A Closer Look at Membranes
Chapter 5
Cystic Fibrosis
• Caused by defective protein channel
(CFTR) in epithelial cells
• Not enough Cl- and water cross
membranes
• Thick mucus clogs airways and ducts
throughout body
• Usually fatal by age 30
Lipid Bilayer
• Main component of
cell membranes
• Gives membrane its
fluid properties
• Fatty acid tails
sandwiched
between hydrophilic
heads
lipid
bilayer
fluid
fluid
Figure 5.2c
Page 82
Fluid Mosaic Model
• Membrane is a mosaic of
– Phospholipids
– Glycolipids
– Sterols
– Proteins
• Most phospholipids and some proteins can
drift through membrane
Membrane Proteins
adhesion
protein
receptor
protein
communication
protein
recognition
protein
passive
transporter
active transporters
Figure 5.5
Page 85
Membrane Experiments
• Split membranes reveal embedded
proteins
In-text figure
Page 86
Membrane Experiments
• Hybrid human-mouse cell shows some
proteins drift within membrane
human cell
mouse cell
hybrid cell
Figure 5.6
Page 86
Selective Permeability
O2, CO2,
glucose and other large, polar,
and other small, nonpolar water-soluable molecules; ions,
molecules; some water
water molecules
molecules
Membrane Crossing Mechanisms
Diffusion across lipid bilayer
Passive transport
Active transport
Endocytosis
Exocytosis
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 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)
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
Example of Diffusion
Transport Proteins
• Span the lipid bilayer
• Interior is able to open to both sides
• Change shape when they interact with
solute
• Move water-soluble substances across
a membrane
Passive and Active Transport
Passive Transport
Active Transport
• Doesn’t require
• Requires ATP
• Protein is an ATPase
•
•
energy inputs
Solutes diffuse
through a channel
inside the protein’s
interior
Net movement is
down concentration
gradient
•
pump
Pumps solute against
its concentration
gradient
Passive Transport
glucose transporter
solute (glucose)
high
low
Stepped Art
Figure 5.10
Page 88
higher calcium
concentration
Active Transport
lower calcium concentration
ATP
Pi
ADP
Stepped Art
Figure 5.11
Page 89
Osmosis
• Water molecules tend to diffuse down
water concentration gradient
• Total number of molecules or ions
dictates concentration of water
• Tonicity - relative solute concentrations
Tonicity
2% sucrose
solution
distilled water
10%
sucrose
solution
2%
sucrose
solution
Hypotonic
Conditions
Hypertonic
Conditions
Isotonic
Conditions
Figure 5.13
Page 90
Fluid Pressure
• Hydrostatic pressure
• Turgor pressure
• Osmotic pressure
Exocytosis
Endocytosis and
Exocytosis
plasma membrane
cytoplasm
Endocytosis
cytoplasm
Endocytosis Pathways
• Bulk phase
• Receptor-mediated
• Phagocytosis
clathrin
Figures 5.17, 5.18
Pages 92, 93
Membrane
Cycling
clathrin
Exocytosis and
endocytosis
continually replace
and withdraw
patches of plasma
membrane
Golgi
body
Figure 5.19
Page 93
lysosome