Passive Transport

Passive transport is the most common way to transport material in and out of a cell. Molecules are in constant random motion. Because of this random motion, molecules possess the energy of motion, called kinetic energy. Passive transport occurs as a result of this kinetic energy driving certain molecules across the membrane. Because the energy to move these molecules comes from the molecules themselves, passive transport does not require the cell to expend its own energy in their movement. Small molecules that can pass through the cell membrane on their own do so through the process of diffusion. Diffusion is the random movement of molecules along a concentration gradient, a difference in concentration between two locations. Concentration gradients are established because molecules move from an area of high concentration to an area of low concentration. In nonliving systems, the molecules would eventually be evenly concentrated throughout the space, reaching a state of dynamic equilibrium in which the rate of loss is equal to the rate of gain. An example would be a drop of dye in a glass of water. Eventually, the dye molecules would be evenly distributed among the water molecules in the glass. In living systems, the cell membrane and other body control systems can operate to prevent equilibrium from being established, effectively maintaining a concentration gradient indefinitely. The constant movement of O₂ into cells is achieved because the cells use the O₂ continuously, maintaining a sharp deficit inside the cell. Facilitated diffusion occurs when materials diffuse across the cell membrane with the help of membrane proteins. A concentration gradient drives the diffusion of these items into the cell; however, their chemical nature prevents them from easily diffusing across the cell. Most often these materials, such as charged compounds and polar molecules, are resistant to the hydrophobic region of the cell membrane. Channel and carrier proteins shield such molecules from the repulsive forces of the membrane and allow them to diffuse quickly into the cell. Facilitated diffusion is a specific process. Channel or carrier proteins are able to transport specific ions or molecules into or out of the cell. For example, a group of channel proteins facilitates the passage of glucose, but not other molecules, across the membrane. The process of facilitated diffusion still follows a passive mechanism, even though a transport protein is used. That is, the direction of net movement occurs from an area of high concentration to an area of low concentration. The rate of diffusion is limited by the number of specific transport proteins in the membrane. A cell may alter the density of certain transport proteins, in response to environmental cues or changing cellular requirements, in order to maintain the necessary concentration of materials inside the cell.

Diffusion describes the passive transport of any material across a membrane. Osmosis is a type of diffusion that only involves the movement of water molecules across a semipermeable membrane from an area of high concentration to one of low concentration. Water molecules can diffuse directly through a cell membrane via osmosis, and it is constantly occurring in living things.

Water dissolves substances, so it is a solvent. The substance that is dissolved is called a solute. The cell membrane limits the diffusion of solutes dissolved in water. However, the water molecules themselves can freely diffuse across the cell membrane through channel proteins called aquaporins. An aquaporin is a transport protein in a cell membrane that allows for osmosis, the movement of water back and forth. When osmosis occurs, the direction of water movement is based on the relative concentration of solutes on either side of the membrane. During osmosis, water moves from an area of low solute concentration to area of high solute concentration. In a cell, water will always move to reach an equal concentration of solute on both sides of the cell membrane. For example, if the salt concentration inside a cell is higher than the salt concentration outside of the cell, water will move into the cell until the salt concentration is the same on both sides of the membrane. Cell volume may change due to osmosis when the extracellular environment changes. Osmolarity refers to the concentration of solutes in a solution. The osmolarity of the cell can be compared to the osmolarity of the extracellular fluid that surrounds the cell. This relationship is described by three different conditions. In isotonic conditions, the extracellular fluid has the same osmolarity as the cell. Movement of water into the cell exactly balances the amount of water moving out of the cell. These are the ideal conditions for most animal cells.

When a cell is in a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell. In this case, solute concentration in the extracellular fluid is lower than the solute concentration inside the cell. Water flows to the region with the highest solute concentration, inside the cell. In hypotonic conditions, there is a net water movement into the cells and as a result, cells will swell. If the concentration difference is extreme and excess water is not removed, cells may burst, or lyse.

In hypertonic conditions, the extracellular fluid has a higher osmolarity than the inside of the cell. Because there is more solute outside the cell, water will flow in this direction until equilibrium is achieved. As a result, the cell shrinks as it loses water. This impairs a cell's ability to function or divide. If the solute concentration difference is extreme, the cell may lose so much water that it "dies."