Cell Membranes

Passive Transport

Passive transport is the movement of material across the cell membrane from an area of high concentration to an area of low concentration, through diffusion or facilitated diffusion, without the expenditure of cellular energy.
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.

Establishing Equilibrium in Passive Transport

During passive transport (the most common way to transport material into and out of a cell), a concentration gradient is established in which molecules move from regions of higher concentration to regions of lower concentration, before reaching dynamic equilibrium (where the rate of loss is equal to the rate of gain).
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 in Cells

During facilitated diffusion, carrier proteins embedded in the membrane are used to facilitate the transport of insoluble molecules across the cell membrane.
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.

Factors That Influence Diffusion

Several factors affect the rate of diffusion: the nature of the diffusing substance, the physical environment in which diffusion is occurring, the cellular conditions in which diffusion is occurring, and the concentration gradient.
Several factors affect the rate of diffusion. One factor is the nature of the diffusing substance. Heavier molecules diffuse much more slowly than lighter molecules. Nonpolar substances, those that have the same charge on both sides, diffuse across a membrane at a higher rate than polar substances, which are less soluble in the cell membrane's phospholipid bilayer.

Another factor is the physical environment in which diffusion is occurring. If the temperature or pressure of the environment increases, the kinetic energy of the molecules increases, causing the rate of diffusion to increase.

The cellular conditions in which diffusion is occurring can influence the rate of diffusion. The higher the density of the solvent, the slower diffusion will progress. When cells are dehydrated, the density of the cytoplasm is higher, reducing the ability of material to diffuse. Thick membranes, or membranes with a higher density of glycoproteins and glycolipids, can impede diffusion. A larger membrane surface area will increase the rate of diffusion because there are more places where the molecules can diffuse across.

When the concentration gradient increases (i.e., the difference in concentration on either side of the cell membrane is high), the rate of diffusion increases. The degree to which the rate of facilitated diffusion can increase may be limited by the density of transport proteins in the membrane. If all transport proteins are active and operating at maximum efficiency, an increase in concentration will not increase the rate of diffusion.

Osmosis

The passage of water across the cell membrane, through osmosis, is based on relative concentrations of solutes on either side of the membrane. Cellular integrity can be compromised if too much water enters or leaves 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.
Osmosis, a type of diffusion, involves the movement of water, not insoluble molecules, across the cell membrane from a region of low to high concentration.
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."
Osmolarity refers to the concentration of solutes in a solution. This is also referred to as its tonicity. 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. In a hypotonic solution, the extracellular fluid has a lower osmolarity than the fluid inside the cell, so there is a net water movement into the cells and as a result, cells will swell. In hypertonic conditions, the extracellular fluid has a higher osmolarity than the inside of the cell.