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Membrane Structure and
Function
Plasma Membrane
• Boundary that separates the living cell
from nonliving surroundings.
• About 8 nm thick, and is selectively
permeable
• Controls traffic into and out of the cell
• Ability of the plasma membrane to
move materials is due to its structure.
Phospholipid
Structure of the
Plasma membrane
Membrane Structure
• Membranes are composed mainly of
phospholipids, which are amphipathic
molecules (having hydrophilic and
hydrophobic areas). Proteins are
embedded in the membrane.
• Fluid mosaic model
Fluidity of Membranes
• Membrane held together by hydrophobic
interactions
• Phospholipids and proteins can move about
within the membrane
• Elements of the cytoskeleton may hold some
proteins in place
• Membranes remain fluid as temp. decreases
up to the point the phospholipids settle into a
close-packed arrangement.
• Cholesterol acts as a buffer to resist changes
in temperature.
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Experiment: Membrane protein movement
• Researchers labeled the
plasma membrane
proteins of a mouse and
a human cell, then
fused them.
• Results: Mixing the
mouse and human
proteins produced a
hybrid, showing that
some proteins move
sideways within the
membrane.
Membrane proteins and their
functions
• Phospholipids form the main fabric of the
membrane, but proteins determine its specific
function.
• Integral proteins-penetrate the
hydrophobic core of the lipid bilayer. Many
are transmembrane proteins that completely
span the membrane.
• Peripheral proteins- not embedded in the
membrane. Appendages loosely bound to
membrane surface.
Structure of a
transmembrane
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protein
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Six major functions of
membrane proteins
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Transport
Enzymatic activity
Signal transduction
Cell-cell recognition
Intercellular joining
Attachment of the cytoskeleton and
ECM
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Membrane surfaces
• Membranes are bifacial, having a side facing
inside the cell and a side facing the
cytoplasm.
• Two lipid bilayers may vary in composition.
• Membrane synthesis and modification by the
ER and Golgi determines the distribution of
lipids, proteins, and carbohydrates.
Cell-Cell Recognition
• Cell recognition is the ability of a cell to
determine if other cells it encounters are alike
or different from itself.
• Essential for sorting embryonic cells into
tissues and organs, and rejection of foreign
cells by the immune system
• Carbohydrates, glycolipids and glycoproteins,
on the external surface of the cell membrane
act as “markers”.
• These can vary between species, individuals,
and among the cells of the same individual.
Traffic Across Membranes
• Membrane’s molecular organization
results in selective permeability.
• Selective permeability of a membrane
depends on solubility characteristics of
the phospholipid bilayer and the
presence of integral transport proteins.
Permeability of the Lipid bilayer
• Nonpolar (hydrophobic) molecules
dissolve in the membrane and cross it
easily (hydrocarbons, O2, and CO2)
• Small, polar (hydrophilic) uncharged
molecules such as water pass freely
• Large uncharged polar molecules and
all ions have difficulty penetrating the
hydrophobic layer.
Transport Proteins
• Polar molecules and specific ions can pass
through the hydrophobic layer of the cell
membrane through transport proteins that
span the membrane.
• Some materials pass through protein
channels, while others bind to the protein and
are physically moved across. Ex) glucose
enters liver cells by very selective transport
proteins
Passive Transport
• Passive transport is diffusion of a
substance across a biological membrane.
• Diffusion- net movement of molecules from
high concentration to low concentration
(down the concentration gradient)
• Results from kinetic energy of molecules.
Does not require energy input.
• Diffusion continues until a dynamic
equilibrium is reached (no net directional
movement)
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Osmosis
• Osmosis is the passive transport of water
across a selectively permeable membrane.
• Relative concentration terms:
a) Hypertonic- solution with greater solute
concentration than inside the cell.
b) Hypotonic- solution with lower solute
concentration than inside the cell.
c) Isotonic- solution with equal solute
concentration compared to that inside a cell.
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Movement of Water
• If two solutions are separated by a selectively
permeable membrane that is permeable to
water but not to solute, water will diffuse
from the hypoosmotic solution to the
hyperoosmotic solution.
• Water move to DILUTE!
• Direction of osmosis is determined by the
difference in total solute concentration,
regardless of the types of solutes in the
solutions.
• At equilibrium, water molecules move in both
directions at the same rate. (True for isotonic
solutions also)
Cell Survival
• Balance of water between the cell and its
environment are crucial to organisms.
• Osmoregulation- control of water balance.
• Animal cells (no cell walls) have adaptations
for osmoregulation. Ex) Paramecium have
contractile vacuoles
• Plants can take in water and become turgid
(firm), while in isotonic solutions cells become
flaccid (limp). In hypertonic solutions, the
plant cell shrivels and plasmolysis occurs.
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Facilitated Diffusion
• Diffusion of polar molecules and ions
across a membrane with the aid of transport
proteins.
• Proteins have a specialized binding site for
the solute they transport.
• Some proteins have gated channels that
open or close in response to a stimulus.
Facilitated Diffusion
Active Transport
• Pumping of solutes against the
concentration gradient, requiring energy
from the cell.
• Sodium-potassium pump allows cells to
exchange Na+ and K+ across animal cell
membranes.
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Ion Pumps
• Because anions and cations are unequally
distributed across plasma membranes, all
cells have voltages across their plasma
membrane (membrane potential).
• Forces that drive passive transport of ions
across membranes include: concentration
gradient of the ion (chemical force), and
effect of membrane potential (electrical force)
on the ion.
• The combination of these forces is called the
electrochemical gradient.
Electrogenic Pumps
• Transport proteins that generate voltage
across a membrane.
• Na+/K+ ATPase is the major electrogenic
pump in animal cells
• A proton pump (H+) is the major electrogenic
pump in plants, bacteria, and fungi.
• Mitochondria and chloroplasts use a proton
pump to drive ATP synthesis.
• Voltages created by electrogenic pumps are
sources of potential energy available to do
cellular work.
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Cotransport
• Process where a single ATP-powered pump
actively transports one solute and indirectly
drives the transport of other solutes against
their concentration gradients.
• Example: Plants use the mechanism of
sucrose/H+ cotransport to load sucrose
produced by photosynthesis into specialized
cells in the veins of leaves. Transport proteins
can move sucrose into the cell against the
concentration gradient only if it travels with
the H+ ion.
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Movement of Large molecules
• Water and small molecules cross
membranes by passing through the
phospholipid bilayer or being carried by
a transport protein.
• Large molecules such as proteins and
polysaccharides cross membranes by
the processes of endocytosis and
exocytosis.
Exocytosis
• Process of exporting macromolecules from a
cell by fusion of vesicles with the plasma
membrane.
• Vesicle usually budded from the ER or Golgi
and migrates to plasma membrane
• Used by secretory cells to export products,
such as insulin in the pancreas or
neurotransmitters from neurons.
Endocytosis
• Process of importing macromolecules
into a cell forming vesicles derived from
the plasma membrane.
• Vesicle forms from a localized region of
plasma membrane that sinks inward,
pinches off into the cytoplasm.
• Used by cells to incorporate
extracellular substances.
Types of Endocytosis: Phagocytosis
• Endocytosis of solid particles.
• Cell engulfs particle with pseudopodia,
and pinches off a food vacuole.
• Vacuole fuses with a lysosome
containing hydrolytic (digestive)
enzymes that break down the particle.
Phagocytosis of a
bacterial cell by an
amoeba
Pinocytosis
• Endocytosis of fluid droplets
• Extracellular fluid is engulfed in small
vesicles
• Nonspecific in the substances it
transports. Cell takes in all solutes
dissolved in the droplet.
Pinocytosis of
fluid into cell
lining a blood
vessel
Receptor-mediated Endocytosis
• More discriminating process than pinocytosis
• Coated pits form vesicles when specific
molecules (ligands) bind to receptors on the
cell surface.
• After ingested material is released from the
vesicle for metabolism, the receptors are
recycled to the plasma membrane.
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Receptor-mediated endocytosis
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