Active transport of substances across cell membranes

2021-06-04 03:00 PM

Various substances are actively transported across membranes including Na, K, Ca, H, I, urea, several other sugars, and most amino acids.

At the same time, high concentrations of the degraded substance must remain inside the cell while the concentration outside is very low. This is true, for example, a cell must always keep the K concentration inside the cell much higher than outside and conversely the Na concentration inside is lower than outside. So there needs to be a mechanism to maintain such a thing, and that's done by active transport, the cell actively taking the substances or ions needed for themselves despite outside concentrations of these substances. This substance is very little. That is, it went against the concentration scale.

Various substances are actively transported across membranes including Na, K, Ca, H, I, urea, several other sugars and most amino acids.

Primary and secondary active transport

Active transport is divided into two types according to the source of energy used to induce transport: primary and secondary active transport.

In primary active transport, energy is used directly from the breakdown of ATP molecules or from some compounds containing high-energy phosphate bonds. In secondary active transport, the secondary energy received from stored energy is in the form of different ion concentrations between the two sides of the cell membrane, the source of the difference being primary active transport. . Both examples, transmembrane carrier protein-dependent transport, are facilitated diffusion. However, inactive transport, the function of a carrier protein that differs from that of a carrier in tns amplification is facilitated by its ability to transfer energy to the carrier to move against the electrochemical gradient.

Primary active transport


Figure. Mechanism of the sodium-potassium pump. ADP, adenosine diphosphate; ATP, adenosine triphosphate; Pi, phosphate ion.

Substances that are transported by primary active mechanisms: Na, K, Ca, H, Cl, and a few other ions

The active transport mechanism studied in the most detail is the Na-K pump, which transports Na out of the cell, and K from outside to inside. This pump is responsible for maintaining the different concentrations of Na and K between the two sides of the cell membrane, establishing a negative potential inside the cell membrane and a positive potential outside the cell.

The carrier protein is composed of two subunits in the figure above: the large α subunit and the β smaller subunit. The large subunit has 3 receptors for Na binding and 2 for K binding, within the division of this protein near the Na binding site where ATPase is active.

Mechanism: When 2 K ions are attached to the outside of the carrier protein and 3 Na ions are attached to the inside, the function of ATPase starts to work. It cleaves a high-energy phosphate bond of ATP to ADP and 1 phosphate, which changes the configuration of the pump and helps bring 3Na out and 2K into the cell. And like other enzymes, the Na-K-ATPase Pump can also generate ATP from ADP and phosphate when the electrochemical scale of Na and K is large enough to overcome the normal process of the pump.

An important function of this pump is to regulate cell volume: without this function, everybody cell would expand in volume until it exploded. In the cell there are many proteins and other molecules that cannot escape from the cell, which increases the osmotic pressure of the cell and draws water into the cell, but when this pump mechanism is present, The amount of positive ions lost in each pump is higher than 1 molecule, so it also reduces the osmotic pressure of the intracellular fluid. If the cell begins to take in water and expand for any reason, the Na-K pump automatically becomes more active, pumping more ions out to pull water with it and maintain cell volume.

Primary active transport of Ca ions: Ca ions are maintained at very low concentrations intracellularly, about 1000 times lower inside than outside. Therefore, two primary active transport processes are required. One stays in the cell membrane and pumps Ca out of the cell. Another pump is within the cell of the endoplasmic reticulum with muscle cells and mitochondria in all cells.

Primary active transport of H ions: in two places, where the transport of H ions is most important, they are (1) the gastric gland of the stomach (2) and the distal convoluted tubule and collecting duct of the kidney.

In the gastric gland, located in the wall of the parietal cells, there is primary active transport. In the secretion of gastric juice, with H and Cl ions

In the renal tubules, many H ions are secreted from the blood into the urine to eliminate H ions also by primary active transport.

Secondary active transport - synergistic and antagonistic

When Na is transported across the cell membrane by primary active transport, the concentration of Na is much higher outside than inside. This concentration gradient creates an energy wave, and that diffuses Na back inside the cell. Under such action, Na diffuses and can drag other substances along. It is a co-transport mechanism and is a secondary active transporter.

In the antagonist, Na ions try to diffuse into the cell because the concentration outside is too high. However, at the same time, there is a substance that is also transported from the inside out. Thus, the Na ion binds to the carrier protein at the receptor site outside the cell, while the agonist attaches to the internal site of the carrier protein. Once the two are bound, a conformational change occurs in the protein, and the energy released by the Na ion moves inward causing the other substance to move outside.

Figure. Mechanisms for sodium co-transport glucose. Synergists of Glucose and some amino acids with Na ions

Na-Ca, Na-H antagonists: two important antagonistic mechanisms as shown below h├Čnh


Figure. Sodium is the reverse transport of calcium and hydrogen ions

Active transport through cell arrays (Cellular Sheets)

In other parts of the body, substances can be transported all the way through an array of cells instead of through a simple cell membrane. Transport in this fashion occurs in: (1) the intestinal epithelium, (2) the epithelium of the renal tubules, the epithelium of all exocrine glands, (4) the epithelium of the gallbladder, (5) the choroid plexus of the brain and several other membranes.

The basic mechanism of transport of a substance across a cell array is (1) active on one side of the cell array and (2) diffusion on the opposite side.

Figure. The basic mechanism of active transport across a cell layer 

The figure above depicts the mechanism of Na ion transport across an epithelium (gastrointestinal tract, renal tubule, or gallbladder). Epithelial cells are tightly bound together at the poles by the interstitial space of the junction known as the soft touch. The brush edge of the cell's contact surface (lumen) is permeable to both Na and water. Thus, Na and water diffuse readily from the lumen into the cytoplasm. And then at the basal and lateral membranes, Na is actively transported into the interstitial fluid of adjacent connective tissue and blood vessels. This causes a high ion concentration gradient across the cell membrane, which leads to better water permeation. Thus, active transport of Na at the brush border of epithelial cells results in Na and water carrying.

This mechanism is present in most nutrients, ions, and other substances that are absorbed into the bloodstream from the intestines; It is also how some substances are reabsorbed in the renal tubules.