A Closer Look at Cell Membranes
BOZEMAN VIDEO—CELL
MEMBRANES
http://www.youtube.com/watch?v=y31DlJ6uGgE
http://www.youtube.com/watch?v=S7CJ7xZOjm0
http://www.youtube.com/watch?v=y31DlJ6uGgE
http://www.youtube.com/watch?v=S7CJ7xZOjm0
Lipid Bilayer
 Cell membranes consist of a lipid bilayer containing
different proteins
 Membrane is a continuous boundary layer that
selectively controls the flow of substances across it
hydrophilic
parts
hydrophobic
parts
fluid
fluid
one layer of lipids
b
one layer of lipids
cross-section
through lipid bilayer
a
Fig. 5.3, pg. 76
Fluid Mosaic Model
 Every cell membrane has a mixed composition of
phospholipids, glycolipids, sterols, and proteins
 KNOW THE CAMPBELL DIAGRAM “CELL
MEMBRANE MOSAIC”
Fluid Mosaic Model
passive
transporter
adhesion
protein
recognition
protein
phospholipid
cholesterol
receptor
Lipid bilayer
active transporter
(calcium pump)
cytoskeletal proteins just
beneath the plasma
membrane
active transporter
(ATPase pump)
Cytoplasm
Plasma
Membrane
Fig. 5.4, pg. 77
Studying Membranes
Stepped Art
Fig. 5.5a, pg. 77
Overview of
Membrane Proteins
Adhesion
Proteins
Communication
Proteins
Fig. 5.6, p.78
Overview of
Membrane Proteins
Receptor
Proteins
Recognition
Proteins
Passive
Transporters
Active
Transporters
Fig. 5.6, p.79
Transport Proteins
 Span the lipid bilayer
 Interior is able to open to both sides
 Change shape when they interact with solute
 Play roles in active and passive transport
Concentration Gradient
 Means the number of molecules or ions in one region
is different than the number in another region
 In the absence of other forces, a substance moves from
a region where it is more concentrated to one one
where it’s less concentrated - “down” gradient
Diffusion
 The net movement of like molecules or ions down a
concentration gradient
 Although molecules collide randomly, the net
movement is away from the place with the most
collisions (down gradient)
Diffusion
Stepped Art
Fig. 5.7a, p.80
Factors Affecting
Diffusion Rate
 Steepness of concentration gradient
 Steeper gradient, faster diffusion
 Molecular size
 Smaller molecules, faster diffusion
 Temperature
 Higher temperature, faster diffusion
 Electrical or pressure gradients
Membrane Crossing Mechanisms
Diffusion across lipid bilayer
Passive transport
Active transport
Endocytosis
Exocytosis
Cell Membranes Show Selective
Permeability
oxygen, carbon
dioxide, and other
small, nonpolar
molecules; some
water molecules
glucose and other large,
polar, water-soluble
molecules; ions (e.g.,
H+, Na+, K+, Ca++,
Cl–); water molecules
Fig. 5-8, p.80
Membrane Crossing: Overview I
High
Concentration
gradient across
cell membrane
ATP
Diffusion of
lipid-soluble
Substances
across bilayer
Passive transport of watersoluble substances
through channel protein;
no energy input needed
Low
Active transport
through ATPase;
requires energy
input from ATP
Fig. 5-9, p.81
Passive Transport
 Flow of solutes through the interior of passive
transport proteins down their concentration gradients
 Passive transport proteins allow solutes to move both
ways
 Does not require any energy input
TYPES OF PASSIVE TRANSPORT
 DIFFUSION
 FACILITATED DIFFUSION—diffusion that is “helped
out” by membrane transport proteins (such as helping
larger molecules pass)
Passive Transport
glucose transporter
solute (glucose)
high
low
Stepped Art
Fig. 5.10, p.80
Active Transport
 Net diffusion of solute is against concentration
gradient
 Transport protein must be activated (requires an
integral membrane protein)
 ATP gives up phosphate to activate protein
 Binding of ATP changes protein shape and affinity for
solute
TYPES OF ACTIVE TRANSPORT
 SODIUM-POTASSIUM PUMP—3 Na move out and
allow 2 K to enter; needs net 1 ATP molecule to dothis
 PROTON PUMP– pushes H ions out of the membrane
allowing them to eventually diffuse back in; generates
ATP for the cell; used in cell respiration
This uses ATP synthase enzyme
 COTRANSPORT– when molecules travel together
across the membrane
Types of active transport
Continued
 Endocytosis—”engulfing “things into cell membrane;
difficult for plants to do
Ex: white blood cells engulfing bacteria—to fight
infections
phagocytosis (large food)
pinocytosis (small things)
Receptor-mediated endocytosis—use of receptor
proteins to help pull things into cell
Exocytosis—”spitting” things out of cell
Osmosis
 Diffusion of water molecules across a selectively
permeable membrane from high concentration to low
concentration (moves with the concentration
gradient)
Osmosis
p.84
Water Potential
 Bozeman Video Link
http://www.bozemanscience.com/water-potential
Also see questions that accompany this video
Tonicity
Refers to relative solute concentration of two fluids
Hypotonic - having fewer solutes
Hypertonic - having more solutes
Isotonic - having same amount
Tonicity and
Osmosis
2%
sucrose
solution
1 liter of
distilled water
1 liter of
10% sucrose
solution
1 liter of
2% sucrose
solution
Hypotonic
Conditions
Hypertonic
Conditions
Isotonic
Conditions
Fig. 5-13, p.85
DIALYSIS BAG % MASS CHANGE
Pressure and Osmosis
 Hydrostatic pressure
 Pressure exerted by fluid on the walls that contain it
 The greater the solute concentration of the fluid, the
greater the hydrostatic pressure
 Osmotic pressure
 Amount of pressure necessary to prevent further
increase of a solution’s volume
Increase in Fluid Volume
first
compartmen
t
hypotonic
solution
second
compartmen
t
hypertonic
solution
membrane
permeable to
water but not to
solutes
fluid volume
rises in
second
compartmen
t
Fig. 5.14, p.85
ENDOCYTOSIS AND EXOCYTOSIS
VIDEO
http://www.youtube.com/watch?v=DuDmvlbpjHQ
Endocytosis and Exocytosis
 Exocytosis: A cytoplasmic vesicle fuses with the plasma
membrane and contents are released outside the cell
3 types: phagocytosis—engulfs large molecules; difficult
for plant cells to do (WHY??)
pinocytosis—engulfs small molecules
receptor-mediated—uses receptor membrane
proteins to pull things in
 Endocytosis: A small patch of plasma membrane sinks
inward and seals back on itself, forming a vesicle
inside the cytoplasm – membrane receptors often
mediate this process
Macrophage engulfing Leishmania
mexicana
parasite
macrophage
Fig 5.17, p.87
Phagocytosis
bacterium
phagocytic vesicle
Fig. 5-17b, p.87
Contractile Vacuole
contractile vacuole filled
contractile vacuole emptied
Fig. 5.21, pg. 89
Plasmolysis
Plasmolysis
adhesion
protein
passive
transporter
recognition
protein
receptor
protein
lipid
bilayer
cytoskeletal
proteins
cytoplasm
active transporter active transporter
(calcium pump)
(ATPase pump)
Fig. 5-19, p.88
BOZEMAN VIDEO—CELL
TRANSPORT ACROSS MEMBRANES
AND ANIMATION
http://www.youtube.com/watch?v=RPAZvs4hvGA
http://www.youtube.com/watch?v=RPAZvs4hvGA