Download Membrane_Structure_and_Function

Membrane Structure
and Function
Fluid Mosaic Model
The plasma membrane is composed
of a phospholipid bilayer with
embedded proteins. Carbohydrates
are also attached to the outer surface
of the membrane.
The accepted model of the plasma
membrane is that the membrane is a
fluid structure composed in the above
form with a “mosaic” of proteins
Fluid Mosaic Model
Membranes are not
static
Held together by
hydrophobic
interactions
Lipids and proteins
can shift within the
membrane (usually
laterally)
Membrane Proteins
Two major types
Integral Proteins
Many are transmembrane
proteins (span through the
membrane) others only
extend midway
Have a hydrophobic
region composed of one
or more nonpolar amino
acids, usually in α helices
Many also have
hydrophilic channel for
hydrophilic substances to
pass
Membrane Proteins
Peripheral Proteins
Not imbedded in
bilayer, but
attached to the
surface of the
membrane – via
the cytoskeleton or
ECM
Often attached to
exposed areas of
integral proteins
Protein Functions
Membrane Carbohydrates
Cellular recognition often relies on
carbohydrates because these are
often binding sites
Usually short, branched chains and can
be covalently bonded to lipids
(glycolipids) or proteins (glycoproteins)
Glycoproteins are more common
Carbohydrate variations on cell surfaces
allow for marking of cells
Example: Blood typing of A, B, AB, O is
designated by carb. variation
Synthesis of Membranes
1.
2.
3.
4.
Membrane proteins and
lipids begin in the ER.
Carbs are added to proteins.
In GA, glycoproteins
undergo carb modification.
Carbs added to lipids.
Transmembrane proteins,
glycolipids, and secretory
proteins are transported via
vesicles to the membrane
Vesicles fuse with the
membrane, realeasing the
secretory proteins and
positioning the glycoproteins
and glycolipids outside the
membrane
Selective Permeability
Nonpolar molecules are hydrophobic and
can therefore dissolve in the lipid bilayer
without membrane proteins
Includes: hydrocarbons, carbon dioxide, and
oxygen
Ions and polar molecules (ie. hydrophilic)
are blocked and/or slowed by the
hydrophobic core of the membrane
therefore they often use transport proteins
Water often uses a specialized channel protein
called an aquaporin
Transport Proteins
Channel Proteins – transport proteins with
a hydrophilic channel that allow certain
polar molecules and ions to pass
Carrier Proteins – change shape to
specifically hold whatever is crossing
**Transport proteins are specific to each
substance and will not allow other
substances via that route
Passive Transport
Diffusion – the movement of molecules of any
substance so that they spread out evenly in an
available space
In the absence of other forces, a substance will diffuse
from where it is more concentrated to where it is less
concentrated
It diffuses DOWN the concentration gradient and does not
use energy
Passive Transport
Osmosis – The
diffusion of water
across a selectively
permeable membrane
Water diffuses across
the membrane from
the region of lower
solute concentration
to that of higher
solute concentration
Water Balance (No Walls)
Tonicity – the ability of a solution to
cause a cell to gain or lose water;
related to the concentration of solutes
that cannot cross membrane
Isotonic – no NET movement of water
Hypertonic – cell loses water to the
environment; greater solute
concentration outside the cell
Hypotonic – cell gains water from the
environment; greater solute
concentration inside the cell
Water Balance (Walls)
Cell wall protects against excess
water uptake
Cells can be turgid (full of water) or
flaccid (limp)
Plasmolysis – in a hypertonic
environment, a plant cell can lose
water, causing the plasma membrane
to pull away from the cell wall and the
plant to wilt
Passive Transport
Facilitated Diffusion –
diffusion using
transport proteins
Used by water and
other polar molecules
May also include
gated channels that
are opened via a
chemical or
mechanical stimulus
Active Transport
The pumping of solute across a
membrane AGAINST the
concentration gradient, thus using
energy (ATP)
Enables cells to maintain solute
concentrations that are different from
their environment
Example: Sodium-Potassium Pump
Active Transport
Ion Pumps – The inside
of the cell is more
negative in relation to
the outside of the cell,
favoring the transport of
cations into the cell and
anions out of the cell
This can be a form of
diffusion; however, if
movement goes against
the electrochemical
gradient it can be active
Active Transport
Cotransport – the
transport of several
solutes driven by a
single ATP-powered
pump
The 2nd step in this is
actually diffusion, but
indirectly relies on the
ATP from step 1
Ex: Plant Cells use a
single proton pump for
hydrogen ions but this
also drives the transport
of amino acids, sugars
and nutrients
Active Transport
Exocytosis - Cell secretions are
released via vesicles
Endocytosis – Cell takes in
items from outside the cell by
creating a vesicle
Phagocytosis – cellular eating
Pinocytosis – cellular drinking; the
extracellular fluid engulfed also
contain a variety of solutes
Receptor-Mediated Endocytosis –
receiving specific substances via
binding proteins in the plasma
membrane which forms a vesicle
(Ligand with specific receptor)