Question

Compare and contrast facilitated diffusion and passive diffusion. Explain how the sodium-potassium ion pump works. include an explanation of why ATP is involved and how sodium and potassium ions are “chemically moved” across the membrane.

Answer 1

Diffusion through the cell membrane is divided into two subtypes, called simple diffusion and facilitated diffusion. Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane opening or through intermolecular spaces without any interaction with carrier proteins in the membrane. The rate of diffusion is determined by the amount of substance available, the velocity of kinetic motion, and the number and sizes of openings in the membrane through which the molecules or ions can move.
Facilitated diffusion requires interaction of a carrier protein. The carrier protein aids passage of the molecules or ions through the membrane by binding chemically with them and shuttling them through the membrane in this form.
Simple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of the lipid bilayer if the diffusing substance is lipid soluble and
(2) through watery channels that penetrate all the way through some of the large transport proteins, as shown to the left in Figure

sodium-potassium (Na+-K+) pump,
a transport process that pumps sodium ions outward through the cell membrane of all cells and at the same time pumps potassium ions from the outside to the inside. This pump is responsible for maintaining the sodium and potassium concentration differences across the cell membrane, as well as for establishing a negative electrical voltage inside the cells.

The carrier protein is a complex of two separate globular proteins—a larger one called the ? subunit, with a molecular weight of about 100,000, and a smaller one called the ? subunit, with a molecular weight of about 55,000. Although the function of the smaller protein is not known (except that it might anchor the protein complex in the lipid membrane), the larger protein has three specific features that are important for the functioning of the pump:
1. It has three binding sites for sodium ions on the portion of the protein that protrudes to the inside of the cell.
2. It has two binding sites for potassium ions on the outside.
3. The inside portion of this protein near the sodium binding sites has adenosine triphosphatase (ATPase) activity.
When two potassium ions bind on the outside of the carrier protein and three sodium ions bind on the inside, the ATPase function of the protein becomes activated. Activation of the ATPase function leads to cleavage of one molecule of ATP, splitting it to adenosine diphosphate (ADP) and liberating a high-energy phosphate bond of energy. This liberated energy is then believed to cause a chemical and conformational change in the protein carrier molecule, extruding the three sodium ions to the outside and the two potassium ions to the inside.

As with other enzymes, the Na+-K+ ATPase pump can run in reverse. If the electrochemical gradients for Na+ and K+ are experimentally increased to the degree that the energy stored in their gradients is greater than the chemical energy of ATP hydrolysis, these ions will move down their concentration gradients and the Na+-K+ pump will synthesize ATP from ADP and phosphate.

The phosphorylated form of the Na+-K+ pump, therefore, can either donate its phosphate to ADP to produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the cell. The relative concentrations of ATP, ADP, and phosphate, as well as the electrochemical gradients for Na+ and K+, determine the direction of the enzyme reaction. For some cells, such as electrically active nerve cells, 60 to 70 percent of the cells’ energy requirement may be devoted to pumping Na+ out of the cell and K+ into the cell.

Figure2---The postulated mechanism of the sodium-potassium pump. ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion.

Sodium-Potassium Pump Transports Sodium Ions Out of Cells and Potassium Ions Into Cells