The Cell Membrane
Studying Membranes –
The Electron Microscope
• Understanding the structure and the function of
the phospholipid bilayer cell membrane
depended on the invention of the electron
microscope.
• Beams of electrons have a very short wavelength
and can resolve cell parts to 0.2 m in size.
• U of T graduate students James Hillier and Albert
Prebus built the first electron microscope in 1938.
EM Photo of a Membrane
Cell Membrane and Homeostasis
• The phospholipid
bilayer is responsible
for maintaining
homeostasis.
• Homeostasis is the
maintenance of a
steady state inside
the cell regardless of
external changes, to
ensure survival.
Liver cells - hepatocytes
Cell Membrane Functions
• In order to maintain homeostasis the cell must
regulate the movement of molecules:
1. The transport of raw materials into the cell.
2. The removal of wastes from the cell.
3. Transfer materials produced in the cell to
outside the cell.
4. Prevent entry of unwanted molecules.
5. Prevent the escape of needed molecules.
• All cell organelles are also surrounded by the
same phospholipid bilayer membrane.
Cell Membrane Structure
• There are two phospholipid layers with the
hydrophobic ends facing each other on the
inside of the bilayer farthest away from the
fluids.
• Outside the cell is the extracellular (EC) fluid.
• Inside the cell is the intracellular (IC) fluid.
• The fatty acid bilayer is selectively permeable.
Phospholipids in bilayer
Hydrophilic
head
Hydrophobic
tail
Intracellular Fluid
Extracellular Fluid
The Membrane is Flexible and Fluid
More Fluid
Unsaturated hydrocarbon
tails with kinks
More Viscous
Saturated hydrocarbon tails
When the membrane is colder it is less fluid and
a warmer membrane is more fluid. This is due
to cholesterol molecules in the membrane.
The Membrane is Dynamic
Lateral movement
(~107 times per second)
Movement of phospholipids
Flip-flop
(~ once per month)
Fluid Mosaic Model
• Our current understanding of cell membranes is
called the Fluid Mosaic Model.
• There are many molecules within the membrane
and other molecules connected to the
membrane.
• Some small non-polar molecules, like oxygen and
carbon dioxide, can pass through the
hydrophobic part of the cell membrane.
• Even water, which is slightly polar, can pass
through the hydrophobic cell membrane at a low
rate.
Structure of the Cell Membrane
Cell Membrane Proteins
• Carrier proteins are transmembrane proteins
that help move neutral water soluble
molecules, like glucose, from outside to inside
the cell.
• Channel proteins are also transmembrane and
allow ions to pass through the membrane.
• Aquaporins are special proteins that help
water pass through the cell membrane.
Cell Membrane Carbohydrates
• The lipids and the proteins often have unique
carbohydrates attached to them.
• These carbohydrates are used as recognition
sites for molecules attaching to the cell
membrane.
• Glycoproteins also serve as markers for
anchors to other internal structures like the
cytoskeleton.
• Glycolipids also serve as recognition sites for
other molecules as well as joining cells to form
tissues.
Videos and Animations
• http://www.youtube.com/watch?v=Qqsf_UJcfBc
• http://www.youtube.com/watch?v=moPJkCbKjBs
&feature=related
• http://www.youtube.com/watch?v=Rl5EmUQdku
I&feature=related
• http://www.teachersdomain.org/resource/tdc02.
sci.life.cell.membraneweb
• (also includes molecule movement)
• http://www.teachersdomain.org/resource/tdc02.
sci.life.cell.membraneweb