1. AP Biology Ch. 7 Membrane Structure & Function

2. The Edge of Life
The plasma membrane of a cell is the boundary that separates the living cell from its surroundings.
It is only about 8nm thick—it would take over 8,000 of them to equal the thickness of a piece of paper.
It is essential to maintain the internal environment of the cell.

3. Selective Permeability
The plasma membrane exhibits selective permeability—it allows some substances to pass through more easily than others.
What needs to come into a cell? (oxygen, water, nutrients, ions)
What needs to leave a cell?(carbon dioxide, water, wastes, proteins)

4. Cell membranes are fluid mosaics made of lipids and proteins
The most abundant molecules in cell membranes are phospholipids and proteins. Phospholipids are amphipathic. What does this mean?
Phospholipids make a bilayer around the cell with water inside the cell kept separate from water outside the cell.

5. The Fluidity of Membranes
The membrane is a fluid structure with a “mosaic” of various proteins embedded in or attached to the phospholipids.
See video:

6. Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate
protein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Figure 7.7 The detailed structure of an animal cell’s plasma membrane, in a cutaway view
Cholesterol
Microfilaments
of cytoskeleton
Peripheral
proteins
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE

7. Movement of Molecules within the Plasma Membrane
Membranes are not locked into place.
Most of the lipids and some of the proteins can shift laterally side-to-side.
It is rare for them to flip-flop transversely.
Adjacent phospholipids move within the membrane rapidly—107 times/second
Unsaturated fatty acid chains in the phospholipids makes membranes more fluid since the “kinks” in the “tails” don’t pack together tightly. This keeps it fluid even in cold temperatures.

8. Cholesterol—important for cell membrane structure
The steroid cholesterol is wedged between phospholipids in the plasma membrane of animal cells.
At higher temperatures, cholesterol makes the membrane less fluid by restraining phospholipid movement.
Because cholesterol keeps the membrane from packing tightly, it lowers the temperature required for the membrane to solidify (keeping animals from freezing)
Membranes must be fluid to work properly; they are usually about as fluid as salad oil.

9. Membrane Proteins

10. Membrane Proteins and Their Functions
A mosaic of different proteins is embedded in the plasma membrane. Proteins determine most of the membrane’s functions.
Enzymes
Signal transduction
Active transport of materials

11. Functions of Membrane Proteins
Inter-cellular Joining
Attachment to the cytoskeleton and the extra-cellular matrix
Cell-Cell Recognition

12. Two Major Kinds of Membrane Proteins:
Integral Proteins: penetrate the hydrophobic core of the lipid bilayer
Peripheral Proteins:
Not embedded in the lipid bilayer at all
They are appendages

13. The Structure of a trans-membrane protein:
EXTRACELLULAR
SIDE
N-terminus
Figure 7.8 The structure of a transmembrane protein
C-terminus
CYTOPLASMIC
SIDE
 Helix

14. The Role of Membrane Carbohydrates in Cell-to-Cell Recognition
A cell’s ability to distinguish one type of neighboring cell from another is crucial.
It helps sort cells into different tissues and helps the organism recognize foreign cells.
Membrane carbohydrates may be bonded to proteins (Glyco-proteins) or to lipids (Glyco-lipids)
Cells recognize other cells by binding to surface molecules, often to carbohydrates.

15. Synthesis and Sidedness of Membranes
Membranes have distinctive inside and outside faces.
When a vesicle fuses with the plasma membrane, the OUTSIDE of the vesicle membrane fuses with the INSIDE of the plasma membrane.
Therefore, molecules that start on the INSIDE of the ER end up on the OUTSIDE of the plasma membrane.

16. ER
Synthesis of membrane components and their orientation in the resulting membrane.
The plasma membrane has distinct cytoplasmic (orange) and extracellular (aqua) faces, with the extracellular face arising from the inside face of the ER, Golgi, and vesicle membranes. 1
Transmembrane
glycoproteins
Secretory
protein
Glycolipid
Golgi
apparatus 2
Vesicle
Figure 7.10 Synthesis of membrane components and their orientation on the resulting membrane 3
Plasma membrane:
Cytoplasmic face 4
Extracellular face
Transmembrane
glycoprotein
Secreted
protein
Membrane glycolipid

17. Membrane Structure Animations

18. Membrane structure results in selective permeability
Most important function of the cell membrane: to regulate what goes in/out of the cell
Form fits function: the fluid mosaic model helps explain how membranes regulate the cell’s molecular traffic

19. The Lipid Bilayer
Nonpolar molecules such as hydrocarbons, CO2 and O2 are hydrophobic and can pass easily through the lipid bilayer. Why?

20. Permeability of the Lipid Bilayer
Ions and polar molecules cannot pass easily across the lipid bilayer. Why not?
Polar molecules such as glucose and other sugars pass only slowly across the membrane.
Even water passes more slowly than expected, since it is such a small molecule. Why?

21. Transport Proteins
Cell membranes are permeable to certain ions and other polar substances such as water.
These hydrophilic substances can avoid contact with the “Hydrophobic Zone” by passing through transport proteins that span the membrane.

22. Channel Proteins
Channel proteins function by having a hydrophilic channel that certain molecules can use as a tunnel through the membrane
Aquaporins help water molecules move through the membrane easily. Each one allows up to 3 billion water molecules/second!

23. Carrier Proteins
Carrier proteins hold onto their “passengers” and change shape in such a way that shuttles them across the membrane.
A transport protein is specific for the substance it moves across.

