Cell Membrane Permeability

The cell membrane is often described as being semipermeable because it allows certain materials to pass through while keeping others out. This ability to regulate the passage of materials is important because a cell needs to remain in a state of homeostasis, which is the body's processes that physiologically regulate its internal environment with response to fluctuations that occur in the internal or external environment. Generally, charged molecules, or ions, cannot pass freely through the membrane. However, some materials can easily pass through the cell membrane, such as small, nonpolar molecules including carbon dioxide and uncharged polar molecules including water, without expending any energy. This is known as passive transport, and in the absence of other forces, materials move from an area of greater concentration to an area of lesser concentration. This is known as diffusion, a general movement that does not need to occur across a cell membrane. When the cell membrane is involved, it creates a barrier that causes the number of molecules on one side of the membrane to be different from the number of molecules on the other side of the membrane. This is known as the concentration gradient; when the process of materials is moving from an area of greater concentration to an area of lesser concentration, the process is referred to as moving down gradient. There is also a difference between net movement and overall movement regarding cell transport. The net movement of particles follows the direction toward areas of low concentration to establish equilibrium in cells. Overall movement refers to the various directions of particle movement in cell transport. There are some factors that affect the speed at which diffusion occurs. The steeper the concentration gradient, the faster the rate of diffusion. Smaller molecules diffuse faster, and higher temperatures result in faster diffusion rates. Movement and electrical or pressure gradients can also affect rates of diffusion. Some materials need assistance from proteins to diffuse across the cell membrane, and this is known as facilitated diffusion. In the process of facilitated diffusion, molecules are transported either into or out of the cell. A carrier protein with a central channel acts as a channel to move molecules across the membrane. The carrier protein binds with a specific molecule, such as a carbohydrate or an amino acid, to start the transport mechanism. When the molecule binds to the protein, the protein alters it shape and moves the molecule down the concentration gradient. This process does not require any energy input and is solely dependent on the aid of the carrier protein. Transport may happen in either direction. For example, a high concentration of a carbohydrate exists in the extracellular fluid. The carbohydrate binds to the carrier protein and is transported to and released into the cytoplasm, or from a high concentration to a lower concentration.
Both water and solutes may involve transport through a semipermeable membrane via a channel protein. A solute is one example of a polar molecule that may require assistance from channel proteins, and water is another, although some water can diffuse across the membrane itself. Osmosis is the movement of water molecules across a semipermeable membrane from an area of lower concentration of solute to higher concentration of solute. Because the cell membrane limits the movement of some materials, it is necessary for solutes to move into or out of the cell. This in turn affects the size and shape of the cell. 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 this state, water may move across the cell membrane, but there is no net electrical change, and solute concentration is in a type of balance on either side of the cell membrane. 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 this type of environment, water will diffuse out of the cell and moving down its concentration gradient, causing it to shrink. Conversely, a hypotonic solution is a solution in which the concentration is less than that of another solution or less than the concentration inside the cell. Water will diffuse into the cell, causing it to expand and possibly burst. Hypertonic and hypotonic may also be used to refer to two solutions, as opposed to inside versus outside cells.

It is sometimes necessary for materials to pass through the cell membrane, moving against their concentration gradient. For this to occur, the cell must expend energy. This is called active transport. Active transport is also required to move larger molecules, such as a glucose molecule, across the cell membrane but against the concentration gradient. Because energy is expended by the proteins involved in active transport, they are often referred to as pumps. An example found in all animal cells is a carrier protein called a sodium-potassium pump, which is an ion channel that allows sodium and potassium ions to move into and out of the cell.

Sodium (Na⁺) and potassium (K⁺) ions cannot pass freely through the cell membrane, so ion channels are needed. The function of the Na⁺/K⁺ pump is so that the cell does not reach equilibrium. These ion channels, which are sodium-potassium pumps, allow the ions to move into and out of the cell. For every two potassium ions that enter the cell, three sodium ions are pumped out, thereby reducing the positive charges inside of the cell. This creates an electrical potential difference in which the negative charge inside the cell is greater than outside the cell. The difference in electrical potential inside and outside a cell membrane is called the membrane potential. In some cells, such as a cardiac cell, the membrane potential facilitates communication between cells. This occurs at gap junctions, which are essentially tunnels that consist of proteins. The junctions allow current transmission from cell to cell, which is what happens with cardiac cells. Two cells connect to form a channel, called a gap junction, which allows ions to flow from one cardiac cell to the other. These ions carry electrical charges that cause the cardiac cells to respond and pass along the "message" to the next cardiac cell.

Some molecules, organisms, and other materials are too large to be moved into or out of the cell by transport proteins and are instead moved via vesicles. An example is the ingestion of a bacterium by the white blood cells of the immune system. The bacterium would be moved into the cell via bulk transport. Bulk transport, which is movement of material into or out of the cell through a phospholipid bilayer and cell membrane, is done by vesicles. Endocytosis is a form of bulk transport that moves material into a cell by an infolding of the cell membrane around the material, forming a vesicle (small sac) that moves into the cell. Pinocytosis, a form of endocytosis, is a method by which cells ingest liquid through small vesicles budding from the cell membrane. Conversely, exocytosis is a form of bulk transport used to move material outside of the cell by fusion of a vesicle with the plasma membrane and the release of the contents outside the cell. The immune system requires the body to capture and engulf foreign bodies that cause disease or infection. This process employs various types of white blood cells, called phagocytes, to identify foreign bodies, such as bacteria, or dead cells. The cell membrane extends and surrounds the foreign body using an extension of the cytoplasm called a pseudopod (plural, pseudopodia), or false foot. The pseudopod extends and engulfs the foreign body. In the case of bacteria, the phagocyte digests the material. This is called phagocytosis, the process through which a cell takes in and digests a particle, such as another cell or bacterium.