Download THE MEMBRANE AND TRANSPORT

THE MEMBRANE AND
TRANSPORT
BEGINS
Hey boss. Da plane! Da plane!
We have many privileged guests arriving on the Island today Tattoo.
First, we have a school group of biology students who seek to learn about cells and
the cell membrane. I think they may be surprised about what they discover.
Second we have the beautiful Miss Aqua. She has two suitors, but she can’t decide
which she wants. She needs direction and I hope she finds it
Third, we have Mr . Trans. All his life he has been treated like a doormat in the game
of life. We have to show him how important he is. Perhaps then he will see his true
vocation
Enough of this, let us welcome our guests to the
island Tattoo.
Welcome guests. I am your
host, Mr. Rourke. I hope to
make your stay here an
enjoyable one. So let us raise
our glasses in a toast. May all
your dreams come true here
on Fantasy Island!
Hey boss when are we going to let them in
on your secret.
All in good time Tattoo. All in good
time.
Come have a seat all you biology students. Lets talk
about the cell membrane.
Now first let me say that the membrane acts as a
protective barrier for the cell much like the ocean
around this Island protects us from unwanted guest.
It also functions to allow necessary substances into and
out of the cell. On this Island, where the land meets
the sea, we select what enters and what exits. You see
here on Fantasy Island I control everything.
Like our Island is surrounded by the water of the great ocean, so are the cells
bathed in water. Also like the ocean, cells are surrounded by a solution of salty
ion containing water not pure water. This creates an equivalent environment
both inside and outside the cell called an ISOTONIC ENVIRONMENT. In this
isotonic environment, cells neither gain nor lose water and so they remain
stable. It however doesn’t mean that water cannot move across the membrane.
Quite the contrary, water moves across the membrane but just equally in both
directions. When water moves across this membrane it is called OSMOSIS.
OSMOSIS – the diffusion of water across a selectively permeable membrane
DIFFUSION – movement of a substance from an area of high to an area of low concentration
Since neither of the above require an expenditure of energy by the cell when
they diffuse across the membrane, they are called PASSIVE TRANSPORT
So you probably think you know what osmosis is now don’t you.
Well try this switch. What if I say that a substance moves down
its CONCENTRATION GRADIENT due to random thermal motion. Is
that also a definition for OSMOSIS?
YES it means the same! It also adds
why it happens. -RANDOM
THERMAL motion is why diffusion
occurs faster in hot water than cold.
Molecules move faster when hot
Very good class. You realize that the concentration gradient is just an
expression to describe a series of more and more dilute regions progressing
away from an original source.
Here is an example of concentration of colour. You could pretend that the darker
regions represent higher concentrations, the lighter regions, lower concentrations
source
Diffusion down the concentration gradient
In addition, the diffusion of one solute is unaffected by
concentration gradients of other solutes
Okay here is another tough one for you class. What
does the term TONICITY mean?
Once again, I must say that I am
impressed. TONICITY is the tendency
of a cell to gain or lose water in a
given solution. This is affected by the
permeability of the plasma
membrane and whether the cell has a
wall.
Are you saying that since I’m an
animal cell –no cell wall- I can lose
or gain water.?
Well we’re not really sure,
but the root word TON is
found in the previous term
isoTONic. It would make
sense that it is related to
water balance.
Ha, ha, sucker.
That would be correct, Mr. Cell. Only an animal cell in an isotonic
environment will remain the same size and neither swell up nor
shrink.
There are names to describe those conditions.
Hypertonic solutions will cause you to shrink or shrivel due to water
loss.
Hypotonic solutions will cause you to swell up as you gain water. Too
much water, and you can possibly lyse( burst ).
Think of a hypodermic needle injecting the cell and adding
water causing it to swell. HYPOTONIC – cell swells
Think of an angry hyper face. It’s wrinkled and
shrivelled. HYPERTONIC –cell shrivels - water is lost
shrinking the cell
So I have to live in an isotonic environment such as salt water or isotonic
body fluids.
That’s right Mr. Cell and if you don’t, you have to have
adaptations for OSMOREGULATION – The control of water
balance.
Ha, ha. I’ve got a cell wall
I’d also like to introduce another
member of the gang. Say hello to
Mr Fungus cell
Hello there. I’m Mr. Fungus and even I
have a cell wall!
I got a cell wall too
You three are not excluded from the affects of tonicity. In the
hypotonic environment your cell walls prevent you from bursting Your
cells are described as TURGID when the water puts pressure on your
cell wall. Cells absorb water by endosmosis, so that the increased
volume of water in the cell will increase pressure, making the
protoplasm push against the cell wall.
I need to speak
with you in
private, Mr Plant.
See. I told ya. Stick by me and everythin’
will be fine. Just keep your cell wall on!
Turgid- cell filled with water cell swelled
Flaccid – cell water loss- shrivels
In non- woody plants, turgid cells provide
mechanical support causing the plant stems
to be stiff. Hey more water spray. Keep the
cells turgid. I hate wilted (flaccid) celery. If
there is enough water for the celery, it stays s
hypotonic!
Mr. Plant cell. Do you realize that you could die as a result of being
placed in hypertonic environment. In this case, water would leave
your cell and your cell membrane will pull away from the cell wall
causing it to tear as the cell shrivels. For you this fatal situation is
called PLASMOLYSIS!
Hey, So what happens
to Mr Cell in this
hypertonic
environment?
I’m sorry to say it Mr. Plant,
but he is right. Lets look at
a comparison chart on the
next slide.
The reverse process, deplasmolysis,
can occur if the cell is in a hypotonic
solution (water moves in) resulting in
a higher external osmotic pressure
(that’s the pressure on the
membrane/wall). – CELL SWELLS
Ha, ha. Right back at you. I
might shrivel up (called
crenation) in hypertonic
environments, but at least I
don’t break open!
Hey leave him alone unless
you want our gang on ya’
Plant cell
BEFORE
Plant cell
AFTER
I guess we’re about
even
Mr Badbacter, I need to see you
in private now
Uh oh.
Environment
Plant cell
Animal cell
isotonic
Stay same
Stay same
Hypotonic (water
moves into cells)
Swells to TURGID
Swells –LYSIS
possible
Hypertonic (water
moves out of cells)
Shrivels
PLASMOLYSIS
possible
Shrivels
Mr. Badbacter, please indulge, I have
some honey for you.
What’s the matter Mr.
Badbacter. You usually
love to grow in sugary
media. Are you afraid
of something?
Excuse me Mr. Rourke,
but could I please have
my honey pot back.
Hey. What’s
the deal here.
I hate honey!
Ya Rourke,
give him back
his honey.
Kids, you know that honey does not go bad even
if it is not put in the refrigerator. The reason is
that it is hypertonic to the bacteria cells and
these cells will lose water and plasmolyse. Bye
bye Mr. Badbacter.
HELP!
The Paramecium
DID YOU KNOW?
Freshwater protists like the
paramecium face a daily if not
hourly challenge of
osmoregulation. They
constantly are gaining water
from their hypotonic
environment. In order to deal
with this excess water, they
have a specialized structure
called the contractile vacuole
that expels water from their
bodies.
These freshwater
protists also have
membranes that are
less permeable to
water
Now the next thing I would like to deal with is how to
determine whether water will enter or leave a cell. How do
you know if the environment is hypertonic hypotonic or
isotonic?
That brings me to our guest, Miss Aqua. Now she needs
direction. She needs to know in what direction to go. So when
we determine the environment, we can follow the water to
determine the solution.
TONICITY = The tendency for a cell to GAIN or LOSE water
Me too.
Water movement
Plasma membrane
Inside the
cell
I need to know the
tonicity
Outside the
cell
Water movement
HYPOTONIC SOLUTION (ENVIRONMENT)
There is a GREATER CONCENTRATION of SOLUTES INSIDE the cell
Lots of salt of sugar inside the cell will cause more water to move in through the membrane.
Fresh (pure) water sources would be considered hypotonic solutions to cells
Cells must contain dissolved salts etc.
When more water enters the cell the salts in the cell become more dilute
This is hypotonicity
Plasma membrane
Sometimes cell interior
described as a negative water
potential (more solute here)
Inside the
cell
water will move from
an area of higher
water potential to an
area that has a lower
water potential.
Water in
Freshwater would have
a positive or high water
potential ( lack of solute)
Outside the
cell
Water movement
Osmotic Pressure increases
HYPOTONIC
ENVIRONMENT
More water inside causes the
plasma membrane to stretch. This
leads to a turgid cell. If it stretches
and cannot withstand the osmotic
pressure, it will burst – cell lysis
Inside the
cell
The cell wall of
plants and bacteria
prevent these cells
from bursting in
these conditions
More solute here
Plasma membrane
putting pressure on the cell
membrane
HYPO means LOWER or low solute
(salt or sugar) concentration. Pure
water would have a very low
solute concentration –being mostly
just water
Outside the
cell
Yes
hypotonicity I
can feel water
entering
Water in
Water movement
HYPERTONIC SOLUTION
(ENVIRONMENT)
Plasma membrane
OSMOTIC PRESSURE
DECREASE as water leaves
the cell. The membrane
moves inward and the cell
shrivels
Inside the
cell
Plant cells can undergo plasmolysis as the
cell membrane shrinks
away from the rigid
cell wall it is attached
to. Result - Lysis
This is
hypertonicity
Outside the
cell
I can feel
water
moving
out
Water movement
The DEAD SEA
– notice the
white- it is not
snow but salt
crystals in the
water which is
extremely
hypertonic.
