Biochemical Foundations of Human Vital Activity - Volkov, N. I., & Nesen, E. N. 2000

Biochemical Foundations of Human Vital Activity
Acid-Base Balance of the Organism
Mechanisms of Substance Transport

There are four main Mechanisms of Substance transport that facilitate the movement of molecules within biological fluids and across Cell membranes: diffusion, osmosis, Active Transport, and exocytosis and endocytosis (Fig. 26). Diffusion and osmosis represent passive transport, as substance movement occurs without Energy Expenditure; the latter two mechanisms are forms of active transport, driven by ATP energy.

Diffusion is the thermal movement of molecules or ions from a region of high concentration to a region of low concentration, i.e., along the concentration gradient. As a result of diffusion, the concentrations of substances in contacting solutions equalize (Fig. 27, a). Each substance moves along its own concentration gradient. For instance, oxygen molecules (O₂) diffuse from the Lungs into the Blood, while carbon dioxide molecules (CO₂) diffuse from the blood into the pulmonary alveoli during Respiration, thereby accomplishing gas exchange. During muscular activity, metabolic intensity increases, and consequently, The rate of substance diffusion rises as well.

Two Types of substance diffusion across cell membranes are distinguished: passive (carrier-free) and facilitated (involving a carrier molecule). Passive diffusion involves the random movement of substances through pores (channels) in cell membranes or directly through Membrane Lipids. Many metabolic products (H₂O, CO₂, NH₃, etc.) as well as oxygen diffuse through pores. Pores are present not only in the Plasma Membranes of The Cell but also in the nuclear membranes (Fig. 28). Through these pores, Proteins destined for ribosome assembly and NUCLEOTIDES required for nucleic acid synthesis enter The Nucleus. Conversely, Ribosomes and certain Types of Nucleic acids exit the nucleus into the cell Cytosol. Fats and fat-soluble substances, such as Vitamins, penetrate cell membranes by dissolving within their lipid bilayer. In Facilitated Diffusion, the movement of a substance across the membrane is mediated by a carrier. The carrier either rotates within the membrane or forms a channel specific to a particular substance, thereby enabling its diffusion along the concentration gradient. Small solute molecules, such as Metal Ions and glucose, are transported into the cytosol through The cell membrane in this manner.

The rate of substance diffusion depends on numerous factors. Specifically, it is directly proportional to the magnitude of the concentration gradient, Temperature, and the cross-sectional area of the system through which the molecules are transferred, and inversely proportional to molecular size and solution viscosity. Large molecules (Polysaccharides, proteins, Nucleic Acids) exhibit low diffusion rates and tend to be stored (depositioned) within Cells. Small molecules (glucose, urea, ATP) diffuse rapidly and undergo fast turnover within cells (for example, ATP crosses a Cell Cytoplasm with a diameter of 10 µm in 0.2 s).

Class="center">

Fig. 26 Mechanisms of substance transport

Fig. 27 Equalization of substance concentrations along the concentration gradient in contacting solutions — diffusion (a) and in solutions separated by a membrane — osmosis (b)

Fig. 28 Cell Nucleus and nuclear pores. × 30,000

Fig. 29 Diagram of the osmotic Movement of Water molecules across The Plasma Membrane of an erythrocyte placed in different solutions: isotonic (a), hypotonic (b), and hypertonic (c)

The movement of many molecular substances into the cell is restricted because cell membranes are semipermeable. The equilibration of soluble substance concentrations inside and outside the cell is achieved through the osmotic movement of water across cell membranes.

Osmosis is a specialized type of diffusion in which water molecules move through a semipermeable cell membrane toward a region of higher solute concentration (Fig. 28, b). This type of diffusion occurs due to the osmotic pressure generated within the solutions separated by the membrane, which is determined by the concentration of osmotically active substances such as NaCl, Na₂CO₃, Na₃PO₄, etc. Osmosis plays a vital role in maintaining the shape and function of all body cells. This can be illustrated by erythrocytes suspended in Blood Plasma, where the content of water and dissolved salts varies (Fig. 29).

