Cell Membrane Structure and Function
Chapter 3
Lipid Bilayer
Main component of cell membranes
Gives the membrane its fluid
properties
Two layers of phospholipids
Fluid Mosaic Model
Membrane is a mosaic of
w Phospholipids
w Glycolipids
w Sterols
w Proteins
Most phospholipids and some proteins
can drift through membrane
Membrane Proteins
Transport proteins
Receptor proteins
Recognition proteins
Adhesion proteins
Cell Membranes Show
Selective Permeability
Membrane Crossing
Mechanisms
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
its 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)
What Is Diffusion?
Molecules in Fluids Move in Response
to Gradients
A Drop of Dye in Water Illustrates
Diffusion
Factors Affecting
Diffusion Rate
Steepness of concentration gradient
w Steeper gradient, faster diffusion
Molecular size
w Smaller molecules, faster diffusion
Temperature
w Higher temperature, faster diffusion
Electrical or pressure gradients
Cell Membranes Show
Selective Permeability
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
Passive Transport:
Facilitated Diffusion
Flow of solutes through the interior
of passive transport proteins down their concentration gradients
Proteins form channels or carriers
Passive transport proteins allow
solutes to move both ways
Does not require any energy input
Passive Transport
Osmosis
Diffusion of water molecules across
a selectively permeable membrane
Direction of net flow is determined
by water concentration gradient
Side with the most solute molecules
has the lowest water concentration
Tonicity
Refers to relative solute concentration of two fluids
Hypertonic - having more solutes
Isotonic - having same amount
Hypotonic - having fewer solutes
Tonicity and Osmosis
Increase in Fluid Volume
Active Transport
Net diffusion of solute is against
concentration gradient
Transport protein must be activated
ATP gives up phosphate to activate
protein
Binding of ATP changes protein shape
and affinity for solute
The Role of ATP
Cells earn ATP in energy releasing
reactions
Cells spend ATP in energy spending
reactions
Active Transport
ATP gives up phosphate to activate
protein
Binding of ATP changes protein shape
and affinity for solute
Bulk Transport
Cytomembrane system
Membrane is not solid
Some Plasma Membranes Are
Surrounded by Cell Walls
Back to schedules:
MWF
TTH
Biology Home
Merced College Home