Introduction
The miracle of human life is made possible by the cell membrane. It is the defining feature of all cellular life forms. It acts to compartmentalise tissue into distinct structures each with a specialised biological function. The human body is home to over 200 different cell types, each with a unique role (see figure 1). The muscle cell provides movement, the nerve cell communicates with the muscle cell to produce that movement, cells of our skin protect us from the ravages of the external environment and the blood cell acts to transport oxygen throughout our body.
The role of the cell membrane
It was once believed that functionally the cell membrane was a static, semipermeable envelope around cells. However, new imaging techniques have greatly enhanced our understanding of the structure and function of biological membranes. The cell membrane has dynamic functions, with all kinds of changes constantly taking place in the membrane. The cell membrane plays an important role in a number of things your body does. For example, it:
- Defines cellular boundaries and provides protection for the cell.
- Within the cell, organelles also have boundaries defined by a membrane.
- It controls the entry of nutrients and transports toxins out of the cell.
- It can organize complex reactions in response to drugs or hormones.
- It communicates with other cells via receptors.
- It has dynamic structures.
Membrane structure and function.
The cell membrane, also called the plasma membrane, is an intricate assembly of lipids, proteins, cholesterol, and other components. The membrane structure consists of a phospholipid bilayer (see figure 2). Housed within this bilayer are a wide range of proteins, cholesterol, and other components.
A phospholipid is two fatty acids (these could be omega 3 or 6 or something else) attached to a phosphate head. In figure 2 the phospholipid looks like a head with two legs. As you can see one leg is bent and one is straight. The kink is where a double bond has been inserted into the chain making it unsaturated, omega 3 has three double bonds. The straight leg has no double bonds and is a saturated fatty acid.
The cell membrane is in a constant state of dynamic flux, continually monitoring its external environment and changing in response to events taking place inside the cell and its surroundings. Critical to its structural and functional integrity are the essential fatty acids (EFA’s) omega 3 and 6.
Why are EFA’s (omegas 3 & 6) so important for the cell membrane?
Lipids i.e., omegas 3 & 6 provide the fundamental building blocks of the cell membrane. The membrane allows the cell to separate itself from its surrounding environment allowing it to import, retain and export a variety of molecules. It is the ‘gate keeper’. Within the cell are the organelles (see figure 3), these subunits each with a specialised role are also surrounded by a lipid-based membrane. The cell membrane allows each cell to carry out metabolic functions in an efficient and well-regulated manner.
In a typical membrane, these lipids usually have two fatty acid parts (see figure 2). The length of these parts can be 16, 18, or 20 carbon atoms long, and they can have zero to three double bonds. There are also some shorter and longer fatty acid chains in membranes, but they're not as common.
There are also saturated fatty acids (SFAs) present in most membranes. In certain specialized parts of the membrane called lipid rafts, SFAs can be a major component.
Membrane omega 3’s provide fluidity.
If you were a fish (a cold-blooded creature) living in cooler temperatures most of your phospholipid bilayer would consist of omega 3 fatty acids. They do have some omega 6 but this is less than 10 percent. Fish need this organisation of abundant omega 3 levels to keep their cell membrane fluid. If it consisted of saturated fatty acids, remember the straight leg in figure 2, the membrane would be too stiff (think how hard butter gets in your fridge) for the functions of the cell membrane to occur.
What does ‘fluidity’ mean? Fluidity refers to how objects within the membrane can move around relative to one another. Think of fluidity in terms of being a passenger on a commuter train. If there is space in the carriage passengers can move around. If, however, people are packed in like sardines (how apropos) then it becomes very difficult to move even within the carriage. Essential fatty acids provide this ‘space’ for objects embedded within the membrane to move around relative to one another (see figure 4). This is a critical function if the cell is to work at its optimal capacity.
Membrane dynamics and Eicosanoids
Cell membranes are not static barriers but dynamic structures in a state of flux. They interact with the environment through molecular messengers. Protein receptors on the membrane's surface receive incoming messages and communicate with other proteins on the inner surface. These interactions lead to changes in response to the signals. An enzyme called phospholipase A2 (PLA2) is often involved in these processes. It removes a specific fatty acid from a phospholipid molecule in the membrane, such as arachidonic acid one of the most common PUFA in membranes of many cells.
Arachidonic acid, commonly found in cell membranes, can undergo chemical transformations by enzymes. Phospholipase A2 initiates this process, resulting in arachidonic acid that can be converted into potent messengers within the cell. These messengers, such as prostaglandins, thromboxanes, leukotrienes, lipoxins, and cannabinoids, have distinct chemical structures and play various roles in cellular communication.
Conclusion
Nature encompasses various lipids with diverse biological functions. Understanding the impact of saturated, monounsaturated, and polyunsaturated fatty acids on health is crucial for making informed dietary choices. These fatty acids are found in triglycerides, which constitute storage fats, as well as in the phospholipids of cell membranes and lipoproteins. Cell membranes serve not only as barriers but also as specialized structures that facilitate the transportation of nutrients and waste in and out of cells.
Moreover, cell membranes play a vital role in communication through the influence of hormones, neurotransmitters, drugs, and other stimuli. They are in a constant state of dynamic change, responding to events occurring both within the cell and in its surroundings. Additionally, cell membranes act as reservoirs for long-chain polyunsaturated fatty acids. These fatty acids can be released from the membranes and metabolized into a diverse array of potent signaling agents, including bioactive eicosanoids and other lipids. Recognizing the multifaceted functions of lipids within cell membranes provides insight into their significance in various biological processes.
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