2. CELL MEMBRANE & CELL TRANSPORT

3. Homeostasis: Maintaining a Balance
Organisms must adjust to changes in their environment.
If not…DEATH!
A formal definition is—maintaining a stable internal state despite what is going on externally.

4. What Maintains Homeostasis?
The PLASMA or CELL MEMBRANE maintains the proper concentrations of materials by controlling the passage of molecules in and out of the cell.
Therefore, the cell membrane’s function is to maintain HOMEOSTASIS through passive transport, active transport and cell communication!

5. This electron micrograph of the cell membrane shows the appearance of the phospholipid bilayer using several staining processes. The magnification on this structure is 53,260 X
Cell Membrane
X 53, 260

6. Characteristics of the Cell Membrane

7. The cell membrane is Selectively Permeable aka Semi-permeable
Allows some things in/out and not others…
Oxygen, nitrogen, carbon dioxide, and other small, nonpolar molecules can diffuse directly
Water was once thought to move directly through, but it is now understood to travel through aquaporins (a type of transport protein).
Ions, sugars, and larger molecules move through transport proteins or in vesicles.

8. Plasma/Cell Membrane-Structure
Composed of a
Phospholipid Bilayer with proteins embedded/floating in it
Phospholipid Structure:
Polar Head (hydrophilic—”water loving”)
Nonpolar Tails—(hydrophobic—”water fearing”)

9. Phospholipid Bilayer Arrangement Polar Head Nonpolar Tails
Outside of cell
Inside cell

10. Proteins are embedded/floating in the lipid bilayer.

11. Protein in membrane

12. Cell Membrane Structure-Proteins
These proteins are needed
for the membrane to function
properly.
3 Types of Membrane Proteins
1. Transport—regulate what enters or leaves cell
2. Marker—identify the cell
3. Receptor—allow cells to communicate

13. Transport Proteins—Channel Proteins
Function as “gates/passageways”
Allow polar sugars, amino acids, and ions to cross the membrane.
Special channel proteins:
Gated ion channels—gates that open/close
Carrier proteins—change shape to allow specific molecule to pass
Aquaporins—allow water to diffuse through (called osmosis)
Carrier Protein

14. Marker Proteins Cell’s “Name Tag”
Protein sticks out of phospholipid layer
Often has carbohydrates attached to outside end
Functions in cell identification to identify the cell to other cells and molecules
Important in
immunity—so various white blood cells in your body do not mistake your cells for foreign cells
- blood typing – so you can’t receive just any ol’ type of blood
Marker Protein

15. Receptor Proteins Function as “messenger/receiver”
Receive information from the environment (extracellular fluid, blood, interstitial fluid) and transmit that info to the inside of the cell
Protein has specific shape/charge to only allow certain molecules (like hormones) to bond
Triggers a response in cell
Ex) epinephrine can bind to a receptor protein and send a message inside that says “break down glycogen”
Receptor Protein

16. CELL TRANSPORT
Concentration gradient - the difference in the concentration of a particular substance across a space.
Equilibrium is reached when the molecules become even throughout a space.

17. Cell Transport Types
Passive Transport—no energy required, molecules move from high to low concentration (down or with concentration gradient)
Diffusion
Osmosis
Facilitated Diffusion
Active Transport—energy (ATP) required, molecules move from low to high concentration (up or against the concentration gradient)
Pumps
Vesicles

18. Diffusion, Osmosis, Facilitated Diffusion
Passive Transport
Diffusion, Osmosis, Facilitated Diffusion

19. Diffusion Diffusion - the process by which molecules spread from
areas of high
concentration,
to areas of low concentration
Molecules are said to go “Down” or “with” the concentration gradient.
Passive Transport-requires no energy

20. Osmosis Osmosis - the diffusion of water molecules through a
semi-permeable
membrane requires no energy
Ex. Water will move in the direction
where there is a higher concentration
of solute (and hence a lower
concentration of water).

21. About this diagram: this represents a cell in a solution
About this diagram: this represents a cell in a solution. The cell will not let the red particles pass through the membrane. The green molecules can pass through, as can water molecules. The arrows show the direction of particle movement. The green particles are moving in to the cell where their concentration is lower, and water is moving out of the cell because its concentration is higher inside
diffusion
osmosis

23. 3 Solution Types— Isotonic, Hypertonic, Hypotonic
Solvent—the substance that is doing the dissolving (ex. Water)
Solute—the substance that is being dissolved in the solvent (ex. Sugar, salt, etc.)

