Membrane permeability is a quality of a cell’s plasma membrane that allows substances to pass in and out of the cell, so that the cell can expel waste products and ship out the chemicals it assembles for the body. At the same time, the nutrients that the cell needs can pass through the membrane to the inside. Cell membranes have selective permeability, meaning that the membrane will allow certain substances to pass while forming a barrier against others.

Cells are like 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 — such as nutrients from food — inside its workshop. Once the cell 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 cell membrane is a flexible plasma that envelopes the exterior boundaries of the cell. It separates the intracellular fluid — the fluid within the cells — from the extracellular fluid, which is the fluid outside of the cells. The cell membrane is not a passive or insurmountable wall however, as there is a constant and dynamic exchange of substances between the two fluids.

Membrane permeability is dependent upon the bi-layer structure of the cell membrane. The cell membrane is composed of two back-to-back layers of phospholipids. Phospholipids are composed of phosphorus and lipids — essentially, fats.

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. 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 polar heads will repel them. 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.

Membrane permeability is an important feature of the plasma membrane, because it facilitates continuous but controlled traffic in and out of the cell. Due to the permeability of this membrane, the cells can extract the amino acids, fatty acids, sugars, and vitamins it needs to carry out its daily functions. Membrane permeability 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, membrane permeability allows these substances to exit the cell. Waste products can also pass through the cell membrane.

The plasma membrane also acts a gatekeeper. 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 precious fluids and proteins at any one time.