In This Lesson:
Cell Membranes
and Transport
(Lesson 4 of 5)
Today is Tuesday,
th
October 20 , 2015
Stuff You Need:
Guided
Reading On Your
Desk
Pre-Class:
Have a seat. When your partner arrives, say hi, ask how he/she
is doing, and then tell them all about the cell membrane.
Anything you recall.
This works even better if you don’t tell them why you’re doing it.
Today’s Agenda
• The Cell Membrane
– Structure
– Function
• You know…osmosis? Diffusion?
• It’s gonna get “insane in the membrane…”
• Where is this in my book?
– Chapter 7
By the end of this lesson…
• You should be able to describe in detail the
structure of the cell membrane and link it to
its functions.
• You should be able to predict the outcome of
an osmotic process.
• You should be able to use the water potential
equation.
So now then…
• Okay, you giant piles of cells you…
• Where do we go from here?
• Well, let’s think about our last two units:
– Evolution turned into speciation when we
considered the “interactive” perspective.
– Ecology turned into community ecology when we
considered the “interactive” perspective.
– Cells are going to be turning into cell membranes
and transport – it’s what allows multicellular
organisms to be so darn interesting.
• And yes, it’s how cells interact with their environments.
Cell Membrane Structure Overview
• The cell membrane is around 8 nm thick.
– For perspective, the thickness of human hair is around
99,000 nm.
• It’s composed of:
– Lipids
• Mainly phospholipids and some cholesterol.
– Carbohydrates
• Signal molecules attached to…
– Proteins
• Embedded in the membrane.
• How was it discovered?
– TED: Ethan Perlstein – Insights into Cell Membranes Via Dish
Detergent
The Phospholipid Bilayer
• The cell membrane is primarily
composed of phospholipids.
• The cell is sitting in a water-based
environment, therefore:
– Each phospholipid has a polar
(hydrophilic) head…
Polar
• It’s a phosphate group.
– …and a non-polar (hydrophobic)
pair of tails.
• They’re fatty acids.
• Because it has hydrophilic and
hydrophobic parts, it’s
amphipathic.
– Note: Amphoteric = acid/base.
Different.
Nonpolar
The Phospholipid Bilayer
• These phospholipids are
then arranged in a bilayer.
– Key: Don’t confuse a bilayer
for a “double membrane.”
– A bilayer is one membrane
with two layers.
Polar
Nonpolar
Aside: Soap
• Ever wonder how soap works?
• Soap, like the cell membrane, is amphipathic.
• Unlike the membrane, soap molecules form a
micelle (basically a phospholipid monolayer).
• Because it’s got a lot of non-polar regions,
soap can dissolve cell membranes of other
cells.
– Like pathogens’.
http://0.tqn.com/y/chemistry/1/S/L/5/1/micelle.jpg
The Phospholipid Bilayer
• The bilayer acts as a semi-permeable barrier.
• Polar molecules can’t get in or out.
Sugar
Polar
Heads
H2O
Salt
NonPolar
Tails
Polar
Heads
Lipids
Waste
Quick Note: Permeability
• Some things are “impermeable:”
– Raincoats, balloons, brick walls.
• Some things are “permeable:”
– Air, water.
• Some things are “semi-permeable:”
– Nets, gates, cell membranes.
• Semi-permeability is sometimes called selective
permeability.
Back to the Phospholipid Bilayer
• Importantly, the composition of
the bilayer is not constant.
• A certain percentage is composed
of phospholipids with unsaturated
fatty acid tails; the rest with
saturated tails.
– Unsaturated hydrocarbons lead to
increased fluidity.
• The lower the temperature, the more
unsaturated the membrane needs to
be to prevent freezing.
– Cholesterol is also in the membrane
and acts to increase viscosity except
at low temperatures.
The Phospholipid Bilayer
• And why the fluidity? To allow for movement of
embedded membrane proteins.
• This view of the membrane is the Fluid Mosaic
Model:
Another View
Membrane Proteins
• Membrane proteins provide the bulk of the cellspecific (or organelle-specific) functions.
• There are two main types:
– Peripheral Proteins
• They’re stuck to the outside of the cell.
• Example: Antigens (cell markers)
– Integral Proteins
• They’re stuck within and usually span the membrane.
• Example: Transmembrane Proteins or Transport Proteins
So why proteins?
• What do you see in the
picture to the right?
• What are the blue things
with two tails?
Polar areas
of protein
– Phospholipids
• What’s the yellow thing
wedged in there?
– Cholesterol
• What are the red squiggly
lines?
– -Helices
Non-polar areas of protein
So why proteins?
• Remember how amino
acids can be polar or
non-polar?
• That makes proteins (also
amphipathic) a great
candidate for
transmembrane proteins.
Polar areas
of protein
– The hydrophobic regions
act as anchors to the
membrane.
Non-polar areas of protein
Fluid Mosaic Model
• Those anchors are needed because the
phospholipids move frequently.
– It’s the Fluid Mosaic Model, remember?
• Studies of hybrid cell membranes made of a
combination of human and mouse cells confirmed
this:
What do they do?
Example:
Channel
Protein
Signal
Transduction
Protein
Enzymes
Cell Surface
Receptor
Cell-Cell
Recognition
Cell
Cohesion
Attachment to
cytoskeleton
Example:
Antigen
Transport
Cell Surface Proteins
• Cell surface proteins play a key
role in recognition between
cells.
– This aids in development of
organs and tissues.
• Antigens are proteins on the
cell surface that cause a
response from the immune
system.
– They’re how the body “rejects”
cells that are foreign.
Cell Surface Proteins
• Take a look at the image to the
right. See those two orangey
things?
– They’re carbohydrate chains.
• One’s coming from a lipid,
making it a glycolipid.
• The other is coming from a
protein, making it a
glycoprotein.
• These carbohydrate chains make
the cell identifiable to other
cells.
Cell Membrane Function Overview
• Cells must take in and release substances:
– Food in, products and waste out.
• They can do it with one of two general modes:
– Passive Transport (does not require energy)
• Diffusion
• Facilitated Diffusion
• Osmosis
– Active Transport (requires energy)
• Endocytosis
• Exocytosis
• Molecular Transport
• To fully understand these, we need to understand
concentration gradient.
Concentration Gradient
• Concentration refers to the amount of a substance in
a certain area.
• Particles diffuse down their concentration gradient.
– What does that mean?
• In passive transport, particles always go from an
area of high concentration to an area of low
concentration.
• Fun Fact: Passive transport occurs in part to satisfy
the second law of thermodynamics, AKA entropy.
Concentration Gradient
High
Warning:
Steep
Grade
Low
Concentration Gradient
High
Concentration
In Passive Transport, particles
move from areas of high
concentration to areas of low
concentration.
Low
Concentration
Diffusion Demo
• Diffusion in Air
What can diffuse?
• Can diffuse through the membrane:
– Lipids
– CO2
– O2
• Can’t diffuse through the membrane:
– H2O and other polar molecules
– Ions and other charged particles
– Large molecules (like starches and proteins)
Facilitated Diffusion
• Simply put, it’s diffusion
with help.
• Those particles that can’t
diffuse can get through
channel proteins.
• No energy needed.
• This leads to semipermeability for molecules
that can’t otherwise
diffuse.
HIGH
LOW
inside cell
– There are specific channels
for specific molecules, too.
NH3
salt
H2O
aa
sugar
outside cell
Summary of Passive Transport
Osmosis
• Osmosis is basically the same thing as diffusion,
only with water molecules and some form of a
barrier.
– Osmosis is another form of passive transport.
• Just like in diffusion, in osmosis, water moves
from areas of high water concentration to low
water concentration.
• Or, water moves from areas of low solute
concentration to areas of high solute
concentration.
Osmosis
• Which drink has more liquid in it?
Drink A
Drink B
Osmosis in a U-Tube
Side A
Side B
Which side has more water on it?
http://www.biologycorner.com/resources/osmosis.jpg
Tonicity
• Hypertonic solution
– Relatively more solute than surroundings.
– Relatively less free water than surroundings.
• Free water is water not busy hydrating a dissolved solute.
• Hypotonic solution
– Relatively less solute than surroundings.
– Relatively more free water than surroundings.
• Isotonic solution
– The same amount of solute as the surroundings.
• No net water change.
Isotonic Solutions
• Water does not experience a net
movement in isotonic solutions.
– There is no concentration gradient.
No concentration gradient
No net movement of water
And now, I present to you…
• …the key to EVERYTHING!!!!!!*
–
*osmosis-related.
• Draw this in your notebook. Make it BIG.
Hypotonic
H2O Flow
Hypertonic
Osmosis in Plant Cells
• As we have learned, plant cells are good at
holding water.
• If they’re placed in a hypertonic solution,
however, they lose water and wilt.
– Their cells undergo plasmolysis.
• Place them in a hypotonic solution and they will
swell slightly, like a garden hose with water.
– Their cells become turgid.
– In animal cells, without a cell wall, the cell may burst
in a process called cytolysis.
Osmosis – The Big Idea
http://upload.wikimedia.org/wikipedia/commons/thumb/a/ab/Turgor_pressure_on_plant_cells_diagram.svg/2000pxTurgor_pressure_on_plant_cells_diagram.svg.png
Osmosis – The Big Idea
Blood hypertonic,
Blood hypotonic,
surroundings
surroundings
hypotonic
hypertonic
Isotonic solutions
http://upload.wikimedia.org/wikipedia/commons/thumb/e/e3/Erythrozyten_und_Osmotischer_Druck.svg/450px-Erythrozyten_und_Osmotischer_Druck.svg.png
Managing Water Balance
• Animals:
– Kidneys.
– Methods to either remove
salt or pump in water.
• Unicellular Organisms:
– Contractile Vacuoles
• Pump water out at a cost of
ATP (energy).
• Maintaining water balance
is just another aspect of
homeostasis.
Osmosis in Kidneys
http://classes.midlandstech.edu/carterp/Courses/bio211/chap25/Slide18.GIF
Osmosis in Kidneys
• The proximal Loop of Henle is the part of the
nephron (kidney component) responsible for
re-absorbing water from urine.
• With this in mind, would you guess that desert
animals have larger or smaller Loops of Henle
than other animals?
Osmosis in Kidneys
http://www.answersingenesis.org/assets/images/articles/cm/v26/i3/rats.jpg
Osmosis in
Merriam’s Kangaroo
Rats
http://www.bio.davidson.edu/Courses/anphys/1999/Chisholm/nephron1copy.wc2.jpg
Osmosis Supplements
• Egg Osmosis
• Gummi Bear Osmosis
• Woman Dies After Water Drinking Contest
• CrashCourse – In Da Club – Membranes and
Transport
Stop everything, you liar!
• “You said at the beginning of this PowerPoint that
polar substances like H2O can’t diffuse into the
cell through the membrane, yet now you’re
talking about osmosis being like water diffusion.
How could that be?” said the student.
• “For a while scientists noticed the same thing.
Water clearly efficiently enters a cell, but how?”
replied the teacher, quietly appreciative of his
student’s skepticism.
Aquaporins
Roderick
MacKinnon
Rockefeller
Peter Agre
Johns Hopkins
• Aquaporins are channel proteins that
move water rapidly into the cell through
facilitated diffusion.
– They were discovered by these two in 1991.
– They shared the 2003 Nobel Prize in
Chemistry.
Equilibrium
• For things like diffusion and osmosis,
eventually the solutes reach a point where
there is no net change in molecule movement.
– This is equilibrium.
• We call it “dynamic equilibrium” because the
molecules are still moving, but there is no net
change in concentration or movement.
Equilibrium
• When dynamic equilibrium is reached,
diffusion and osmosis stop.
