Cell membrane: Definition, structure and functiom

Cell membranes are the set of lipid and protein envelopes that enclose the cellular content. They are basically made up of two layers of superimposed lipids, such as bags or closed bags. In this way, the cell separates from the environment that surrounds it and can fulfill its survival functions.

Both prokaryotic and eukaryotic cells have a plasma membrane that envelopes and concentrates the components of the cell. In addition, eukaryotic cells have organelles inside them, which are compartments wrapped in membranes, such as mitochondria, nucleus, and vacuoles.

The biological membranes model currently recognized as a fluid mosaic was proposed by SJ Singer and GL Nicholson.

Characteristics of cell membrane

The cell membranes have the following characteristics:

Dynamics: the components of the membrane are not fixed and are immobile in their position. Phospholipids can pass from one side of the membrane to the other, they also move in the same plane.
Fluid: the fluidity of the membrane depends on the composition and temperature.
Semipermeable: the cell membrane is permeable to certain molecules, depending on its affinity for lipids. Small non-polar molecules such as oxygen O ₂ and carbon dioxide CO ₂ are easily diffused through the membrane. Ions such as sodium Na ⁺ , hydrogen H ^+, or calcium Ca ²⁺ cross the membrane through proteins.
Selective: the proteins of the membrane allow the passage of some molecules. For example, the glucose transporter does not pass through galactose, which is a molecule very similar to glucose.
Asymmetric: the outer extracellular monolayer of the lipid bilayer is different from the intracellular internal monolayer. This asymmetry is important, especially when converting extracellular signals into the cell.
Diversity: Plasma membranes can have from 500 to 1000 different types of lipids, and approximately 30% of the proteins of eukaryotic cells are found in membranes.
Electrical potential: when differences in ion concentrations occur on one side of the membrane and another, potential energy is stored in the form of electrochemical gradients.

Structure of the cell membrane

The biological membranes consist of a continuous double layer of lipids, the lipid bilayer. These lipids are the phospholipids, which are characterized by having a hydrophilic end or polar head and another extreme hydrophobic. This structure measures 5 nm wide.

While the lipid bilayer provides the basic structure for all cell membranes, it is the proteins that perform the specific functions such as transport and recognition. The proteins can cross the membrane, or settle on the surface.

Lipid rafts

They are known as lipid rafts to certain regions of the plasma membrane that move through it as if they were “rafts in the sea”. These regions are known to be rich in sphingolipids and cholesterol and contain proteins that give them stability.

Functions of the plasma membrane

The membrane fulfills various functions.

Selective barrier

Due to the hydrophobic interior of the lipid bilayer, the passage of polar molecules through the membrane is restricted. This function as a barrier allows the cell to maintain certain compounds within the cytoplasm or organelles.

However, the cell needs to allow the entry of soluble molecules and ions in water as nutrients and the output of waste products. For this purpose, conveyors and membrane channels are found. For example, the potassium sodium pump is a transporter that inserts two potassium K ⁺ ions and removes three Na ⁺

sodium ions using the energy of the ATP.

Intercellular communication

In higher organisms, cells have the property of communicating with each other through protein receptors and signaling molecules. There are four known mechanisms of communication between cells:

Contact-dependent communication: two cells establish direct physical contact. For example, the cells of the immune system.
Paracrine communication: a cell releases a compound that reaches another cell very close. For example, the signal is transmitted from a nerve cell to a muscle cell.

Endocrine communication: in this case, the cells release signaling molecules or hormones through the bloodstream to act on cells at a long distance. For example, beta cells in the pancreas secrete the hormone insulin that travels through the blood to the rest of the body.
Synaptic communication: when a neuron is stimulated, it sends an electrical signal through the axon, and at the end of it, there is the release of neurotransmitters that bind to receptors in the postsynaptic terminal of another neuron.

Cell motility

The locomotion of certain cells is a function of the plasma membrane, either by cilia, flagella or amoeboid movement. The mechanisms of endocytosis and exocytosis are produced by the movement of membrane portions to trap or release material.

Cell shape

The variety of forms presented by the cells is in accordance with their function. The cell membrane and the cytoskeleton contribute to maintaining the cellular form.

Cell signaling

In the plasma membrane, there are elements that allow the retransmission of extracellular signals inside the cell. For example, phospholipase C is an enzyme in the membrane that acts on phosphatidylinositol 3 phosphate. This produces new elements in the signaling chain called “second messengers”.

Composition of the plasma membrane

The membrane is composed of lipids and proteins with specific functions. Some of these lipids and proteins may have fewer carbohydrate chains attached.

Lipids of the plasma membrane

The most abundant lipids in the membrane are phospholipids. These have an extreme or polar head, with an affinity for water, and an apolar end, composed of two tails of hydrophobic fatty acids. Among the main phospholipids, we have phosphoglycerides (phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol) and sphingomyelin.

In addition to phospholipids, the other lipid of the membranes is cholesterol, which is found mainly in animal eukaryotic cells. In plants there is no cholesterol in the membranes; instead, there are other types of sterols, such as stigmasterol and sitosterol. In the plasma membrane of fungi, ergosterol is obtained.

In animal cells, glycolipids are found in the outer monolayer of the membrane. These glycolipids are lipid and carbohydrate compounds and play an important role in cell recognition. Examples of these are gangliosides and galactocerebrosides.

Plasma membrane proteins

The proteins of the membrane can be inserted into the lipid bilayer in different ways. Transmembrane proteins cross from one side to the other; they are also called integral proteins. Examples of integral proteins are receptors and transporters.

Other proteins are “sitting” on the surface without crossing the membrane; these proteins are known as peripheral proteins. Examples of peripheral proteins are the cytochrome c of the mitochondria and the spectrin of the erythrocytes.

The channels are integral membrane proteins that allow the passage of molecules from one side to the other. For example, aquaporins are water channels.

Enzymes are also found in the membrane, such as phospholipase C, which is important in intracellular signaling.

The cellular receptors are proteins with the ability to bind molecules or compounds outside the cell and send a signal to the interior thereof. It is like a bell that, when touched, announces to the cell that something outside is waiting for an answer. For example, insulin, by binding to its receptor on the membrane, activates the cellular mechanism that allows glucose to enter the cell.