The Plasma Membrane


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The plasma membrane is composed of a phospholipid bilayer. In the bilayer, the two long hydrophobic tails of phospholipids face toward the center, and the hydrophilic head group faces the exterior. Integral membrane proteins and protein channels span the entire bilayer. Protein channels have a pore in the middle. Peripheral membrane proteins sit on the surface of the phospholipids, and are associated with the phospholipid head groups. On the exterior side of the membrane, carbohydrates are attached to certain proteins and lipids. Filaments of the cytoskeleton line the interior of the membrane.
The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.

Source: OpenStax Biology 2e

OpenStax Biology 2e

Like prokaryotes, eukaryotic cells have a plasma membrane, a phospholipid bilayer with embedded proteins that separates the internal contents of the cell from its surrounding environment. A phospholipid is a lipid molecule with two fatty acid chains and a phosphate-containing group. The plasma membrane controls the passage of organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon dioxide and ammonia) also leave the cell by passing through the plasma membrane.

– What is a thin polar membrane made of two layers of lipid molecules?

The plasma membranes of cells that specialize in absorption fold into fingerlike projections that we call microvilli (singular = microvillus). Such cells typically line the small intestine, the organ that absorbs nutrients from digested food. This is an excellent example of form following function. People with celiac disease have an immune response to gluten, which is a protein in wheat, barley, and rye. The immune response damages microvilli, and thus, afflicted individuals cannot absorb nutrients. This leads to malnutrition, cramping, and diarrhea. Patients suffering from celiac disease must follow a gluten-free diet.

– What is known as the pericellular matrix which is a glycoprotein and glycolipid covering that surrounds the cell membranes of some bacteria, epithelia, and other cells?

The left part of this figure is a transmission electron micrograph of microvilli, which appear as long, slender stalks extending from the plasma membrane. The right side illustrates cells containing microvilli. The microvilli cover the surface of the cell facing the interior of the small intestine.
Microvilli, as they appear on cells lining the small intestine, increase the surface area available for absorption. These microvilli are only on the area of the plasma membrane that faces the cavity from which substances will be absorbed. (credit “micrograph”: modification of work by Louisa Howard)
The volume of cells can vary dramatically. Similarly to an inflating balloon, the volume increase of growing cells pushes on the plasma membrane — the lipid envelop that surrounds the cell. This “turgor” pressure increases the tension of the membrane, which, if left uncorrected, will ultimately cause the cell to burst. To prevent this from happening, cells have evolved mechanisms to monitor the tension of their plasma membrane. When tension is too high, cells respond by increasing the amount of lipid in the membrane. Conversely, when tension is too low, cells remove lipid from the membrane to “tighten” it.


Clark, M., Douglas, M., Choi, J. Biology 2e. Houston, Texas: OpenStax. Access for free at: