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Biological Membranes
Chapter 5
Fluid Mosaic Model
 Amphipathic molecules: nonpolar,
hydrophobic portion (2 fatty acid chains in
a phospholipid) linked to a polar,
hydrophilic portion (glycerol head in a
phospholipid)
 Cylindrical shape – allows the phospholipid
to orient itself with tails toward the center
of the bilayer and heads out
 The embedded proteins in the bilayer are
free to move about like icebergs floating on
the sea
 This can be thought of as a liquid crystal
Figure 5-4
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Lateral movement only
Time
Other membrane lipids:
 Cholesterol – in animal cell
membranes. At low temperatures it
helps prevent solidifying. At higher
temps it helps maintain stability
 Glycolipids – carbohydrates
combined with lipids; found on cell
surfaces of animal cells; allow cells to
recognize and interact with each
other
In addition to phospholipids
 Integral proteins – tightly bound to the
membrane. Some do not extend all the way
through the membrane. Transmembrane
proteins do
 Peripheral proteins – not embedded in
the lipid bilayer and are located on the
inner or outer surface
 Glycoproteins – carbohydrates and
proteins; on the outer surface of cells; may
allow cells to adhere or provide protection
Carbohydrate
chains
Glycoprotein
Carbohydrate
chain
Extracellular fluid
Hydrophobic
Hydrophilic
Glycolipid
Cholesterol
Hydrophilic
a helix
Cytosol
Peripheral
protein
Integral
proteins
Functions of membrane proteins
 Anchoring: anchor the cell to the
extracellular matrix and connect to
microfilaments within the cell
 Passive transport: for passage of
certain ions or molecules
 Active transport: used to pump
solutes across the membrane
opposite to diffusion
Functions of membrane proteins…
 Enzymatic activity: catalyze
reactions that occur along the surface
 Signal transduction: bind to signal
molecules such as hormones
 Cell recognition: identification tags
 Intercellular junctions: attach
membranes of adjacent cells
Passage through the cell membrane
 Diffusion concepts:
 Kinetic energy of all matter
 Concentration gradient
 Equilibrium
 Cellular diffusion:
 Passive transport
 Osmosis (water + plasma membrane)
 Facilitated diffusion
Membranes are selectively
permeable
 Physical processes
Diffusion
Osmosis
 Carrier-mediated processes
Channel proteins
Carrier proteins
Diffusion
1
2
3
Osmosis:
water
passes
through
selectively
permeable
membrane
from region of
higher
concentration
to lower
Osmosis
 Isotonic
 Equal solute concentration
 Cell remains in equilibrium
 Hypertonic
 High in solute, low in water
 Water moves out of cell
 Cell shrinks
 Hypotonic
 Low in solute, high in water
 Water moves into cell
 Cell swells
Effects of osmosis
 In plant cells:
 Turgor pressure – cells push against
rigid cell walls
 Plasmolysis – cell contents pull away
from cell wall  wilting
 In animal cells:
 Cells may swell to bursting - cytolysis
 Some have contractile vacuoles to
remove excess water to prevent bursting
(a)
Plasma
membrane
(b)
Nucleus
(c)
Vacuole
Vacuole
Vacuolar
membrane
(tonoplast)
Cytoplasm
Plasma
membrane
Outside
cell
Inside
cell
Outside
cell
Outside
cell
Inside
cell
Inside
cell
(a)
No net water
movement
Net water movement
out of the cell
Net water movement
into the cell
(b)
10µm
(c)
Isotonic solution
Hypertonic solution
Hypotonic solution
Carrier-mediated transport
 Uses carrier proteins to allow substances
to cross the nonpolar interior of the
phospholipid bilayer
 Ions, large polar molecules (glucose, amino
acids)
 Two types:
 Facilitated diffusion – passive transport; does
not require an additional source of energy;
works with the concentration gradient
 Carrier-mediated active transport – requires
an additional energy source, usually ATP; works
against the concentration gradient
Facilitated Diffusion
 Makes use of the potential energy of the
concentration gradient
 As a molecule moves from a high
concentration to a low concentration –
energy is released
 Specific carrier proteins are involved –
the shape of the protein determines the
solute particle it can transport and changes
as it moves the particle across the
membrane
 This is a form of passive transport
Carrier-mediated active
transport
 For materials that the cell requires in
high concentrations
 The cell must ‘pump’ the material
from low to high concentration – up
the concentration gradient
 An example is the sodiumpotassium pump found in animal
cells
Sodium-potassium pump
 Uses energy in the form of ATP
 Pumps two K+ ions into the cell for every
three Na+ ions it pumps out
 This causes an electrical as well as chemical
gradient across the cell membrane – an
electrochemical gradient
 This gradient stores energy for the cell and
can be used to help drive other transport
systems
Na+/K+ pump
Transportation in vesicles
 Exocytosis – cells eject wastes or
specific products such as hormones
 The vesicle fuses with the cell membrane
 Endocytosis – extracellular materials
are incorporated into the cell:
 Phagocytosis
 Pinocytosis
 Receptor-mediated endocytosis
Exocytosis
Phagocytosis
 ‘cell eating’
 large, solid materials such as other
cells
 White blood cells ingest whole
bacteria
Phagocytosis
Pinocytosis
 ‘cell drinking’
 Droplets of fluid collect in a fold of the
plasma membrane which pinches off
into the cytosol in a vesicle
 Contents of the vesicle diffuse into
the cytosol and the vesicle shrinks
Pinocytosis
Receptor-mediated endocytosis
 Specific molecules combine with
receptor molecules on the plasma
membrane
 Cholesterol is taken up this way
Receptor-mediated endocytosis