Bi 12
B9 – Cell Membrane and Transport
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1. Label the diagram below.
The lipid section of
the phospholipids bilayer – the interior of the membrane.
The HYDROPHILIC regions? Both surfaces (interior and exterior)
of the membrane. Label these regions on the diagram.
2. Where are the HYDROPHOBIC regions of the membrane?
3. On the diagram, label the two types of molecules that make up the structure that is labeled “E”.
Actin filaments
+ microtubules
4. Permeability of Our Membranes
a. Our membranes are said to be selectively
permeable. What does this mean?
b. What types of molecules find it easy to
move through our membranes? Why?
c. What types of molecules find it difficult to
move through our membranes? Why?
A. carbohydrate
chain
B. glycoprotein
C. glycolipid
D. cholesterol
E. cytoskeleton
(actin filaments and
microtubules)
F. membrane
(integral) proteins
Only specific molecules are permitted to enter and exit
the cell. It is controlled as much as possible.
Small, non-charged and non-polar molecules are able
to move through the membrane easier because of the
hydrophobic lipid region in the interior of the bilayer.
These types of molecules tend to be more soluble in
lipids, making it easier to slide through. Small
molecules are just small, making it physically easier to
move through. WATER seems to move through easily
despite its polarity because it is so small, and it
interacts so well with the hydrophilic regions on both
surfaces of the membrane.
(opposite from above!) Large molecules are physically
harder to move through a membrane. Charged or
polar molecules are not soluble in the lipid bilayer,
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B9 – Cell Membrane and Transport
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and therefore do not move through easily.
WATER is the exception – it seems to leak through
membranes despite its polarity because it is so small.
5. Biological Molecules in the Membrane:
Molecule
Skeleton (simple) diagram (sketch)
Glycoprotein
Function in
Membrane
Often used for cell
recognition and
communication.
Usually protrudes
outside the cell
membrane.
Same as above.
Glycolipid
(not found alone in the membrane, but you need to know this for later
anyway – might just as well add it here!)
(Carbohydrate)
This is starch,
a
polysaccharide)
(Function with
membrane AND in
body)
Most of the time, we
use carbohydrates in
cellular respiration
to release their
stored energy. In the
membrane,
carbohydrate chains
can be found
attached to
glycoproteins or
glycolipids to be used
for cell recognition
and communication.
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Cholesterol
Phospholipids
Fatty Acids
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In the membrane,
cholesterol reduces
the permeability and
makes the membrane
less “fluid”. It makes
it more rigid and
firm.
In the body,
cholesterol is used in
a number of
different ways
including
transporting fats and
producing steroid
hormones.
Phospholipids line
up in 2 layers with
their lipid portions
together to make up
the “lipid bilayer” of
the cell membrane.
Fatty acids are part
of the builiding
blocks of lipids,
including
phospholipids and
triglycerides.
2 FA + phosphate =
1 phospholipid
3 FA + glycerol = 1
triglyceride.
Fatty acids can be
saturated
(containing max.
number of H atoms
and NO double
bonds) or
unsaturated (double
bonds)
Bi 12
B9 – Cell Membrane and Transport
Proteins
Lipids
(triglycerides)
(for comparison to phospholipids)
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In membranes,
proteins are used for
cell recognition,
receptors, channels,
carriers and
attachment sites for
the cytoskeleton.
In our body, protein
can be used as
enzymes, for
structure (like
muscles) and can be
broken down for
energy and amino
acids.
(Function in body)
Lipids (fats) are used
mainly for energy
storage (long term)
and insulation,
although there are
several other
functions throughout
the body.
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Channel protein
Carrier protein
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These act as a pore
in the membrane, but
will only allow
SPECIFIC
molecules to move in
or out of the cell.
Each molecule that
that is transported
has a different
channel. This is to
facilitate PASSIVE
transport.
Similar to channel
proteins, but these
proteins must attach
to the specific
molecule it is to
transport. The
attachment causes a
conformational
change in the carrier
protein, which allows
the molecule to pass
into (or out) of the
cell. Often a form of
ACTIVE Transport.
