Okay, we did exactly 80 facts about cells broken down into 4 different articles (links are given below). Those cell facts covered both plant and animal cells. However, in case you are looking for detailed information, they are just not enough. So, we decided to go in depth, covering every component of a typical cell in as much details as possible. So, we decided to start cell membrane facts. Study carefully as there is a wealth of knowledge awaiting you in this article.
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But before we start with cell membrane facts and start delving deep into its structure and its functions, it is essential that we start with the understanding of a few things. Because we will like to keep this article simple and key it down in layman’s terms, let’s start by defining a few things.
|Lipids||Lipid is basically an organic compound which is water insoluble. It is usually made of sterols, waxes, oils, fats and triglycerides. There are different types of lipids. Of these various types, two are ‘Phospholipid’ and ‘Cholesterol’. Both of these are present in cell membrane.|
|Proteins||They are basically complex and large molecules that are produced naturally. They have at least one long amino acids chain in which peptide bonds are used for holding together amino acid groups.|
|Carbohydrates||These are basically food sources that are used for energy by plants and animals. They can be cellulose, starch or sugar.|
|Hydrophilic||Something that likes or loves water.|
|Hydrophobic||Something that dislikes or hates water.|
|Phospholipids||They are lipids but are made of two tails of fatty acids, head-group containing glycerol that is linked to a phosphate.|
|Cholesterol||This is also a lipid that is made of four carbon rings that are fused together.|
We have given the definitions that we will be needing for this article. We acknowledge the fact that there are some technical terms here that are still require some explanation but will leave them for now and we will cover them in separate articles.
So, let’s start with cell membrane facts…
Amazing Cell Membrane Facts: 1-5
1. Cell membrane is a very specific name. It simply implies that it is a membrane of a cell. However, cell membrane is also known by its generic name – “Plasma Membrane”.
2. The cell membrane is a semipermeable membrane. This means that selectively allows things to pass through it (either enter through it or move out through it).
3. The job of the cell membrane is to protect the components that are present inside the cell (called intracellular components) from the components that are present outside the cell (called extracellular components).
4. The cell membrane of a cell contains specific markers. These markers are more like cellular ID cards which are used by cells to recognize each other. These markers are extremely important from the perspective of our immune system. There are immune cells in our body that attack and destroy foreign tissues and cells that get inside our body. The only way these immune cells can do so is when they are aware of which are foreign and which are important part of the body. These markers help immune cells to identify the important ones and selectively destroy the foreign bodies.
5. We wrote about cytoskeletons on one of our generic cell facts list. The cytoskeleton is attached to the cell membrane. So basically, the cell membrane, along with the cytoskeleton helps a cell to maintain its shape.
Amazing Cell Membrane Facts: 6-10
6. The cell membrane has a unique structure which has been explained by what is known as Fluid Mosaic Model. This model was proposed back in 1972. Over time, the model became more advanced and today, it is the most widely accepted model of plasma membrane.
7. Talking of plasma membrane, do you remember from our previous articles on cell facts that there are organelles present inside a cell and that these organelles are bounded by a membrane? These membranes are also plasma membranes. So, unless we don’t want to get confused about which membrane we are talking about, it is better to use Cell Membrane when we are talking about the membrane that wraps the entire cell (including the organelles inside).
8. Just like the cell membrane works as a base where cytoskeleton get attached, in case of plant cells, the cell membrane doubles as the base where not only the cytoskeleton gets attached but also where the cell wall gets attached.
9. Coming back to the Fluid Mosaic Model, this model states that the plasma membrane has a mosaic structure with several components in the mosaic. These components move around fluidly and freely in the membrane’s plane. The components of the membrane primarily include proteins, cholesterol and phospholipids.
10. This plasma membrane consists of two phospholipids layers. Each phospholipid has tails and head. The phospholipids are so arranged that the tails face or point each other while the heads point outwards. Take a good look at the image below. It shows the structure of the plasma membrane as explained by the Fluid Mosaic Model. The phospholipid layer that you see in the image is known as phospholipid bilayer.
Amazing Cell Membrane Facts: 11-15
11. In the phospholipid bilayer, cholesterol – a different type of lipid – stays embedded. The cholesterol never comes outside the bilayer.
12. Then, there are membrane proteins. There are three variants that you can see here. There is one variant that will extend partly into the bilayer. There will be a second variant that will cross the entire bilayer right through it. Finally, there is a third variant in which the protein is loosely attached to the surface of the bilayer (can be on the inside surface and or can be on the outside surface). This third variant is known as the peripheral membrane protein while the previous two are called the integral membrane proteins.
