HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY, AND PATHOLOGY - Ya.I. Fedonyuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.3. MOLECULAR-GENETIC AND CELLULAR LEVELS OF LIFE ORGANIZATION

1.3.2. Structural, chemical, and functional organization of eukaryotic cells

Cytoplasmic Organelles

Organelles are permanent cytoplasmic structures with a specific architecture that perform specialized Functions. Some organelles (microscopic) are visible under a Light Microscope, while others (submicroscopic) require an Electron microscope. Based on the presence of a biological membrane, they are classified into membranous and non-membranous. Membranous organelles include The Endoplasmic reticulum, Cell/35.html">Mitochondria, Lysosomes, Golgi apparatus, and Peroxisomes; non-membranous ones include Ribosomes, centrosomes, microfilaments, and microtubules. These nine organelles are termed general-purpose organelles because they are present in all Cells. There are also organelles specific to specialized cells, known as special-purpose organelles: myofibrils in Muscle cells and fibers, neurofibrils in Nerve Cells, cilia in the epithelium of the Trachea and Bronchi, etc.

The endoplasmic reticulum (vacuolar network, reticulum) is a submicroscopic general-purpose organelle that forms a system of membranes comprising an interconnected network of tubules, vesicles, and cisternae. These permeate the hyaloplasm in all directions and contact both the nuclear and Plasma Membranes, forming a single enclosed Circulatory system. The Main Functions of the endoplasmic reticulum are the synthesis and transport of organic substances. There are rough (granular) and smooth (agranular) types of endoplasmic reticulum. Ribosomes are located on the cytoplasmic surface of the rough endoplasmic reticulum membranes, whereas they are absent on the smooth endoplasmic reticulum. Ribosomes synthesize Proteins that are transported via the channels of the rough endoplasmic reticulum to the Golgi apparatus, where they undergo chemical modifications and membrane packaging. The membranes of the smooth endoplasmic reticulum synthesize Lipids and CARBOHYDRATES

(Glycogen). It is believed that The formation of Introduction/36.html">Biological Membranes begins in the agranular reticulum.

Ribosomes are submicroscopic non-membranous general-purpose organelles where METABOLISM/35.html">Protein Biosynthesis takes place. The diameter of a ribosome is about 20 nm. They are located freely in the Cytoplasm or attached to the membranes of the granular Endoplasmic reticulum and The Nucleus. A ribosome consists of two subunits: large and small, which are capable of reversible dissociation (they can spontaneously disintegrate and reassemble). Each subunit is built from a single ribonucleoprotein strand. During Protein Synthesis, ribosomes assemble into complexes called Polysomes (polyribosomes) on an mRNA molecule.

Lysosomes (Gr. lisis — dissolution, soma — body) are submicroscopic general-purpose organelles whose primary function is The breakdown of Biopolymers of various chemical compositions (cellular Digestion). They appear as single-membrane enclosed vesicles with a diameter of 0.2–0.4 μm containing a set of hydrolytic Enzymes. Lysosomes digest foreign particles entering The Cell (heterophagy), as well as the cell's own structures that have died or lost their functional purpose (autophagy). Under normal conditions, digestion occurs entirely within the lysosomes. If the lysosomal wall is ruptured, lysosomal enzymes are released into the cytoplasm, causing the entire cell to undergo self-digestion and death (autolysis). Lysosomes exist as prelysosomes, true lysosomes, and postlysosomes. Prelysosomes (primary lysosomes) are inactive lysosomes that contain enzymes but lack substrates for digestion. Primary lysosomes are formed in the Golgi apparatus. True lysosomes (secondary lysosomes) are active lysosomes containing both enzymes and substrates slated for degradation. They are formed by the fusion of prelysosomes and phagosomes. Phagosomes refer to pinocytic, phagocytic, and other vesicles; all of them contain substrates for digestion. Secondary lysosomes in which digestion is complete are called postlysosomes or residual bodies; they contain only undigested residues. Some residual bodies migrate to The Plasma Membrane, fuse with it, and expel their contents, while others remain within the cell. The deficiency or absence of a particular lysosomal enzyme leads to the intracellular accumulation of undigested substances, causing so-called lysosomal storage diseases, which are predominantly hereditary in nature. The Development of atherosclerosis and obesity is also linked to a deficiency of lysosomal enzymes.

Mitochondria (Greek mitos — thread, chondros — grain) are microscopic membranous general-purpose organelles whose main function is to extract energy from organic substances through oxidation and to store the released energy in ATP molecules. Energy is utilized in this ATP form for cellular activities. The size of mitochondria ranges from 1 to 10 μm. They can be spherical or rod-shaped, and they are capable of changing shape, fusing, migrating, and concentrating predominantly around the Cell Nucleus during pathological states.

