sheet of paper
Functions of Cell Membrane
1. Control what enters and exits the cell
2. Help cell keep its shape
3. Recognize substances that might harm
the cell.
4. Communicate with other cells
• Cell membrane separates living cell
from nonliving surroundings
• Controls traffic in & out of the cell
– Selectively permeable - allows some
substances to cross more easily than
others.
– WHY? Hydrophobic tails vs hydrophilic
heads
What has to get in and out of
the cell?
•
•
•
•
•
Oxygen
Glucose
Amino acid
Vitamins
water
CO2
Waste
new proteins
Parts of the Cell Membrane
1. Phospholipids
2. Cholesterol
3. Proteins
4. Carbohydrates
Phospholipids
Phosphate
1. Fatty acid tails
– hydrophobic
2. Phosphate group
3. Glycerol
Fatty acid
– Hydrophilic
Aaaah,
one of those
structure–function
examples
Why are the phosolipids
arrange this way?
• Heads - hydrophilic face outward
• Tails - Hydrophobic face inward
• Water surrounds the cell and is inside the
cell. Hydrophilic must face outward.
• Arranged as a bilayer
The Arrangement of the Phosolipids
determine how particles pass through
the cell membrane?
• If a particle is small and not charged then
it can move straight through the
membrane.
• If the particle is large and charged, then it
has to move through the cell proteins.
• Aquaporins - protein channel that water
moves through.
3 Functions of Proteins – read page 10
1. Pumps material through the membrane
- large and charged particles
channels (pumps)
2. Proteins on the inside of
cell. Attach to cytoskeleton.
Help with structure!!
3. Receptors – Carbo. Are
attached to proteins!
4. Enzymes!
Membrane carbohydrates
• Play a key role in cell-cell recognition
– ability of a cell to distinguish one cell from
another
• antigens
– important in organ &
tissue development
– basis for rejection of
foreign cells by
immune system
Function of Cholesterol
• Strengthen cell membrane – phosolipids
are not bonded so membrane is not
strong!
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Fluid Mosaic Model
Describes the characteristics of the cell
membrane
1. Cell membrane is flexible not rigid. Acts
as a fluid.
2. Cell membrane is constructed for many
different parts.
Selective Permeability
• Cell membrane allows some materials in
and others not.
• Homeostasis: Cell maintenance of stable
internal conditions in a changing
environment.
• How a molecules moves across a
membrane depends on the size, polarity,
and concentration of the molecules.
Molecules are constantly moving!!
• You must understand that molecules are
constantly moving.
• The more energy added the faster they
move.
• Move from an area of a higher
concentration to an area of lower
concentration.
• Video molecular movement!!!!!
• Air freshener!!! Perfume!!! Farts!!!!!
• Do you Agree that Molecules
Must Move Across Membrane?
Concentration Gradient
• The difference in the concentration of
molecules in two different regions.
• Can be a room or a cell!!!!
• Equilibrium: molecules will continue to
move until equal on both sides.
3 Ways Particles Move!!
1. Small and non charged particles
can just move through the
membrane.
2. Small polar molecules move
through the protein channels.
3. Large particles move in vesciles.
Types of Cellular Transport
•
Weeee!!
!
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
•Animations of Active
Transport & Passive
Transport
high
low
•
Active Transport
cell does use energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
This is
gonna
be hard
work!!
high
low
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
3 Types of Passive Transport
1. Diffusion
2. Facilitative Diffusion – diffusion with the
help of transport proteins
3. Osmosis – diffusion of water through
aquaporin.
Passive Transport
1. cell uses no energy
2. molecules move randomly
3. Molecules spread out from an area of
high concentration to an area of low
concentration.
• (HighLow)
• CAN BE THROUGH A
CHANNEL!!!
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 stay spread out.
http://bio.winona.edu/berg/Free.htm
Diffusion
• Move from HIGH to LOW concentration
– “passive transport”
– no energy needed
– Small and non charged
– Straight through membrane
movement of water
diffusion
osmosis
Diffusion through phospholipid bilayer
• What molecules can get through directly?
– fats & other lipids
inside cell
NH3
lipid
salt
• What molecules can
NOT get through
directly?
– polar molecules
• H 2O
– ions
outside cell
sugar aa
H 2O
• salts, ammonia
– large molecules
• starches, proteins
Passive Transport:
Facilitated Diffusion
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
A
B
Facilitated
diffusion
(Channel
Protein)
Diffusion
(Lipid
Bilayer)
Carrier Protein
Facilitated Diffusion
• Diffusion through protein channels
– channels move specific molecules across
cell membrane
facilitated = with help
– no energy needed
open channel = fast transport
high
low
“The Bouncer”
Passive Transport: Facilitated Diffusion
Glucose
molecules
Cellular Transport From aHigh Concentration
High
• Channel Proteins
animations
Cell Membrane
Low Concentration
Through a 
Go to
Section:
Transport
Protein
Protein
channel
Low
Passive Transport:
3. Osmosis
• 3.Osmosis: facilitated
diffusion of water
through a selectively
permeable membrane
• Water moves from
high to low
concentrations
• AQUAPORIN
Osmosis
animation
•Water moves freely
through pores.
•Solute (green) to large
to move across.
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.
Concentration of water
• Direction of osmosis is determined by the
amount of solution in the cell. If there is more
solute then less water!
– Hypertonic - more solute, less water
– Hypotonic - less solute, more water
– Isotonic - equal solute, equal water
water
hypotonic
hypertonic
net movement of water
3 Types of Solutions For a Cell
1. Hypertonic - more solute, less water. More
water inside cell; it leaves
2. Hypotonic - less solute, more water. More
water outside cell; water enters
3. Isotonic - equal solute, equal water
Equal – net movement equal!
Managing water balance
• Cell survival depends on balancing water
uptake & loss
freshwater
balanced
saltwater
•
Hypotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than
inside the cell. (Low solute; High water)
Result: Water moves from the solution to inside the
cell): Cell Swells and bursts open (cytolysis)!
Managing water balance
• Hypotonic
– a cell in fresh water
• example: Paramecium
• problem: gains water,
swells & can burst
– water continually enters
Paramecium cell
• solution: contractile vacuole
ATP
– pumps water out of cell
– ATP
– plant cells
• turgid
freshwater
•
Hypertonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypertonic: The solution has a higher concentration
of solutes and a lower concentration of water than
inside the cell. (High solute; Low water)
shrinks
Result: Water moves from inside the cell into the
solution: Cell shrinks (Plasmolysis)!
Managing water balance
• Hypertonic
– a cell in salt water
• example: shellfish
• problem: lose water & die
• solution: take up water or pump
out salt
– plant cells
• plasmolysis = wilt
saltwater
•
Isotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Isotonic: The concentration of solutes in the solution
is equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and
the cell remains same size! (Dynamic Equilibrium)
Managing water balance
• Isotonic
– animal cell immersed in
mild salt solution
• example:
blood cells in blood plasma
• problem: none
– no net movement of water
» flows across membrane equally, in
both directions
– volume of cell is stable
balanced
What type of solution are these cells in?
A
B
C
Hypertonic
Isotonic
Hypotonic
Active Transport
• Cells may need to move molecules against
concentration gradient
– shape change transports solute from
one side of membrane to other
– protein “pump”
conformational change
low
– “costs” energy = ATP
ATP
high
“The Doorman”
Active Transport
1. cell uses energy
2. actively moves molecules to where they are
needed
3. Movement from an area of low
concentration to an area of high
concentration
•AGAINST THE CONCENTRATION GRADIENT!
•(Low  High)
How Organisms Deal
with Osmotic Pressure
•
Paramecium
(protist) removing
excess water
video
•Bacteria and plants have cell walls that prevent them
from over-expanding. 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 over-expanding.
•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.
Aquaporins
1991 | 2003
• Water moves rapidly into & out of cells
– evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
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!
Types of Active Transport
• 2. Endocytosis: taking
bulky material into a cell
• Uses energy
• Cell membrane in-folds
around food particle
• “cell eating”
• forms food vacuole &
digests food
• This is how white blood
cells eat bacteria!
Endocytosis
phagocytosis
pinocytosis
receptor-mediated
endocytosis
fuse with
lysosome for
digestion
non-specific
process
triggered by
molecular
signal
How about large molecules?
• Moving large molecules into & out of cell
– through vesicles & vacuoles
– endocytosis
• phagocytosis = “cellular eating”
• pinocytosis = “cellular drinking”
– exocytosis
exocytosis
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