Transport Across the Cell Membrane 11/5/07 Cell Biology 1 Transport Across the Cell Membrane "The difference between the internal and external chemical composition of a cell represents a degree of order, that can be maintained only by a barrier to free movement into and out of the cell. And since cells must also assimilate matter from their external environment, to grow and reproduce, they must be able to selectively allow certain molecules and ions across this barrier, often against concentration gradients, while restricting and excluding others." A. Plasma Membrane - The plasma membrane surrounds the cell and functions as an interface between the living interior of the cell and the nonliving exterior. - All cells have one. - It regulates the movement of molecules into and out of the cell. Membrane Structure - The membranes of a cell are phospholipid bilayers that contain numerous proteins embedded within them. Some of the proteins extend all the way through the membrane; others do not. Source: http://en.wikipedia.org/wiki/Cell_membrane Phospholipids - Most of the lipids in a membrane are phospholipids. Notes
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Transport Across the Cell Membrane 11/5/07
Cell Biology 1 Transport Across the Cell Membrane
"The difference between the internal and external chemical
composition of a cell represents a degree of order, that
can be maintained only by a barrier to free movement
into and out of the cell.
And since cells must also assimilate matter from their
external environment, to grow and reproduce, they must
be able to selectively allow certain molecules and ions
across this barrier, often against concentration gradients,
while restricting and excluding others."
A. Plasma Membrane
- The plasma membrane surrounds the cell and
functions as an interface between the living interior of
the cell and the nonliving exterior.
- All cells have one.
- It regulates the movement of molecules into and out of
the cell.
Membrane Structure
- The membranes of a cell are phospholipid bilayers
that contain numerous proteins embedded within them.
Some of the proteins extend all the way through the
- A Mole of a substance may be defined as the molecular
weight of that substance expressed in grams. o The molecular weight (m wt) of glucose is 180 so
180g of glucose = 1mole. The m wt of NaCl is about 58 so 58g of NaCl = 1mole. If 58g of NaCl are placed into a beaker and water added to a volume of
1l then the result will be a 1 molar solution. Which may be shown as 1mol/l or 1mol l-1 or 1M.
o Moles are used as a mass unit by biologists (and
Chemists) in preference to grams because a mole of any substance contains the same number of particles as a mole of any other substance.
o This greatly simplifies calculations that deal with chemical reactions (including biologically relevant reactions such as buffering) and osmotic effects.
o The number of particles, atoms or molecules in a
mole of any substance is given by Avagadro's number which is about 602204500000000000000000 or 6 x 10
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Transport Across the Cell Membrane
Cell Biology 8 Transport Across the Cell Membrane
- Osmolarity is a measure of the osmotic pressure exerted by a solution across a perfect semi-permeable membrane (one which allows free passage of water and completely prevents movement of solute) compared to pure water.
o Osmolarity is dependent on the number of particles in solution but independent of the nature of the particles.
o For example, 1 mole of glucose dissolved in 1 litre of water has an osmolarity of 1 osmole (osm) /l. If 1 mole of another sugar, such as sucrose were added to the same litre of water, the osmolarity would be 2
osm/l. o It doesn't matter that the solution contains 1 mole of
glucose and 1 mole of sucrose. o If 1 mole of NaCl were dissolved in 1 litre of water it
would produce a 1 mol/l NaCl solution with an
osmolarity 2 osm/l because NaCl dissociates into Na+ and Cl- (two particles) in solution. This is true
of all compounds that dissociate in solution. Na2SO4, which dissociates into Na
+, Na
+ and SO4
2-, to give 3
particles per molecule produces 3 osm/l for every mole dissolved in 1 litre.
o If two solutions contain the same number of particles
they may be said to be iso-osmotic (isosmotic) with
respect to each other. o If one solution has a greater osmolarity than another
solution it is hyperosmotic with respect to the weaker solution. If one solution has a lower osmolarity than another solution then it is hypo-osmotic (hyposmotic) with respect to the stronger solution. Iso, hyper and hypo osmolarity should always be
stated with respect to another solution. For example, a 1 mol/l NaCl solution is hyperosmotic with respect to 1 mol/l glucose solution.
- Because there are more Na+ and Cl-
particles relative to glucose particles
- Tonicity is nearly the same as osmolarity.
o For substance that cannot cross cell membranes, tonicity is practically identical to osmolarity.
o Tonicity is a measure of the osmotic pressure that a substance can exert across a cell membrane, compared to blood plasma. Plasma has an osmolarity of about 0.3 osm/l, therefore a 0.15 mol/l NaCl solution may be said to be isotonic with plasma (Assuming that neither Na+ nor Cl- can cross cell membranes, which is nearly true).