Water out
More solute here
HYPERTONIC SOLUTION (ENVIRONMENT)
There is a GREATER CONCENTRATION of SOLUTES OUTSIDE the cell
Too much salt or sugar or alcohol in the bloodstream could cause cells this problem.
Fresh (pure) water sources would be considered hypotonic solutions to cells
Cells must contain dissolved salts etc.
When more water enters the cell the salts in the cell become more dilute
Inside the cell
water potential is
higher
Water will move from
an area of higher
water potential to an
area that has a lower
water potential.
Plasma membrane
This is hypertonicity
Water moves from high
water potential to low
I’m going to
water you with
loads of fertilizer
in this water to
get you growing
better than ever!
Outside the cell water potential
is lower more solutes like salt
or even too much fertilizer
(fertilizer contains mineral ions
Don’t get confused by the term WATER
POTENTIAL. Compare these two solutions to
clarify the meaning
When you look at a hypotonic solution there is
MORE WATER= HIGHER WATER POTENTIAL
PLASMA MEMBRANE
Don’t forget that
WATER POTENTIAL
can be positive zero
or negative
SOLUTION 1
5% SOLUTE
95% WATER
More water
More water
potential
PLASMA MEMBRANE PLASMA MEMBRANE PLASMS MEMBRANE
When you look at a hypertonic solution, there is
LESS WATER = LOWER WATER POTENTIAL
SOLUTION 2
10% SOLUTE
90% WATER
It is obvious that water would move from solution 1 to
solution 2 through the membrane to reach equilibrium.
Thus water would move from an area of high to low
WATER POTENTIAL
Less water
potential
Less water
Adding more
solutes will
decrease the
water potential
even more!!
Now there are other terms that are involved that can
really confuse things so you have to get an understanding
of two other potentials. These two other potentials added
together make up the water potential
WATER POTENTIAL
=
PRESSURE POTENTIAL
+
SOLUTE POTENTIAL
If you can determine the water potential for cells and the water potential for a
solution, you can determine how the water will move. Thus the above
equation( often used by botanists) is useful to do this calculation.
PRESSURE POTENTIAL is a measure of the amount of water in the cell
exerting an outward pressure that is retained by the structural rigidity
of the cell wall. It is usually positive but in plasmolysed cells, pressure
potential is almost zero and in dead xylem cells it is often negative.
SOLUTE POTENTIAL is when the presence of a solute causes water
molecules to remain with the solute rather than moving away by osmosis.
This solution will have a lower and thus more negative water potential
than that of pure water. The more solute molecules present, the more
negative the solute potential.
Water molecules
K+
attraction
The important thing to remember is that MORE PRESSURE
POTENTIAL lead to more WATER POTENTIAL
And the GREATER THE SOLUTE POTENTIAL the
LESS WATER POTENTIAL.
Tattoo, I think our guests have had enough. Why
don’t you bring them some snacks
Sure , boss. Right
away
DID YOU KNOW?
Water can rise up to 100m through the xylem
vessels of a tree without a pump. Cohesion,
adhesion and root pressure will pull the water only a
few meters. Transpiration pull causes it to rise the
rest of the way.
Transpiration pull provides a negative pressure that is
found in the dead xylem cells that line the pipeline
leading up to the leaves. This water pipeline provides
the leaves with a higher water potential that the air
(low water potential. Water vapour in the airspaces
of leaves diffuse down the gradient into the opening
the plant leaf called the stomata.
Pressure potential is almost zero and in
dead xylem cells it is often negative.
So Miss Aqua. I think you understand the importance of
your decisions regarding movement. All the cells on Earth
are dependent. Whether you move into or out of a cell can
mean life of death for that cell.
This is isotonic
Water Potential same
Plasma membrane
Inside the
cell
There is no NET
movement of water
Outside the
cell
Water Potential same
Aqua, your movement affects us greatly. In our blood if water moves out
of Red blood cells they shrink., but it water moves into blood cells they will
burst open (lyse) once the osmotic pressure becomes too great.
CRENATION of
eurythrocytes
LYSIS (CYTOLYSIS) of
eurythrocytes
To be continued
See part 2