– Molecular motion continues, though.
Net Water Flow Inward
No Net Water Flow
WATER
WATER
0.50%
Sugar
1.0%
Sugar
WATER
0.75%
Sugar
0.75%
Sugar
WATER
Osmosis Practice Problems
• We’re going to talk about water potential
soon, but for now, let’s get the concept of
osmotic movement under our belts.
• Following this slide are four osmosis practice
problems, all multiple choice.
• Get them all correct on the whiteboards and,
uh…
– …um…
• 
Osmosis Practice Problem SAMPLE
• Suppose a human blood cell (saline concentration
0.9%) is sitting in a beaker of 2% NaCl. Will it
shrink, expand, or remain unchanged?
– Make a sketch!
Hyper
Hypo
2%
0.9%
The blood
cell will
shrink.
Osmosis Practice Problem #1
• If you soak your hands in dishwater, you
may notice that your skin absorbs water
and swells into wrinkles. This is because
your skin cells are _______________ to the
_______________ dishwater.
A.
B.
C.
D.
E.
hypotonic…hypertonic
hypertonic…hypotonic
hypotonic…hypotonic
isotonic…hypotonic
hypertonic…isotonic
Osmosis Practice Problem #1
• If you soak your hands in dishwater, you
may notice that your skin absorbs water
and swells into wrinkles. This is because
your skin cells are _______________ to the
_______________ dishwater.
A.
B.
C.
D.
E.
hypotonic…hypertonic
hypertonic…hypotonic
hypotonic…hypotonic
isotonic…hypotonic
hypertonic…isotonic
Osmosis Practice Problem #2
• You decide to buy a new fish for your
freshwater aquarium. When you introduce
the fish into its new tank, the fish swells up
and dies. You later learn that it was a fish
from the ocean.
Osmosis Practice Problem #2
• Based on what you know of tonicity, the most
likely explanation is that the unfortunate fish
went from a(n) _______________ solution
into a(n) _______________ solution.
A.
B.
C.
D.
E.
isotonic…hypotonic
hypertonic…isotonic
hypotonic…hypertonic
hypotonic…isotonic
isotonic…hypertonic
Osmosis Practice Problem #2
• Based on what you know of tonicity, the most
likely explanation is that the unfortunate fish
went from a(n) _______________ solution
into a(n) _______________ solution.
A.
B.
C.
D.
E.
isotonic…hypotonic
hypertonic…isotonic
hypotonic…hypertonic
hypotonic…isotonic
isotonic…hypertonic
Osmosis Practice Problem #3
• In osmosis, water always moves toward the
____ solution: that is, toward the solution
with the ____ solute concentration.
A.
B.
C.
D.
E.
isotonic…greater
hypertonic…greater
hypertonic…lesser
hypotonic…greater
hypotonic…lesser
Osmosis Practice Problem #3
• In osmosis, water always moves toward the
____ solution: that is, toward the solution
with the ____ solute concentration.
A.
B.
C.
D.
E.
isotonic…greater
hypertonic…greater
hypertonic…lesser
hypotonic…greater
hypotonic…lesser
Osmosis Practice Problem #4
• The concentration of solutes in a red blood cell is
about 2%. Sucrose cannot pass through the
membrane, but water and urea can. Osmosis
would cause red blood cells to shrink the most
when immersed in which of the following
solutions?
A.
B.
C.
D.
E.
a hypertonic sucrose solution
a hypotonic sucrose solution
a hypertonic urea solution
a hypotonic urea solution
pure water
Osmosis Practice Problem #4
• The concentration of solutes in a red blood cell is
about 2%. Sucrose cannot pass through the
membrane, but water and urea can. Osmosis
would cause red blood cells to shrink the most
when immersed in which of the following
solutions?
A.
B.
C.
D.
E.
a hypertonic sucrose solution
a hypotonic sucrose solution
a hypertonic urea solution
a hypotonic urea solution
pure water
Water Potential
• Last thing before the lab: Water potential.
