Transcript Basic Structure of a Cell - Mexico Central School District

1
Question:
Are the cells in an elephant
bigger, smaller, or about the
same size as those in a
mouse?
2
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Surface area (plasma membrane
surface) is determined by
A=LxW
Volume of a cell is determined by
V=LxWxH
Therefore, volume increases FASTER
than the surface area
3
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When the surface area is no longer
great enough to get rid of all the
wastes and take in enough nutrients,
food and water…
then the cell MUST divide
Therefore, the cells of an organism
are close in size
4
Question:
Are the cells in an elephant
bigger, smaller, or about the
same size as those in a mouse?
About the same size, but …
The elephant has MANY MORE cells
than a mouse!
5
•Osmosis is a special case of diffusion
•Osmosis involves the diffusion of water
through a membrane
•The membrane may be:
Artificial (non-living) - Cellophane
Natural - Cell Membrane
•The membrane must allow water
molecules to diffuse through.
The cell membrane is permeable to H2O.
•If a dilute solution is separated from a
concentrated solution...
water will pass from the dilute to the
concentrated solution.
dilute
solution
concentrated
solution
level
falls
level
rises
membrane
More water passes from
dilute to concentrated ...
...until concentrations
become equal
Common misconception:
Water does happen to pass in
BOTH directions...
it just passes faster from the
dilute to the concentrated
solution.
Remember...
Water molecules undergo diffusion.
H2O
water
membrane
sugar solution
There are as many water molecules on the
right as there are on the left. Many of them
on the right are attached to sugar molecules
and are not free to move.
Because there are more freely moving water
molecules on the left,
more H2O diffuses through the pores of the
membrane from left to right.
Next slide
Osmosis in Animal Cells
Cytoplasm functions as a
fairly concentrated solution.
There is a greater
concentration of free water
molecules outside the cell
than inside…
...so water diffuses into
the cell by osmosis
and the cell swells up
If osmosis continued,
the animal cell would burst!
This would be bad news for animals 
Consequently, there are processes in the
animal’s body which control osmosis...
This is done by keeping the
concentration of body fluids inside the cell
the same as it is outside
Trout
Perch
In fresh water fish,
blood is more concentrated than the surrounding water
As a result the fish takes in water by osmosis
Not so much through the skin - (too) thick –
but through the gills, which have a very thin membrane
The fish gets rid of the excess water by means
of its kidneys
blood carries water
to kidneys
gills absorb
water by
osmosis
kidneys expel
excess water ...
heart
...in the form of
dilute urine
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Found in unicellular
protists like paramecia
Regulate water intake by
pumping out excess
H2O(homeostasis)
Keeps the cell from
lysing (bursting)
Contractile vacuole animation
20
A solution can be…
1.
2.
3.
Hypotonic
Isotonic
Hypertonic
21
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Hypo = less, (think hypothermia)
Tonic = dissolved particles
Hypotonic Solution =
Solute concentration lower outside than inside the cell
The outside of the cell has LESS solute,
MORE free H2O


Water moves into cell from solution.
Cell expands (and may burst)
http://www.chem.ufl.edu/~itl/4411/colligative/F
G13_022.GIF

The concentration of solutes is the same on
either side of the membrane.
Both sides have the same amount of solutes
and free H2O!

No NET water movement.

This is the ideal situation!

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Hyper = more (think hyperactive)
Tonic = dissolved particles
Hypertonic Solution: Solute concentration
higher outside the cell than inside the cell.
The outside of the cell has MORE solute,
LESS free H2O
http://cdn.innovateus.net/preset_4/hypertonicbloodcell.png
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Water moves out of cell into solution.

Cell shrinks and becomes shriveled
Comprehension Question:
What would happen if a nurse
gave a patient an IV with a pure
water solution?
Draw it, explain it!
Comprehension Question:
Survival Tip: If stranded at sea,
people are cautioned not to drink
the ocean water, no matter how
thirsty they may become.
Draw it, explain it!
Osmosis and Plant Cells
Cytoplasm functions as a
fairly concentrated solution.
Plant cells are surrounded by a strong,
rigid cell wall which prevents the cell
from taking on too much water and
exploding.
•Cell membrane acts as a selectively permeable
membrane
•Cell wall is freely permeable to water
•The vacuole contains a solution of salts and sugars
•If there is water outside the cell, it will diffuse by
osmosis through the plasma membrane into the vacuole
The vacuole will expand, pushing the
cytoplasm outwards against the cell wall
•The cellulose cell wall cannot stretch, so the
vacuole cannot continue to expand
•But the pressure of the vacuole against the cell
wall makes the cell very firm
A plant cell in this condition is said to be turgid
If all the cells in the plant are turgid, the plant will
be firm and upright and the leaves would be
expanded
cell wall
vacuole
The cell absorbs water
by osmosis ....
cytoplasm and
cell membrane
....but the cell wall stops
the cell from
expanding any more
These cells are short of
water; the tissue is limp
and the plant is wilting
The cells have taken up
water by osmosis; the
cells are turgid and the
tissue is firm
(1) Limp/
Plasmolyzed
(2) turgid
The color of cell sap in these two cells differ.
What does this say about the concentrations of the
cell sap?
What will happen?
If the concentration of the cell sap is greater in one
cell than in its neighbor, water will pass by osmosis
from the less concentrated to the more concentrated.
cell sap
more concentrated
cell sap
less concentrated
Plasmolysis in Red Onion Cell
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http://www.youtube.com/watch?v=Sw87Q53
NZLI
http://www.youtube.com/watch?v=gYbt7hhI
xPo
http://www.youtube.com/watch?v=x4il3cHhq
hY
Exposure to extremely hypertonic
environments can kill a plant.
EX:
•Your dog creates spots of dead lawn
where he pees.
•The urine is hypertonic to the interior of
the cells that make up the blade of grass
•Large amounts of water are drawn out of
the grass, killing it!
1.
2.
3.
4.
5.
6.
7.
Osmosis
Contractile Vacuole
Hypotonic Solution
Hypertonic Solution
Isotonic Solution
Plasmolyzed
Turgid