Biology 107
Cellular Membranes
September 19, 2005
Cells are
Microscopic
Structures
All cells are delimited by a
membrane
The size of cells is limited by
the surface area of plasma
membrane available for
transport of materials into
and out of the cell
Phospholipids are Amphipathic
Molecules
The Nature of
Phospholipid
Molecules Directs
the Spontaneous
Assembly of
Bilayers in
Aqueous
Environments
Polar (hydrophilic) head
region
Non-polar (hydrophobic)
tail region
Lipid Bilayer
The Cellular Membrane Bilayer
is Associated with Proteins
Computer Model of Lipid Bilayer
Functions of
Membrane
Proteins
Transport – Active or passive
Enzymatic
Signal transduction
Cell-cell recognition
Cell-cell adhesion
Cell-matrix adhesion
Transmembrane Protein
Example of Transmembrane
Protein
Movement Across Membranes
Passive – driving force is diffusion gradient. May
use membrane protein(s) to facilitate movement
through bilayer, but some molecules (e.g.,
water) cross membranes unaided by proteins.
Active – driving force is cellular energy supply (e.g.,
ATP). Uses membrane proteins.
Transport Across Membranes
Osmosis Across Cell Membranes
Experiment that Shows the
Fluid Nature of Membranes
Fluid Nature of
Membranes
Phospholipids not covalently
linked together
Lipid composition (e.g.,
cholesterol amount/location;
degree of saturation of fatty
acids) affects the degree of
fluidity and “flexibility” of
membranes
Lipid “sea” forms the
environment in which
membrane proteins may be
embedded
Fluid Mosaic Model of
Membranes
Cellular Membranes
Student Objectives: As a result of this lecture and the assigned
reading, you should understand the following:
1.
Membranes confine the contents of cells.
2.
Membranes provide the structural basis for metabolic order in
organisms. They partition and compartmentalize functions, such
as different enzyme systems. Membranes also form the physical
matrix on which enzymes may be positioned.
3.
Membranes are selectively permeable. Control of exchanges
necessary to: 1) protect the cell's integrity; 2) maintain the
conditions at which optimal metabolic activities take place; and 3)
coordinate the activities of different cells in a multicellular
association.
Cellular Membranes
4.
The main structural components of membranes are lipids, mainly
phospholipids containing a hydrophilic polar head region and a
hydrophobic non-polar tail region, that spontaneously form stable
bilayers.
5.
Much of the selective permeability of membranes depends upon
the proteins present.
6.
The two mechanisms of crossing a membrane are: 1) passive
transport (e.g., diffusion) or 2) active transport.
Cellular Membranes
7.
Membrane proteins may be peripheral proteins or integral proteins
depending upon the degree of interaction with the membrane.
Integral membrane proteins often span the thickness of the
bilayer.
8.
The model that best explains the organization of membranes is
the fluid mosaic model.
Fluid Mosaic Plasma Membrane as
an Interface Between Intracellular
and Extracellular Environments