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Cell Membranes
PLO’s
 B9 - analyse the structure and function of the cell
membrane
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Identify molecules in the membrane and how they contribute
the characteristics of the membrane
Describe hydrophobic and hydrophilic regions
CELL MEMBRANE STRUCTURE
Fluid Mosaic Model:
• Plasma membrane consists of a phospholipid bilayer
with protein molecules embedded in it
• Phospholipids give a “fluid” consistency
• Proteins form the “mosaic”
http://en.wikipedia.org/wiki/Image:Cell_membrane_detailed_diagram_edit2.svg:
http://www.bioteach.ubc.ca/Bio-industry/Inex/
Phospholipids
 Phosphate end is
polar and
hydrophilic
(attracted to water)
 Fatty acid chains are
non polar and
hydrophobic
http://bioinfo.bact.wisc.edu/themicrobialworld/Structure.html
 Since the membrane is surrounded by water
inside and out, the phospholipids arrange
themselves like this:
Membrane Proteins
 Peripheral proteins - on the inner or outer surface
 Integral proteins - extend right through
 Many of the proteins are free to move around
laterally within the membrane
Membrane Protein Functions
 Channels to allow materials in and out of the cell
 Transport proteins that carry specific molecules in
and out of the cell
 Receptors for hormones, growth factors and other
chemicals that bind to the outside of the cell
 Enzymes to catalyze reactions in the cell
 Connecting cells to other
cells at junctions
 Ex. Gap junctions (allow
passage of materials
from cell to cell), tight
junctions (connect cells
together)
Carbohydrates
 Carbohydrate chains are attached to some lipid and
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protein molecules
(glycolipids & glycoproteins)
Usually attached to the outer surface
These are different in each individual
Cell recognition - “ID cards”, enable the body to
recognize its own cells
Cholesterol
 Embedded in the membrane
 Helps to maintain the fluidity
and flexibility of the membrane
 Helps to hold the proteins in
place
 Reduces permeability of the
membrane
Membrane Permeability
 Cell membranes are
differentially (selectively)
permeable
 Certain substances can move
through freely (water, gases,
small molecules, uncharged
(non-polar) molecules
 Others cannot, and require
special processes to get
through (larger molecules,
ions)