24. Sodium-Potassium Pump

25. Passive Transport
Passive transport is diffusion of a substance across a membrane with no energy investment required by the cell. Materials move by themselves! How?
Diffusion = the movement of a substance from where it is more concentrated to where it is less concentrated.
Molecules move DOWN a concentration gradient. They move from HIGH concentration to LOW.

26. Diffusion
Molecules move by themselves due to kinetic energy, the energy of motion. This is also called thermal energy.
Each molecule moves randomly, in all directions.
Diffusion occurs as molecules move randomly, but the NET movement of a population of molecules is to move AWAY from high concentration to lower concentration until it reaches equilibrium.

27. Osmosis
Osmosis is a special kind of diffusion. It is the diffusion of water across a selectively permeable membrane.
Water moves from where it is MORE concentrated to where it is LESS concentrated.

28. Effect of Osmosis on Water Balance
If there is a difference in the concentration of solutes in the water on either side of a membrane, the water level may change as it diffuses across the membrane.
It is the difference in free water concentration that makes the difference.
Why does the level of water go up on the right-hand side of the tube?

29. 3 Solutions a Cell Could be in:
Isotonic Solution– a solution where the concentration of solutes is EQUAL inside and outside the cell
Hypertonic—a solution where the concentration of solutes is GREATER in the water outside the cell. (More solutes = less water)
Hypotonic—a solution where the concentration of solutes is LESS in the water outside the cell. (Less solutes= more water)
Hypertonic
Hypotonic

30. What happens to cells in the 3 conditions?
“Iso” = equal
“Hyper” = Greater than
“Hypo” = Less than

31. Water Balance in Cells with Cell Walls ( mostly Plants!)
The cells of plants, prokaryotes, fungi and some protists have cell walls.
When such a cell is placed in a hypotonic solution, the wall helps maintain the cell’s water balance.

32. Facilitated Diffusion: Passive Transport Aided by Proteins
Facilitated diffusion occurs when transport proteins embedded in the membrane aid the movement of substances. (Note: it is still moving DOWN the concentration gradient!)
Two types of transport proteins:
Channel protein
Carrier Proteins

33. Passive Transport Animation

34. Active Transport
Active transport uses the cell’s energy (ATP) to move materials across the membrane.
Materials that must move AGAINST the concentration gradient must be moved with active transport.

35. The Need for Energy in Active Transport
To pump a solute across a membrane against its gradient requires work; the cell must expend energy.
Carrier proteins embedded in the membrane can move these solutes.

36. One Example: Sodium-Potassium Pump
The sodium-potassium pump exchanges sodium ions (Na+) for potassium (K+) across the cell membrane.
Each ion is in high concentration on one side of the membrane. The cell needs to maintain that higher concentration.

37. Sodium-Potassium Pump Animations

38. How Ion Pumps Maintain Membrane Potential
All cells have voltages across their plasma membranes.
Voltage is electrical potential energy—a separation of opposite charges.
The cytoplasm is negative (-); the extracellular fluid is positive (+).

39. Membrane Potential
The voltage across a membrane is called membrane potential. It ranges from about –50 to –200 millivolts.
The membrane potential acts like a battery, an energy source that affects the traffic of all charged substances (either + or -).
The membrane potential favors + charged ions. Why?

40. Membrane Potential
Two forces drive the diffusion of ions across the membrane:
1)a chemical force (the concentration gradient)
2)an electrical force (the membrane potential)
Together, these are called the electrochemical gradient
In the above example, there are more negative charges inside the cell than outside. Ions such as Na+ are more likely to move inside. Why? (2 reasons!)

41. Electrogenic Pumps
Some membrane proteins that actively pump ions contribute to the membrane potential.
The Sodium-Potassium pump is an example because it pumps out 3 Na+ ions for every 2 K+ ions, creating a difference in charge.
An electrogenic pump creates voltage across a membrane. With each “crank” of the pump, there is a net transfer of one + charge, a process that stores energy as voltage.

42. Proton Pumps
The main electrogenic pump of plants, fungi, and bacteria is a proton pump.
A proton pump actively pumps out hydrogen ions (protons, H+).
The pumping of H+ transfers + charge from the cytoplasm, generating voltage.

43. Cotransport
A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of other solutes as well.
A substance that has been pumped across the membrane can do work as it moves back across by diffusion.
An example of cotransport: A carrier protein such as this sucrose-H+ transporter is able to use the diffusion of H+ down its electrochemical gradient to drive the uptake of sucrose.

44. Exocytosis
Exocytosis = the cell secretes (transports out of the cell) certain biological molecules by fusing vesicles with the plasma membrane.
The contents of the vesicle then spill to the outside of the cell and the vesicle membrane becomes part of the cell membrane.

45. Endocytosis
In endocytosis, the cell takes in biological molecules (such as food) by forming new vesicles from the plasma membrane.
A small area of the plasma membrane sinks inward to form a pocket.
As the pocket deepens, it pinches in, forming a vesicle.

46. Phagocytosis– “Cell Eating”
In Phagocytosis, a cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac that can be large enough to be called a vacuole.
The particle is then digested by a lysosome.

47. Pinocytosis “Cell Drinking”
In pinocytosis, a cell “gulps” in droplets of extracellular fluid into tiny vesicles.
It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplets.

48. Receptor-Mediated Endocytosis
Receptor-mediated endocytosis enables the cell to bring in bulk quantities of specific substances, even if they are not in high concentrations outside the cell.

49. Coated Pits
Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid.
The receptor proteins are usually clustered in regions of the membrane called coated pits.
These coated pits are lined on the inside of the cell with fuzzy proteins.