Mass transport in a biological system

The transformations can be represented simply as a term of production or consumption, while in the second, since the transformation takes place on the contour surfaces of the system, the corresponding reaction is introduced as a boundary condition for the system.

- The reactions that take place within a single-phase, be it a gas or a liquid, are called homogeneous reactions. They, therefore, take place anywhere within the fluid mass, so they can be considered as evenly distributed within the control volume. Therefore, they can be represented analytically as a source term or well in the differential equations that describe the system;

- The reactions which, however, involve more than one phase are called heterogeneous reactions. They take place at the contour surfaces of the fluid volume and, therefore, must be represented analytically through conditions at the source or well boundary in the differential equations that describe the system.

These reactions determine the consumption of nutrients and the production of biomass and are also accompanied by production or consumption of energy. A classic example of biological reaction is that of the irreversible type of decomposition of the organic substance, which can be expressed, in general form, as:

Here C₆H₁₂O₆ is glucose which can be taken as an example of an organic substance to biological decomposition. This reaction takes place when the slurry is introduced into an aquatic ecosystem. In fact, the organic substance contained in the slurry is oxidized by bacteria. In particular, under aerobic conditions, due to the action of aerobic microorganisms, proteins are transformed first into amino acids and then into CO _2, H₂O and nitrogen and sulfur salts, carbohydrates into CO ₂ and H₂O, the fats first in fatty acids and then also in CO₂ and H₂O.

Another classic biological reaction, in a sense inverse to the previous one, is that of photosynthesis which produces glucose starting from CO₂, with the mediation of sunlight h v :

The two reactions of respiration and photosynthesis are extremely important in the cycle of dissolved oxygen in a water body, which is, as is known, a very important quality parameter for the aquatic ecosystem. However, it should be noted that these two inverse reactions do not usually take place in the same place and that they are rather slow compared to the time scale of some Environmental Hydraulics problems

The mass transport through the plasma membrane

To meet its biological needs of adaptation to the environment and reproduction, the cell must introduce the materials it needs for nutrition and respiration and, at the same time, eliminate waste products (excretion).

The passage of materials between the internal environment of the cell and the extracellular environment is regulated by the plasma membrane. Its lipid bilayer constitutes an insurmountable barrier for most of the large biological molecules that are polar and water-soluble and ions, to allow the passage of the large molecules and ions necessary for the life of the cell, membrane proteins intervene, some of them which form pores or channels, while others have binding sites for specific molecules. Due to its selective characteristics regarding the passage of molecules and ions, the plasma membrane is called semipermeable.

The passage of substances through the membrane can take place according to two mechanisms. The first one does not involve energy consumption and is called passive transport; the second involves energy consumption by the cell and is distinguished by active transport, endocytosis, and exocytosis.

Passive transportation

Passive transport is the movement of substances in solution through the membrane that does not require energy expenditure by the cell. The membrane does not interfere with the direction of movement of the particles since this occurs thanks to the existence of gradients, i.e., concentration differences. The particles move spontaneously from areas of high concentration to areas of low concentration, such as those existing outside respectively and inside the membrane itself (movement proceeds until equilibrium conditions are established). In other words, the movement of the particles takes place by exploiting the potential energy due to the presence of the concentration gradient. There are three forms of passive transport: simple diffusion facilitated diffusion and osmosis.

Simple diffusion is the movement of gas or fat-soluble molecules through the phospholipid double layer of the plasma membrane. The diffusion speed is influenced by the concentration of the substance it diffuses, the temperature and the pressure. Diffusion continues until the concentration gradient is canceled, that is when the concentration of the gas or molecule becomes equal to the two sides of the membrane.

The facilitated diffusion is the transport of ions, amino acids, and monosaccharide mediated by membrane proteins. Some of these (channel proteins) can form permanent channels across the membrane. Other proteins (transporters) have specific binding sites for certain molecules; once the bond has taken place, the transporter changes its shape and, consequently, the bound molecule is transferred and released from the other side of the membrane. Since the plasma membrane has a limited number of transporters, the speed of facilitated diffusion is lower than that of simple diffusion.

Osmosis is the diffusion of water through the plasma membrane. The passage of water is allowed by the presence of pores formed by the assembly of a certain number of intrinsic proteins. Osmosis ends when osmotic equilibrium is reached. Under these conditions, the speed of the water flow on both sides of the plasma membrane is the same in both directions, and there is no longer a clear passage of molecules from one side to the other. The three forms of passive transport, depending on differences (gradients) in concentration, are reversible, that is, they occur in one direction or the other according to the gradient.

Active transport

Active transport is the movement of materials across the plasma membrane against the concentration gradient (i.e., from low concentration areas to high concentration areas) and with energy consumption by the cell. The substances transported from the inside to the outside can be waste products (the presence of which, even in very small quantities, could damage the cell). They transported from the outside to the inside are nutrients present in low concentration in the extracellular environment and ions that must be present in the cell at lower concentrations than those of the extracellular liquid.

Active transport is regulated by intrinsic membrane proteins (called pumps because they move molecules against the concentration gradient), equipped with binding sites for a specific molecule to be transported and for ATP. Hydrolysis (splitting) of ATP provides the energy necessary for the carrier protein to change shape and transport the molecule across the membrane. Once the molecule is released, the transporter protein regains its original configuration.

Endocytosis is the transport of materials inside the cell by means of vesicles. The cell surrounds the particle to be introduced with its plasma membrane until it is incorporated into a vesicle, which then merges with a lysosome; inside the latter, the assumed particle is demolished by proteolysis enzymes. Endocytosis represents the method of food intake of some protozoa, such as amoeba. In this case, the process takes the name of phagocytosis, the amoeba surrounds the food with extensions of the cytoplasm called pseudopods, then englobes it and digests it using its own lysosomes.

Exocytosis is the transport of materials outside the cell. In practice, it is the opposite of endocytosis. The substances that must be removed from the cell are enclosed in a vacuole, which moves to the periphery of the cell so as to merge with the plasma membrane.

Author: Vicki Lezama