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Membrane permeability is essentially the ability of certain substances to pass in and out of an organic or semi-organic barrier. It is most commonly talked about in terms of biology, where the membrane is a cell wall and the substances passing in and out are elements of chemical production, cell food, and waste products. The concept is broader than this, though, and does have some applications in industry, scientific experimentation, and manufacturing. Membranes can take many shapes and play a variety of roles when it comes to keeping some elements out while holding others in. A membrane that is permeable isn’t usually open like a sieve, but is more or less penetrable by certain substances under certain conditions.
Cell membranes have selective permeability, meaning that they will allow certain substances to pass while forming a barrier against others. It is essentially a quality of a cell’s plasma membrane that allows substances to pass in and out of it, so that the cell can expel waste products and ship out the chemicals and other refined nutrients that it assembles for the body. At the same time, the nutrients that the cell needs can pass through the membrane to the inside.
Sometimes the concept is easiest to understand by thinking about cells as microscopic factories: they design, produce, and package the substances the body needs to survive each day. Just like a factory, a cell needs a way to bring the raw materials for its products, like nutrients from food, inside its workshop. Once it has assembled and packaged a substance, the cell needs a way to ship the finished product out into the bloodstream so that the body can make use of it.
The membrane can be thought about as the shipping and receiving department, where some things are approved for entry and exit while others are not. In most cases the membrane is a flexible plasma that envelopes the exterior boundaries of the cell. It separates the intracellular fluid, which is the fluid within the cells, from the extracellular fluid, which is found just outside of the cells. The membrane is not a passive or insurmountable wall, however, as there is a constant and dynamic exchange of substances between the two fluids.
Permeability is largely dependent on the structure of the cell membrane, which is composed of two back-to-back layers of phospholipids. Phospholipids, in turn, are made of chains of phosphorus and lipids, which are essentially fats. It is the chemical attributes of these chains that make passage possible for certain elements.
Each phospholipid has a polar head that is electrically charged and hydrophilic — which means water-loving — and a non-polar, uncharged tail that is hydrophobic — which means water-fearing. The heads and tails arrange themselves so that the heads face out into the water, while the tails avoid the water by lining up in the center. The result is like a sandwich where the hydrophilic heads are the two slices of bread and the hydrophobic tails are the peanut butter in the middle.
Substances that are non-polar and lipid-soluble can diffuse in and out a plasma membrane. Lipid soluble means able to dissolve in fats. Polar substances are not able to pass through the cell membrane because the heads will repel them; the charged substances repel other charged substances, much like two magnets. The cell membrane is also impermeable to substances that are not lipid-soluble, as they are unable to pass through the lipids of the membrane.
Permeability is an important feature because it facilitates continuous but controlled traffic in and out of the cell. As a result, cells can extract all the amino acids, fatty acids, sugars, and vitamins they need to carry out their daily functions. It also enables chemical messengers, such as hormones, to enter the cell and either trigger or inhibit some function of the cell as necessary. After cells create hormones, neurotransmitters, proteins, and other substances required by the body the membrane allows these substances to exit the cell. Waste products can pass through in most cases, too.
The plasma membrane also acts a gatekeeper, so not just any substance can drift in and out the cell. The plasma membrane features selective permeability, meaning that it allows some substances to enter but excludes others. This prevents harmful substances from infiltrating the cell and ensures that the cell does not lose too much of its fluids and proteins at any one time.
Different kinds of membranes are used in a variety of applications, from chemical manufacturing to engineering and pharmacology. In these settings, membrane permeability is usually about allowing certain elements, chemicals, or foods into a closed environment while keeping others out. The concept is really similar to that of cells in most cases, but the coverings can be quite large, often sealing off entire containers or tanks. They also are usually man-made, though often contain a number of organic ingredients. A lot depends on what is being kept out, what is being pulled in, and the needed chemistry of the finished product.
Since cells are selectively permeable, does that mean they can be selective and allow prescribed medications into the blood stream? For example, I take high blood pressure medicine. Is the reason it works is because the cells are able to let it in?