Carrier-Mediated Transport

Question 0

Michaelis Menten equation

J = J_max*[S]/(K_t + [S])

Answer 0

J = flux at substrate concentration [S]
J_max = maximum flux at infinite [S]
K_t = Michaelis constant at which half maximal transport occurs

Question 1

What are two important diagnostic characteristics of carrier-mediated transport proteins?

Answer 1

• saturability

- specificity

Question 2

competitive inhibition

Answer 2

occurs when two or more substrates of similar structure attempt to simultaneously access a limited population of transporters

*increases K_t

Question 3

What are the two phenomenon related to saturability and specificity?

Answer 3

• competitive inhibition

- the specific effect, usually inhibitory, some agents (toxins) can exert on particular transport processes

Question 4

What are the two general classes of mediated transport?

Answer 4

• facilitated diffusion

- active transport

Question 5

What is facilitated diffusion?

Answer 5

this process is only capable of supporting a net movement of solute across a membrane in response to a trans-membrane electrochemical gradient of the substrate molecule

Question 6

What classifies primary transport in regards to ATP use?

Answer 6

They directly use ATP; they are ATPases

- it is directly influenced by inhibition of metabolism (due to decrease of ATP in cytoplasm)

Question 7

There are two categories of active transport and what are its two classifications?

Answer 7

There are two categories of active transport that are differentiated on the basis of the immediate source of energy each uses to transport substrate against an electrochemical gradient: 1) primary active transport, and 2) secondary active transport.

Question 8

What are the five categories of primary active transport?

Answer 8

i) the Na,K-ATPase found in virtually all animal cell plasma membranes
ii) the Ca-ATPase of plasma membranes (PMCA)
iii) the Ca-ATPase of endoplasmic reticulum (SERCA)
iv) the H(K)-ATPase of a few cell types (incl. parietal cells of the stomach and intercalated cells of the kidney)
v) the “V”-type, or “vacuolar,” H-ATPase found in the membrane of several intracellular organelles (incl. lysosomes and endosomes) and in the plasma membrane of some cells

Question 10

What is ouabin, what does it effect?

Answer 10

• a cardiac glycoside
• selectively inhibits the transport activity of the Na,K-ATPase by interfering with the binding of K+ and the subsequent hydrolysis of ATP by the transporter

Question 10

What is the most widely used criterion for establishing the presence of a carrier-mediated process?

Answer 10

saturability

Question 11

phloretin

Answer 11

• specific toxic

- selective inhibitor of the GLUT family of Na-independent D-glucose transporters

Question 12

What are the concentrations of K+ inside and outside of the cell?

Answer 12

inside: 120-140 mM
outside: 3.5-5.0 mM

Question 13

What are typical values for Na+ concentration inside and outside of the cell?

Answer 13

inside: 10-20 mM
outside: 125-145 mM

Question 14

What is the function of a primary active transport process?

Answer 14

the maintenance of this steady state condition
(of the non-equilibrium conditions of Na and K gradients across the membrane and constant tendency for K to leave and NA to enter)
Na-K pump
3 Na out, 2 K in

Question 15

What are the reactants of the Na,K-ATPase?

Answer 15

• intracellular Na+
• extracellular K+
• intracellular ATP (goes to ADP and Pi)

Question 16

What are the products of the Na,K-ATPase?

Answer 16

• extracellular Na+
• intraceullular K+
• intracellular ADP and Pi

Question 17

Which ion gradient is responsible for defining the transmembrane difference in electrical voltage that is characteristic of all cells?

Answer 17

the K+ gradient

Question 18

Which ion serves as the immediate source of energy for many of the secondary active transport processes?

Answer 18

Na+

Question 19

What are two important results of the Na,K-ATPase?

Answer 19

• net removal of positive charge from cell; plays modest role in generation and maintenance of transmembrane electrical potential difference
• maintenance of normal cell volume (because of removal of cation); cytoplasm contains large concentration of impermeant anions (ex proteins) forces arise, that if not checked, tend to result in
  –> accumulation of small, permeant cations
  –> development of a significant osmotic gradient
  this can cause cell swelling

Question 20

What does secondary active transport make use of?

Answer 20

the potential energy stored in trans-membrane ion gradients

coupled transport processes

Question 21

What are two types of secondary active transport?

Answer 21

• cotransport (symport)

- counter-transport (antiport)

Question 22

Na-D-glucose cotransport

Answer 22

• most cells transport glucose by facilitated diffusion using GLUT family of transporters
• cotransporters used for transporting glucose against gradient (ex, in intestinal mucosa and renal proximal tubule)
• direct physical coupling of the flows of Na+ and glucose, with the energy for the process being derived from inwardly directed gradient for Na+

driving forces:

• chemical gradient for Na+
• PD across membrane

but intake of Na+ gradient needs to be maintained–by what? Na,K-ATPase

Question 23

Na/H exchange

Answer 23

• movement of Na+ ions into the cell results in pumping of protons out of cell
• 1:1
• therefore, no net movement of charge
• the principal mechanism used to regulate cytoplasmic pH

Question 24

What is the principal mechanism used to regulate cytoplasmic pH in most cells?

Answer 24

Na/H exchangers

*regulation of cell pH influences a wide variety of metabolic processes, ranging from aspects of energy metabolism to protein and DNA synthesis

Question 25

What is the Na/C exchanger?

Answer 25

3Na down its gradient into the cell, for one Ca2+ out of the cell (against gradient)

• secondary active countertransport
• vital role in beat-to-beat function of cardiac tissue

Question 26

Cl/HCO3 exchanger

Answer 26

a family of exchangers that influence cytoplasmic pH and play a critical role at shuttling HCO3- (and, therefore, CO2) into and out of RBCs.
more about this later in CPR block