– Yes, it has to do with potential energy.
• Water flows toward the hypertonic solution.
– Or…
• Water flows toward the lower water potential
value ().
• Let’s discuss this further.
– Heads-up: In your book, water potential is discussed
in Chapter 36, pages 782–785.
Water Potential Conceptual View
+, +
+ and
The
nearby
waterin
molecules
create
a hydration
shell
around
the
Na
Suppose
you side,
have
aNaCl,
semi-permeable
membrane
separating
some
water.
You
dissolve
some
whichhas
dissociates
into Na
Cl-()
(just
the
Na
So
the
right
this case,
a lower
water
potential
than
the
decreasing
amountshown).
ofand
free
water
in
thatexcept
location.
membrane
impermeable
to
everything
water. it.
leftThe
side,
making the
itishypertonic,
making
water
flow
toward
H H
O
Na+
left >  right
Water Potential Units
• Water potential is measured in units called
megapascals (MPa) or (more commonly) bars.
– Remember kPa from chemistry? Kilopascals?
• Out of curiosity: 1 MPa ≈ 1000 kPa
• 1 MPa = 10 bars
• 1 bar ≈ standard atmospheric pressure
• For perspective, the pressure of a plant cell is 0.5
MPa, or about twice as much as a car tire’s air
pressure.
Water Potential Equation
• Water potential has two components:
– Solute Concentration (S – Solute Potential)
– Physical Pressure (P – Pressure Potential)
• Together, they make up the water potential
equation:
•  = S + P
Water Potential Forces
• Think of water as being able to be pushed away
from an area or to be pulled toward an area.
– For example, dissolved solutes in an area pull the water
toward that area.
– Since water always moves toward the lowest water
potential area, solutes make  go down.
• S, which is solute potential, is thus familiar:
– The more solute in an area, the greater the pull to bring
water near, the lower the value of S.
– You can almost think of S as a “pull magnitude.”
Solute Potential: S
• Pure water has a S of 0.
– Makes sense: no dissolves solutes in the water,
therefore no pull.
• Key: Solute potential (S) is always either 0
or negative. More negative = more pull.
– It can’t be positive. There’s no way to dissolve
particles and have water go away from them.
Solute Potential: S
• Solute potential is formally calculated this way:
• S = -iCRT
– C is concentration in molarity (M).
– R is the ideal gas constant:
• 0.0831 liter  bars / mol  K (it’s given).
– T is the temperature in Kelvin.
• K = °C + 273
– i is the ionization constant.
• This one needs some more explanation, starting with a
diagram.
Dissociation
Bound ions in…
…component ions out.
Ca
Cl
Cl
Cl-
Ca2+
Cl-
S = -iCRT
• Ionic compounds break down into their components in
solution.
• Let’s say you put a mole of NaCl into water.
– It’ll dissociate into one mole of Na+ and one mole of Cl-, or
two moles total.
• The ionization constant (i) is 2.
• Let’s say you put a mole of C12H22O11 (sucrose) into
water.
– It won’t dissociate. It stays one mole of C12H22O11.
• The ionization constant (i) is 1.
• Key: i = how many particles into which a solute
breaks.
Water Potential Forces
• P is the pressure potential and it’s a little
different – there’s no direct formula for it.
• In the same way S = 0 for pure water, P = 0 for
normal atmospheric pressure in an open
container.
• However, P can be positive or negative.
– Positive would be like squeezing a water balloon –
you’re pushing water away.
– Negative would be like sucking on a straw – you’re
pulling water closer.
Pressure Potential: P
• When would P be negative?
– Perhaps in plants, when transpiration at the leaf
draws water up in a column through the plant
stem.
• When would P be positive?
– Perhaps in plants, when the cell becomes so
turgid and swollen that the cell wall begins to
push back down.
Water Potential Summary
• Water moves from high water potential areas to low
water potential areas. So:
– Relatively high water potential = hypotonic.
– Relatively low water potential = hypertonic.
