TRANSPORT MECHANISMS - DR. SHRIER

Question 1

Which compartment are we talking about?

1. Materials enter the body, can get absorbed, and exit the body
2. Materials get absorbed from the GI tracht, and can move through lungs, skin and kidneys, and back up to the GI tracht
3. Materials go through the capillary wall from the plasma, and can cross the cell membrane and back up
4. Last compartment that is reached by a material entering from the GI tracht

Answer 1

1. Gastro-intestinal tracht
2. Plasma
3. ISF
4. ICF

Question 2

Cell membranes are highly permeable to__

Answer 2

Water, Lipid-soluble substances, Dissolved Gases (O2, CO2), Small uncharged molecules

Question 3

Cell membranes are less permeable to___

Answer 3

Larger molecules, charged particles

Question 4

Cell membranes are impermeable to__

Answer 4

Very large molecules

Question 5

Whats the cell membrane composed of?

Answer 5

Phospholipid bilayer.

Question 6

What are characteristics of the phospholipid bilayer

Answer 6

It’s ampiphatic (polar ends on the outside surface with hydrophilic heads; nonpolar on the inside with the hydrophobic tails)
Phospholipids comprise 40-50% of the plasma membrane by weight

Question 7

How does Cholesterol act within the phospholipid bilayer? (3 actions)

Answer 7

1) Reduces packing of fatty acid tails and tends to increase membrane fluidity.
2) Involved in formation of vesicles that pinch off the plasma membrane and in lipid rafts.
3) Acts to stabilize cell membrane and adds firmness when at high temperatures; Inserts itself into phospholipids and prevents aggregation when at low temperatures

Question 8

Is Cholesterol hydrophobic, hydrophilic or amphipatic

Answer 8

amphipatic

Question 9

Which proteins are found in cell membranes

Answer 9

Integral: closely associated with phospholipids, mostly cross the membrane (transmembrane, amphipatic)

Peripheral: more loosely associated, mostly on the cytoplasmic side

Question 10

What percentage of weight of the cell membrane do proteins take up

Answer 10

25 to 75 per cent

Question 11

what is glycocalyx and what is it attached to

Answer 11

layer of carbohydrate formed by a chain of monosaccharides that extend from the extra cellular surface of the cell membrane. it is bound to proteins.

Question 12

what is glycocalyx role (3)

Answer 12

provides protection from infection, enables cells to identify each other and interact

Question 13

what are 6 functions of plasma membrane proteins

Answer 13

selective transport channel
enzyme
cell surface receptor
cell surface identity marker
cell adhesion
attachment to the cytoskeleton

Question 14

What is enzymes role in membrane

Answer 14

Catalyze membrane associated rx

Question 15

what is receptors role in cell membrane

Answer 15

receiving and transducing chemical signals from the cell environment

Question 16

what are the two transmembrane transport pathways

Answer 16

1. via phospholipid layer
2. via interaction with protein cluster (channel or carrier)

Question 17

What are the two types of transport mechanisms across cell membrane

Answer 17

passive and active

Question 18

what are 3 passive transport mechanisms across cell membrane

Answer 18

diffusion, carrier-mediated facilitated diffusion, osmosis

Question 19

what are 2 active transport mechanisms across cell membrane

Answer 19

1. carrier-mediated active transport (a. primary b. secondary)
2. pino/phagocytosis

Question 20

whats the difference between active and passive transport mechanisms

Answer 20

active = energy dependant
passive = energy independant

Question 21

whats diffusion

Answer 21

movmt of molecules from one location to another as a result of random thermal motion

Question 22

what is flux

Answer 22

amount of particles crossing a surface per unit time

Question 23

net flux is from ___ [C] to ___ [C]

Answer 23

high to low

Question 24

at equilibrium, diffusion fluxes are ___ and net flux is ___

Answer 24

equal, zero

Question 25

At equilibrium, net movement =

Answer 25

0

Question 26

When a cell is exposed to a solute in a large extracellular solution, the Ci rises over time and only plateau's when?

Answer 26

when Ci = Co

Question 27

The rate of diffusion may be calculated by Fick's Law of Diffusion, which is given by

Answer 27

J = PA (C0 - Ci)

Question 28

In J = PA (C0-Ci), what do the variables mean

Answer 28

J = net flux = rate of diffusion P = permeability coefficient, constant based on the ease with which a molecule moves through a membrane A = surface area of the membrane C0-Ci = concentration gradient of the diffusing molecule across the membrane

Question 29

Diffusion time increases in proportion to the ____________ travelled by the solute molecules. (1 um = 1 msec, 10 um = 100 msec, etc.)

