Membrane Transport Proteins: Channels, Transporters, and Pumps

Flashcards covering the functional differences between channels and transporters, the discovery of aquaporins, types of active transport, and the mechanism and function of the Na+/K+-ATPase pump.

What is the primary function of a channel in membrane transport?

A channel provides an open passageway for the facilitated diffusion of ions or molecules across a membrane.

How quickly can solutes move through an open channel?

When a channel is open, the transmembrane movement of solutes can be extremely rapid, up to 100 million ions or molecules per second.

What is the key difference in function between channels and transporters?

Channels provide open passageways for rapid facilitated diffusion, whereas transporters bind solutes within a hydrophilic pocket and undergo conformational changes for movement, typically at a much slower rate.

Describe the first step in the pumping mechanism of Na+/K+-ATPase.

Three Na+ bind to the E1 conformation from the cytosol, ATP is hydrolyzed, phosphate is covalently bound to the pump (phosphorylation), and the pump switches to the E2 conformation.

Why is Na+/K+-ATPase considered a critical ion pump in animal cells?

It maintains critical Na+ and K+ gradients across the plasma membrane, which are essential for many cellular processes such as neuronal signaling and muscle contraction.

What makes Na+/K+-ATPase an electrogenic pump?

It pumps three Na+ out and two K+ into the cell per ATP hydrolyzed, resulting in the net export of one positive charge and thus producing an electrical gradient across the membrane.

How do ion electrochemical gradients contribute to the transport of ions and molecules?

Symporters and antiporters utilize H+ and Na+ gradients to actively transport nutrients into cells and export waste products.

What is the role of H+ gradients in energy production within the mitochondrion and chloroplast?

In both the mitochondrion and chloroplast, H+ gradients are used to synthesize ATP.

What is the fundamental difference between primary active transport and secondary active transport?

Primary active transport directly uses energy (often from ATP) to move a solute against its gradient, while secondary active transport uses a pre-existing electrochemical gradient, established by primary active transport, to drive the movement of another solute.

Compare and contrast uniporters, symporters, and antiporters.

Uniporters bind and transport a single ion or molecule across the membrane. Symporters (cotransporters) bind two or more ions or molecules and transport them in the same direction. Antiporters (exchangers) bind two or more ions or molecules and transport them in opposite directions.

How do transporters typically move solutes across the membrane?

Transporters bind one or more solutes in a hydrophilic pocket and undergo a conformational change that switches the exposure of the pocket from one side of the membrane to the other, where the solute is then released.

How does the rate of transport for transporters compare to that of channels?

Transporters are much slower than channels, typically moving 100 to 1000 ions or molecules per second, while channels can move up to 100 million ions or molecules per second.

What observations led researchers, including Peter Agre, to hypothesize the existence of water channels?

Researchers observed that certain cell types, like red blood cells and kidney cells, allowed water to move across their plasma membranes much faster than simple diffusion alone could explain.

How did Agre and colleagues test their hypothesis that CHIP28 functions as a water channel?

They injected CHIP28 mRNA into frog oocytes, allowed the protein to be expressed, and then compared the swelling and lysis rates of these oocytes versus control oocytes when placed in a hypotonic medium.

What were the key results of Agre's experiment indicating that CHIP28 is a water channel?

Oocytes expressing CHIP28 swelled rapidly and lysed within 3-5 minutes in a hypotonic medium, whereas control oocytes did not swell as rapidly or rupture.

What was CHIP28 renamed to, and why?

CHIP28 was renamed aquaporin to reflect its newly identified function of allowing water to diffuse through a channel in the membrane.

What does it mean for most channels to be 'gated,' and how does this benefit cells?

Being 'gated' means channels open to allow solute diffusion and close to prohibit it, allowing cells to regulate the movement of solutes across the membrane.

How can gated channels be controlled?

Gating can be controlled by the direct binding of small molecules called ligands, such as hormones or neurotransmitters, to the channel protein itself.