Lipid bilayer of the cell membrane (GB#101E01)
● The membrane of fat is wrapping the cell
The cells are closed bags, and the contents and the outside world are completely partitioned by a cell membrane. Assuming that the cell is a sealed room, the room has doors, windows, ventilation fans and various parts. It is an incredibly thin sheet where the molecules of phospholipid, which is the basis of the cell membrane, are arranged regularly, corresponding to just the wall.
● Phospholipids side by side to make sheets
In the figure describing the cell membrane it is a standard to represent one phospholipid in such a form. ◯ is a structure containing a phosphoric acid group and || of two legs corresponds to two fatty acid molecules made of carbon and hydrogen. This is staggered and it is lined up on the plane. Since it is a double layer with the back side and the front side, it becomes a structure called “lipid bilayer” collectively. If you think that adjacent lipid molecules and molecules on the front and back are molecularly bonded to each other and become macromolecules, it is hard to realize that it is just a close-up side by side. However, it seems to be that. You will worry that it will tear if it is this.
● There is a degree even if it is thin
It is mysteriously thin how truly this film will not be torn. Anyway lipid molecules are only in two layers. Although the actual thickness of the cell membrane varies depending on the constituent components and is not definitely determined, it is about 10 nm (nanometer) even if it is thicker, and about 3 nm if it is thinner. The unit of nanometer means 10-9 m, which is the level of 1/100-million meters as 10 nm, and 1/100,000 mm thickness in terms of milli. There are various cell sizes, but if the diameter is 100 μm, it is ten thousandth of that thickness. A slightly thick copying paper with a thickness of 100 μm is regarded as a cell membrane, and when we make a box of a cell model by the paper, it becomes about a box with a side of 1 meter. This cell will hardly keep its shape on the floor. At last you could have realized the thinness.
● There are things that pass through the cell membrane and some that do not pass through it
As a fundamental property of lipid, it is familiar with fat by repelling water. Just a certain part of the phosphate group of the phospholipid is facing the outside all together because it has a familiar hydrophilic property with water. Because of this situation, the phospholipid membrane is not just a wall to block anything but a slightly complicated nature. First, gas molecules such as oxygen and carbon dioxide pass freely. Lipid molecules such as steroids and fatty acids can also easily pass through. It seems that the outer hydrophilic part does not interfere much with this. They go through the membrane along a concentration gradient from a higher concentration section to a lower section. Such movement is called diffusion, regardless of whether or not there is a film in between, regardless of the speed being slow. The lipid bilayer membrane is unexpectedly said that even though it is a membrane of lipids, water molecules can pass through relatively easily.
Then what is the wall against what, the cell membrane does not pass anything big molecules, but even small molecules do not like water soluble substances. In particular it is good to block the entry and exit of water-soluble substances such as carbohydrate, peptides, amino acids and ions with electricity.However, these are indeed going through in living cell membranes. Cells can not be used unless glucose passes through.
After all it is not a meaning of the membrane if it goes through? Not to say, these substances, which the lipid bilayer membrane is blocking, are selected through a special mechanism, with the timing and amount of movement and the direction selected. In order to properly control its flow, blocking by a lipid bilayer membrane is important. Because there are walls that can not pass through even in and out of the room, there will be meaning of doors and windows. Since some actual lipid bilayer membranes pass gases and others well, images like mesh partitions may be closer rather than hard walls. Imagine a room with a door and windows on the wall of the mesh. Bug basket!! The mechanism of cell membranes corresponding to doors and windows is various devices such as channels, transporters, and pumps.
● Cell membrane generates osmotic pressure
However, in the image of insect basket, the difference is that the actual cell membranes will certainly pass water, but that does not leak so much. Frequently the cell membrane is described as “freely” passing water, but if so it is with a bug basket. Take a bug and water in the water. Then water will come in and go through the mesh. It really is that water really “passes freely”. This is quite different from the appearance of cells. Actually it is hard to pass as much as there is a membrane. Still it is easy to pass through as far as ions and carbohydrates etc? Quantitative values are hit hardly at the textbook level, but they are physiologically important, as the properties of water molecules are much easier to pass through when compared to substances that do not clearly pass through cell membranes. Due to this nature osmotic pressure is active in the cells. Basically, the concentration of cell contents is somewhat higher than outside the cell, and water is drawn into the cell from outside the cell (by subtraction), so when considering only with osmotic pressure, cells tend to expand slightly. So the organization is in a state of constant tension. As water enters and the cells swell with that pressure, here, water will not go out freely. (← This explanation is strange as it is thought that pressing the membrane by dispersing particles that do not permeate with water molecules rather than pushing the cell membrane directly with water molecules.) In the actual cell membrane, there is a water channel that selectively passes water molecules called aquaporin, and the permeation rate of water molecules is tens of times higher than that of the elemental lipid bilayer membrane. However, there is no way that water can indefinitely enter and leave cells.
* In fact, if it is a pure lipid bilayer membrane it is weak and it will be torn soon. However, the cell membrane has a backing structure, which increases the mechanical strength, and this backing structure seems to support the shape of the cell.
* According to the molecular biology of cells (4th edition), the water permeability coefficient is approximately 10-2 cm/sec. It is said that 109 times as easy to pass through membranes as compared with ions.
* Phospholipids making up cell membranes are synthesized in the liver and supplied to the whole body by blood flow.
○ Related articles
◆ 糖質の吸収 absorption of carbohydratel
◆ Cation and anion, attraction and repulsion
◆ Voltage-dependent sodium channel
◆ Ligand-gated ion channel
◆ Resting membrane potential
◆ Osmotic pressure
◆ 膠質浸透圧 colloid osmotic pressure
◆ Glucose and the hydrolysis of sucrose
◆ Body acid-base reaction and carbon dioxide gas
◆ エンドサイトーシスと細胞内消化 endocytosis and intracellular digestion
○ Referenced books
・なっとく解剖生理学〈1〉やりとりする細胞と血液 文光堂 (2013/11)
・細胞の分子生物学， ニュートンプレス; 第5版 (2010/01)
・カラー図解 人体の正常構造と機能 全10巻縮刷版，坂井 建雄，日本医事新報社