Similar to having a
key open a door.
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B9 – Cell Membrane and Transport
Receptor
Protein
These proteins have
active sites on them
that will receive a
specific messenger
chemical (eg. A
hormone). When the
messenger attaches
to the active site, it
causes a
conformational
change in the
receptor protein
which that triggers a
chemical reaction (or
series!) within the
cell. The receptor
“tells” the cell what
to do according to
the messenger
chemical in this way.
6. Describe the differences
Processes
Endocytosis
Endo = moving in
vs Exocyosis. Exo = moving out
pinocytosis
vs.
phagocytosis
diffusion vs
osmosis
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Differences
Pinocytosis – endocytosis to move small molecules into the cell by pinching off a piece of
membrane around the incoming molecule.
Phagocytosis is very similar, but on a larger scale. Often, phagocytosis appears to have
“arms” (pseudopods) of cell membrane that move out and surround the molecule or
organism being engulfed.
Diffusion is the general term. It is the net random movement of molecules from an area of
high concentration to an area of low concentration.
(this is what it means if to be “following the concentration gradient”)
Osmosis IS diffusion, but it is a specific type.
It is the diffusion of WATER, and it must happen across a semi (or selectively) permeable
membrane.
9. For each of the next three examples, identify whether the solution is isotonic, hypertonic or
hypotonic compared to the “cell”, DESCRIBE what will happen to the cell and explain WHY.
B9 – Cell Membrane and Transport
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a. The “cell” is in a hypotonic solution. That will cause the cell to “lyse” (burst)
because water will move into the cell following its concentration gradient. NaCl
would love to LEAVE the cell, but it is not able to move through the membrane,
so it stays inside the cell.
b. The cell is now in a hypertonic solution, so it will “crenate” (shrink). Water will
leave the cell, following its concentration gradient. If it could, the glucose
would move into the cell, but it is too large a molecule to move through the
membrane without a carrier protein. Those proteins do not open unless
stimulated but a hormone (insulin!).
c. This cell is not going to gain or lose any water because it is in an isotonic
solution (as far as glucose is concerned). Water will still move in and out of the
cell, but there will not be any net loss or gain.
10. Complete the following table:
Do the molecules
Is the use of
Transporter
follow their
energy
process
concentration
required for
gradients with this this process?
process?
Diffusion
Yes
No
Osmosis
Yes
No
Facilitated
Yes
No
Transport
Active
Transport
No
Yes
Endocytosis
Sometimes
Yes
Is a channel
or carrier
protein used
in this
process?
No
No
Yes
What types and/or sizes of
molecules are transported using
this process?
Small, non-polar molecules
Water
Larger, polar or charged
molecules following their
concentration gradient
Yes
Molecules you need to move
AGAINST the concentration
gradient
No (unless it is Pinocytosis – liquid and small
receptor
molecules
mediated)
Phagocytosis – larger molecules
(food or enemies)
B9 – Cell Membrane and Transport
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All kinds – usually molecules that
don’t pass through the membrane
on their own.
11. Describe how the RATE of diffusion across a cell membrane is affected by each of the following factors:
exocytosis
sometimes
yes
Factor
No
How it affects the rate of diffusion.
a. temperature
Increased temperature increase the rate of diffusion.
b.
Increasing the size of the molecule DECREASES the rate of diffusion
size of molecule
c. charge of molecule
Increasing the charge on the molecule decreases the rate
d. concentration
gradient
e. pressure gradient.
Increasing the concentration gradient increases the diffusion rate
Increasing the pressure gradient in the direction of the concentration
gradient will increase the rate. If the pressure gradient is against the
concentration gradient, the diffusion rate will decrease.
12. Identify the type of membrane transport in the diagrams below, and classify them as either passive or
active transport.
b.
a.
c.
e.
d.
f.
Bi 12
B9 – Cell Membrane and Transport
Transport
Type
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Active
Or
Passive?
a.endocytosis active
b. exocytosis
active
c. phagocytosis active
d. osmosis
passive
e. carrier protein
passive
f. diffusion
passive