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

Biochemical Foundations of Human Vital Activity
Metabolism in the Body
Cellular Structures and Their Role in Metabolism

The strict Organization, sequential nature, and independence of metabolic processes within Cells are ensured by their compartmentalization into distinct cellular Organelles. Each Cell can be viewed as an independent chemical system with its own METABOLISM, allowing it to finely respond to fluctuations in the concentration of specific substances within both the internal and external environments.

When examining a living cell under an Electron microscope, one can observe a gel-like substance—the Cytoplasm—and a multitude of diverse, rapidly moving particles known as cellular organelles. All cellular organelles are enclosed by individual membranes, possess a specific set of Enzymes, and perform dedicated Functions in cellular metabolism. Let us examine The Role of individual cellular structures in supporting Intermediary Metabolism.

Most human body cells are characterized by the presence of eight major intracellular compartments: the Cytosol, Endoplasmic reticulum, Nucleus, Mitochondria, Golgi apparatus, Ribosomes, Lysosomes, and Peroxisomes. The Structure of a cell and its individual organelles are illustrated in Fig. 9.

The Cell is enclosed by a cell or Plasma Membrane, which separates the cell's contents from the extracellular environment and plays a crucial role in metabolism. The Plasma Membrane is a trilaminar structure consisting of a lipid bilayer—which creates an impermeable barrier to Water-soluble molecules—and a layer of Proteins "embedded" within The Lipid Bilayer (Fig. 10). Many protein molecules span the membrane entirely and function as pores or channels through which specific substances are transported into and out of the cell. Other proteins may reside on one surface of the lipid bilayer and participate in metabolic processes. Membrane Proteins also serve as receptors for numerous chemical signals.

On the outer surface of the plasma membrane of all Eukaryotic cells, there are CARBOHYDRATES linked to proteins (Glycoproteins) or Lipids (Lipoproteins). It is believed that these carbohydrates are involved in cell-Cell Recognition processes.

The plasma membrane exhibits selective permeability to small ions and simple molecules. Furthermore, it can maintain specific ion concentrations inside the cell, particularly Na⁺ and K⁺, thereby establishing a concentration gradient of these ions relative to the extracellular fluid, as well as a membrane electrical potential.

The primary role in generating and maintaining the Membrane Potential is played by the enzyme Na⁺-K⁺-ATPase. Utilizing energy from ATP, this enzyme pumps Na⁺ out of the cell and K⁺ into the cell against their concentration gradients (see Chapter 5). The existence of Na⁺ and K⁺ concentration gradients is essential for the electrical excitability of cell membranes and The transport of glucose, Amino Acids, and other substances. Plasma Membranes are also involved in secretion, the uptake of large molecules, intercellular interactions, and the recognition of external signals.

Class="center">

Fig. 9. Cell Structure and individual organelles involved in cellular metabolism:

1 — mitochondrion; 2 — Chromatin; 3 — nucleolus; 4 — nucleus; 5 — Golgi apparatus; 6 — cytoplasm; 7 — cell membrane; 8 — peroxisomes; 9 — lysosomes; 10 — ribosomes; 11 — endoplasmic reticulum

The cytoplasm (cytosol) represents the portion of the intracellular space that is not occupied by membrane-bound structures, or organelles. The cytoplasm surrounding the cellular organelles is called the cytosol. The cytosol accounts for about 55% of the total cell volume and is a gel-like mass containing approximately 20% proteins. In addition, the cytosol houses thousands of enzyme proteins and cytoskeletal proteins—which maintain cell shape and the independence of metabolic processes—as well as complex carbohydrates (Glycogen) and lipid droplets. The majority of intermediary metabolism reactions take place in the cytosol, including carbohydrate breakdown (Glycolysis) and the Biosynthesis of Proteins on ribosomes, as well as the synthesis of carbohydrates and Fatty acids. Some of the synthesized proteins are utilized by cellular organelles.

