Lipid bilayer

A lipid bilayer is a biological membrane consisting of two layers of lipid molecules. Each molecule lipid molecule, or phospholipidcontains a head hydrophilic and a tail hydrophobic. The tail regions, repelled by water and slightly attracted to each other, come together. This exposes the head regions to the outside, creating a barrier between two bodies of water. A lipid bilayer is the fundamental part of all cellular membranes, typically completed by integral proteins specific to the species and other functional aspects.

A lipid bilayer functions through the actions of polarity. The interior of the lipid bilayer no en polarwhile the heads are molecules polar and create links of hydrogen with other polar molecules. This also means that polar molecules such as water and ions cannot easily pass through the non-polar tail region of the lipid bilayer. The cell membranes of most organisms are created with lipid bilayers, as is the nuclear membrane and various organelles.membranes. The various functions of these membranes are then specified by a variety of proteins that allow or prevent certain substances from crossing the membrane. In doing so, individual cells and organelles can create an ideal environment for biochemical reactions to take place, allowing them to remain in the membrane for a long period of time. homeostasis.

Structure of the lipid bilayer

A lipid bilayer consists of two sheets of amphiphilic phospholipidsas seen in the image below. Amphiphilic describes a molecule that is partly hydrophobic and partly hydrophilic. Often there are atoms of phosphorus in the heads of the molecules, which gives them polarity. The tails of the molecules are apolar and hydrophobic. In the image below, the polar parts of the molecules are marked in red.

As seen in the animation, the molecules are not glued rigidly in place. In a single sheetmolecules are actively moving again and again. In fact, a better analogy is that of people crowded into a lift. They mostly stay still, but they can slide side by side if someone needs to get out of the lift and is standing at the back. Put two of these layers together and you have a lipid bilayer.

In living systems, a lipid bilayer is never by itself. It is associated with a number of integral and surface proteins, as well as extracellular and intracellular elements that have specific functions in the cell. An all-encompassing model the cell membrane is the fluid mosaic modelwhich assumes that proteins within the lipid bilayer act like icebergs in the sea, floating but not attached to anything. The specific properties of the protein and the lipid bilayer keep them bound within the layers, but not stationary. This can be seen in the image below.

Function of the lipid bilayer

A lipid bilayer serves many functions both in the organism unicellular as in multicellular organisms. Regardless of whether a cell lives freely in the water of a pond or is confined to its body performing a function, it needs to maintain different conditions for the various reactions it needs to carry out in order to survive. In all applications, the lipid bilayer acts as a filter between the inside and the outside. However, depending on the conditions, the exact functions of the lipid bilayer may change.

Imagine two cells, one in the ocean and one in a pond. The pond water is fresh, while the ocean water contains a lot of dissolved salts. In the pond, the water will want to pass into the cell more hypertonic or saltier. In the ocean, the salts in the water will draw water out of the cell. These two different situations show the importance of proteins in a lipid bilayer. While each bilayer stops ions and slows the movement of water, it can only hold so much pressure. Water will continuously leak into or out of the cell. Different types of organisms have different strategies for dealing with water loss, most relying on proteins within the lipid bilayer or extracellular support structures (cell walls) to help mitigate water and ions appropriately.

Of these membrane proteins, ion pumps, ion channels y aquaporins. Ion pumps are based on cellular energy sources (e.g., ATP ) to actively move unwanted ions across a lipid bilayer. Ion channels, on the other hand, respond to a signal (electrical or chemical) and open accordingly. Aquaporins are a type of ion channel that allows large amounts of water to pass through the membrane at the right time.

The lipid bilayer and its associated proteins provide another function for cells, in the form of cell signalling. They can be involved in several ways. At signal transductiona signal passes through the lipid bilayer using a series of integral and surface proteins, creating a reaction internally. Lipid bilayers are also directly involved in the transmission of signals. nerve impulses. When a nerve impulse arrives at the end of a nerve, called the synapsesends a signal for special vesicles to fuse with the lipid bilayer of the cell membrane. The vesicles, filled with molecules of neurotransmittersrelease their contents when they fuse. This sends the neurotransmitter across thesynaptic cleftwhere the next nerve cell can receive it. In this nerve cell, the binding of the neurotransmitter to special proteins causes the formation of a action potential which moves as an electrical wave through the lipid bilayer.

Another function of the lipid bilayer is cell stiffness and support. The composition of the lipid bilayer is such that, at different temperatures and compositions, it acts differently. Depending on the species and the environment in which it lives (hot, cold, etc.), the lipid bilayer will be composed of different types and kinds of lipids. For example, human beings produce a lipid called cholesterolwhich influences the rigidity of the cell membrane. With more cholesterol among the other lipid molecules in the bilayer, the whole structure becomes stiffer. This becomes a problem when there is too much cholesterol, as the cells can no longer bend and flex as they should. In humans and other animals, this leads to tears in the walls of the arteries, which are under immense pressure from the cholesterol. heart. If these arteries rupture, it can bleed internally.

Finally, in various species, the lipid bilayer is involved in the processes of endocytosis y exocytosis. Ingesting food and excreting substances, respectively, are the simple definitions of these terms. During these events, the lipid bilayer folds (or unfolds) to take up (or excrete) substances. While there are several types of endocytosis, the most common is the phagocytosis is the act of enveloping a prey or food item by folding the lipid bilayer around it and forming a vesicle internal vesicle in which the article can be digested. This method is practised by several unicellular species in feeding.