1. Lecture 25 Diffusion and Osmosis Ozgur Unal
NIS - BIOLOGY
Lecture 25
Diffusion and Osmosis
Ozgur Unal

2. Diffusion
Imagine a source of bad smell
How is it possible to smell a bad
odor from a long distance?
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.

3. Diffusion 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.

4. Diffusion 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.

5. Diffusion Across the Plasma Membrane
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.

6. Diffusion Across the Plasma Membrane
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.

7. Osmosis: Diffusion of Water
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.

8. Cells in an Isotonic Solutions
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.

9. Cells in an Hypotonic Solutions
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.

10. Cells in an Hypotonic Solutions
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.

11. Cells in an Hypertonic Solutions
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.

12. Lecture 26 Active Transport Ozgur Unal
NIS - BIOLOGY
Lecture 26
Active Transport
Ozgur Unal

13. Active Transport
Does this process require energy input?
If you wanted to put all the particles back into the box, would you need to spend energy?

14. Active Transport
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

15. Na+/K+ ATPase Pump
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.

16. Transport of Large Particles
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.

17. Transport of Large Particles
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.

18. Transport of Large Particles