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The Cell Membrane
AP Biology
Fluid Mosaic Model
Time to learn
about 7.3 and 7.4
AP Biology
Arranged as a Phospholipid bilayer
 Serves as a cellular barrier / border, Selective
Permeability
sugar
H 2O
salt
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
impermeable to polar molecules
polar
hydrophilic
heads
waste
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lipids
Cell membrane is more than lipids…
 Transmembrane proteins embedded in
phospholipid bilayer

create semi-permeabe channels
lipid bilayer
membrane
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protein channels
in lipid bilyer membrane
Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Classes of membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 ex: cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 ex: transport proteins
 channels, permeases (pumps)
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Many Functions of Membrane Proteins
“Channel”
Outside
Plasma
membrane
Inside
Transporter
Cell surface
identity marker aka Cell-cell
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recognition
Enzyme
activity
Cell adhesionIntercellular joining
Cell surface
receptor aka Signal Transduction
Attachment to the
cytoskeleton
Movement across the
Cell Membrane
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2007-2008
Two Types of Transport
1) Passive Transport- 2) Active TransportNO ENERGY
ENERGY
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Three Types of Passive Transport
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
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Simple Diffusion
 Particles move from HIGH to LOW
concentration
“passive transport”
movement of water
 no energy needed

diffusion
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osmosis
Diffusion of Liquids
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Cell membrane
-Solute moves DOWN concentration gradient (HIGH
to LOW) until reaches equilibrium
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Diffusion through phospholipid bilayer
 What molecules can get through directly?

fats & other lipids, hydrocarbons, CO2,
Oxygen, hydrophobic molecules
inside cell
NH3
 What molecules can
lipid
salt
NOT get through
directly?

polar molecules
 H 2O

outside cell
sugar aa
ions (charged)
 salts, ammonia
H 2O

large molecules
 starches, proteins
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Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules (ex.glucose)
across cell membrane
facilitated = with help
open channel = fast transport
no energy needed
Channel proteins are embedded in the cell membrane &
have a pore for materials to cross
Carrier proteins can change shape to move material
from one side of the membrane to the other
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Osmosis is just diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
HIGH concentration of water to
LOW concentration of water

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across a
semi-permeable
membrane
Diffusion of H2O Across A
Membrane
High H2O potential
Low solute concentration
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Low H2O potential
High solute concentration
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Three Types of Solutions
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Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
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hypotonic
hypertonic
net movement of water
Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
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Managing water balance
 Hypotonic

a cell in fresh water

high concentration of water around cell
 problem: cell gains water,
swells & can burst (cytolysis)
KABOOM!
 example: Paramecium
 ex: water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 Requires ATP

plant cells
No problem,
here
 turgid = full (turgor pressure)
 cell wall protects from bursting
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freshwater
Cell in Hypotonic Solution
10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
-HYPOtonic – water moves inside of cell
-Turgor pressure- pressure that builds up, gives plant
shape
-Contractile Vacuole- forces out excess water
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2
Managing water balance
 Hypertonic
I’m shrinking,
a cell in salt water I’m shrinking!
 low concentration of water
around cell

 problem: cell loses water &
can die
 solution: take up water or
pump out salt

plant cells
 plasmolysis = wilt
 can recover
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I will
survive!
saltwater
Cell in Hypertonic Solution
15% NaCL
85% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
-Hypertonic- water moves OUT
-plasmolysis- loss of water, decreasing turgor
pressure causing wilting
-Crenation- animal cell shrivels up
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3
Managing water balance
 Isotonic
That’s
perfect!

animal cell immersed in
mild salt solution

no difference in concentration of
water between cell & environment
 problem: none
 no net movement of water
flows across membrane equally, in
both directions
I could
 cell in equilibrium
be better…

 volume of cell is stable
 example:
blood cells in blood plasma
 slightly salty IV solution in hospital
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balanced
Cell in Isotonic Solution
10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
NO NET
MOVEMENT
-Isotonic- amount of solute is equal on both
sides, there is no net movement
- Cell has reached EQULIBRIUM
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1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

evidence that there were water channels
 protein channels allowing flow of water
across cell membrane
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Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Three Types of Solutions
90% H2O
10% solute
85% H2O
15% solute
95% H2O
5% solute
90% H2O
10% solute
90% H2O
10% solute
90% H2O
10% solute
Isotonic
(no net movement
of water)
Hypotonic
Hypertonic
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Fig. 7-UN4
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Three Types of Passive Transport
1) Diffusion
2) Facilitated Diffusion
3) Osmosis
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Active Transport
 Cells may need to move molecules against
concentration gradient



conformational shape change transports solute
from one side of membrane to other
protein “pump”
“costs” energy = ATP LOW conformational change
ATP
HIGH
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“The Doorman”
Sodium-Potassium Pump
3 Na+ pumped in for every 2 K+ pumped
out; creates a membrane potential
http://www.youtube.com/watch?v=yz7EHJFDEJs
http://www.youtube.com/watch?v=7EyhsOewnH4
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How about large molecules?
 Moving large molecules into & out of cell

through vesicles & vacuoles

Endocytosis- bulk transport of materials into the
cell
 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”
Exocytosis- bulk transport of materials out of the
cell.

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http://www.youtube.com/watch?v=kfy92hdaAH0
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
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ATP
Getting through cell membrane
 Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 HIGH  LOW concentration gradient

Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 HIGH  LOW concentration gradient
 Active transport

diffusion against concentration gradient
 LOW  HIGH


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uses a protein pump
requires ATP
ATP