1. NOTES: Cell Membrane & Diffusion

2. The Cell membrane helps organisms maintain homeostasis by controlling what substances may enter or leave cells. The cell membrane is known as being selectively permeable, because not all substances can easily pass through. It is also known as fluid- mosaic model because the membrane moves!

3. The structure!!!!
The Cell membrane is a continuous double- layer of phospholipids interwoven with carbohydrates and proteins. Phospholipids are composed of two hydrophobic tails attached to a hydrophilic compound as a head.

4. The structure
The phosphate head is water-soluble, also called “hydrophilic” (water-loving) and the fatty-acids are water-insoluble, or “hydrophobic” (water-fearing). Since outside the cell is a water containing, or aqueous environment, and inside the cell is also aqueous, the phosphate heads of the phospholipids face both the cell's inside and the environment outside the cell, while the fatty acids face the inside of the membrane.

5. SKETCH and LABEL!!!
The phospholipid tails are flexible, causing the lipid bilayer to be fluid. This makes the cells flexible. SKETCH AND LABEL a phospholipid coloring the heads red and the tails blue.

6. The structure!
Embedded in the bilayer are proteins that aid in diffusion and cell recognition. The purpose of the proteins is to transport materials in and out of the cell. There are special proteins used as carriers, others as channel proteins

7. The Structure!!!
There are also cholesterol molecules in the membrane. This helps slightly immobilize the outer surface of the membrane and make it less soluble to very small water-soluble molecules that could otherwise pass through more easily. Without cholesterol, cell membranes would be too fluid, not firm enough, and too permeable to some molecules.

8. In other words, it keeps the membrane from turning to mush
In other words, it keeps the membrane from turning to mush. Large molecules like glucose or carbohydrates use proteins to help move across cell membranes. On top of the cell membrane are receptor carbohydrates that are used to identify various chemicals outside the cell.

9. LABEL & COLOR the bilayer
Phospholipid
A1. head = RED
A2 = tail = BLUE
Glycolipid = BLACK
Glycoprotein = BROWN
Carbohydrate receptor = GREEN
Cholesterol = ORANGE
Protein = PURPLE
Transport protein = YELLOW
Outside of the cell
Inside of the cell

10. The cell membrane is semi or selectively permeable – only allowing certain molecules to pass through. Phospholipids allow small, gases, ions and nonpolar molecules to pass through into or out of the cell. This is known as diffusion because it does not require energy. Energy is not required because molecules are moving across a concentration gradient.

11. DIFFUSION
The movement of substances across the membrane is known as diffusion. Diffusion is a type of passive transport. It is the movement from an area of high concentration to an area of low concentration. Smaller molecules diffuse faster than larger ones.

12. DIFFUSION….
Diffusion continues until the concentration of the molecules is the same on both sides of a membrane. When a concentration gradient no longer exists, equilibrium has been reached BUT molecules will continue to move equally back and forth across a membrane.

13. FACILITATED DIFFUSION
Facilitated diffusion is considered passive transport because extra energy is not used. Facilitated diffusion involves transport proteins embedded in a cell’s membrane to help move across certain solutes such as glucose. There are two types of transport proteins: Channel proteins and Carrier proteins.

14. FACILITATED DIFFUSION….
Carrier proteins open to the outside of the membrane & change shape to accommodate the solute, then close and reopen to the other side of the membrane to release the solute.

15. FACILITATED DIFFUSION….
Channel proteins or pores in the cell membrane form tunnels across the membrane to move materials. Channel proteins may always be open or have gates that open & close to control the movement of materials; called gated channels. Gates open & close in response to concentration of the solute inside & outside the cell.

16. ACTIVE TRANSPORT
Active transport requires the use of energy (ATP). It moves materials against their concentration gradient from an area of low to high concentration. Special carrier proteins are used to help movement in and out.

17. Active Transport
The sodium-potassium pump in the body is an example of how energy is used to transport molecules of sodium and potassium in and out of the cells. The sodium-potassium pump uses active transport to move 3 sodium ions to the outside of the cell for each 2 potassium ions that it moves in.

18. SODIUM-POTASSIUM PUMP

19. ENDOCYTOSIS & EXOCYTOSIS
Endocytosis is a process in which a substance gains entry into a cell without diffusing through the cell membrane. This process is subdivided into different types: phagocytosis and pinocytosis.

20. ENDOCYTOSIS & EXOCYTOSIS….
Phagocytosis is a process in which large molecules enter the cell. The membrane will surround the particle enclose the particle and package them in a vesicle. Inside the membrane the vesicle will transport the particle to its specific location. Pinocytosis is the same process but with a liquid instead of a solid.

21. ENDOCYTOSIS & EXOCYTOSIS…
Exocytosis is the same process but instead the particles are released outside of the cell. Exocytosis is used to remove large products from the cell such as wastes, mucus, & cell products.

22. ENDOCYTOSIS & EXOCYTOSIS…

23. OSMOSIS
The diffusion of water across a semipermeable membrane is called osmosis. Movement of water is down its concentration gradient & doesn’t require extra energy. Since water is polar, it will not diffuse freely across the membrane (remember those phospholipids- they don’t mix!).

24. Osmosis
Water uses protein channels called aquaporins to enter the cell. The cytoplasm is mostly water containing dissolved solutes (salts and other small molecules). Therefore water will diffuse into the cell to where there is more solute and therefore a lower water concentration.

25. OSMOSIS
Think of a solution as being 100%. The more water in a solution, the less the solute. For example: 75% water, 25% solute. Water molecules will cross membranes until the concentrations of water & solutes is equal on both sides of the membrane, once again called equilibrium.

26. HYPERTONIC SOLUTIONS
If concentrations of dissolved solutes are higher outside the cell, the concentration of water outside is correspondingly lower. As a result, water inside the cell will flow outwards to attain equilibrium, causing the cell to shrink. The cell will die if too much water is lost. Plant cells become flaccid (wilt) if the vacuole loses too much water.

27. HYPOTONIC SOLUTION
If concentrations of dissolved solutes are lower outside the cell than inside, the concentration of water outside is correspondingly higher.

28. Hypotonic solutions cont.
When a cell is exposed to hypotonic conditions there is net water movement INTO the cell. Cells without walls will swell and may burst (lyse) if excess water is not removed from the cell. Cells with walls will often benefit from the turgor pressure that develops.

29. ISOTONIC SOLUTION
Concentrations of dissolved solutes are equal inside & outside cell. Water moves into & out of the cell at an equal rate so there is no net movement of water. Animal cells do best in isotonic solutions.

30. Osmotic solutions