If a substance can cross a plasma membrane, then it cannot exert an osmotic pressure across that membrane. The solute will equilibrate across the membrane instead of forcing water to move. Urea behaves like
this, so a 0.3 mol/l urea solution may be said to be iso-osmotic with plasma but it is not isotonic.
center of the lipid bilayer (the fatty acid tails) is
nonpolar and does not readily interact with polar
molecules.
The following substances can pass through the cell
membrane:
- Nonpolar molecules (example: lipids)
- Small polar molecules such as water
o Via porins, specialized protein channels
Porins are proteins which cross the cellular membrane and
act as a pore through which molecules can diffuse.
- Unlike other membrane transport proteins, porins are
large enough to allow passive diffusion - i.e. they act
as channels.
- Porins are composed of beta sheets (2° proteins)
these form a cylindrical tube, called a beta barrel.
- The amino acid composition of the porin beta
sheets is unique in that polar and nonpolar
residues alternate along them.
- This means the nonpolar residues face outwards so
as to interact with the nonpolar lipid membrane,
whilst the polar residues face inwards into the
centre of the beta barrel to interact with the
aqueous channel.
- And water can pass through
The following substances cannot pass through a cell
membrane without help:
- Ions and charged molecules (example: salts
dissolved in water) – via porins/channels/carrier
The lipid bilayer is permeable to small, uncharged (nonpolar), molecules like oxygen (O2), carbon dioxide (CO2 (these diffuse freely in and out of the cell), and ), other lipids.
Lipid bilayers are not permeable to charged particles and really big particles: - Water (polar) - ions such as
- K+, Na
+, Ca
2+ (called
cations because when subjected to an electric
field they migrate toward the cathode [the negatively-charged electrode])
- Cl-, HCO3- (called anions
because they migrate toward the anode [the positively-charged electrode])
- small hydrophilic molecules
like glucose - macromolecules like proteins
and RNA Note though that the plasma membrane (cf fluid mosaic model) is permeable to water and
other small polar molecules because of porin channels, openings in the phospholipids bilayer.
Transport Across the Cell Membrane
Cell Biology 10 Transport Across the Cell Membrane
- chief difference from “free” diffusion is that the
membrane is impermeable to the molecule except
for passage thru the carrier channels,
- e.g., the cell membrane, composed of the
phospholipids is impermeable to the passage of
water…
6. “Active Transport” - Against the Gradient
- the other half of Facilitated Transport
- the pumping of molecules or ions across a cellular
membrane thru a carrier protein
- from a region of lower conc. to one of higher
conc.
- therefore against the “current” or concentration
gradient
- such a process requires energy
- = Active transport
source: http://www.accessexcellence.org/AB/GG/
Water and lipids are the two major types of solvent in the body. The lipid cell membrane separates the intracellular fluid from the extracellular fluid. Substances which are water soluble typically do
not cross lipid membranes easily unless specific transport mechanisms are present. It might be expected that water would likewise not cross cell membranes easily. Indeed, in artificial lipid bilayers, water does not cross easily and this
is consistent with our expectation…
but paradoxically, water crosses
nearly all the membranes in the
body with ease!
How? Via aquaporins
These aquaporin proteins form complexes that span the membrane and water moves through these channels passively in response to osmotic gradients. These channel proteins are present in highest concentrations in tissues where
rapid transmembrane water movement is important (e.g., in renal tubules). Source: http://wilkes-fs1.wilkes.edu/~terzaghi/BIO-226/lectures/13.html
Carrier proteins – bind to a substrate to assist it thru the lipid bilayer - facilitated and active
transport Channel proteins – are “pores” through which a particle simply
glides thru - facilitated diffusion
Transport Across the Cell Membrane
Cell Biology 11 Transport Across the Cell Membrane
C. Bulk (Vesicular) Transport
- also a form of active (ATP using) transport
1. Endocytosis
- material is engulfed by the plasma membrane and
deposited in the cytoplasm in pockets that are
“pinched off” as vesicles toward the interior of the
cell
- the vesicle is then digested by the cell
- two variations on this theme:
i. Pinocytosis
- Greek “drink cell”s
- cell takes in dissolved material
ii. Phagocytosis
- Greek “eat cell”
- cell takes in solids
- basically large particulate matter is tightly enclosed
by the membrane bound arm of cytoplasm, and
most extracellular fluid is excluded, e.g., amoebae
eating a paramecium
2. Exocytosis
- like endocytosis only the vesicle is not digested,
but transported to and fused with, another part of