In plasma, erythrocytes, like other cells, exist in a state of osmotic equilibrium. In this state, the concentration of dissolved osmotically active substances and the osmotic pressure inside the cells and in the extracellular environment are identical. Such an environment is termed isotonic. In an isotonic medium, water molecules move in both directions at equal rates, allowing cells to maintain their shape and function (see Fig. 29, a). For The Human Body, a 0.9% NaCl solution is isotonic and is commonly used clinically for diluting medications. If the salt concentration in the plasma decreases, the erythrocytes find themselves in a hypotonic environment. Water enters the erythrocytes at a higher rate, which can cause them to swell or even rupture the plasma membrane (see Fig. 29, b). Consequently, Hemoglobin is released into the plasma, a phenomenon known as hemolysis. Hemolysis can occur due to severe fluid retention in the body or a deficiency in osmotically active salts, primarily table salt (NaCl).

When the salt concentration in the plasma increases, erythrocytes are exposed to a hypertonic environment (Fig. 30, c). Water flows out of the erythrocytes, leading to cell shrinkage, cytoplasmic compression, and loss of function. This phenomenon is called plasmolysis. Plasmolysis is one of the causes of Organism death due to dehydration.

These phenomena must be taken into account by athletes during intense training, particularly endurance training under high ambient temperature conditions when the body loses significant amounts of fluid and salts.

Active Transport of substances is the movement of molecules across cell membranes against the concentration gradient, utilizing ATP energy and carrier molecules. Chemical energy drives the movement of substances toward regions of higher concentration, from which they tend to diffuse passively afterwards. Active transport maintains the ionic concentration gradients between the intracellular and extracellular fluids (Fig. 30).

One of the most crucial active transport systems in body cells is the plasma membrane Na⁺-K⁺-ATPase system, which pumps Na⁺ ions out of the cell into the extracellular space against their gradient while driving K⁺ ions into the cell. This system is known as the Na⁺-K⁺ pump. It ensures the simultaneous movement of these ions toward areas of their higher concentration. Operating the Na⁺-K⁺ pump, which maintains the intracellular and surface gradients of Na⁺ and K⁺, consumes approximately 40% of the ATP utilized by the body at rest. The Hydrolysis of a single ATP molecule drives the export of three Na⁺ ions and the import of two K⁺ ions.

Fig. 30 Diagram of The active transport of Na⁺ and K⁺ ions (right) and glucose (left) across the plasma membrane mediated by Na⁺-K⁺-ATPase

Active transport of Na⁺ and K⁺ ions holds great physiological significance because it generates the electrical potential across the plasma membrane, which regulates the electrical excitability of nerve and Muscle cells. It also drives the active transport of glucose and Amino Acids into body cells, including during intestinal absorption. The active transport of glucose into cells is powered by the Na⁺ gradient. Sodium enters the cell and facilitates the cotransport of glucose (see Fig. 30).

There is also an active transport system for Ca²⁺ ions across membranes. For instance, in the sarcoplasmic reticulum of Skeletal Muscle, such systems include the Ca²⁺-ATPase and the Ca²⁺ pump, which transport Ca²⁺ back into the reticulum, leading to muscle relaxation.

Endocytosis and Exocytosis are membrane transport processes for large molecules, Bacteria, and foreign particles entering (endocytosis) or leaving (exocytosis) the cell. These processes involve the plasma membrane, which either invaginates inward to engulf macromolecular substances or fuses with intracellular vesicles to release their contents into the extracellular space (Fig. 31). In this manner, bacteria and microorganisms are neutralized, and Hormones and neurohormones are secreted.

Fig. 31. Scheme of endocytosis (invagination of the cell membrane) and exocytosis (its evagination and rupture)