The cell membrane is semi-permeable:
Certain substances can move across,
while others cannot.
Transport Across Cell
Membranes
Section 2.2
Transport across cell membranes can be
passive or active:
Passive transport:
• substances move down
their concentration gradients
• energy is not required
Active transport:
• substances move against
their concentration gradients
• energy is required
Passive transport
Down the concentration gradient
Passive Transport
• Substances move down their concentration gradients.
• Diffusion occurs until the concentration is equal
throughout.
• Three types:
• simple diffusion
• osmosis
• facilitated diffusion
http://highered.mcgrawhill.com/sites/0072495855/student_vie
w0/chapter2/animation__how_diffusion
_works.html
Against the concentration gradient
Active transport
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Simple Diffusion
• Particles move across the membrane from an area of high
to low concentration, until equilibrium is achieved
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Factors affecting rate
of diffusion through
membrane:
• molecule size
• molecule polarity
• molecule or ion charge
Other factors (non-specific):
• temperature
• pressure
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Facilitated diffusion
• “Facilitated”: assisted
• polar molecules or ions diffuse through integral
proteins embedded in the membrane
• allows them to pass through the hydrophobic
membrane
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Proteins can be channels
or carriers:
• Channel –
molecule/ion just
passes through (like a
tunnel)
• Carrier – molecule/ion
temporarily binds to
the protein for
transportation
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Channel Proteins
• tubular shape
• forms a passage for small molecules or ions
• some channels are gated
• gated – opens only in response to external signals
(chemical, tactile, electrical)
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Example: Aquaporins
(water channels)
• water is small, but is polar
• simple diffusion alone can’t
account for observed rates of
osmosis across cell membranes
• it diffuses through channel proteins
called aquaporins
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Carrier Proteins
• the polar molecule to be transported binds to the protein on
one side of the membrane
• binding causes the protein to change its shape
• the molecule is released on the other side
Osmosis
• Movement of free water
molecules from areas of high to
low concentration.
• Concentration of “free” water
molecules is directly related to
solute concentration.
Example:
glucose
transporter
How carrier
proteins work:
http://highered.
mcgrawhill.com/sites/00
72495855/stude
nt_view0/chapte
r2/animation__h
ow_facilitated_d
iffusion_works.h
tml
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Side B contains solute (Na+ ions).
Many water molecules are associated
with each ion, and are not “free”
Glucose binds
to binding site
open to outside
Transport protein
shifts to alternative
shape
Glucose is released
and protein returns to
original shape
Side A has a higher concentration of
free water molecules.
Water will diffuse from
Side A to Side B.
Side A
Lower:
Higher:
Hypotonic
Hypertonic
Higher:
Lower:
Solute concentration
Free water
concentration
Side B
Higher osmotic pressure Lower osmotic pressure
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Osmosis will continue until solute concentrations are equal.
At this point, the solutions are isotonic to one another.
iso = equal
Water diffuses:
• from hypotonic  hypertonic
• from high osmotic P  low osmotic P
How osmosis works:
http://highered.mcgrawhill.com/sites/0072495855/stude
nt_view0/chapter2/animation__h
ow_osmosis_works.html
Passive transport: simple diffusion ● facilitated diffusion ● osmosis
Biological significance
• Animal cells have no walls
• Placing cells in hypotonic or
hypertonic solutions will cause
osmosis in/out of the cell
• water in  bursting
• water out  shriveling
For plant cells…
You may be familiar with what
happens when you pour salt on
an earthworm… can you
explain it in terms of solute
concentration and osmosis?
Learning Checkpoint
In hypotonic solution:
• Plants’ cell wall prevents them from bursting.
In hypertonic solution:
• The wall can’t protect the cell from shrinking
(visible as wilting, then death)
Homework
pg. 74 #25-30
• Cell membranes are semi-permeable.
• permeability to a solute is determined by two factors:
polarity and size
• Two types of transport occur across cell membranes:
passive and active.
• Passive transport does not require energy input.
• It occurs when substances move along their concentration
gradients, as is their natural tendency.
• includes diffusion (solutes) and osmosis (water)
• If solutes cannot diffuse across membranes, water
molecules will move, in order to balance out solute
concentration.
Active Transport
• substances are transported against their concentration
gradients (low concentration to high)
• requires input of energy
Why move against the gradient? It is often important to
cellular functioning to maintain differing concentrations of
molecules or ions inside compared with outside.
ATP is the molecule used to release energy, in a cell
• adenosine + three phosphate groups
• hydrolysis of the terminal (last) phosphate releases energy
Two types of active transport: primary and secondary
• Primary – ATP is directly used
• Secondary – ATP is indirectly used
ATP
+
H2O 
adenosine
triphosphate
ADP +
adenosine
diphosphate
Pi
+
energy
phosphate
(PO43-)
Active transport: primary ● secondary
Active transport: primary ● secondary
Sodium-Potassium Pump
verb of hydrolysis
Primary Active Transport
• a transporter protein hydrolyzes ATP
• ATP’s released energy is used to directly power transport
through the protein
Example:
Sodium-potassium pump
ECF:
High [Na+]
(ICF)
When a cell is at rest...
[Na+] is high outside the cell
[K+] is high inside the cell
The Na/K pump is responsible for
these concentration gradients:
ICF:
High [K+]
pumps out 3 Na + for every 2 K+ in
Active transport: primary ● secondary
Active transport: primary ● secondary
Secondary Active Transport
• Step 1: ATP is used to transport ions against their gradients
(via primary active transport)
• Step 2: The build-up of ions on one side produces a build-up
of electric charge (potential) in the form of an
electrochemical gradient.
Chemical:
Difference in
concentration
Electrical:
Difference
in charge
(since ions are charged)
Step 3: This electrochemical gradient is a store of potential energy.
The energy can be released to drive secondary transport.



Example: H+/sucrose pump
1) Primary active transport:
H+ is pumped out of the cell
2) Electrochemical gradient:
+ charge builds up outside of cell
3) Secondary active transport:
H+ diffusing along its gradient
provides the energy to
transport sucrose into the cell,
against its gradient
Summary of molecule/ion transport:
Passive
▫ simple diffusion (no protein)
▫ facilitated diffusion (channel or carrier)
▫ osmosis (aquaporin channel)
Learning Checkpoint
Active
▫ primary (ATP)
▫ secondary (electrochemical
Homework
pg. 77 #31-36
• Active transport requires an input of energy.
• Molecules or ions are transported against their
concentration gradients.
gradient)
• Primary active transport uses ATP as a direct source of
energy.
• Secondary active transport uses ATP as an indirect
source of energy.
• ATP is used to actively pump ions against their gradients.
• The resulting electrochemical gradient is used to actively
transport other molecules.
Membrane-Assisted Transport uses
vesicles to transport things in and out.
• uses energy to move large, complex molecules across
the cell membrane
• proteins, polysaccharides, nucleic acids, bacteria
In exocytosis,
molecules are transported out of the cell.
• vesicle fuses with membrane
• contents are expelled into ECF
• release enzymes, hormones, proteins, and other cell products for
use elsewhere in the body
• secretion of neurotransmitters
• expel cellular waste
• substances are
packaged into vesicles
and sent out/into cell
Endocytosis: Brings things in
Exocytosis: Sends things out
In endocytosis,
molecules and liquids are brought into the cell.
a) Phagocytosis (“cell eating”):
Membrane wraps pseudopodia around a particle; membrane
folds inwards and forms an interior vesicle
Examples:
• unicellular organisms eat this way
• immunity in white blood cells
b) Pinocytosis (“cell drinking”):
•
•
•
same as phagocytosis, but for liquids and dissolved particles
happening continuously
allows cells to maintain water content, and internally recycle
“lost” phospholipids that have joined membrane during
exocytosis
c) Receptor-mediated
endocytosis:
• form of endocytosis that is
specific for certain molecules
• molecules outside cell bind
with receptors on cell
membrane surface 
stimulates endocytosis
Summary
• Transport across cell membrane can be passive or active.
• Passive – Moves substances down their concentration
gradients; no energy is required
• Active – Move substances against their concentration
gradients; energy is required
• inner surface of “pit” is
coated with a layer of a
protein called clathrin
• clathrin stabilizes the
outside of the vesicle
• Three types of passive transport:
• simple diffusion – small solutes
• facilitated diffusion through carriers or channels – ions,
small polar molecules
• osmosis – diffusion of water through channels (aquaporins)
• Two types of active transport:
• primary active transport – Hydrolysis of ATP directly powers
transport against a concentration gradient
• secondary active transport – An electrochemical gradient
(established by primary transport) powers transport of
another substance
Homework
• Read 2.2 – Membrane Transport
• pg. 74 #25-30 (Passive)
• pg. 77 #31-36 (Active)
• pg. 81 #3, 4, 6, 8, 9-12 (Overview)
• Membrane-assisted transport moves large, bulky
substances across the cell membrane
• Exocytosis – Expels substances from the cell
• Endocytosis – Brings things in
• Phagocytosis – Large particles are engulfed
• Pinocytosis – ECF and dissolved solutes are brought into cell
• Receptor-mediated endocytosis – Specific molecules bind to
membrane receptors, and trigger endocytosis