What Is Cell Transport?

The process of absorption, distribution, and excretion of foreign chemicals from outside the body is called biotransportation, including absorption, distribution, metabolism, and excretion processes ^[1] . Foreign chemicals can be absorbed by the body through the skin, digestive tract, respiratory tract, and some other ways or means; transported to the tissues and organs of the body via blood transport, they are stored or biotransformed in tissue cells and converted into metabolites; finally, It is excreted by its original substance or its metabolites through various routes. Because the mechanisms of the absorption, distribution, and excretion processes have common points, they are collectively referred to as biological transport or configuration.

Biotransportation is a general term for processes in which environmental pollutants are absorbed, distributed, and excreted through contact with the body through various means and methods. These processes have a similar mechanism, that is, each process of environmental pollutants being absorbed, distributed and excreted by the body needs to pass through the membrane structure of the cell. Cell membranes include the outer cell membrane (endoplasmic reticulum membrane, mitochondrial membrane, and nuclear membrane in plasma membrane cells, etc. These membranes are also called biofilms. Generally biofilms are composed of lipid molecules and protein molecules. Lipid molecules are mainly Phospholipids. The hydrophilic phosphate and base portions of the phospholipids face the inner and outer surface of the membrane; the hydrophobic fatty acid portion faces the center of the membrane; protein molecules are embedded in the lipid molecule layer; hydrophobic amino acids are mostly in the membrane; hydrophilic amino acids Then exposed outside the membrane. ^[2]

General biofilm consists of lipid molecules and

The biological transport process of environmental pollutants in contact with the body through the biofilm is mainly divided into two forms: passive transport and special transport.

Passive transfer. It is characterized by the fact that the biofilm does not act actively and does not consume the metabolic energy of the cell. This form of transport includes both simple diffusion and filtration. The simple diffusion process is that the environmental pollutants are transported from the high-concentration side of the biofilm to the low-concentration side through the biofilm, which is the main transport mode of fat-soluble organic compounds. Filtration is the transport of environmental pollutants through the hydrophilic pores on the biofilm. The hydrophilic pores are composed of the hydrophilic amino acids of protein molecules in the biofilm. The diameter is about 4 angstroms (such as intestinal epithelial cells). ), The diameter is 40 angstroms (such as glomeruli and capillary epithelial cells). Filtration is the main mode of transport of water-soluble compounds whose molecular diameter is smaller than the diameter of the hydrophilic pores of the biofilm.

Special transfer. Its characteristic is that the carrier composed of environmental pollutants with specific structures and proteins in biofilms forms reversible complexes for transport, and biofilms have active selectivity. This form of transport includes two forms of active transport and facilitation of diffusion. Active transport is the transfer of environmental pollutants from the low concentration side of the biofilm to the high concentration side, which requires consumption of cellular metabolic energy, and is the main form of transport of water-soluble macromolecular compounds. Facilitated diffusion, also known as promoting diffusion or carrier diffusion, is a combination of environmental pollutants and the carrier of a biofilm, which is transported from the high concentration side of the biofilm to the low concentration side. This transport cannot reverse the concentration gradient and does not consume cellular metabolic energy ^[1]

The process by which environmental pollutants that come into contact with the body enter the blood through biofilms through a variety of pathways. The route of absorption is mainly through the skin, lungs and gastrointestinal tract. The skin is a very good barrier for the human body, which can isolate environmental pollutants from the outside, but there are also many toxic environmental pollutants that can be absorbed through the skin, causing systemic poisoning. The alveolar epithelial cell layer of the lung is extremely thin, has a large surface area, and is rich in blood vessels. Many gases, volatile liquids, and aerosols, especially fat-soluble environmental pollutants, can be quickly and completely absorbed by the lung through simple diffusion. The gastrointestinal tract is the main way to absorb environmental pollutants, and its method is simple diffusion, and a few are active transport. Environmental pollutants with a small molecular weight, which are not easily soluble in lipids but soluble in water, can be absorbed through hydrophilic pores with a diameter of 4 angstroms on the small intestinal epithelial cells. The small intestine is an important absorption site in the intestine and stomach.

The process of the dispersion of environmental pollutants with the flow of blood or other body fluids to cells and tissues throughout the body. After some environmental pollutants enter the blood, some of them can be combined with plasma proteins (mainly albumin) and cannot easily pass through the biofilm; the other part is in a free state and can reach certain tissue cells, showing a certain biological effect. The combination of environmental pollutants and plasma proteins is reversible, and under certain conditions, it can be transformed into a free state. This combined state and free state are in dynamic equilibrium, and their toxicological effects are also different.

Some of the absorbed environmental pollutants can accumulate and deposit in adipose tissue or bone tissue. For example, 90% of lead is deposited in bones; organochlorine compounds such as DDT and 666 are accumulated in fat tissues. Environmental pollutants deposited in fat or bone are generally less toxic to the body, but under certain conditions, they can be re-released and enter the systemic circulation. For example, when starved, the stored fat in the body will be catabolized again, and the organochlorine compounds accumulated in the fat will also be released. There are also some barriers in the body that can prevent or slow down the distribution of foreign pollutants from the blood to tissues and organs, such as the blood-brain barrier and placental barrier. They can prevent or slow down environmental pollutants from entering the central nervous system from the blood and from the mother through the placenta to the fetus, which is a defense function of the human body. When the animal is born, the blood-brain barrier is not yet fully established, so there are many environmental pollutants, which are more toxic to newborn animals than adult animals. For example, some brain lesions caused by lead on newborn rats do not appear in the brains of adult animals.

The process by which the environmental pollutants entering the body and their metabolic transformation products are removed by the body. The main route of excretion is through the kidneys into the urine and through the liver's bile into the feces, and some can be excreted through other excretion routes:

Kidney excretion. The kidney is the most important excretory organ for environmental pollutants. Its transport mode is glomerular filtration and active tubule transport. In addition to environmental pollutants with a molecular weight of more than 20,000 or binding to plasma proteins, environmental pollutants that generally enter the body can be filtered into the urine through the glomerulus. Some environmental pollutants present in the plasma can be actively transported through the proximal tubule epithelial cells of the renal tubules, enter the renal tubules, and are excreted with urine.

Excreted with bile. This is also a major excretion route. Environmental pollutants absorbed by the gastrointestinal tract enter the liver through the portal circulation and are metabolized. Its metabolites and unmetabolized environmental pollutants mainly enter bile through active transport and are excreted with feces.

other excretion routes. Some environmental pollutants can also be expelled from the body with exhaled gases, sweat, etc. In addition, some pollutants can be excreted through breast milk, which may have adverse effects on infants ^[3] .