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Lecture 25
Diffusion and Osmosis
Ozgur Unal
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Imagine a source of bad smell
How is it possible to smell a bad
odor from a long distance?
http://www.biosci.ohiou.edu/introbioslab/Bios170/di
ffusion/Diffusion.html
Bad smelling gas moves to areas where its concentration is
low.
Diffusion is the net movement of particles from an area
where there are many particles of the substance to an area
where there are fewer particles of the substance.
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Check out Figure 7.20.
Blue and red inks eventually mix together, because blue
(red) particles move to areas where there is less blue (red)
particles in the water.
After a long time, the blue and red ink concentrations
eventually become uniform in the water.
The color turns purple.
The ink particles keep moving randomly, but there is no
more overall change.
The particles reach dynamic equilibrium.
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The rate of diffusion increases as the
temperature increases.
This is because the number of collisions
between particles increase as it gets hotter.
The size and the charge of a substance
also affect the diffusion rate.
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In addition to water, cells need certain ions and small
molecules.
Water can diffuse across the plasma membrane, but most
other substances cannot.
Another form of transport, called facilitated diffusion, uses
transport proteins to move ions and small molecules into the
cells.
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Channel proteins open and close to allow the substances to
diffuse through the plasma membrane.
Carrier proteins change shape help substances diffuse by
changing shape as the diffusion process continues.
Diffusion of water and facilitated diffusion do not require
energy. Therefore, this is also known as passive transport.
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The diffusion of water across a selectively permeable
membrane is called osmosis.
Regulating the movement of water across the plasma
membrane is important for homeostasis.
Remember mixtures..
Water is a solvent and sugar in water is the solute.
Water molecules are small enough to pass through the
selectively permeable membrane.
Water molecules move from high sugar concentration to
low concentration through the membrane to reach dynamic
equilibrium.
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When a cell is in a solution that has the same concentration
of water and solutes (ions, sugars, proteins etc) as its
cytoplasm, the cell is said to be in an isotonic solution.
In cells in isotonic solutions, water moves in and out of the
cell at the same rate.
The cells reatin their normal shape in isotonic solutions.
Example: Blood cells in blood.
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If a cell is in a solution that has a lower concentration of
solute, the cell is said to be in a hypotonic solution.
There is more water outside of the cell than inside.
Due to osmosis, water moves into the cell through the
plasma membrane.
Pressure generated as water flows through the plasma
membrane is called osmotic
pressure.
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In an animal cell in a hypotonic solution as water moves into
the cell, the pressure increases and the plasma membrane
swells.
If the solution is extremely hypotonic, the animal cell might
be unable to withstand the osmotic pressure and the cell
bursts.
However, plant cells do not burst like animal cells, because
they have a rigid cell wall that supports the cell.
As the pressure inside the cel increases, the plant’s central
vacuole fills with water, pushing the plasma membrane
against the cell wall.
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When a cell is placed in a hypertonic solution, the
concentration of the solute outside of the cell is higher than
inside.
During osmosis, the net movement of water is out of the cell.
Animal cells in hypertonic solutions shrivel because of
decreased pressure in the cells.
Plant cells in hypertonic
solutions lose water mainly
from vacuole and the plasma
membrane moves away from
the cell wall.
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Lecture 26
Active Transport
Ozgur Unal
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http://www.biosci.ohiou.edu/introbioslab/Bios170/
diffusion/Diffusion.html
Does this process require energy input?
If you wanted to put all the particles back into the box,
would you need to spend energy?
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Sometimes substance must move from a region of lower
concentration to a region of higher concentration.
This movement of substances across the plasma membrane
against a concentration gradient requires energy, therefore it
is called active transport.
Active transport requires energy input.
Due to active transport, the cell maintains the proper
balance of substances it needs.
Figure 7.26
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One common active transport pump is called the
sodium/potassium ATPase pump, which is found in the
plasma membrane of animal cells.
This pump maintains
the Na+ and K+ levels
inside and outside the
cell and it consumes
energy.
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Some substances are too large to move through the plasma
membrane by diffusion or transport proteins.
These large particles get inside the cell by a different
process.
Endocytosis is the process by which a cell surrounds a
substance in the outside environment,
enclosing the substance in a portion
of the plasma membrane.
The membrane then pinches off and
leaves the substance inside the cell.
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Exocytosis is the secretion of materials at the plasma
membrane.
Exocytosis is the reverse of endocytosis.
Cells use exocytosis to expel wastes and to secrete
substances such as
hormones produced
by the cell.
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