Unit 8 Cell Transport
• In this unit, we observe the careful and purposeful
transport of substances in and out of the cell.
Cell Membrane - Review
• Remember that the membrane of a cell is made up
of lipids.
• These lipids have a charged “head” and a neutral
“tail”.
• The charged head likes being in contact with water
(hydrophilic).
• The neutral tails dislike water (hydrophobic).
• This causes the lipids to form a bilayer.
Lipid bilayer
• The hydrophilic parts form the outside and inside of
the bilayer, with a hydrophobic core.
Crossing the Bilayer
• Crossing the bilayer is difficult.
• Any hydrophobic chemical will not like the
environment near the head groups of the lipids.
• Any hydrophilic chemical will not like the
environment near the tails of the lipids.
• Basically, few chemicals like the two environments
they need to pass through to get across the
membrane.
Permeability
• If a barrier does not let anything through, it is
impermeable.
• Barriers that let anything pass through are
permeable.
• Barriers that only allow certain particles through are
selectively permeable. The cell membrane is this
type.
Passive Transport
• The most basic way a
substance can cross
the membrane is by
simple diffusion.
• In this case, small
molecules are able to
slip in between the
gaps and get through.
Simple Diffusion
• This is a passive process
– the cell spends no
energy during simple
diffusion.
• Oxygen, CO2, water
and small molecules
like ethanol can diffuse
unaided.
Simple Diffusion - Factors
• The size/mass of a
substance affects
permeability. Small
molecules like oxygen
are small enough to
sneak through the gaps
in the lipids.
• Smaller particles also
move quicker and with
more energy, allowing
them to plow through.
Simple Diffusion - Factors
• The shape of molecules
affects diffusion.
• A flat, bulky mole like
cholesterol would have
a much tougher time
than the small,
condensed carbon
dioxide molecule.
Simple Diffusion - Factors
• Temperature will also
affect diffusion.
• Molecules move faster
at higher temperatures,
allowing the substance
to have more energy
to “push through” the
membrane.
Simple Diffusion - Factors
• At the same time, the
membrane becomes
more fluid at higher
temperatures, allowing
an easier trek through
the bilayer.
Simple Diffusion - Factors
• Remember that the
inside of the bilayer is
neutral and
hydrophobic.
• Highly charged
particles (potassium,
calcium, sodium) will
not readily diffuse
through.
Simple Diffusion - Factors
• The distance to cross
affects diffusion.
• If the lipids have longer
“tails” then diffusion is
harder.
• Shorter “tails” would
allow easier diffusion.
Simple Diffusion - Factors
• Diffusion is hindered
during pneumonia.
Mucous covers the
surface of the lung
cells, creating a
greater distance from
the interior of the cell
and the air in the lungs.
• This makes breathing
less effective.
Simple Diffusion - Factors
• The amount of surface
area a cell has will
affect how fast
substances can diffuse
in/out of it.
• High surface area cells,
like the highly ciliated
cells of the lungs and
intestines maximize SA
to absorb as much as
possible.
Simple Diffusion - Factors
• A large driving force for
simple diffusion is the
concentration
gradient.
• This is the ratio of a
substance outside the
cell (extracellular)
versus inside
(cytoplasm).
Simple Diffusion - Factors
• Substances will always
move down their
concentration
gradient.
• This means from the
more “crowded” side
to the more “roomy”
side.
Simple Diffusion - Factors
• Substances will try and
reach a balance –
equilibrium – where
there is equal
concentrations on the
outside and inside of a
cell.
• Each substance will
reach its own
equilibrium.
Simple Diffusion - Factors
• The greater the
difference in
concentration on each
side of the membrane,
the stronger the driving
force of diffusion.
Simple Diffusion - Factors
• Simple diffusion cannot
force a substance to
go against its
concentration gradient
as it does not consume
any energy.
• Energy must be spent
to create a
concentration deficit.
Facilitated Diffusion
• Certain proteins in the
membrane help
substances cross over.
• This allows substances
that normally cannot
get through the bilayer
to cross over.
Facilitated Diffusion
• Facilitated diffusion
can be done with a
channel protein.
• This protein form a
specifically
sized/charged path
through the bilayer
allowing substances
through.
• These may have a gate
to keep them open or
closed.
Facilitated Diffusion
• Alternatively, a carrier
protein may bind or
envelop a substance,
and then later release
it on the other side.
• Facilitated Diffusion is
still passive transport –
substances will NOT go
against their
concentration
gradient.
Facilitated Diffusion
• A channel protein
designed to allow
water to easily flow
between the
membrane is
Aquaporin.
• Certain immune cells
will implant these in the
cell membranes of
invading bacteria,
killing them.
Active Transport
• Remember that passive
transport cannot go
against the
concentration gradient
of a substance.
• In order to create a
concentration
gradient, the cell must
spend energy.
Active Transport
• ATP is a high-energy
molecule where cells
store energy.
• Primary active transport
membrane proteins
consume ATP’s energy
to force substances
across the membrane.
Active Transport
• The sodium-potassium
pump membrane
protein uses ATP to
move both sodium and
potassium against their
concentration
Gradients.
• It first pumps 3 sodium
out of the cell, then
deposits 2 potassium
into the cell, burning
one ATP along the
way.
Active Transport
• By regulating the
number of transporters
on a cell’s membrane,
the cell can control the
rate of transport.
• Because of the many
variables influencing
secondary transporters,
they can be very finely
regulated.
Osmosis
• Water moves in osmosis
much like the dissolved
substances do in
diffusion.
• If there is a difference
in the amount of water
on each side of a
membrane, water will
move from an area
with lots of water to
areas with less water.
Osmosis
• An area with lots of
dissolved substances in
it is hypertonic. This also
means it has little water
in it.
• An area where there is
low amounts of
dissolved substances is
hypotonic. This means it
was lots of water in it.
Osmosis
• If water can cross the
membrane, it will go
from an area with lots
of water (hypotonic) to
an area with less water
in it (hypertonic.
• The two will even out
and become isotonic.
Osmosis
• Controlling osmosis is
very important for cells
to do to survive.
• If a cell were to lose or
gain to much water, it
would be very
damaging.
Osmosis
• If a cell takes on too
much water (if the cell
is in an hypotonic
environment), it make
eventually leak or
break open from all the
water pressure built up
inside.
Osmosis
• If a cell is in a
hypertonic
environment, it will lose
too much water to the
outside. This will cause
the cell to shrink and
shrivel. The cell may dry
out too much and not
have the needed
water for the cell to
work.
Osmosis
• In any case, plants are able to cope with some
changes by losing or storing water in the vacuole.
• However, many animals are not able to tolerate
tonicity changes. Freshwater fish may die when
placed in seawater and vice versa.
• Species that spend time in both kinds of water will
spend some time in a middle-range body of water
to acclimate themselves to the new environment.
Vesicles
• Vesicles are small
membrane-bound sacs
In the cell that are used
for a variety of
purposes.
• When a portion of a
cell membrane is uses
to take something into
the cell, it is
endocytosis.
Vesicles
• Endocytosis has many
functions.
o Absorption of
released vesicles
from other cells
o Engulfing of
foodstuff for
digestion.
Pinocytosis
• Pinocytosis occurs
when cells simply take
in a small potion of
extracellular fluid into
the cell. Once inside, it
is merged with a
Lysosome to sterilize
and break down
anything useful that
was taken in.
Phagocytosis
• Phagocytosis is a
special type of
endocytosis where
receptors on a cell
specifically bind to the
outer walls of bacteria
or other invaders to
engulf it.
• This is how many small
organisms feed.
Phagocytosis
• After enough receptors
bind the invader, it is
taken into the cell in a
vesicle.
• The vesicle combines
with a lysosome and
the invader is
destroyed.
Vesicles
• Well cells expel a
vesicle, it is called
exocytosis.
• Unicellular organisms
may excrete wastes by
exocytosis.
• Fungi can use
exocytosis to release
digestive juices onto its
food.
Vesicles
• Exocytosis is important
in the nervous system.
Neurotransmitters are
released from one
nerve cell by
exocytosis, and then
detected by the next.