Models Of Membrane Structure
• In electron micrographs, all cell membranes have similar structure: an extremely thin
sheet (~5-7 nm) with "railroad track" appearance.
• All models of membrane structure built around notion of lipid bilayer.
o View schematic diagram of lipid bilayer
o View molecular model of lipid bilayer
• Phospholipids spontaneously form bilayers. Similar to soap bubbles: thin, flexible, fluid,
not very durable or strong.
• Animal membranes strengthened by cholesterol.
• Biological membranes include proteins; provide structural integrity, variety of functions.
• View schematic diagram of protein embedded in membrane
• View animation of membrane structure (from the Biology Place)
• Fluid Mosaic model: proteins "float" in a 2-dimensional sheet of lipids.
• Composition of typical membrane:
o ~50% lipid (largely phospholipid; in animal cells, 1/3 cholesterol)
o ~50% protein
• Proteins function in variety of ways: (see Fig. 8.8 in text)
Some are integral; span entire membrane. Include transport proteins (permeases).
• Some are peripheral; include receptor proteins for hormones, matrix of structural
proteins that attach to membrane and provide shape, etc.
DiffusionAnd Size Limits
• Diffusion is the driving force for substances to move around in cells.
• Diffusion results from random motion of molecules. If some regions more concentrated
than others, diffusion will tend to cause equilibrium (see text Fig. 8.9).
• Einstein's equation for diffusion in 2 dimensions: d = 6Dt
(d = distance traveled; t = time; D = diffusion coefficient characteristic for each type of
• Sample calculations: for sucrose, D = 2.38 x 10 cm /sec. How long will it take for
sucrose to diffuse length of 3 cells: 1 µm (typical bacterium); 10 µm (small animal cell,
e.g. lymphocyte); 1 meter (length of spinal neuron)?
1. Rearrange equation to solve for t: t = d /6D
2. Convert distance to cm so units are compatible: 1 µm = 10 cm; 10 µm = 10 cm; -3
1 m = 10 cm
3. Plug in values and solve:
For 1 µm cell: t = 0.7 x 10 sec, or 0.7 millisecond
For 10 µm cell: t = 0.07 sec, or 70 millisecond
For 1 m cell: t = 7 x 10 sec, or 2.2 years
1. diffusion rapidly moves molecules around in small cells, but rate increases as
the square of distance, so expanding cell diameter by 10 slows down diffusion
through cell by factor of 100. 2. Large cells cannot rely on diffusion to transport material throughout cell;
nerve cells must have some other mechanism (motor molecules such as kinesin
that transport materials attached to microtubules) to move materials throughout
OsmosisAnd Water Balance
• water flows smoothly across cell membranes without needing any carrier = osmosis.
Other polar or charged molecules (unless lipid soluble).
• View osmosis movie
• Three situations can result from water movement.
1. Isotonic environment. Water concentration outside = water conc. inside cell.
Since most cells contain about 0.9% dissolved salts + solutes, isotonic
environments must contain 0.9% salt. In this situation, water flow out =
water flow in. For human cells, this is desirable state.
Laboratory and clinical workers often use Ringer's solution to bathe
exposed tissues, provide isotonic environment.
View animation of membranes in isotonic environment (from the Biology
2. Hypotonic environment. Water concentration outside cell is higher (e.g. pure
water) than inside cell. Or, solute concentration outside cell is lower than inside
Result: water moves in at greater rate that moves out.
2 possible results:
1. if cell lacks a wall, will swell up. Can cause lysis (swelling leading
to breakage) if no way to remove excess water (e.g. in blood cells).
Freshwater protists (e.g. paramecium) have contractile vacuoles
to pump water back out, prevent lysis.
2. if cell has a wall, water pressure will push membrane tightly
against wall, lead to turgor. This is desirable state for walled cells.
View animation of hypotonic effects on plant and animal cells (from the
3. Hypertonic environment. Water concentration outside cell is lower (e.g. brine,
syrup) than inside cell. Or, solute concentration outside cell is higher than inside
Result: water moves out at greater rate that moves in.
2 possible results:
1. if cell lacks a wall, will shrivel up like a raisin. Causes crenation
in blood cells. Cells stop metabolizing, but not immediately killed.
Can restore activity by placing back in isotonic environment.
2. if cell has a wall, membrane will shrink away from wall as water
leaves cell, rigid wall remains where it is, leads to plasmolysis.
This is undesirable state for walled cells, cells stop metabolizing.
View animation of hypertonic effects on plant and animal cells (from the
View red blood cell crenation movie Membranes separate compartments of different
• Many substances occur at very different concentrations across cell membranes.
• Examples: ion gradients across a human cell
Ion Extracellular Intracellular Difference
Na + 140 mM 10 mM 14x
K 4 mM 140 mM 35x
Ca ++ 2.5 mM 0.1 microM 25,000x
Cl- 100 mM 4 mM 25x
• These gradients are maintained by membrane transport of each ion. How?
Movement Of Small MoleculesAcross Membranes can
involve simple diffusion or protein-mediated transport
• Cell membranes are selectively permeable.
• Lipophilic solutes cross the membrane freely by dissolving in the lipid bilayer. This is
Examples: ethanol (alcohol, contains both polar and non-polar regions); also fatty acids,
glycerol, steroids, etc.Also nonpolar gases like