Membrane

Question 1

Structure of a Phospholipid

Answer 1

1. Hydrophilic head
   - Polar
2. Gylcerol backbone
3. Hydrophobic Tail
   - Fatty acid tail
   - “Kink” caused by the presence of a double bond (unsaturated hydrocarbon) which bends but does not rotate well

Question 2

Phospholipid bilayer

Answer 2

Hydrophilic heads face the inside and outside of the cell. Hydrophobic tails create the inside, middle layer.

Question 3

Fluid Mosaic Model

Answer 3

Membranes have a mosaic of proteins in a phospholipid bilayer. Proteins have a hydrophilic and hydrophobic region so they can be membrane-bound.`

Question 4

What does fluid mosaic mean?

Answer 4

1. Membrane fluidity: membrane lipids drift laterally (frequent), and even “flip-flop” (rarely).

Question 5

How does one visualize the lateral movement of lipids?

Answer 5

Fluorescence Photobleaching:

1. Label lipids with a fluorescent tag
2. Focus a strong beam on a cell surface to bleach the label
3. Watch how fast the label comes back from unbleached parts.

Question 6

How do we visualize protein movement

Answer 6

1. Label two membrane proteins, one with phodamine (red) and the other with fluorescence (green)
2. Fuse both types of cells
3. Mixed proteins after a few hours.
   - Proteins move also, but at a much slower rate; some of them are bound to the cytoskeleton

Question 7

Are membranes always fluid?

Answer 7

1. No, they solidify at low temperatures
2. Unsaturated fatty acid residues solidify at lower temperatures (as compared to saturated residues)
3. Due to its bulky structure, cholesterol has a dual effect:
   - High temp: restricts the movement of phospholipids, reducing fluidity
   - Low temp: prevents close packing of phospholipids, increasing fluidity.
4. Environmental conditions can cause changes in membrane structure (in winter, plants have more unsaturated fatty acid residues in their phospholipids to prevent solidification of their cell membranes)

Question 8

Fluid vs Viscous

Answer 8

Fluid membrane has unsaturated hydrocarbon tails with kinks. Viscous membrane has saturated hydrocarbon tails.

Question 9

Membrane Proteins

Answer 9

Membrane proteins = integral + peripheral.

1. Integral Protein: are at least partly inserted into membranes; most completely span it (even several times)
2. Peripheral proteins: are attached to the membrane surface, but not inserted.

Question 10

Functions of the cell membrane

Answer 10

1. Cell-cell recognition and communication involve specific molecules on the cell surfaces (ex: blood groups reflect variations in cell surface oligosaccharides)
2. One of the most important properties of biological membranes is the ability to regulate transport into and out of the cell.
   - Via channels (Na+, K+, etc), endocytosis
   - Cell-cell connections
3. Enzymatic activity
4. Signal transduction
5. Intercellular joining
6. Attachment to the cytoskeleton and extracellular matrix

Question 11

Extracellular Matrix (ECM)

Answer 11

ECM helps the cell adhere and communicate. Integrins bind ECM outside and transmit info from ECM to cytoskeleton inside.

Question 12

Transport of Small molecules

Answer 12

• Transport is selective and often unidirectional
• The lipid bilayer is impermeable to charged molecules (even small ions Na+, K+, Cl-) and larger charged and/or polar molecules (sugar, amino acid). However, these molecuels are still transported
• They are transported by hydrophilic membrane proteins that act either as channels or as shuttles.

Question 13

Types of transport across membranes

Answer 13

*Transport can be Passive (down concentration gradient, no energy required) or Active (against concentration gradient, requires energy)
1. Passive transport: cell does not spend energy
(While movement of an individual molecule is random, movement of a large number of molecules can be directional. Molecules will spread into an available space down a concentration gradient)
- Usually hydrophobic and small uncharged molecules move passively across membranes
- Diffusion: transport of a solute down a concentration gradient
- Osmosis: transport of water down ITS concentration gradient

Question 14

Diffusion

Answer 14

Diffusion stops at equilibrium.

- Note that, at equilibrium, individual molecules do not stop moving, but their concentrations remain constant.

Question 15

Osmosis

Answer 15

If the solutes are too large to diffuse, water molecules will then move through the membrane to dilute the more concentrated solute.
- Relative measure of the ability for osmosis (based on relative concentrations of solute):
Hypersomotic (hypertonic)
Isoosmotic (isotonic)
Hypoosmotic (hypotonic)

Question 16

Osmosis Characteristics

Answer 16

1. Osmostic pressure: the tendency for a solution to take up water when separated from pure water by a selectively permeable membrane
2. Cells have many solutes (mostly macromolecules) that cannot diffuse through membranes => they are hyperosmotic to pure water.

Question 17

Osomotic Environments

Answer 17

Moves from distilled water (lowest salinity) to ocean water (highest salinity).
- Water moves from hypo-osmotic environment to hyper-osmotic environment to dilute the more concentrated solute.

Question 18

3 Types of Cellular osmotic environments

Answer 18

1. Hypotonic solution
   - Water enters
   - Animal cells lyse
   - Plant cell (optimal) turgid
2. Isotonic
   - Water in = water out
   - Animal cell normal
   - Plant cell flaccid
3. Hypertonic
   - Water in cell goes out
   - Animal cell shriveled
   - Plant cell plasmolyzed

Question 19

Facilitated diffusion

Answer 19

• An intermediate step between diffusion and active transport.
• Like diffusion, it occurs down a concentration gradient (usually hydrophilic substances)
• Like active transport, it is facilitated by proteins
• > Bind specifically to transport substrate (cargo)
• > Exhibit saturation by transport substrate

Question 20

Active Transport

Answer 20

• Works against a concentration gradient, and requires energy
• Ex: sodium-potassium pump. Cells have high concentrations of potassium ions and low concentrations of sodium ions.
• Active export of sodium and import of potassium

Question 21

Na+ K+ pump

Answer 21

• A four protein complex that spans the membrane
• Binds 3 ions of sodium inside the cell, receives energy by phosphorylation
• Changes conformation and expels sodium ions to outside
• Binds 2 ions of potassium outside the cell, is dephosphorylated and returns to original conformation, releases potassium ions inside the cells and again binds 3 ions of sodium.
• Because sodium-potassium channel causes hydrolysis of ATP, it is also called sodium-potassium ATPase

Question 22

Chlorine Channel

Answer 22

• Ion transport channel regulated by phosphorylation (cAMP dependent)
• Defects in this channel cause cystic fibrosis
• The channel is called CFTR = CF transmembrane conductance regulator

Question 23

Co-transport

Answer 23

• Transport of H+ can occur through an ATP-dependent pump
• It increases membrane potential by removing protons
• Co-transport = H+ gradient can drive another active transport
  (H+ “leaking” into the cell helps import other molecules against their gradient)

Question 24

How are large molecules transported across membranes?

Answer 24

1. Phagocytosis = non-specific engulfing and internalizing a particle
2. Pinocytosis = same as above for liquid droplets
3. Receptor-mediated endocytosis = specific internalization (the major type of transport of macromolecules across membranes)

Question 25

Receptor-mediated endocytosis

Answer 25

• Macromolecules bind to a specific receptor on cell surface
• These receptors are in specific areas (called coated pits)
• Coated pits are coated with specific protein (clathrin) on the inner side of the membrane
• Binding results in internalization and formation of the transport vesicles (endosomes)

Question 26

Membrane transport of large molecules Examples

Answer 26

1. Uptake of cholesterol
   - lack of receptors results in genetic disease called hypercholesterolemia (high cholesterol level in blood)
2. Virus infection by endocytosis
   - Influenza
   - Vesicular stomatits virus