Lipid Bilayer Definition
A lipid bilayer is a biological membrane consisting of two layers of lipid molecules. Each lipid molecule, or phospholipid, contains a hydrophilic head and a hydrophobic tail. The tail regions, being repelled by water and slightly attracted to each other, congregate together. This exposes the head regions to the outside, creating a barrier between two bodies of water. A lipid bilayer is the foundational part of all cellular membranes, typically completed with species-specific integral proteins and other functional aspects.
A lipid bilayer functions through the actions of polarity. The inside of the lipid bilayer is non-polar, while the heads are polar molecules and create hydrogen bonds with other polar molecules. This also means that polar molecules like water and ions cannot as easily cross through the nonpolar tail region of the lipid bilayer. The cellular membranes of most organisms are created with lipid bilayer, as well as the nuclear membrane and various organelle membranes. The various functions of these membrane are then specified with a variety of proteins which allow or disallow certain substances to cross the membrane. In doing so, cells and individual organelles can create an ideal environment for biochemical reactions to occur, allowing them to stay in homeostasis.
Structure of the Lipid Bilayer
A lipid bilayer consists of two sheets of amphiphilic phospholipids, as seen in the image below. Amphiphilic describes a molecule which is part hydrophobic, part hydrophilic. There is often phosphorus atoms in the heads of the molecules, giving the heads polarity. The tails of the molecules are nonpolar 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 stuck rigidly in place. In a single sheet, the molecules are actively moving around and past each other. In fact, a better analogy is that of people crammed in an elevator. They mostly stay put, but can slide past one another if someone needs to get off the elevator and is standing in 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 surface and integral proteins, as well as extracellular and intracellular elements that have specific functions in the cell. An encompassing model of the entire cellular membrane is the fluid mosaic model, which assumes that proteins within the lipid bilayer act as icebergs within the sea, drifting around but not bound to anything. The specific properties of the protein and of 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 within unicellular organism and multicellular organisms alike. Regardless if a cell is living freely in pond water or confined in your body serving a function, it needs to maintain different conditions for the various reactions it needs to conduct to survive. In all applications, the lipid bilayer acts as the filter between the inside and outside. However, depending on the conditions the exact functions of the lipid bilayer can change.
Imagine two cells, one in the ocean and one in a pond. Pond water is fresh, whereas the ocean water contains many dissolved salts. In the pond, water will want to move into the more hypertonic, or saltier, cell. In the ocean, the salts in the water will draw water out of the cell. These two differing situations show how important the proteins in a lipid bilayer are. While each bilayer stops the ions and slows the movement of water, it can only hold back a certain pressure. Water will continually leach into or out of the cell. Different types of organisms have different strategies for dealing with water loss, most depending 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, and aquaporins. Ion pumps rely 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 allowing larger quantities of water to pass through the membrane at the appropriate time.
The lipid bilayer and its associated proteins provide another function for cells, in the way of cellular signaling. They can be involved in a number of ways. In signal transduction, a signal is passed 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 nerve impulses. When a nerve impulse reaches the end of a nerve, called the synapse, it sends a signal for special vesicles to fuse with the lipid bilayer of the cell membrane. The vesicles, filled with neurotransmitter molecules, release their contents upon fusing. This sends the neurotransmitter across the synaptic cleft, where the next nerve cell can receive it. On this nerve cell, the binding of the neurotransmitter to special proteins causes the formation of an electrical action potential, which moves as an electrical wave down the lipid bilayer.
A further function of the lipid bilayer is of cellular rigidity and support. The makeup of the lipid bilayer is such that at different temperatures and compositions, it acts different. According to the species and environment it lives in (hot, cold, etc.), the lipid bilayer will be made from different kinds and types of lipids. For example, humans produce a lipid called cholesterol, which influences the stiffness of the cellular membrane. With more cholesterol between the other lipid molecules in the bilayer, the entire structure becomes more rigid. This becomes a problem when there is too much cholesterol, as cells can no longer bend and flex as they are meant to. In humans and other animals this lead to tears in the walls of arteries, which are under immense pressure from the heart. If these arteries tear, you can bleed out internally.
Finally, in a number of species the lipid bilayer is involved in the processes of endocytosis and exocytosis. Taking in food and excreting substances, respectively, are the simple definitions of these terms. During these events, the lipid bilayer is folded (or unfolded) to take in (or excrete) substances. While there are several types of endocytosis, phagocytosis is the act of enveloping a prey or food item by folding the lipid bilayer around it and forming an internal vesicle in which the item can be digested. This method is practiced by a number of unicellular species in feeding.
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