Membrane Structure and
Function
Chapter 8
• Plasma membrane of cell selectively
permeable (allows some substances
to cross more easily than others)
• Made mostly of proteins and lipids
(phospholipids).
• Phospholipids and proteins create
unique physical environment (fluid
mosaic model)
Phospholipid
• Membrane - bilayer - hydrophilic
(water loving) heads pointing
outwards, hydrophobic (water
fearing) tails pointing inwards.
• Proteins help membrane to stick to
water.
• Fluid because lipids and proteins
can move laterally.
• As temperatures drop, liquid
membrane can solidify.
• Saturated fatty acid tails - more
solid than unsaturated fatty acid
tails.
• Cholesterol found in membrane
helps with fluidity of membrane.
• Membranes need to be fluid to
work properly - systems in place to
help keep it fluid.
• Two different types of proteins are
found in membrane.
• 1Peripheral proteins not in
membrane, bound to surface of
protein.
• 2Integral proteins in membrane
often spanning entire membrane.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/M/MembraneProteins.gif
• Membrane helps keep cell’s shape.
• Also aids in cell-to-cell recognition
(ability of a cell to distinguish one
type of neighboring cell from
another)
• Some substances move steadily
across membrane (sugars, ions, and
wastes like CO2)
• Hydrophobic molecules (i.e.
hydrocarbons, CO2, and O2) can
dissolve in lipid bilayer and cross
easily.
• Charged particles and polar
molecules have more difficulty
passing.
• Specific ions and polar molecules
can cross lipid bilayer by passing
through transport proteins that
span membrane.
• Diffusion - tendency for substance
to spread out in open area.
• Permeable membrane separating a
solution with dye molecules from
pure water, dye molecules will cross
barrier randomly.
• No force acting upon it - substance
will tend to move down it’s
concentration gradient from where
it is more concentrated to less
concentrated (passive transport).
• Diffusion of molecules with limited
permeability through lipid bilayer
may be assisted by transport
proteins.
http://library.thinkquest.org/27819/media/protein_channel.gif
• Difference in concentration - ions
move from one area to other.
• Solution with higher [ ] solutes hypertonic.
• Solution with lower [ ] solutes hypotonic.
• [ ] equal - isotonic.
http://www.biologycorner.com/resources/hypertonic.gif
http://www.biologycorner.com/resources/hypotonic.gif
• Solution hypertonic - higher solute
[ ] but lower H2O [ ].
• H2O moves into solution and solute
moves out.
• Movement of H2O across
selectively permeable membrane osmosis.
• 2 solutions isotonic, H2O molecules
move at equal rates from one to the
other, (no net osmosis)
• Cell placed in hypertonic solution –
H20 rushes out of cell (cell shrinks).
• Cell placed in hypotonic solution –
H2O rushes into cell (cell swells).
• Organism does not have rigid walls
must have ability to osmoregulate
and maintain internal environment.
• Plant cells expand when watered
causing pressure to be exerted
against cell wall.
• Allows plant to stand up against
gravity (turgid cell); not watered,
plant will begin to wilt (flaccid cell).
• Plant loses enough water, plasma
membrane will pull away from cell
(plasmolysis).
http://faculty.southwest.tn.edu/jiwilliams/plasmolysis.gif
• Charged particles that cannot pass
through membrane - proteins to
pass through (facilitated diffusion
- diffusion of substance down it’s
[ ] gradient with help of transport
protein)
• Some channel proteins (gated
channels) open/close depending on
presence/absence of physical or
chemical stimulus.
In this case, the protein actually rotates
to dump the materials to the inside
of the cell.
• Sometimes materials need to be
moved against [ ] gradient (active
transport)
• Active transport requires energy of
cell to move substances from an
area of low [ ] to an area of high [ ]
(i.e. sodium-potassium pump in
animal cells)
http://www.sp.uconn.edu/~terry/images/anim/antiport.gif
• Sodium-potassium pump actively
maintains gradient of sodium (Na+)
and potassium ions (K+) across
membrane.
• Sodium-potassium pump uses
energy of 1 ATP to pump 3 Na+ ions
out and 2 K+ ions in.
• Cells maintain voltage across plasma
membranes.
• Cytoplasm negative compared to
opposite side of membrane
(membrane potential - ranges from
-50 to -200 millivolts)
http://bioweb.wku.edu/courses/Biol131/images/neuronions.GIF
• Membrane potential favors passive
transport of cations (positive ions)
into cell and anions (negative ions)
out of cell.
• Creates an electrochemical
gradient across membrane.
• Some organisms have proton pumps
that actively pump H+ out of cell
(i.e. plants, bacteria, and fungi)
• Materials leave membrane through
lipid bilayer or through transport
proteins.
• Exocytosis - transport vesicle buds
from Golgi apparatus - moved by
cytoskeleton to plasma membrane.
• When membranes meet - fuse material is let out to outside of cell.
• Endocytosis - cell brings in
macromolecules and matter by
forming new vesicles from plasma
membrane.
• Membrane is inwardly pinched off
and vesicle carries material to
inside of cell.
http://www.kscience.co.uk/as/module1/pictures/endoexo.jpg
• 1Phagocytosis (“cell eating”) - cell
engulfs particle by extending
pseudopodia around it, packaging it
in a large vacuole.
• Contents of vacuole are digested
when vacuole fuses with lysosome.
• 2Pinocytosis (cell drinking) - cell
creates vesicle around droplet of
extracellular fluid.
• 3Receptor-mediated endocytosis specific in transported substances.
• Extracellular materials bind ligands
(receptors) - causes vesicle to
form.
• Allows materials to be engulfed in
bulk (i.e. cholesterol in humans)