Easy diffusion across the cell membrane

2021-06-06 10:51 AM

Diffusion is facilitated with the help of carrier proteins. Carrier proteins help a molecule or ion pass through a membrane by chemically bonding with them.

Diffusion across the cell membrane is divided into two forms called simple diffusion and facilitated diffusion.

Diffusion simply means that the movement of molecules or ions occurs when cell membranes are open or they pass between intermolecular spaces without any influence from carrier proteins. The rate of diffusion is determined by the amount of substance in effect, the velocity of the kinetics of motion, and the number of opening sizes of the cell membrane that the molecule or ion can cross.

Diffusion is facilitated with the help of carrier proteins. Carrier proteins help a molecule or ion pass through a membrane by chemically bonding with them.

As shown above, simple diffusion can occur in 2 ways: (1) by crossing the crevices of the lipid membrane if the substances are lipid-soluble; (2) bypassing by the transport proteins when the molecules are solubilized. water-soluble cannot pass through the membrane directly.

Diffusion of lipid-soluble substances across cell membranes: an important factor determining whether a substance diffuses rapidly across lipid membranes by some mechanism is to determine whether the substance is lipid-soluble or not. For example, lipids are highly soluble in oxygen, nitrogen, CO2, and alcohol, so they can pass through cell membranes directly.

Diffusion of water and lipid-insoluble molecules across protein channels: although water is insoluble in lipids, it crosses membranes by channels of protein molecules. The rapid passage of water across the cell membrane is astonishing, for example, the total amount of water that diffuses across the red blood cell membrane per second is 100 times the volume of the red blood cell. Other lipid-insoluble substances can also pass through protein channels in some of the same ways as water if they are soluble and small enough. However, when they are larger substances, penetration is no longer rapid. For example, the diameter of urea molecules is only 20% larger than that of water, but they pass through cell membranes by 1/1000 times that of water.

Diffusion through protein holes and protein channels, channel selection: Using 3D imaging techniques, it has been shown that holes or protein channels have tubular passages for molecules to pass through. Substances can simply diffuse through these holes. Such holes are made of whole membrane proteins by opening the transmembrane tubules, and they are always open. However, the diameters of these pores are selective for molecules. For example, an aquaporin channel, also known as a water channel, allows water to pass through quickly but blocks other molecules. There are at least 13 different types of aquaporin channels found on the cell membranes of the human body.

Protein channels differ in two important factors: (1) they are often highly selective, and (2) many channels can be opened and closed by related signals, such as voltage-gated signals. channels) or ligand-gated channels.

Protein Channel Selectivity: Many protein channels are highly selective for one or more specific ions. This is the result of many factors: the diameter, the specific, natural shape arrangement of the electrical difference or the surface chemical bonding.

Figure. Transport of sodium and potassium ions through protein channels. Also shown are conformational changes in protein molecules that open or close gates, guarding the channels.

Signal-activated protein channel: This means that the protein channel is controlled by the signal received by the channel. For example, Na and K channels as above.

Figure. A, A flow record through a single voltage sodium channel, demonstrating the all-or-nothing principle to open and close the channel. B, Clamp patch method for recording flow through a protein channel. On the left, recordings are made from a patch of a living cell membrane. On the right, the recording is from a membrane patch that has been torn away from the cell.

The opening and closing mechanism are controlled in two main ways:

Voltage gating: protein channels have very large areas of charge, when the voltage between the two sides of the cell membrane changes abnormally, the mechanism will cause chemical bonds to change the structure in space, making the channel open due to voltage change.

Chemical(ligand) gating: Some ligand-directed protein channels are opened upon binding to chemical(ligands).

Open state versus closed state: look at the figure below we see that when open, the channels are open maximum, all open and when closed they all close; this is referred to by the general mechanism of all or none action potential channels.

Diffusion made easy

Ease of diffusion is also referred to as carrier-mediated diffusion because a substance is transported by diffusion across a membrane using a specific carrier protein to help.

Ease of diffusion differs from simple diffusion in several ways: although the rate of simple diffusion through open channels increases in proportion to the concentration of the diffusion, facilitated diffusion is closely related to the concentration of the diffusor. maximum concentration, called Vmax as the concentration increases. This difference is best demonstrated in the figure below.

Figure. Effect of concentration of a substance on the rate of diffusion across a membrane by simple diffusion and facilitated diffusion. This graph shows the favourable diffusion reaching the maximum rate known as Vmax.

So the question, in particular, is what makes the diffusion limit easy?

Figure. Postulate mechanisms for facilitated diffusion

The answer is: facilitated diffusion depends entirely on the number of protein channels, when a molecule binds to a signal region (receptor) of the carrier protein, causing them to change conformation and give the substance. pass. As the concentration of substances increases, the binding capacity of the substances to the channel increases and increases the diffusion capacity, but here there are pauses, which is when all the protein channels have tried to molecularly break down. this is the maximum speed at which they can diffuse, if the concentration increases much, it will not bring about an increase in the rate of diffusion.

Of most substances that cross cell membranes by facilitated diffusion, the most important are glucose and most amino acids. In the case of glucose, at least five types of glucose channels are found in many tissues. Some of them can also pass through other structurally similar monosaccharides, including galactose and fructose. An important channel is insulin-activated GLUT4, which causes a 10- to 20-fold increase in glucose diffusion when tissue is stimulated by insulin. This is the basic mechanism by which insulin regulates blood glucose levels.

Selective Permeation of Cell Membrane - True Diffusion of Water

Figure. Osmosis at the cell membrane when sodium chloride solution is placed on one side of the membrane and water is placed on the other side.

The most abundant substance that diffuses across the cell membrane is water. Water is diffused from an area of ​​high water potential to an area of ​​low water potential, or so to speak, from an area of ​​low concentration to an area of ​​high solute concentration. And the diffusion of water under such conditions is called osmosis. Osmotic refers to the permeability of the cell membrane to water (osmotic).

Osmotic pressure: as shown above, if osmosis is blocked, stopped or reversed. The correct pressure to prevent osmosis is the osmotic pressure of a solution.

"Osmolality"- osmole: to clarify the concentration of a solution within the limit of the number of particles, a unit called osmole is used. 1 osmole is 1 gram of osmotic molecule, so 180 grams of glucose is equivalent to 1 osmole because glucose does not break down into other ions. But if a molecule in solution dissociates into 2 ions, then 1 gram of its molecule counts as 2 osmoles. For another example, a NaCl solution with 58.5 grams of NaCl would have 2 osmoles.

Relationship between osmolality and osmotic pressure: at 37, a concentration of 1 osmole per litre will cause 19300 mmHg osmotic pressure. Likewise, if the solution has an osmole concentration of 1 milliosmole, it will produce an osmotic pressure of 19.3 mmHg.