Cellular Membranes
• Two main roles
• Allow cells to isolate themselves from the
environment, giving them control of
intracellular conditions
• Help cells organize intracellular pathways into
discrete subcellular compartment, including
organelles
Membrane Structure
Figure 3.20
Lipid Profile
• Lipid bi-layer: phospholipids, primarily
phosphoglycerides
• Other lipids
• Sphingolipids: alter electrical properties
• Glycolipids: communication between cells
• Cholesterol: increase fluidity while decreasing
permeability
Cholesterol reduces permeability while enhancing fluidity.
Membrane Fluidity
• Environmental conditions can affect membrane
fluidity, e.g., low temperature increases the number of
van der Waals forces between lipids and restrict
movement
Membrane Fluidity, Cont.
• Homeoviscous adaptation – keep membrane fluidity
constant by altering the lipid profile
• Three strategies: all reduce the number of van der
Waals forces
– Replace long chain fatty acids with shorter chains
– Introduce double bonds
– Introduce phospholipids with higher mobility of polar head
groups
Figure 3.23
Membrane Proteins
• Can be more than half of the membrane mass
• Two main types
– Integral membrane proteins – tightly bound to the
membrane, either embedded in the bilayer or
spanning the entire membrane
– Peripheral proteins – weaker association with the
lipid bilayer
Figure 3.24
Membrane Transport
•
•
•
•
Three main types
Passive diffusion
Facilitated diffusion
Active transport
• Distinguished by direction of transport, nature
of the carriers, and the role of energy
Figure 3.25
Passive Diffusion
•
•
•
•
Lipid-soluble molecules
No specific transporters are needed
No energy is needed
Depends on concentration gradient
– High  low
– Steeper gradient results in higher rates
Facilitated Diffusion
•
•
•
•
Hydrophilic molecules
Protein transporter is needed
No energy is needed
Depends on concentration gradient
Facilitated Diffusion, Cont.
• Three main types of proteins
– Ion channels – form pores
– Porins – like ion channels, but for larger molecules
– Permeases – function more like an enzyme
Figure 3.25
Ion Channels
Figure 3.26
Active Transport
• Protein transporter is needed
• Energy is required
• Molecules can move from low to high
concentration
Active Transport, Cont.
• Two main types: distinguished by the source of
energy
– Primary active transport – uses an exergonic
reaction
– Secondary active transport – couples the
movement of one molecule to the movement of a
second molecule
Primary Active Transport
• Hydrolysis of ATP provides energy
• Three types
– P-type: pump specific ions, e.g., Na+, K+, Ca2+
– F- and V-type: pump H+
– ABC type: carry large organic molecules, e.g.,
toxins
Figure 3.27
Secondary Active Transport
• Use energy held in the electrochemical gradient of
one molecule to drive another molecules against its
gradient
• Antiport or exchanger carrier: molecules move in
opposite directions
• Symport or cotransporter carrier: molecules move in
the same direction
Electrical Gradients
• All transport processes affect chemical gradients
• Some transport processes affect the electrical
gradient
• Electroneutral carriers: transport uncharged
molecules or exchange an equal number of
charged particles
• Electrogenic carriers: transfer a charge, e.g.,
Na+/K+ATPase  exchanges 3Na+ for 2K+
Membrane Potential
• Difference in charge inside and outside the
cell; electrochemical gradient
• Active transporters help establish this gradient
• Can be determined by Nernst equation and
Goldman equation
• Two main functions
– Provide cell with energy for membrane transport
– Allow for changes in membrane potential used by
cells in cell-to-cell signaling
Depolarization and Hyperpolarization
Figure 3.29