Lectures 21 & 22 Part Four

Front

What is an electrochemical gradient?

Back

An electrochemical gradient, or electrochemical potential, determines the direction in which a charged solute moves across a membrane. It is composed of the chemical gradient and the electrical gradient (Vm).

Front

What is the role of membrane transporters and ion channels?

Back

Membrane transporters and ion channels catalyze the transmembrane movement of specific solutes, allowing cells to move solutes across the lipid bilayer, which is otherwise impermeable to charged or polar solutes.

Front

What is the difference between passive and active transporters?

Back

Passive transporters facilitate movement down a concentration gradient, increasing the transport rate (passive transport or facilitated diffusion). Active transporters move substrates across membranes against a concentration gradient or an electrical potential (active transport).

Front

What are primary and secondary active transporters?

Back

Primary active transporters use energy provided directly by a chemical reaction. Secondary active transporters couple the uphill transport of one substrate with the downhill transport of another.

Front

How do transporter proteins reduce the energy of activation for diffusion?

Back

Transporter proteins reduce the energy of activation (∆G‡) for diffusion by forming noncovalent interactions with the dehydrated solute and providing a hydrophilic transmembrane pathway.

Front

What are ion channels and their characteristics?

Back

Ion channels provide an aqueous path across the membrane for inorganic ions to diffuse at high rates. They have a gate regulated by a biological signal, show specificity for an ion, are not saturable, and flow stops when the gate is closed or there is no electrochemical gradient.

Front

How does the glucose transporter of erythrocytes work?

Back

Glucose enters erythrocytes by passive transport via GLUT1, analogous to an enzymatic reaction where glucose outside is the substrate (Sout), glucose inside is the product (Sin), and the transporter is the enzyme (T).

Front

What is the rate equation for glucose transport?

Back

The rate equation for glucose transport is analogous to the Michaelis-Menten equation: V0 = Vmax [S]out / (Kt + [S]out), where V0 is the initial velocity of glucose accumulation inside the cell, [S]out is the glucose concentration outside, and Kt is a constant.

Front

What is the membrane topology of GLUT1?

Back

GLUT1 is an integral membrane protein with 12 hydrophobic segments forming 12 membrane-spanning helices. The helices are amphipathic, with nonpolar residues on one side and polar residues on the other.

Front

What are the general classes of transport systems?

Back

Transport systems include cotransport systems (simultaneously transport two solutes, either antiport or symport) and uniport systems (carry only one substrate).