Membrane Carriers (Lecture 3, Exam 1)

Front 1

What do carriers transport?

Back 1

Polar and charge solutes (ions, sugars, amino acids and other solutes)

Front 2

How do carriers work?

Back 2

Carriers bind to solute, alter their conformation to move solute across the membrane, release solute on other side.

Front 3

What is the rate of transport for carriers in comparison to channels?

Back 3

Carriers have a slower rate d/t conformation change

Front 4

Are carriers reversible?

Back 4

Yes, most carriers can move solute both in and out of a cell.

Front 5

Main Characteristics of Carriers

Back 5

1. Show chemical specificity

2. demonstrate competition, can saturate

3. Non-competitive inhibition (ie ligand binds, but is not transported)

4. Rate of transport described by enzyme kinetics (for a simple carrier, km = 1/2Vmax )

Front 6

Differences between carriers and channels?

Back 6

1. carriers are slower

2. offer mechanism that can create or maintain an electrochemical gradient b/c they can directly or indirectly utilize metabolic energy

3. Channels are uncoupled, carriers can be coupled

4. channels are driven by electrochemical gradient only

Front 7

How are carriers classified?

Back 7

by how their energy is utilized, either active or passive

Front 8

Characteristics of Passive Carriers

Back 8

1. driving force is the difference in electrochemical potential for the transported solute across the membrane

2. cannot create of maintain a concentration gradient

3. pathway for polar and charged solutes across bilayer

4. no input of metabolic energy

Front 9

Characterisitics of Active Carriers

Back 9

1. Can create or maintain concentration gradients

2. Driven by metabolic energy either directly or indirectly

Front 10

Example of a passive carrier?

Back 10

D-glucose carrier

Front 11

What does Km measure when referring to glucose?

Back 11

Km is a measure of the affinity of the glucose transport protein for glucose.

Front 12

What is KM?

Back 12

KM is the concentration of external glucose at which half maximal velocity (Vmax) is reached

Front 13

Definition of affinity

Back 13

measure of attraction or binding strength between receptor and its ligand and its ligand

Front 14

What primarily determines the value of Km for a carrier?

Back 14

Km for a carrier is primarily determined by the binding affinity of the carrier for the solute. (the higher the value of Km the lower the affinity of the transporter for the solute)

Front 15

Primary Active Transport

Back 15

Directly utilizing energy at the site of transport, normally from ATP hydrolysis

Front 16

3 examples of primary active transporters?

Back 16

1. Na, K-ATPase

2. CA- ATPase

3. H,K-ATPase

Front 17

Important points about Na,K-ATPase

Back 17

1. present in the plasma membrane of almost all cells

2. Most important factor in excluding Na+ from and concentratingK+ in the ICF.

3. pumps at ratio of 3Na:2K for each molecule of ATP hydrolyzed

Front 18

About Ca-ATPase in ER and SR (SERCA) and in the plasmamembrane (PMCA)

Back 18

1. PMCA and SERCA responsible for regulating the conc of intracellular Ca++ and maintaining the resting level of free Ca++ 10^4 lower than present in the ECF

2. Molecular structure of SERCA1 shows how an activetransport protein changes its structure in order to transport asolute.

Front 19

Where would you find a H,K-ATPase (H,K-pump)?

Back 19

in gastric mucosa, kidney, stomach, and intestine

Front 20

What is the pump/leak model?

Back 20

relationship between ion transport and cellular metabolism where metabolic energy is stored in the form of ion gradients that are subsequently used by other transporters to carry out specific functions.

Front 21

What things are the sodium gradient used to do?

Back 21

1. regulate cell pH by actively transporting protons via the Na+/H+ exchanger

2. regulate cell Ca++ via the Na+/Ca++ exchanger

3. actively transport sugars and amino acids via other proteins

4. regulate cell volume via movement of Na+ and K+ ions

5. depolarize membrane potential

Front 22

How do cardiac glycosides such as ouabain and digitalis inhibit Na,K-ATPase?

Back 22

Affect the operation of Na-dependent transportmechanisms by diminishing the Na+electrochemical gradient. In cardiac muscle Na+/Ca++ exchanger is inhibited, ^ intracellular Ca++, and ^ force of contraction

Front 23

Secondary active transport

Back 23

the indirect use of metabolic energy to drive active transport

Front 24

Examples of secondary active transport

Back 24

Na+-glucose cotransporter:

• Important for oral rehydration

Na/Ca exchanger (NCX): smooth and cardiacmuscle

• Regulates [Ca++]i

Na/H exchanger: present in most cells

• Regulates [H+]i

Front 25

Cotransport (symport)

Back 25

solutes transported in the same direction. ex = Na+/glucose cotransporter in epithelial cells of small intestine

Front 26

Countertransport (antiport or exchange)

Back 26

solutes move in opposite direction across cell membrane. ex = CA++/Na+ exchange protein

Front 27

Explain how the cotransporter Na+/glucose transports solutes across epithelial cells of the small intestines

Back 27

1. the Na+/K+ pump maintains the electrochemical gradient thru active transport

2. this maintains low conc of Na+ in cell, therefor Na+ will want to move out of high conc lumen into the cell, and when it does it carries a glucose molecule with it.

3. Now, ^ glucose conc in cell creates a concentration gradient between the cell and the blood, so glucose then passively moves down the concentration gradient out of cell and into blood.

Front 28

Why does water containing sodium and glucose promote rehydration?

Back 28

Water has a tendency to follow solute, therefore by activating the Na+/glucose cotransporter, you are more rapidly moving solute (and water following the solute) from the lumen, into the epithelial cells of the small intestines, and into the blood, therefore more quickly rehydrating than with just pure H2O

Front 29

Rate at which solute cross the plasmamembrane via carrier or channel isdetermined by

Back 29

– the number of transport proteins in the plasmamembrane.

– specific properties of the transport proteins.

– how transport proteins are regulated by cellsignaling mechanisms.

Front 30

Explain how digitalis increases the strength of cardiac muscle contraction

Back 30

digitalis binds to Na,K-ATPase, partially inhibiting receptors, causing intracellular conc of Na+ to ^, which decreases the amount of energy used by the secondary active cotransporter to pump Ca+ out of the cell. therefore conc of Ca+ inside the cell increases. when conc of Ca+ goes above 10^-7, contraction occurs.

Front 31

Explain how the Ca++/Na+ exchange or antiport protein works

Back 31

at resting membrane potential (normally -50 mV to -80 mV) calcium is pumped out of the cell by the NCX pump. Na+ binds to the carrier on outside of cell, and Ca++ binds to site inside. As Na+ goes down its electrochemical gradient via the carrier, it causes a conformational change in the carrier which pumps the Ca++ out of the cell against its electrochemical gradient. When membrane potential becomes less negative and depolarization occurs, there is no more energy in the calcium gradient and the direction of this carrier can reverse. 3Na+ to 1Ca++

Front 32

what % of ATP is used by the Na,K-ATPase pumps?

Back 32

20 to 40%