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Honors Biology
Module 6: The Cell Part 2
November 7, 2013
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How Substances travel in and out
of Cells
A plasma membrane is responsible for
substances travelling in and out of a cell.
A plasma membrane is made up of:
1. Proteins
2. Cholesterol
3. Phospholipids: is a lipid in which one of the
fatty acid molecules has been replaced by a
molecule that contains a phosphate group.
Lipids are molecules that contain three fatty acids
linked together on a glycerol molecule.
A phospholipid has, instead of the third fatty acid
molecule, a small molecule that contains a
phosphate group, which is composed of
phosphorus, hydrogen, and oxygen.
See figure 6.6 (Symbol for lipid)
The interesting thing about a phospholipid is that
the phosphate group gives the molecule a slight
affinity to water.
Since the two fatty acid components are so long
and since the molecule containing the
phosphate group is so short, the affinity to water
exists only on one end of the molecule – the end
that contains the gylcerol.
The other end of the molecule has no affinity for
water.
Phospholipid has a hydrophilic end
(water-loving)
And a hydrophobic end (water-fearing)
The symbol: The top part of the molecule (end
with the circle) is attracted to water,
The bottom part (the wavy lines) is repelled by
water.
The Plasma Membrane
The Phospholipids are arranged in a double
layer called phospholipid bilayer.
The hydorphobic layers are pointing towards
each other and the hydrophilic ends point
out towards the surroundings and other
cells. See figure 6.7
Proteins and cholesterol are interspersed
throughout the phospholipid bilayer. Some
proteins are near the top, and others are
near the bottom.
Some proteins span the entire width of the
plasma membrane, leaving no room for
phospholipds. This protein “bridge” can be
an active transport site.
Active Transport
There are two basic ways that substances enter or
exit a cell:
1. Passive Transport: Movement of molecules
through the plasma membrane according to
the dictates of osmosis or diffusion.
2. Active Transport: Movement of molecules
through the plasma membrane (typically
opposite the dictates of osmosis or diffusion)
aided by a process that requires energy.
In passive transport, no cellular energy is expended as a
result of the substance passing through the plasma
membrane.
Water for example, is a very small molecule and thus
passes easily through the spaces between the proteins
in the plasma membrane. It will travel according to the
dictates of osmosis. If the concentration of solutes in the
solution surrounding the cell is essentially equal to the
concentration of solutes inside the cell, water will
aimlessly wander back and forth across the plasma
membrane, resulting in no net charge in the amount of
water inside or outside the cell.
Isotonic Solution
A solution in which the concentration of
solutes is essentially equal to that of the
cell which resides in the solution.
The red blood cells in your bloodstream are
an in an isotonic solution. Your kidneys
get rid of excess solutes in the blood t
make sure that the bloodstream stays
isotonic with your red blood cells.
Hypertonic Solutions
Solutions that have a higher concentration of
solute than the inside of a cell hare called
hypertonic solutions.
Although a hypertonic solution has the
potential to kill any cell, the implosion that
results from placing cells that have cell
walls in a hypertonic solution is called
plasmolysis.
Plasmolysis
Is the collapse of a walled cell’s cytoplasm due to
a lack of water.
That is why you should not over fertilize a plant.
Too much fertilizer in the ground near a plant,
when the fertilizer mixes with the water in the
soil, a hypertonic solution could be formed. This
will result in plasmolysis of the plant’s cells,
killing the plant you were trying to fertilize.
Osmosis can also cause cytolysis, when a
cell is placed in a hypotonic solution.
Cytolysis: the rupturing of a cell due to
excess internal pressure.
Hypotonic solution: A solution in which the
concentration of solutes is less than that of
the cell that resides in the solution.
In a hypotonic solution, there is greater
solute concentration on the inside of the
cell than on the outside.
As a result, water flows by osmosis into the
cell until the cell bursts from too much
water pressure.
Organisms that live in fresh water
A lot of cells must deal with living in hypotonic
solutions. Fresh water has few solutes
dissolved in it, therefore it is a hypotonic solution
for cells.
Water flows into a a plant cell, it collects in the
central vacuole. This expands the vacuole,
pushing the cytoplasm against the rigid cell wall.
This increases the pressure, called turgor
pressure.
Once the pressure gets large enough, it
counteracts the effect of osmosis, and no
more water can get into the cell. Even
though the water “wants” to get into the
cell due to osmosis, the turgor pressure
keeps the water out. Turgor pressure
counteracts the effects of osmosis,
keeping the cell from rupturing.
See page 180 .
• Dr Wile’s animations
http://www.sciencegroove.org/Now/Glucose.mp3
Mr. Anderson:
Cellular Respiration:
http://youtu.be/Gh2P5CmCC0M
• Mr. Anderson and the Cell Membrane
http://youtu.be/y31DlJ6uGgE
The Plasma Membrane
• http://youtu.be/moPJkCbKjBs
• Water does not easily pass through the
membrane by simple diffusion.
• Proteins called aquaporin facilitate water going
in/out of the cell.
• Ion channels facilitate ion diffusion. This is a
very important point not made clear in this video.
• You can not be 'selectively permeable'and allow
things to just go in and out by simple diffusion
alone!
• Point being: Water and ions do not cross the
plasma membrane by themselves, proteins
are needed.
Experiment 6.2
Cell Structure II
Object: To see how cells vary in structure
and to observe plasmolysis.
A. Study of a living leaf
B. Study of banana cells
Homework
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Complete OYO questions for Module 6
Complete Study Guide question for Module 6
Complete Experiment 6.2 in Lab book
Review Website posts
Take Module 6 Test
Begin reading Module 7 pps. 195-211
Class challenge:
Quiz: