• What is the cell membrane?
• Cell membrane function
• Cell membrane structure
• Active transport and passive transport
• Endocytosis and exocytosis

We explain what the cell membrane is and some of its characteristics. In addition, its function and structure of this lipid layer.

The cell membrane has an average thickness of 7.3 nm3.

What is the cell membrane?

A double layer of phospholipids that surrounds and delimits the cells is called the cell membrane, plasma membrane, plasmalemma or cytoplasmic membrane. cells, separating the interior from the exterior and allowing the physical and chemical balance between the environment and the cytoplasm of the cell. It is the outermost part of the cell.

This membrane is not visible to optical microscope (yes to electronic), since it has an average thickness of 8 nm (1 nm = 10-9 m) and is located, in the vegetables cells and in those of mushrooms, below the cell wall.

The primary characteristic of the cell membrane is its selective permeability, that is, its ability to allow or reject the entry of certain molecules into the cell, thus regulating the passage of Water, nutrients or ionic salts, so that the cytoplasm is always in its optimal conditions of electrochemical potential (negatively charged), pH or concentration.

Cell membrane function

The membrane allows the desired substances to pass through and the unwanted ones to pass through.

The cell membrane fulfills the following functions:

• Delimitation. It defines and mechanically protects the cell, distinguishing the exterior from the interior, and one cell from another. In addition, it is the first defense barrier against other invading agents.
• Management. Its selectivity allows it to give way to the desired substances in the cell and deny the entry to the unwanted ones, serving as communication between the exterior and the interior while regulating said traffic.
• Preservation. Through the exchange of fluids and substances, the membrane allows to maintain stable the concentration of water and other solutes in the cytoplasm, keep its pH level and its electrochemical charge constant.
• Communication. The membrane can react to stimuli from outside, transmitting information to the interior of the cell and setting in motion certain processes such as cell division, movement cellular or the segregation of biochemical substances.

Cell membrane structure

Lipids are primarily cholesterol, but also phosphoglycerides and sphingolipids.

The cell membrane is made up of two layers of lipids amphipathic, whose hydrophilic polar heads (affinity for water) are oriented in and out of the cell, keeping their hydrophobic (water-rejecting) parts in contact, similar to a sandwich. These lipids are primarily cholesterol, but also phosphoglycerides and sphingolipids.

It also owns 20% of protein integral and peripheral, which fulfill functions of connection, transport, reception and catalysis. The integral membrane proteins are embedded in the bilayer with their hydrophilic surfaces exposed to the aqueous environment and their hydrophobic surfaces in contact with the hydrophobic interior of the bilayer.

Transmembrane proteins are integral proteins that completely span the thickness of the membrane. Peripheral membrane proteins associate with the surface of the bilayer, normally bind to exposed regions of integral proteins, and are easily detached without disturbing the membrane structure. Thanks to them there is also cell recognition, a form of biochemical communication.

Finally, the cell membrane has carbohydrate components (sugars), either polysaccharides or oligosaccharides, which are found on the outside of the membrane, forming a glycocalyx. These sugars represent only 8% of the dry weight of the membrane and serve as support material, as identifiers in intercellular communication and as protection of the cell surface from mechanical and chemical aggressions.

Active transport and passive transport

The membranes form compartments within the eukaryotic cells They allow for a variety of separate functions. In addition, they serve as surfaces for biochemical reactions.

Many ions and small molecules move through biological membranes by passive transport (without energy expenditure) and by active transport (with energy expenditure).

Diffusion is the net movement of a substance down its concentration gradient from a region of higher concentration to one of lower concentration.

Passive transport through the lipid bilayer is called simple diffusion and that carried out through ion channels and membrane proteins is called facilitated diffusion.

The osmosis is a type of diffusion in which water molecules pass through a semi-permeable membrane from a region with a higher effective concentration of water to a region where their effective concentration is lower.

In active transport, the cell expends metabolic energy to move ions or molecules across a membrane, against a concentration gradient.

Primary active transport, also called direct active transport, uses metabolic energy directly to transport molecules across the membrane. For example, the sodium-potassium pump uses ATP to pump sodium ions out of the cell and potassium ions into the cell.

In cotransport, also called indirect active transport, two solutes are transferred at the same time. A powered ATP pump maintains a concentration gradient. So a carrier protein cotransports two solutes. A solute moves down its concentration gradient and uses the released energy to move another solute against its concentration gradient.

Endocytosis and exocytosis

In endocytosis the materials are incorporated into the cell.

Some of the larger materials, such as large molecules, particles of food or even small cells, they also move in or out of cells. They are transferred by exocytosis and endocytosis. Like active transport, these processes require an expenditure of energy directly from the cell. This occurs through the formation of vesicles in the cell membrane that, depending on whether they enter or leave, allow the desired material to dissolve in the cytoplasm or on the contrary, in the environment.

• In exocytosis. A cell expels substances waste or secretion products (such as hormones) by fusing a vesicle with the plasma membrane.
• In endocytosis. The materials are incorporated into the cell. Several types of endocytosis mechanisms operate in biological systems, including phagocytosis, pinocytosis, and receptor-mediated endocytosis.
  • In pinocytosis ("cells drinking"). The cell takes up the dissolved materials.
  • In receptor-mediated endocytosis.Specific molecules combine with receptor proteins on the plasma membrane. Receptor-mediated endocytosis is the main mechanism by which eukaryotic cells take up macromolecules.
  • In phagocytosis (literally, "eating cells"). The cell ingests large particles of solids as food or bacteria. The latter is vital in the case of certain cells and single-celled organisms that engulf (wrap in their membrane) the material to nutrition.