13. Finally, there are carbohydrates that can be found only and only on the outer surface of the phospholipid bilayer. The carbohydrates will attach themselves with either the proteins or the lipids. If attached to lipids, they will form glycolipid and if attached to the proteins, they will form glycoproteins.
14. Now a pressing concern! What will be the percentage of carbohydrates, proteins and lipids in the plasma membrane. This proportion varies among various types of cells. In case of typical human cells, the ratio of protein:lipids:carbohydrates is 50:40:10.
15. Wondering why phospholipids are arranged in the way they are arranged in the bilayer and why not in a different way? The reason for this is that the phospholipids are amphipathic by nature. It means that a part of a phospholipid is hydrophilic and another part of the same phospholipid is hydrophobic.
Amazing Cell Membrane Facts: 16-20
16. As we know that phospholipids have a head and two tails. The head contains a phosphate group which has a negative charge. In addition to this, there is another small group, which scientists denote by the letter R. It too is polarized (that is, charged). It is this polarization which allows the head of the phospholipid to interact with water, which too is a polar molecule. The interaction is electrostatic. This means that the head of the phospholipid is hydrophilic.
17. The two tails of the phospholipid are made of fatty acids. These fatty acids are nonpolar by nature. This means that they have a hard time interacting with water molecules which are polar. Any such interaction will mean that a lot of energy will be required and hence, not a very favorable interaction. On the other hand, the nonpolar fatty acids can easily interact with nonpolar molecules. So, it is more logical for the phospholipid bilayer to form with heads on the outer side and tails tucked inwards to prevent interaction of tails with water. The tails are hydrophobic.
18. Taking of proteins, which form a major part of the plasma membrane, they too have hydrophilic and hydrophobic parts. For integral proteins (that are integrated into the plasma membrane), there is at least one region that is hydrophobic and it always remains anchored to the hydrophobic core of the phospholipid bilayer. The parts of the integral protein which remains outside the hydrophobic membrane core are hydrophilic. Those parts remains attached either with extracellular fluid or with cytoplasm.
19. As said earlier in point number 12, an integral protein can actually cross through the entire hydrophobic core of the plasma membrane and show up on the other side of the membrane. These integral proteins are called transmembrane proteins.
20. Then there are carbohydrates that are found only on the outer surface of the cells. This means, they stay only on outer surface of the plasma membrane. These carbohydrates either attach to lipids of proteins and one chain of carbohydrate may contain a minimum of 2 monosaccharide units to a maximum of 60 monosaccharide units. The carbohydrate chains can either be branched or they can be straight. These carbohydrates form molecular markers (like ID badges), allowing cells to recognize each other.
Amazing Cell Membrane Facts: 21-25
21. Ever wondered how things pass through the phospholipid bilayer of the plasma membrane? What is the secret of its fluidity? The answer to these questions comes from the tails of the phospholipids. The tails can be made of saturated fatty acids or they can be made of unsaturated fatty acids.
22. The tails that are made of saturated fatty acids are relatively straight because of the absence of double bonds. Unsaturated fatty acids on the other hand have double bonds and they have a kink or bend in their. The phospholipids in the cell membrane may have both tails of only saturated fatty acids or have one tail of saturated fatty acids and another tail of unsaturated fatty acids.
23. When there is a drop is temperature, the tails that are made of saturated fatty acids will pack in tightly, making a dense and rigid membrane but those phospholipids that have a combination of saturated and unsaturated fatty acid tails will fail to make such a dense packing because of the bends present in the unsaturated fatty acid tails. So, even when temperature drops, the cell membranes that have unsaturated fatty acids in them will remains pretty fluid.
24. In addition to these, animal cells have cholesterol embedded inside the hydrophobic core of the plasma membrane. This means even when the tails of the phospholipids try to form condensed packing with drop in temperature, the embedded cholesterol prevents that from happening, maintaining the fluidity of the membrane. At higher temperatures, the cholesterol prevents excess fluidity by actually reducing the fluidity. So the cholesterol plays a vital role in maintaining healthy and functional fluidity
25. Final question: What really is fluidity? In this context, the fluidity of the membrane simply means its ability to seamlessly part and create space when something enters and when it passes the membrane, the membrane seamlessly flows back in and closes the gap that was created.