Cell Membrane Transport

All cells found in the human body are constantly passing materials back and forth across their membranes. Among these are oxygen, water, carbon dioxide, and several nutrients needed for proper cell functioning. Proteins are found both embedded within the lipid bilayer and associated with the surface of the membrane facing the inside of the cell. Many cell membrane proteins function in the transport of substances across the membrane.

Peripheral and surface proteins of the cell membrane can be either embedded in the membrane or associated with one side of the membrane. An integral protein is a protein that is embedded within the cell membrane. Some integral proteins are inserted into one side of the membrane but do not go all the way across the membrane. Other integral proteins traverse the entire lipid bilayer and are called transmembrane proteins. Transport proteins are classified as a type of integral protein. There are two types of transport proteins: carrier proteins and channel proteins. Channel proteins only transport substances across the cell membrane passively. Carrier proteins can transport ions or molecules across the membrane either passively (via facilitated diffusion) or actively.

A carrier protein is a protein that physically binds to a molecule and facilitates its transport across the cell membrane's lipid bilayer. Carrier proteins have a specific binding site for an ion or molecule. When the binding site is occupied, the carrier protein changes shape. The change in shape allows the bound substance to have access to the opposite side of the cell. One example of a carrier protein is the glucose transporter protein, which moves glucose into the cell. A channel protein creates a pathway that has a hydrophilic interior for ions (polar molecules) to pass through. The hydrophilic interior of these proteins is exposed to the cytoplasm and extracellular fluids. They have a hydrophilic channel within their core that creates an opening in the cell membrane. Some channels can be opened or closed in response to a stimulus, and these are called gated channels. Gated channels are critical to the functioning of nerve cells by allowing the controlled movement of ions that propagate the signal. Sodium and calcium ion channels are examples of channel proteins.

The fluid mosaic model describes the changeable structure of the cell membrane. The term fluid refers to the nature of the lipid bilayer itself. The phospholipids do not form rigid sheets; rather, the lipids form a flexible film of movable components. This is somewhat similar to a layer of soap bubbles, where each bubble can move in relation to the rest of the bubbles but is still connected to the whole. Because the lipids "flow" past each other, the membrane resembles a fluid.

The principle of "like dissolves like" explains how molecules are prevented from crossing the membrane. Polar molecules, such as glucose, are hydrophilic and can interact with the cell membrane exterior because the glycerol heads of the phospholipids are also hydrophilic. However, they experience difficulty in crossing the cell membrane because their movement is impeded by the hydrophobic phospholipid tails. Small, nonpolar molecules such as oxygen (O₂) can be dissolved in the lipid bilayer and freely travel across the cell membrane. In general, charged and large molecules cannot pass through the membrane freely, whereas uncharged and small molecules can.

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 from an area of higher concentration to an area of lower concentration. A concentration gradient is the difference in concentration between two locations. In nonliving systems, molecules continue to move until, resulting in an equilibrium, where there is a uniform concentration between areas. 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.

Several factors affect the rate of diffusion. 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 (energy from being in motion) 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.

Diffusion describes the passive transport of any material across a membrane. Osmosis is the movement of water molecules across a semipermeable membrane from an area of lower concentration of solute to an area of higher concentration of solute. It is constantly occurring in living organisms. 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 an 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 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 because of 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 and determines the direction the water will move. This relationship is described by three different conditions: isotonic, hypotonic, and hypertonic.

An isotonic solution is a solution in which the concentration of dissolved solutes is equal to that of another solution, or equal to the concentration inside the cell. 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 since their overall volume remains the same. A hypotonic solution is a solution in which the concentration of dissolved solutes is less than that of another solution, or less than the concentration inside the cell. 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 will burst, or lyse.

A hypertonic solution is a solution in which the concentration of dissolved solutes is greater than that of another solution, or greater than the concentration inside the cell. 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, or crenates, 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."

Active transport is the movement of material across the cell membrane against its concentration, requiring the cell to expend energy. This is different from passive transport where molecules move without the addition of energy. This energy is usually provided in the form of adenosine triphosphate (ATP), the energy molecule of the cell. Cells rely on active transport mechanisms when molecules are moved against their concentration gradient, while passive transport occurs along the concentration gradient. This means that active transport can move substances from areas of low concentration to areas of high concentration.

The active transport mechanism commonly involves the movement of some substances that are unable to freely cross the cell membrane but are important for cell function. These molecules typically have a small mass, such as ions and amino acids. However, other mechanisms like cotransport can also involve the transport of larger molecules, such as glucose. In cotransport, the movement of one substance with its gradient releases energy used to move another substance against its gradient in a coupled reaction.

During active transport, ions (atoms or molecules with electric charges) cross the cell membrane through proteins, creating an electrical gradient. Typically the inside of the cell is negatively charged when compared to the extracellular fluid. Because of this difference in charge, a voltage, or membrane potential, exists across the cell membrane. Voltage refers to the electrical potential energy because of a separation of opposite charges.

In addition to the electrical gradient, an ion gradient is established during active transport. There are positively charged potassium (K⁺) and sodium (Na⁺) ions within the cell, but K⁺ leaks out faster than Na⁺, which creates a more negatively charged cellular interior. Ion diffusion across the plasma membrane is driven by an electrochemical gradient. An electrochemical gradient is a gradient established by the driving forces of a chemical and electrical change in the cell membrane. This gradient determines the direction in which an ion moves during active transport across a membrane. Three factors affect the electrochemical potential that is generated across a membrane when an ion is moving: 1) an ion concentration difference on either side of the cell membrane; 2) the ion charge; and 3) the membrane potential on both sides of the cell membrane.

Sometimes large amounts of material are moved quickly across the cell membrane. This is done through a bulk transport mechanism. As a general overview of this mechanism, the transported substances are enclosed in a membrane bubble, called a vesicle, created by pinching off a portion of the cell membrane. The vesicle then moves through the cell and fuses with an internal or external membrane. Fusion of the vesicle membrane with another membrane releases the contents of the vesicle. Two specific processes are involved in bulk transport: endocytosis and exocytosis.

Endocytosis is a form of bulk transport that moves material into a cell from the environment. This transport can move parts of cells, particles, or even whole cells into a cell. The substance needing entry clusters at a location of the cell membrane, often because of the presence of specific receptors. The cell membrane then forms a pocket around the substances. This pocket, or invagination, pinches off from the internal surface of the cell membrane, forming a vesicle. The vesicle travels inside the cell and fuses with a membrane-bound structure such as a lysosome. The fusion of the vesicle opens the pocket and allows for the release of the material from the vesicle.

Exocytosis is a form of bulk transport used to move large molecules to the outside of the cell. This can be thought of as a reverse process of endocytosis. Exocytosis involves expelling a substance from the cell into the extracellular fluid that surrounds the cell. This is done first by enclosing the material in an intracellular vesicle that buds from a membrane-bound structure such as the Golgi apparatus. The vesicle travels to the cell membrane and fuses with the interior side of the cell membrane. Fusion opens the vesicle by creating an invagination in the exterior side of the cell membrane. The contents of the vesicle are then released to the environment.