Membrane Channels

From the overview of kidney function above, it is clear that blood components (e.g., water, ions, sugars) must be able to pass between the nephron tubules and the blood-filled capillaries surrounding them. But recall from the Introduction to this experiment (in the lab manual) that phospholipid-bilayer membranes are not permeable to polar molecules, because the interior lipid region of the membrane is nonpolar. Thus, the polar components of blood could not cross the membranes surrounding the tubules (Figure 3), unless these membranes contained special channels to allow the passage of polar species.

Figure 3

This is a schematic diagram of a segment of a nephron tubule with no protein channels (unlike a real tubule segment, which contains channels) in the phospholipid-bilayer membrane surrounding the tubule, shown as a lengthwise slice through the tubule segment. Polar molecules (green) cannot travel out of the tubule to the blood in the capillaries, because they are insoluble in the hydrophobic (nonpolar) lipid interior of the membrane. To permit passage of polar and charged species between the capillaries and the nephron, the membrane must have protein channels embedded in it, as discussed below. Phospholipid-bilayer membranes are discussed further in the Introduction to the experiment in the lab manual.

Note: For simplicity, the tubule is depicted here as being enclosed by a single membrane. In fact, the tubule and capillaries are lined with cells that are surrounded by membranes. Thus, a particle must travel across several membranes in order to move between the interior of the tubule and the blood-containing capillaries. This figure is not drawn to scale.

The channels required to allow the passage of polar blood components are formed by proteins embedded in the phospholipid-bilayer membrane (Figure 4). Proteins that form channels in the membrane typically have membrane-spanning cylindrical shapes: there is a hydrophobic surface that can interact with the "tail" region of the phospholipid-bilayer membrane and a hollow internal core that forms the pore. These proteins form a "tunnel" from the aqueous phase on one side of the membrane to the aqueous phase on the other side of the membrane. The size of the tunnel determines the size of the particles that will be able to pass through the channel.

Figure 4

This is a CPK representation of a potassium channel embedded in a schematic phospholipid-bilayer membrane. (The pale yellow circles represent the polar head groups and the gray lines represent the nonpolar tails.) This channel is composed of four polypeptide chains (shown in different colors) that span the width of the membrane, with a hollow space through which potassium ions may pass (like a tunnel). Some of the amino acids have been removed to reveal the space occupied by the potassium ion as it crosses the membrane from the aqueous phase on one side to the aqueous phase on the other side.

Note: The coordinates for this protein were determined by x-ray crystallography, and the protein component of this image was rendered using SwissPDB Viewer and POV-Ray (see References).

If the internal core of the protein channel is lined with hydrophilic amino-acid residues, then the channel allows passage of polar or charged particles between the two aqueous sides of the membrane. Figure 5 shows a representative ion channel, with hydrophilic residues lining the internal core and hydrophobic residues lining the regions of the protein that contact the lipid tails in the interior of the membrane.

Figure 5

This is a view through the opening of the same potassium channel shown in Figure 4. Notice that the inner core is lined with hydrophilic amino-acid residues (blue) that interact favorably with the charge on the ion (yellow). The outer areas of the channel contain hydrophobic amino-acid residues (plum), which interact favorably with the hydrophobic lipids in the membrane.

Note: The coordinates for this protein were determined by x-ray crystallography, and the image was rendered using SwissPDB Viewer and POV-Ray (see References).

These channels may be left open continuously, or they may be opened and closed by elaborate cellular gating mechanisms, as we will see below for three representative cases in the kidneys. In either case, passage of particles through the membrane is dictated by the size, shape, and polarity of the channel.

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Revised: 2004-08-08