2. Diffusion
I. Diffusion - over time, molecules tend to spread apart and become more disorganized. This increasing disorder is also called entropy.

3. Entropy
From hyperphysics:

4. Diffusion in the Respiratory System
Oxygen is at a higher concentration in the lungs than in the capillaries surrounding the alveoli. As a result, oxygen diffuses through cell membranes and binds to blood cells.

5. Diffusion in the Respiratory System
Oxygen diffuses into the blood, carbon dioxide diffuses out.

6. Diffusion in the Respiratory System
When the blood reaches an area where the oxygen concentration is low, the oxygen molecules diffuse through the capillary wall and into cells. At the same time, carbon dioxide leaves cells and is carried to the lungs to be exhaled.
Cool animation

7. Cell Membranes
The outside covering of a cell is called the plasma membrane.
The plasma membrane is also called a phospholipid bilayer because it contains two layers of molecules called phospholipids.

9. Proteins in the Plasma Membrane
The plasma membrane also contains proteins.
These proteins have many jobs. One of these jobs is transporting, or moving molecules across the membrane so that they can enter or exit the cell.
Since only some molecules can move across the cell membrane, it is called a semipermeable membrane.

10. Passive Transport Osmosis:
Water can passively diffuse across the plasma membrane. This movement of water, called osmosis, DOES NOT REQUIRE ENERGY OR TRANSPORT PROTEINS because diffusion from high to low concentration happens naturally.

12. Facilitated Diffusion
Other types of molecules need help to get across the membrane. Specialized proteins form channels (like tunnels) that allow molecules to pass through them. When molecules pass through these channel proteins, it is called facilitated diffusion.

13. Facilitated Diffusion
Facilitated diffusion DOES NOT REQUIRE ENERGY either, because these molecules are moving from high to low concentration.

14. Active Transport
Sometimes in order to maintain homeostasis, cells need to pump molecules across a membrane against their concentration gradient. (From low to high concentration). Because this does not happen naturally, protein pumps usually DO REQUIRE ATP. This molecule contains stored chemical energy.
Membrane animation

15. Passive and Active Transport in the Excretory System

16. Passive Transport in the Kidney
Blood is filtered in the glomerulus. The liquid part of the blood, known as plasma, is pushed through the capillary walls. Blood cells and proteins are too big to pass through.

17. As the plasma moves through the tubule, salt, sugar, and water are reabsorbed into the blood.

18. Active Transport in the Kidney
One protein pump called the sodium-potassium ATPase uses ATP (chemical energy) to transport sodium and potassium ions across plasma membranes in the tubule cells.

19. As a result, a high concentration of sodium ions builds up on the outside of cells, and a high concentration of potassium builds up on the inside of cells Animation

20. Homeostasis and Cell Transport
Many homeostatic cell processes require certain concentrations of molecules on one side of the cell or the other, so protein pumps are an important part of maintaining homeostasis.

21. Hypertonic and Hypotonic Solutions
Cells and the fluids around them contain solutes (dissolved substances such as salt or sugar). If the solute concentration on the inside of the cell is different than the concentration outside the cell, water will move across the membrane in a process called osmosis.

22. Hypertonic Solutions
If there is more solute outside the cell than there is inside, the cell is in a hypertonic solution. If the membrane is semipermeable (water can pass through but solutes can’t), water will move toward the area with a higher solute concentration. If water moves toward the area with higher solute concentration,
what will happen to the cell? ___________________________________
Water: 7% solute
Cell: 3% solute

23. Hypertonic Solutions TIP: Remember this phrase: SALT SUCKS!
In these cells, water moved toward the more “salty” area.

24. Hypertonic Solutions
If there is less solute outside the cell than there is inside, the cell is in a hypotonic solution. What will happen to a cell in a hypotonic solution? __________________________ ________________________________________.
Water: 2% solute
Cell: 4% solute

25. Hypotonic Solution
These cells were “saltier” than their surroundings, so water got “sucked” in and the cells swelled.

26. Isotonic Solution
When there is an equal amount of solute on both sides of the plasma membrane, the solution is said to be isotonic.
Water: 3% solute
Cell: 3% solute

27. Which Way Will Water Move?

28. To the saltier side!

29. Equilibrium
Over time, cells in any type of solution reach equilibrium. At equilibrium, water is moving in and out of the cell at equal rates.
*NOTE: Water never stops moving across a membrane. It simply moves in and out at equal rates.