Answer 29

square of the distance

Question 30

Diffusion is an effective transport process only over _____ distances.

Answer 30

short

Question 31

what are 5 factors that affect diffusion across the cell membrane

Answer 31

1. mass of molecule 2. concentration gradient across cell membrane 3. lipid solubility 4. electrical charge 5. availability of selective ion channels or membrane carriers

Question 32

Diffusion particles may penetrate either by a) or b)

Answer 32

a) dissolving in the lipid component (for non-polar molecules), e.g., o2, co2, fatty acids b) diffusing through channels (for ions)

Question 33

In either way of penetration, diffusion movement depends upon the existence of a

Answer 33

concnetration gradient

Question 34

Ion channels show selectivity based on _____ as well as on ______

Answer 34

diameter; distribution of charges lining the channel

Question 35

Movement of ions is affected not only by concentration gradient, but by ______. The simultaneous existence of these two gradients for a particular ion is known as_____.

Answer 35

electrical gradient; electrochemical gradient

Question 36

whats the resting membrane potential

Answer 36

-90 mV

Question 37

Ion channels may be gated in which 3 ways?

Answer 37

1) Ligand-gated 2)Voltage-gated 3)Mechanically-gated

Question 38

What is gating of ion channels

Answer 38

Ion channels can exist in open or closed state as they undergo conformational changes

Question 39

Which are the voltage gated ion channels?

Answer 39

Na+ channel, K+ channel, Ca+ channel, Cl- channel

Question 40

The total number of ions that flow through these channels generating ionic current depend upon:

Answer 40

1. channel conductance 2. how often the channel opens 3. how long the channel stays open

Question 41

Mediated transport is the movement of ions and other molecules (glucose, AA's) by integral membrane proteins called ____

Answer 41

transporters

Question 42

Ion movement across membranes via transporters is much slower than through ion channels. Which two types of movmeent require transporters?

Answer 42

Facilitated diffusion (passive) and Active transport (active)

Question 43

what are 3 characteristics of mediated-transport systems

Answer 43

1. specificity 2. saturation 3. competition

Question 44

What is specificity, a characteristic of mediated-transport systems

Answer 44

system usually transports one particular type of molecule only

Question 45

what is saturation , a characteristic of mediated-transport systems

Answer 45

rate of transport reaches a maximum when all binding sites on all transporters are occupied. Thus , a limit - the TRANSPORT MAXIMUM (Tm) - exists for a given substance across a given membrane

Question 46

what is competition - a characteristic of mediated-transport systems

Answer 46

occurs when structurally similar substances compete for the same binding site on a membrane carrier

Question 47

how does flux into cell differ between diffusion and mediated transport?

Answer 47

mediated transport reaches a maximal flux, at the Tm, whereas diffusion does not

Question 48

which 4 factors determine flux magnitude of mediated-transport systems

Answer 48

1. solute concentration 2. affinity of transporter for the solute 3. numbers of transporters 4. rate of transporter conformational change

Question 49

what is facilitated diffusion

Answer 49

involves the presence of a transporter molecule, which enables a solute to penetrate more readily than it would be expected to by simple diffusion.

Question 50

what are 4 steps to facilitated diffusion

Answer 50

a) solute binds to transporter b) transporter changes configuration c) solute is delivered to other side of membrane d) transporter resumes original configuration

Question 51

what are 3 characteristics of facilitated diffusion

Answer 51

1- transporter mediated 2- passive 3- next flux from high to low concentration

Question 52

what can increase the number or affinity of tansporters in some membrnaes; give an example

Answer 52

hormones; ex. insulin increases Glut-4 which transports glucose in muscle

Question 53

what are 4 characteristics of passive transport

Answer 53

1- transporter mediated 2- requires supply of chemical energy (usually derived form of enzymatic hydrolysis of ATP) 3- susceptible to metabolic inhibitors 4- can transport solute agaisnt its concnetration gradient

Question 54

What is primary active transport?

Answer 54

WHen ATP gives a P group to the transporter as part of phosphorylation, the conformation of the transporter changes and lets the solute cross the membrane agaisnt its gradient of concentration. After it passes, the transporter dephosphoylates and pumps the solute on the other side come across the opposite way. This is how it works with Na+/K+-ATPase.

Question 55

What are other primary active transporters?

Answer 55

Ca2+-ATPase: maintain low intracellular Ca2+ levels H+-ATPase: maintain low lysosomal pH H+/K+-ATPase: acifification of the stomach