Lecture 9, 10: Cell Membrane Dynamics

Question 1

fluid mosaic of the cell membrane

Answer 1

Membranes must have correct fluidity to function
- Too slow: proteins embedded in membrane do not work
- Too fast: not enough membrane organization to function right

Question 2

membrane fluidity is affected by:

Answer 2

1. Temperature
2. Saturated vs. Unsaturated hydrophobic tails of phospholipids
3. Presence of cholesterol

Question 3

two main types of proteins suspended in membrane

Answer 3

1. Integral membrane proteins: penetrate the hydrophobic interior of the lipid bilayer
   - Transmembrane proteins go all the way through
2. Peripheral proteins: associated with only one side of the membrane

Question 4

what do transmembrane proteins do

Answer 4

transport
enzymatic activity
signal transduction
cell-cell recognition
intercellular joining
attachment to cytoskeleton/ECM

Question 5

concentration gradient:

Answer 5

high -> low concentration, when gradient is equal, its at equilibrium

Question 6

passive vs. active transport:

Answer 6

passive: doesn’t require energy
active: requires energy

Question 7

osmolarity:

Answer 7

the number of osmoles of all solutes per litre of solution (Osm/L)

Question 8

tonicity:

Answer 8

the ability of the surrounding solution to cause a gain or loss of water within the cell

Question 9

isotonic vs. hypertonic vs. hypotonic

Answer 9

• Isotonic: (iso=same) no net movement of free water across the plasma membrane
• Hypertonic: (hyper=more) concentration of solutes is greater outside the cell
• Hypotonic: (hypo=less) concentration of solutes is greater inside the cell

Question 10

simple diffusion:

Answer 10

Movement from higher to lower concentrations straight through lipid layer

Question 11

facilitated diffusion:

Answer 11

Movement from higher -> lower concentrations via a membrane carrier protein (aka ‘facilitative transporter’)…. Still diffusion

Question 12

simple diffusion through a channel

Answer 12

Movement from higher -> lower concentrations through the pore of a membrane channel protein

Question 13

active transport

Answer 13

• Movement from low -> high concentrations via a protein transporter
• Requires energy, which often comes from ATP hydrolysis

Question 14

4 ways molecules move past membranes

Answer 14

1. Simple diffusion (passive)
2. Simple diffusion through a channel (passive)
3. Facilitated diffusion (passive)
4. Active transport (active)

Question 15

two kinds of energy are stored in the membrane:

Answer 15

Chemical force:
- The ions concentration gradient
- Higher conc of an uncharged molecule on one side of the membrane is a store of energy due to entropy (thermodynamics)

Electrical force:
- Effect of the membrane potential on the ion’s movement
The electrochemical gradient is the combination of the store of energy due to entropy (chemical force) AND the difference in charge cross the membrane (i.e. membrane potential)
-> this energy can be used to do stuff!

Question 16

what does a selectively permeable membrane mean?

Answer 16

Regulates what can pass through membrane, composition of plasma membrane makes this possible (plasma membrane: mosaic of phospholipids & other proteins)

Question 17

what is osmosis?

Answer 17

a process by which molecules of a solvent tend to pass through a semipermeable membrane from a less concentrated solution into a more concentrated one, thus equalizing the concentrations on each side of the membrane

Question 18

what is secondary active transport?

Answer 18

the transport of a solute in the direction of its increasing electrochemical potential coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential