The Transport
of Materials
Across Cell
Membranes:
Part V
The Plasma Membrane
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The Plasma Membrane
The cell membrane is said to be semi-permeable or
selectively permeable because certain molecules can cross
the membrane and others cannot. The membrane is said
to be permeable to those that can cross and impermeable
to molecules that cannot cross.
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Passive Diffusion
• The red molecules are
initially more concentrated
on side B. These molecules
have more free energy.
• The net movement of the
red molecules is from side
B to side A, or from a
higher concentration to a
lower concentration.
• The water molecules (blue) are equally
distributed and will move across at the same rate.
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Diffusion
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Passive diffusion allows small, non-polar molecules to
move directly across the phospholipid bilayer. Results from
the random movement of molecules. Rates of passive
diffusion increase with higher concentration gradients,
increased temperatures, and smaller molecules.
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Free Energy and Diffusion
o Diffusion (and all passive transport) is related
directly to kinetic energy
 Side of membrane that is more
concentrated will have more kinetic
energy; therefore, the molecules will
bounce off of each other more and will
thus diffuse more in the opposite direction
(to the less concentrated side) until
equilibrium is reached.
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Passive Diffusion
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Facilitated Diffusion
• The molecule is impermeable to the
membrane because it is too large, is
polar, or ionic.
• The transport of the molecule
requires a carrier protein or channel
protein (both are transport proteins).
• It requires no ATP. The driving force
is an increase in entropy. The
molecules are moving from an area
of higher concentration to lower
concentration or from higher free
energy to lower free energy.
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Facillitated Diffusion
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Facilitated Diffusion
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Active Transport
• Membrane is impermeable to the molecules
because they are too large, polar, or ionic.
• Molecules are moving against a concentration
gradient (from low to high) or from low free
energy to high free energy.
• A transport protein (also known as
permeases) and ATP are both needed.
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Active Transport
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Electrogenic Pump
• An electrogenic pump pumps
ions against a concentration
gradient AND a charge gradient
to create a separation of charge
across the membrane.
• Sometimes an electrogenic
pump is combined with another
protein carrier involved in
facilitated diffusion.
– The H+ ions are driven back across
the membrane not only by the
difference in concentration by also
by the attraction of the positive H+
ions to the negative charges on the
other side of the membrane.
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Osmosis
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Osmosis
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Osmosis Continued
 Osmosis is the movement of water directly through the
phospholipids bilayer (water moves from areas of high
potential [high free water concentration] to low potential
[low free water concentration])
o Solutes decrease the concentration of free water, since
water molecules surround the solute molecules.
o Therefore, direction of osmosis will be opposite of
diffusion
o Because water is polar, the hydrophobic tails reject water
for the most part so this movement is very slow
o Proteins called aquaporins allow for the movement of
water at a much greater rate.
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Osmosis
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Tonicity
Tonicity refers to the ability of a solution to make a cell gain or lose
water (refers to the solution, not the cell).
3 types of tonicity
1. Hypertonic – the concentration of the solution is above that of the
cell (hyper means above)
• Cell loses water
• Turgor pressure (in cells with cell walls) is resistance to water
movement in the cell by the cell wall
2. Hypotonic – the concentration of the solution is below that of the
cell (hypo means below)
• Cell gains water
3. Isotonic – the extracellular and intracellular concentrations are
equal
• There is equal movement of water into and out of the cell
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Osmosis in Plant Cells and Animal Cells
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Plasmolysis
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Three Types of Endocytosis
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Endocytosis is the movement
of larger particles (too large
to move through transport
proteins) into the cell by use
of membrane vesicles.
• Phagocytosis
• Pinocytosis
• Receptor Mediated
Endocytosis
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Phagocytosis
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• Phagocytosis “cell eating”- Larger molecules or particles
are brought into the cell by engulfing them into a
plasma membrane vesicle.
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Pinocytosis
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• Pinocytosis “cell drinking”- Dissolved molecules
are brought into the cell by engulfing them into a
plasma membrane vesicle.
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Receptor Mediated Endocytosis
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• Receptor mediated- Receptors on the outside of
cell membrane allow for the attachment of a
particular molecule. When a certain number of
receptor sites are filled, endocytosis occurs.
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Exocytosis
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• Exocytosis is just the opposite of endocytosis. Material
to be secreted usually moves through the
compartments of the Golgi apparatus where it may be
modified. The material is then surrounded by
membrane forming a vesicle.
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Review of Endocytosis
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