Mitochondria consist of a matrix (ground substance) and a double-membrane wall. The outer membrane has a smooth contour, whereas the inner membrane forms inward-directed folds called cristae (Fig. 1.32). Cristae increase the membrane surface area and serve as the site for the Enzymes of the Respiratory Chain (Electron Transport Chain) and Oxidative Phosphorylation. Mitochondria possess their own DNA located in the matrix and are capable of self-Replication. They never arise de novo, always forming solely through division. Mitochondrial DNA is circular in shape (like that of prokaryotes). Genes of mitochondrial DNA encode The Structure of certain mitochondrial proteins, the autonomous synthesis of which occurs on mitochondrial ribosomes. However, the majority of mitochondrial proteins are synthesized outside the mitochondria under the control of nuclear DNA. Mitochondrial ribosomes, like prokaryotic ones, have a sedimentation coefficient of 70S. There is a hypothesis that mitochondria were once free-living prokaryotes that penetrated a cell under certain conditions and became symbionts.

Class="center">

Fig. 1.32. Mitochondrion:

a — general structural diagram: 1 — outer membrane; 2 — inner membrane; 3 — cristae; 4 — matrix;

b — internal structure diagram: 1 — double-membrane envelope; 2 — matrix; 3 — cristae.

The Golgi apparatus is a microscopic membranous general-purpose organelle composed of dictyosomes, numbering from several hundred to several thousand per cell. A dictyosome is an aggregate of interconnected cisternae about 25 nm thick, flattened in the central region and expanded at the periphery. Small vesicles (Golgi vesicles) bud off from the expanded edges of the cisternae. Metaphorically, a dictyosome is compared to a stack of plates with their concave side facing the nucleus. The Golgi apparatus accumulates intracellular secretory products destined for export from the cell. Here they undergo certain chemical modifications and membrane packaging, and, as components of Golgi vesicles, are transported into the cytoplasm or discharged outside via the plasma membrane (exocytosis). Primary lysosomes are formed within the Golgi apparatus, Polysaccharides are synthesized, and complexes of these compounds with proteins (Glycoproteins) and lipids (Glycolipids) are produced. The Golgi apparatus is typically located near the nucleus.

The centrosome (cell center) is a microscopic non-membranous general-purpose organelle that plays an active role in Cell Division. During interphase, it is located near the nucleus on the same axis as the centers of the nucleus and the cell; it consists of two centrioles and the surrounding centrosphere. As the cell prepares for division, each centriole duplicates. During cell division, the duplicated centrioles (diplosomes) migrate to

opposite poles and establish the poles of the cell. Centrioles stimulate tubulin proteins, leading to the formation of microtubules of the mitotic spindle. Centrosomes are present in all animal and human cells, except for egg cells, as well as in lower plant cells. Higher plant cells lack centrosomes.

Peroxisomes are submicroscopic membranous general-purpose organelles whose primary function is to rid the cell of toxic metabolic by-products (detoxification). Peroxisomes utilize chemically active atomic oxygen and break down ethyl alcohol, uric acid, and other substances. They appear as vesicles approximately 0.2–0.5 μm in diameter filled with enzymes, among which catalase serves as the marker (defining) enzyme.

Microfilaments are submicroscopic non-membranous general-purpose organelles that function as part of the Cytoskeleton and the cell's contractile apparatus. They are thin fibers approximately 5 nm in diameter, composed of contractile proteins (Actin, Myosin, etc.). They are located predominantly in the cortical (submembranous) zone of the cell and within its cytoplasmic extensions. Their function is contractile and motor-driven. There are also intermediate microfilaments with a diameter of 10–15 nm. The protein composing them is a strictly specific marker of cells of a given type (for instance, keratin is a histochemical marker of epithelial cells). Intermediate microfilaments are primarily responsible for maintaining cell shape. In recent years, their involvement in The regulation of genome activity and cellular differentiation processes has been confirmed.

Microtubules are submicroscopic non-membranous general-purpose organelles that ensure the motility of cellular organelles and form part of the cytoskeleton. They are built from tubulin proteins, whose molecules are capable of polymerization and depolymerization. Microtubules constitute the structural basis of the centrosome, mitotic spindle, cilia, and flagella.

Inclusions, unlike organelles, are not permanent Structural components of the cytoplasm and lack a strictly defined architecture. Inclusions are predominantly products of cellular metabolism that accumulate in the form of droplets, granules, crystals, or vacuoles. According to their purpose, they are conventionally divided into three groups: trophic (egg yolk, glycogen, fat droplets), secretory, and special (Hemoglobin).