Human Anatomy and Physiology - I. V. Gayvoronsky 2011

The human body as an integrated whole. Fundamentals of cytology and histology
Cell

Structure AND Functions. The Cell is the basic structural, functional, and genetic unit of All living organisms.

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Fig. 3.1. Diagram of the microscopic structure of an animal cell:

1 — nucleus; 2 — Plasmalemma; 3 — microvilli; 4 — endocytic vacuoles; 5 — granular Endoplasmic reticulum; 6 — mitochondrion; 7 — Lysosomes; 8 — Ribosomes; 9 — microfilaments; 10 — agranular endoplasmic reticulum; 11 — Golgi apparatus; 12 — centriole and microtubules; 13 — secretion of secretory granules

It was discovered in 1665 by R. Hooke. Cell shape and size vary, yet there are General Principles of their structure (Fig. 3.1). Any cell has a cell membrane, the plasmalemma (cytolemma), which separates it from the extracellular environment or adjacent Cells. The molecular framework of the plasmalemma consists of a phospholipid bilayer with embedded Proteins that function as protein channels or pores.

The most important Functions of the plasmalemma are barrier, biotransforming, transport, and receptor. The barrier function lies in separating the Cytoplasm from the surrounding environment and mediating interaction with it. The biotransforming function involves ensuring biochemical transformations of substances entering the cell, including Pharmaceuticals. The transport function is the translocation of substances across the membrane necessary to maintain Homeostasis of the internal environment. Transport can be passive (filtration, diffusion, osmosis) and active (protein pumps). The receptor function is the cell's ability to selectively interact with specific chemically active substances (Hormones, Neurotransmitters, etc.).

In addition to the membrane (plasmalemma), every cell consists of two main components: The Nucleus and the cytoplasm.

The nucleus is surrounded by a nuclear envelope, the karyolemma (nucleolemma). It separates the nucleus from the cytoplasm, performing structural and transport functions. The nucleus is filled with nuclear sap, or karyoplasm, which contains proteins required for the synthesis of Nucleic Acids. The nucleus is responsible for the storage, transmission, and Selection/27.html">Realization of Genetic information, as well as the Regulation of cellular activity.

The primary unit for storing Genetic information is Chromatin, which consists of a DNA complex and corresponds to Chromosomes that are not distinguishable as individual structures in the interphase nucleus.

The cytoplasm participates in metabolic processes and the maintenance of METABOLISM/37.html">Cellular Homeostasis. It contains permanent structures specialized for performing specific functions, known as Organelles (organoids), and temporary components—inclusions formed As a result of the accumulation of metabolic products. Organelles are divided into general-purpose and specialized. In turn, general-purpose organelles are classified into membranous and non-membranous based on the presence of a membrane. Membranous organelles include The endoplasmic reticulum, Golgi apparatus, lysosomes and Peroxisomes, vacuoles, and Mitochondria; non-membranous ones include ribosomes, the cell center (centrosome), microtubules and microfilaments, and cilia (Table 3.1).

The endoplasmic reticulum (ER) ensures the synthesis of Lipids, CARBOHYDRATES, and proteins, serves as the main storage depot for Ca²⁺ ions, and facilitates Intracellular Transport of Substances. Two Types of ER are distinguished: granular (rough) and agranular (smooth).

Table 3.1 Classification of Organelles

The outer surface of the agranular reticulum membrane lacks ribosomes, which gives it a smooth appearance. The Golgi complex (Golgi apparatus) synthesizes Polysaccharides and Glycoproteins, provides chemical Processing of secretions and their transport outside the cell, and promotes the structural maturation of proteins synthesized by the ER.

Lysosomes and peroxisomes carry out the Digestion of substances engulfed by cells, as well as The breakdown of biogenic macromolecules. They contain Enzymes that ensure the metabolism of various substances, including foreign compounds (such as drugs), and the detoxification of toxic metabolic products. Vacuoles store various substances, including metabolic waste. Mitochondria are involved in ENERGY GENERATION AND accumulation. Ribosomes synthesize proteins. The cell center participates in Cell Division.

Microtubules provide a supporting function; microfilaments perform a contractile function and participate in The formation of intercellular contacts.

In addition to general-purpose organelles, there are specialized ones. For example, the sperm acrosome plays a crucial role in the Fertilization mechanism; the microvilli of the small intestinal epithelium facilitate absorption processes; the microtubules of the receptor cells in the Tongue's taste buds participate in encoding information about The properties of food substances; and the motile cilia of the tracheal and bronchial epithelial cells ensure the drainage function of the respiratory tract.

Furthermore, cells contain optional elements known as inclusions, which are subdivided into trophic (nutritive): fat droplets, Glycogen; secretory: hormones, BIOLOGICALLY ACTIVE SUBSTANCES; excretory (to be eliminated): urea; and pigmentary—endogenous (internal): melanin, and exogenous (acquired from outside): dust, pigments (e.g., in tattoos).

One of the essential properties of a cell is reproduction. Somatic cells divide by mitosis, while Germ Cells divide by Meiosis. As a result of mitosis, the cell receives a complete (diploid) set of chromosomes—23 pairs. As a result of meiosis, germ cells retain a halved (haploid) set of chromosomes.

The lifespan of a cell from one division to another, or from division to death, is called the Cell Cycle. It consists of several periods:

1st — division phase (M);

2nd — presynthetic period (G1) — a period of accumulation of various substances;

3rd — synthetic period (S) — formation of nutrients and duplication of genetic material occur;

4th — postsynthetic period (G₂) — the cell prepares for division.

Chemical composition of the cell. The cell contains about 70 Chemical elements of D. I. Mendeleev's periodic table. In an animal cell, about 98% of the mass is made up of four elements: hydrogen, oxygen, carbon, and nitrogen, which are classified as macroelements. Below is The chemical composition of an animal cell, as a percentage of total cell mass:

In addition to macroelements, cells contain elements present in tenths and hundredths of a percent: sodium, potassium, calcium, chlorine, phosphorus, sulfur, iron, and magnesium, which are collectively known as macro-microelements. Each of them performs an essential cellular function. For example, sodium, potassium, and chloride ions regulate cell membrane permeability for various substances and facilitate Nerve Impulse transmission. Calcium and phosphorus are involved in Bone tissue formation, and calcium also plays a crucial role in Blood clotting. Iron is a key component of Hemoglobin in red Blood Cells, while magnesium is found in a variety of enzymes.

The remaining elements (zinc, copper, iodine, fluorine, etc.) are present in extremely small amounts—accounting for a total of up to 0,02%—and are referred to as Trace Elements (or microelements). In specialized cells, they participate in the synthesis of biologically active substances: zinc is a component of Insulin, a pancreatic hormone, whereas iodine is a constituent of THYROID HORMONES. Most trace-element metals are integral parts of various enzymes. All chemical elements exist within the Organism as ions or as components of diverse inorganic and Organic compounds.

The specific roles of each chemical compound will be discussed in greater detail in the chapter "Metabolism and Energy Conversion".