The endoplasmic reticulum (sarcoplasmic reticulum in Skeletal Muscle) consists of numerous closed Membrane structures in the form of cisternae, tubules, and lamellae that divide the cell into distinct compartments. The synthesis of various lipids and complex carbohydrates takes place within the reticulum. On its rough surface, studded with ribosomes, The biosynthesis of the cell's principal proteins occurs. In the sarcoplasmic reticulum of Muscles, Calcium Ions are sequestered; upon muscle excitation, these ions are released into the cytoplasm to trigger contraction, and during relaxation, with the help of the Ca²⁺-ATPase enzyme, they are pumped back into the reticulum. Consequently, the sarcoplasmic reticulum regulates the concentration of free Ca²⁺ in the muscle cytoplasm.

The Nucleus is the center for storing hereditary information, as it contains DNA molecules that hold the human Genetic Code. Processes such as the synthesis of ribosomes, Ribonucleic Acids, certain Coenzymes, and other substances take place within the nucleus. The nucleus is surrounded by a double nuclear membrane perforated by pores. Nuclear pores ensure the selective transport of various substances.

Mitochondria are complex double-membrane structures where Biological Oxidation of nutrients occurs in the presence of oxygen, accompanied by the release of thermal energy and The formation of chemical energy in the form of ATP. They are often referred to as the powerhouse of the cell (the STRUCTURE AND FUNCTIONS of mitochondria are discussed in more detail in Chapter 3).

Fig. 10. Diagram of the plasma Membrane Structure: 1 — Phospholipids (bilayer); 2 — Cholesterol; 3 — glycoproteins; 4 — Oligosaccharides; 5 — Glycolipids; 6 — proteins

The Golgi apparatus, or complex, is a stack of membrane structures where the structures of proteins and certain other substances are finalized, and where they are sorted prior to transport to various destinations within the cell.

Ribosomes are cellular organelles where Protein Synthesis takes place. Each ribosome consists of a large and a small subunit, which dissociate after the Synthesis of the protein polypeptide chain is complete. The number of ribosomes depends on the intensity of protein synthesis (for instance, in Liver cells, it reaches 10⁷).

Lysosomes are membrane-bound organelles containing hydrolytic enzymes known as acid Hydrolases (which are highly active at a pH of around 5.0). These enzymes break down proteins, Nucleic Acids, and other macromolecules, as well as foreign particles and Bacteria. Lysosomes also participate in regenerative processes that drive cellular HYPERTROPHY AND HYPERPLASIA, which can be observed in certain Tissues As a result of athletic training.

High activity of hydrolases can lead to damage of the lysosomal membrane (lysis). When hydrolytic enzymes escape into the cytoplasm, they can cause cell death. Intense physical exertion triggers the activation of the lysosomal apparatus in skeletal muscle and The Heart, which may serve to adaptively remodel metabolism during strenuous muscular work.

Peroxisomes are small vesicular organelles in which The oxidation of various substances (SH₂) by oxygen takes place, yielding hydrogen peroxides (H₂O₂):

SH₂ + O₂ → S + Н₂O₂

About 10% of the oxygen uptake in Organs such as the liver is utilized by peroxisomes. These organelles are found in virtually all cells. They contain a set of enzymes, including catalase, which utilize the generated H₂O₂ for the peroxidation of various substances According to the following reaction: H₂O₂ + SH₂ → S + 2H₂O. For example, roughly half of all Fatty acids are oxidized to acetyl-CoA in peroxisomes, which also detoxify ethanol, methanol, and other harmful substances.

At low concentrations of oxidizable substrates, catalase breaks down hydrogen peroxide into water and oxygen: 2H₂O₂ → 2H₂O + O₂. This reaction acts as a cellular "rescue" mechanism, preventing the accumulation of the potent oxidizing agent H₂O₂, which is capable of destroying the cell. During intense muscular activity, processes of substance peroxidation become accelerated, potentially causing adverse alterations in cellular metabolism and structural organization.