A Closer Look at Membranes
Chapter 5
5.1 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
5.2 Membrane Proteins
adhesion
protein
receptor
protein
communication
protein
recognition
protein
passive
transporter
active transporters
Figure 5.5
Page 85
5.4 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
5.6 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
energy inputs
• Solutes diffuse
through a channel
inside the protein’s
interior
• Net movement is
down concentration
gradient
• Requires ATP
• Protein is an ATPase
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
5.7 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
normal plant cells
after plasmolysis
5.8 Endocytosis and Exocytosis
plasma membrane
Exocytosis
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
Figure 5.19
Page 93
Golgi
body
lysosome