24. Isotonic Solution
Concentration of solute and solvent outside the cell is equal to/the same as the solution you are comparing it to (often the cytoplasm in cell)
Water is moving in and out at equal rates, no net movement into or out of cell
Cell size would stay the same

25. Red blood cells in isotonic solution
X 1000
Note that all the cells appear normal.

26. Hypertonic Solution
There are more solute (ex. salt) molecules in solution outside the cell when compared to the inside of cell
Water will move out of the cell
Cell would shrink
Plants cells shrink (plasmolysis) because cell membrane pulls away from cell wall, so plant wilts because water has left the central vacuole. Animal cell shrink (crenate). In both cases, the cell may die.

27. Crenated red blood cells in hypertonic salt solution
X 1000
Notice that the cells have shrunk.

28. Hypotonic Solution
There are less solute (ex. salt) molecules in the solution outside the cell than inside the cell.
Water will move into the cell.
Cell will swell
Cell could burst (lyse)
Plant cells have vacuoles to collect extra water

29. Red blood cells in hypotonic solution
X 1000
Note that the pinkish cells have swollen (the little dip in the middle of a normal rbc is not visible and one side bows out).

31. Osmosis Links Osmosis Examples
Hypotonic, Hypertonic, Isotonic Interactive Quiz

32. Facilitated Diffusion (uses Transport Proteins, passive)
Moves substances (from high to low concentration) down the concentration gradient without using cell’s energy using channel/carrier proteins located in membrane

33. Active Transport in Cells

34. Why do cells need active transport?
Cells must transport certain amino acids, sugars, etc. into their cytoplasm from the surrounding fluid.
Some of these substances, however, are already in higher concentrations inside versus outside.

35. How does active transport work?
Some types involve carrier proteins that function as “pumps”.
Other types use vesicles.
Energy is provided by ATP.

36. Active Transport Using Pumps
Sodium-Potassium Pump
Proton Pumps

37. Example of active transport types using pumps
Sodium/Potassium Pump
Na+ pumped out of a cell
K+ pumped into a cell
Important because it prevents cells from bursting by lowering the sodium inside causing less water to enter through osmosis.
Used by many cells, including nerve cells, to send a chemo-electric message.

38. Sodium-Potassium Pump
3 Na+ out of cell (yellow diamonds)
2 K + into cell (red/purple squares)
Here the energy of a phosphate from ATP (shown in pink) is used to exchange sodium atoms for potassium atoms.

40. Proton Pump
Proton (H+) pump – forces protons out of a membrane enclosed space (organelle or cell), often to create a proton gradient down which the protons can flow back in
Why would the cell “waste” energy on a proton pump? *Because the cell needs isolated areas of the cell with different pH for particular functions; ex) lysosomes – have proton pumps to maintain a pH=5
*Because the cell only uses one ATP to pump a proton out, and that proton can be used in co-transport
Co-transport – process cells use to bring large molecules, such as sugars, into a cell with a minimum amount of energy used; usually a proton and a sugar enter a double tunneled protein at the same time; the tunnel only “works” when both molecules are present

41. Active Transport Using Vesicles
Endocytosis & Exocytosis

42. Endocytosis
Substances are moved into a cell by a vesicle that pinches off from the cell membrane
Requires energy (ATP)

43. Types of Endocytosis
Pinocytosis—when the nutrient particles are dissolved in a liquid; “cellular drinking”
Phagocytosis—when the nutrient particles are solids; “cellular eating”

44. Exocytosis
Exocytosis- substances inside a vesicle are released from a cell as the vesicle fuses with the cell membrane
Involves the cell getting rid of waste or the cell secreting cell products (ex. Hormones, insulin)
Requires Energy (ATP)

46. Animations—Exocytosis, Endocytosis
Exocytosis using ER and Golgi:
McGraw-Hill: Cotransport, Na-K pump, Endo/Exocytosis, & Proton Pump

47. Cell Membrane Physiology--Interactive