Outline
Membrane Structure and Function
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Chapter 5
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Mader: Biology 8th Ed.
Membrane Models
– Fluid-Mosaic
Plasma Membrane Structure and Function
– Protein Functions
– Plasma Membrane Permeability
! Diffusion
! Osmosis
! Transport Via Carrier Proteins
Cell Surface Modifications
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Membrane Models
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Fig. 5.1
Fluid-Mosaic Model
– Membrane is a fluid phospholipid bilayer
in which protein molecules are either
partially or wholly embedded.
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Fluid-Mosaic Model
Plasma Membrane Structure and Function
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Plasma membrane separates internal
environment from the external environment.
Hydrophilic polar heads face outside, and
hydrophobic nonpolar tails face each other.
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Plasma Membrane Structure and Function
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Proteins may be peripheral or integral.
– Peripheral proteins are found on the inner
membrane surface.
– Integral proteins are embedded in the
membrane.
Fig. 5.3
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Protein Functions
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Channel Proteins - Involved in passage of
molecules through membrane.
Carrier Proteins - Combine with substance
to aid in passage through membrane.
Cell Recognition Proteins - Help body
recognize foreign substances.
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Protein Functions
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Plasma Membrane Permeability
Receptor Proteins - Allow molecule binding,
causing protein to change shape and bring
about cellular change.
Enzymatic Proteins - Carry out metabolic
reactions directly.
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Plasma membrane is differentially
permeable.
– Passive Transport - No ATP requirement.
! Molecules follow concentration
gradient.
– Active Transport - Requires carrier protein
and ATP.
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Crossing Plasma Membrane
Diffusion
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Diffusion - Movement of molecules from a
higher to a lower concentration until
equilibrium is reached.
– Down concentration gradient
A solution contains a solute (solid) and a
solvent (liquid).
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Diffusion
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Osmosis
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Osmosis
Osmosis - Diffusion of water across a
differentially (selectively) permeable
membrane due to concentration differences.
– Osmotic pressure is the pressure that
develops due to osmosis.
! The greater the osmotic pressure, the
more likely water will diffuse in that
direction.
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Osmosis
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Isotonic Solution - Solute and water
concentrations both inside and outside the
membrane are equal.
Hypotonic Solution - Solution with a lower
concentration of solute than the solution on
the other side of the membrane.
– Cells placed in a hypotonic solution will
swell.
! Lysis
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Transport by Carrier Proteins
Osmosis
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Hypertonic Solution - Solution with a higher
concentration of solute than the solution on
the other side of the membrane.
– Cells placed in a hypertonic solution will
shrink.
! Plasmolysis
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Carrier proteins combine with a certain
molecules which are then transported
through the membrane.
– Facilitated Transport
! Small molecules follow concentration
gradient by combining with carrier
proteins.
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Transport by Carrier Proteins
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Active Transport
! Small molecules move against their
concentration gradient by combining
with carrier proteins.
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Membrane-Assisted Transport
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Membrane-Assisted Transport
Large marcomolecules are transported into
or out of the cell by vesicle formation.
– Exocytosis - Vesicles fuse with plasma
membrane as secretion occurs.
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Endocytosis - Cells take in substances by
vesicle formation.
! Phagocytosis - Large, solid material.
! Pinocytosis - Liquid or small, solid
particles.
! Receptor-Mediated - Specific form of
pinocytosis using a coated pit.
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Cell Surface Modifications
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Mader: Biology 8th Ed.
Cell Surfaces in Animals
– Junctions Between Cells
! Adhesion Junctions
" Intercellular filaments between cells.
! Tight Junctions
" Form impermeable barriers.
! Gap Junctions
" Plasma membrane channels are
joined (allows communication).
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Cell Surface Modifications
Extracellular Matrix
! Meshwork of polysaccharides and
proteins in close association with the cell
that produced them.
Plant Cell Walls
– Plants have freely permeable cell wall, with
cellulose as the main component.
! Joined by plasmodesmata that spans cell
wall and contains strands of cytoplasm
that allow passage of material between
cells.
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Review
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Membrane Models
– Fluid-Mosaic
Plasma Membrane Structure and Function
– Protein Functions
– Plasma Membrane Permeability
! Diffusion
! Osmosis
! Transport Via Carrier Proteins
Cell Surface Modifications
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