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Marieb’s Human
Anatomy and Physiology
Marieb w Hoehn
Chapter 3
Cell Membranes
Movement Across the Cell Membrane
Lecture 7
1
Lecture Overview
• The cell membrane
• Osmotic pressure and tonicity
• Movement of substances into and out of the
cell
2
Cell Membrane
• outer limit of cell; isolates cell
• controls what moves in and out of cell selectively permeable
• self-sealing
•phospholipid bilayer
• water-soluble “heads” form outer surfaces
• water-insoluble “tails” form interior
• permeable to lipid-soluble substances only
• cholesterol stabilizes the membrane
• proteins
• receptors
• pores, channels, carriers
• enzymes
• CAMS
• self-markers
3
Cell Membranes
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
4
A Transmembrane Protein
Figure from: Alberts et al., Essential Cell Biology, Garland Publishing, 1998
Hydrophilic
channel
Membrane
Lipids
5
Movements Into and
Out of the Cell
Passive (Physical) Processes
• require no cellular
energy
• simple diffusion
• facilitated diffusion
• osmosis
Active (Physiological) Processes
• require cellular energy
• active transport
• endocytosis
• exocytosis
• transcytosis
6
Simple Diffusion
Figure from: Hole’s Human A&P, 12th edition, 2010
Solutes will evenly disperse in a solvent with time
by diffusion. This is the lowest energy state.
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Simple Diffusion
• movement of solute from regions of higher concentration to
regions of lower concentration (a physical process)
Figure from: Hole’s Human A&P, 12th edition, 2010
8
Where Would You Rather Be?
“Spread out, would ya!?”
9
Facilitated Diffusion
• diffusion across a membrane with the help of a channel or
carrier molecule
• e.g, transport of
glucose across cell
membrane
BUT…still from a
region of higher
concentration to a
region of lower
concentration
Figure from: Hole’s Human A&P, 12th edition, 2010
10
Factors Influencing Diffusion Rates
• Distance (shorter is faster)
• Gradient size (bigger difference in
concentration is faster)
In the body, diffusion
distances are
typically limited to a
maximum of about
125 µm
• Molecule size (smaller is faster)
• Temperature (warmer is faster)
• Electrical forces (repulsion is better)
11
Diffusion and the Cell Membrane
Carrier/channel
proteins required
for all but fatsoluble molecules
and small
uncharged
molecules
oxygen, carbon
dioxide and other
lipid-soluble
substances diffuse
freely through the
membrane
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Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
Osmosis
• movement of water (solvent) through a selectively permeable
membrane from regions of higher water concentration to
regions of lower water concentration
• *water always moves toward a higher concentration of solutes
A special case of
passive diffusion
Figure from: Hole’s
Human A&P, 12th
edition, 2010
13
Osmotic Pressure/Tonicity
Osmotic Pressure – ability of osmosis to generate enough
pressure to move a volume of water
*Osmotic pressure increases as the number of nonpermeable
solutes particles increases
0.9% NaCl
• isotonic – same
5.0% Glucose
osmotic pressure as a
second solution
Figure from: Hole’s Human A&P, 12th edition, 2010
• hypertonic – higher
osmotic pressure
• hypOtonic – lower
osmotic pressure
Crenation
The O in
o
hyp tonic
14
Filtration
• smaller molecules are forced through porous membranes
• separates large from small molecules
• hydrostatic pressure; important in fluid movement
• molecules leaving blood capillaries
Think
‘sprinkler hose’
Figure from: Hole’s Human A&P, 12th edition, 2010
15
Active Transport
• carrier molecules transport substances across a membrane from regions of
lower solute concentration to regions of higher concentration, i.e., against a
concentration gradient
• sugars, amino acids, sodium ions, potassium ions, etc.
Active transport
is a physiological
process since it
requires cellular
energy, e.g., ATP
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Figure From: Marieb & Hoehn, Human Anatomy & Physiology, 9th ed., Pearson
Endocytosis
• cell engulfs a substance by forming a vesicle around the
substance
• three types
• pinocytosis – substance is mostly water
• phagocytosis – substance is a solid
• receptor-mediated endocytosis – requires the
substance to bind to a membrane-bound receptor
Figure from: Hole’s Human A&P, 12th edition, 2010
17
Endocytosis
Figures from: Hole’s Human A&P, 12th edition, 2010
18
Exocytosis
• reverse of endocytosis
• substances in a vesicle fuse with cell membrane
• contents released outside the cell
• release of neurotransmitters from nerve cells
Figure from: Hole’s Human A&P, 12th edition, 2010
19
Transmembrane Potential
Dependent upon active transport
Cells maintain a
transmembrane
potential, with
the inside of the
cell membrane
being slightly
negative relative
to the outside
Figure from: Martini, Anatomy & Physiology, Prentice Hall, 2001
22
Lecture Review
TRANSPORT
PROCESS
IS
ENERGY
NEEDED?
CONCENTRATION
GRADIENT
GENERAL
DESCRIPTION
EXAMPLE
IN
HUMANS
SIGNIFICANCE
SIMPLE
DIFFUSION
NO
[HIGH]
TO
[LOW]
spreading out of
molecules to
equilibrium
O2 into cells; CO2
out of cells.
Cellular
Respiration
FACILITATED
DIFFUSION
NO
[HIGH]
TO
[LOW]
Using a special
cm carrier protein
to move
something
through the cell
membrane (cm)
Process by which
glucose enters
cells
OSMOSIS
NO
[HIGH]
TO
[LOW]
water moving
through the cm to
dilute a solute
maintenance
of osmotic
pressure of 0.9%.
Same
FILTRATION
NO
[HIGH]
TO
[LOW]
using pressure to
push something
through a cm
(sprinkler hose)
manner in which
the kidney filters
things from blood
removal of
metabolic wastes
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Lecture Review
TRANSPORT
PROCESS
IS
ENERGY
NEEDED?
CONCENTRATION
GRADIENT
GENERAL
DESCRIPTION
EXAMPLE
IN
HUMANS
ACTIVE
TRANSPORT
YES
[LOW]
TO
[HIGH]
opposite of
diffusion at
the expense
of energy
K+-Na+-ATPase
pump
maintenance of the
resting
membrane
potential
ENDOCYTOSIS
YES
[LOW]
TO
[HIGH]
bringing a
substance
into the cell
that is too
large to
enter by
any of the
above
ways;
Phagocytosi: cell
eating;
Pinocytosis: cell
drinking.
Phagocytosed
(foreign)
particles
fuse with
lysosomes
to be
destroyed
help fight infection
EXOCYTOSIS
YES
[LOW]
TO
[HIGH]
expelling a
substance
from the
cell into
ECF
Exporting
proteins;
dumping
waste
Same
SIGNIFICANCE
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