Solutes can be transported across the plasma membrane through a variety of methods, diffusion, osmosis, facilitated diffusion and active transport.

Passive transport does not require the use of energy (ATP). An example of this is diffusion of particles over the plasma membrane. A single substance tends to move from an area of high concentration to an area of low concentration, called a concentration gradient, until the concentration is equal. Generally, only molecules that are small and hydrophobic can diffuse through the plasma membrane. Molecules that are planar are also more likely to diffuse through the membrane. Several factors affect the rate of diffusion of a solute including the mass of the solute, the temperature of the environment, the solvent density, and the distance traveled.

Osmosis is simply the diffusion of water across the membrane. It is important for cells as the movement of water can change the cell’s volume. Osmolarity describes the total solute concentration of the solution, which gives rise to osmotic pressure. A solution with low osmolarity has a greater number of water molecules relative to the number of solute particles; a solution with high osmolarity has fewer water molecules with respect to solute particles. In organisms this gives rise to three situations:  In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell so water enters the cell. In a hypertonic solution, the extracellular fluid has a higher osmolarity than the fluid inside the cell; water leaves the cell. In an isotonic solution, the extracellular fluid has the same osmolarity as the cell; there will be no net movement of water into or out of the cell.

Facilitated transport is another example of passive transport.  A concentration gradient exists that would allow these materials to diffuse into the cell without expending cellular energy. However, these materials are ions or polar molecules that are repelled by the hydrophobic parts of the cell membrane. Facilitated transport proteins such as channels shield these materials from the repulsive force of the membrane, allowing them to diffuse into the cell.

Active transport requires energy to move substances against a concentration gradient, from an area of low concentration to high concentration. The most common example of this in the cell is the sodium-potassium pump. The sodium-potassium pump moves two K⁺ into the cell while moving three Na⁺ out of the cell.

There are six steps to the sodium-potassium pump:

1. With the enzyme oriented towards the interior of the cell, the carrier has a high affinity for sodium ions. Three sodium ions bind to the protein.
2. ATP is hydrolyzed by the protein carrier, and a low-energy phosphate group attaches to it.
3. As a result, the carrier changes shape and re-orients itself towards the exterior of the membrane. The protein’s affinity for sodium decreases, and the three sodium ions leave the carrier.
4. The shape change increases the carrier’s affinity for potassium ions, and two such ions attach to the protein. Subsequently, the low-energy phosphate group detaches from the carrier.
5. With the phosphate group removed and potassium ions attached, the carrier protein repositions itself towards the interior of the cell.
6. The carrier protein, in its new configuration, has a decreased affinity for potassium, and the two ions are released into the cytoplasm. The protein now has a higher affinity for sodium ions, and the process starts again.

At this point, there are more sodium ions outside of the cell than inside and more potassium ions inside than out. This results in the interior being slightly more negative relative to the exterior and requiring active transport with the movement of ions against the respective concentration gradients.

The ATP-binding cassette (ABC) transporter is another example of active transport. ABC transporters use ATP to transport a variety of different substrates across cell membranes, most commonly out of the cell.

Both the Sodium-potassium pump and ABC transporters are examples of primary active transport. Primary active transport uses a chemical energy source like ATP to move solutes against their concentration gradient. Secondary active transport, on the other hand, uses one electrochemical gradient to move different molecules against their own concentration gradients. Secondary active transport is also known as cotransport.

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Key Points

• Types of passive transport (no ATP required) include diffusion, osmosis, and facilitated transport.

• The osmolarity of the cell and its external environment is important for its volume (hypotonic, hypertonic, isotonic).

• Types of active transport (energy required) include the sodium-potassium pump (which moves two K⁺ into the cell while moving three Na⁺ out of the cell) and ABC transporters.

• The sodium-potassium pump and ABC transporters are examples of primary active transport.

Key Terms

Passive transport: does not require ATP, moves from high concentration to low concentration

Diffusion: substance moves from an area of high concentration to an area of low concentration

Osmosis: diffusion of water

Isotonic: the extracellular fluid has the same osmolarity as the cell; there will be no net movement of water into or out of the cell

Hypertonic: the extracellular fluid has a higher osmolarity than the fluid inside the cell; water leaves the cell

Hypotonic: the extracellular fluid has a lower osmolarity than the fluid inside the cell so water enters the cell

Osmolarity: describes the total solute concentration of the solution

Facilitated transport: proteins move polar molecules in or out of the cell depending on its concentration gradient

Active transport: requires energy (often ATP), moves from low concentration to high concentration

Plasma membrane: the semipermeable barrier that surrounds the cytoplasm (inside contents) of a cell

Solute: the solid component dissolved in a solvent

ABC transporter: class of membrane proteins that use ATP to actively transport molecules outside of the cell

Primary active transport: directly uses ATP to move molecules against their concentration gradient