• S is the solute potential and gets lower as solute
concentration rises. It’s never positive.
• P is the pressure potential and is (+) for pushing
events and (-) for pulling events. (0) for neutral,
isotonic events.
• Key: Most problems will require you to find 
outside and  inside, then compare.
Water Potential Practice Problem
• A cell in an open beaker is in equilibrium with its
environment. Its P = 2.3 bars, the temperature is
24 °C, and the beaker contains a 0.25 M NaCl
solution. What is the cell’s concentration of NaCl?
•
•
•
•
•
Equilibrium means cell = beaker and 24 °C = 297 K.
beaker = P + S
beaker = 0 + (-iCRT)
beaker = 0 + -(2) (0.25 M) (0.0831) (297 K)
beaker = -12.34 bars
Water Potential Practice Problem
• A cell in an open beaker is in equilibrium with its
environment. Its P = 2.3 bars, the temperature
is 24 °C, and the beaker contains a 0.25 M NaCl
solution. What is the cell’s concentration of NaCl?
•
•
•
•
•
•
beaker = -12.34 bars = cell
cell = P + S
-12.34 bars = 2.3 + (-iCRT)
-12.34 bars = 2.3 + -(2) (C) (0.0831) (297 K)
-14.64 bars = -49.36(C)
C = 0.296 M
For more practice…
• Water Potential Practice Questions worksheet
• Need help with water potential?
• Visit my website and head to the Fact Sheets
section for a video review of the concepts of
water potential courtesy a different AP
teacher.
– Note: Chapter numbers referenced therein are
now outdated.
Another way to think of it…
Most equations require you to use this
or compare the two at some point.
cell (inside) =? container (outside)
S + P
-iCRT
If you don’t immediately
have all that information, try
finding the components for
each side.
S + P
Don’t have all of those? Use
either the S formula or
return to the text of the
problem.
-iCRT 0?
Ade: Poseidon and Water Potential
• Who’s the Greek/Roman
mythological god of the
sea/water?
– Poseidon/Neptune
• And what does Poseidon/Neptune
carry?
– A trident
• And what does a trident look like?
–
• Oooooh! Podon!
http://markandrewholmes.com/poseidon_sculpture.jpg
Okay then…I think you’re ready.
• Lab 4!
Active Transport
• What happens when a cell gets greedy?
– What I mean is, what happens when a cell has
within it a higher concentration of a certain
molecule than is present outside the cell, yet still
wants more?
• This is where active transport comes in –
we’re going to need to expend a little energy
to get what we want.
Concentration Gradient
Active Transport
Low
Concentration
High
Concentration
ENERGY
NEEDED!
Quick Note: Transport Proteins
• I’ve been mentioning transport proteins quite
loosely this whole lesson. Here’s something
concrete about them:
• Channel proteins are basically just tunnels for polar
stuff to diffuse in/out. They’re simple.
• Carrier proteins are a bit slower, but they allow for
active transport and the movement of nonpolar
stuff.
– They also typically undergo shape changes to do their
work.
– They’re usually glycoproteins.
Channel vs. Carrier
Carrier Protein
Channel Protein
Transport Proteins
Back to Active Transport
• Active transport costs ATP to move molecules
against their concentration gradient.
• Proteins in the membrane that do this undergo a
conformational change in the process:
Active Transport Classic Example
• The Proton Pump (chloroplasts)
Active Transport Classic Example
• Cotransport Proton Pump (moves in two
molecules)
Active Transport Classic Example
• The Sodium-Potassium Pump (neurons)
Sodium-Potassium Pump
• We’re going to look at this one in greater
detail as it’s very important to know.
– Fun fact: The mere fact that you’re reading these
words means many of your cells are using this
pump right now.
• It’s such a good example of multiple forms of
cell transport that you should expect related
questions from both the AP Exam and me.
• Anon!
Sodium-Potassium Pump
1. Three sodium ions inside the cell bind to a carrier
protein.
2. ATP causes a conformational change in the protein,
releasing the Na+ ions to the ECM.
3. Two potassium ions then bind to the carrier, causing
another conformational change that releases the K+
ions to the cytoplasm.
4. Repeat.
• Key: Both ions are moving against their gradients.
• Key: Not only are these concentration gradients, we
can also call them electrochemical gradients because
the particles are ions and thus electrically charged.
Na-K Pump
• Na-K Pump animation
Active Transport: Three Forms
#toomanynotes
• Exocytosis – Removing stuff from the cell.
• Endocytosis – Bringing stuff into the cell.
– Phagocytosis – “Cell Eating” – when a cell engulfs a large
particle/other cell. The vesicle fuses with the lysosome.
• Amoeba Eats Two Paramecia video
– Pinocytosis – “Cell Drinking” – a continuous intake of small
dissolved particles in the nearby solution.
– Receptor-Mediated Endocytosis – Pinocytosis except the
cell is bringing in particles that have bonded to receptors
on the outside of the membrane.
• Molecular Transport – a general term for using
protein pump-like structures embedded in the
membrane.
Exocytosis
• Easy one:
Endocytosis
Phagocytosis
Will fuse with
lysosome for
digestion.
Pinocytosis
Non-specific
process.
ReceptorMediated
Endocytosis
Triggered by receptors
outside the cell.
Helps for lowconcentration
“targets.”
Cell Contact Points
• One last thing – it’s a straggler cell membrane
structure detail.
• When two cells neighbor one another, you are
bound to get one of several kinds of
connections:
– Tight Junctions
– Desmosomes
– Gap Junctions
– Plasmodesmata
Tight Junctions
• Tight junctions are the tightest
connections between cells in nature.
• They are only found in epithelial cells
(cells that form the skin and other
coverings).
– Makes sense – they need to form a barrier,
so being really closely associated is logical.
• They form a tight seal and prevent
passage of fluids.
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/13t.html
Aside: Tight Junction Regulation
• Tight junctions can be
“adjusted” to allow
certain molecules
through.
• There are a rather
large number of
proteins that regulate
tight junctions:
http://www.bio.davidson.edu/people/kabernd/BerndCV/Lab/Epith
elialInfoWeb/Tight%20Junctions.html
Desmosomes
• Desmosomes form an anchoring
connection between cells.
• While tight junctions prevent
passage of molecules,
desmosomes help cells gain
structure and strength.
• Intermediate filaments of one
cell link with intermediate
filaments of another cell.
– Adherens junctions are similar
(but different) junctions to
desmosomes.
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/13t.html
Gap Junctions
• Gap junctions have pores
allowing for signaling and
coordination between cells.
• Where tight junctions served
to prevent movement of
molecules, gap junctions
promote it.
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/13t.html
Cell Junctions
http://www.biology.arizona.edu/cell_bio/problem_sets/membranes/13t.html
Plasmodesmata
• Plasmodesmata are small
openings between plant
cells.
– Plant cells.
• Allow for communication
through cell walls.
http://biology.kenyon.edu/edwards/project/greg/pd.htm
https://en.wikipedia.org/wiki/Plasmodesma#mediaviewer/File:Plasmodesmata_en.svg
Cell Junctions Summary
• Tight Junctions
– Barriers that prevent flow of materials.
• Desmosomes
– Anchors to make tissue stronger.
• Gap Junctions
– Spaces that promote communication between
cells.
• Plasmodesmata
– Pores for communication between plant cells.
Cell Transport Summary
Closure
• So what’s the point of the cell membrane?
– By now you should have lots of answers.
• Perhaps one we haven’t discussed enough is
homeostasis.
• All these membrane functions, all these pumps
and structures…they all serve to help regulate the
cell’s “chemical balance.”
• As you know, homeostasis is “maintaining
internal balance,” and you should now be able to
see how well the cell membrane can do that.
Closure
• Turn to your partner (say hello), and think of
an real-world example for each of the
following:
– Diffusion
– Facilitated Diffusion
– Osmosis
– Active Transport (any form)