Human Anatomy Part 1 - K. A. Dyubenko, A. K. Kolomiytsev, Yu. B. Chaykovsky 2002

General Part
Structure of the Human Body
Cell - Cytoplasm

The Cytoplasm acts as the primary metabolic apparatus of The Cell.

It is composed of the hyaloplasm, Organelles, and inclusions.

1. The hyaloplasm forms the internal environment of the cell. It is a complex colloidal system containing Biopolymers such as Proteins, Polysaccharides, and Nucleic Acids, along with low-molecular-weight compounds, various Enzymes, and ATP (adenosine triphosphate). It is involved in maintaining METABOLISM/37.html">Cellular Homeostasis and Intracellular Transport processes, and it can transition from a liquid state (sol) to a more solid state (gel) and vice versa.

2. Organelles are Structural components of the cytoplasm essential to all Cells, performing vital life Functions. They are divided into membranous (those containing membranes) and non-membranous (lacking membranes) categories. The former include The Endoplasmic reticulum, Mitochondria, internal reticular apparatus (Golgi complex), Lysosomes, and Peroxisomes. The latter include free Ribosomes, microtubules, microfilaments, Intermediate filaments, and centrioles.

The endoplasmic reticulum is formed by tubules, flattened sacs, and vacuoles bounded by membrane walls. There are two types of endoplasmic reticulum: granular (rough), whose membranes are studded with ribosomes on the outer surface, and agranular (smooth), which lacks this coating. Both types participate in synthesis: the granular type synthesizes proteins, including those intended for export from the cell, while the agranular type synthesizes Lipids and polysaccharides. The granular endoplasmic reticulum also contributes to The formation of certain structural components of other cellular membranes.

Mitochondria appear as oval, rod-shaped, and sometimes branched structures of varying sizes. Their wall consists of two membranes: the outer membrane separates these organelles from the hyaloplasm, while the inner membrane surrounds the mitochondrial interior (matrix). The inner membrane forms folds (cristae) that project into the matrix. The function of mitochondria is associated with the synthesis of adenosine triphosphate (ATP), an energy-rich molecule consumed during energy-requiring cellular work.

The internal reticular apparatus (Golgi complex) is formed by clusters of membranous structures (dictyosomes), each comprising flattened sacs (cisternae) narrowed at the center and expanded at the ends, along with small vesicles located along their periphery. The Golgi complex performs functions such as the accumulation and chemical modification of substances synthesized in the endoplasmic reticulum, the synthesis of polysaccharides, the formation of lysosomes, and the removal of substances from the cell via exocytosis.

Lysosomes appear as spherical structures enclosed by a membrane and typically filled with an electron-dense content. They contain a set of hydrolytic enzymes (Hydrolases) that break down biopolymers. These organelles are involved in the breakdown (Digestion) of products entering the cell from the outside (exogenous origin) or formed within the cell itself (endogenous origin).

Peroxisomes are small, spherical or oval structures enclosed by a membrane and filled with finely granular material containing an electron-dense core, sometimes referred to as a nucleoid. One of the primary functions of peroxisomes is The breakdown of peroxide compounds (such as H₂O₂), which are toxic to the cell.

Ribosomes are non-membranous organelles that appear as granules composed of large and small subunits. Ribosomes may occur singly or form complexes called Polysomes, in which they are linked by Messenger RNA. The function of ribosomes is Protein Synthesis. They may float freely in the hyaloplasm or be attached to the membranes of the endoplasmic reticulum, which is then referred to as the rough (granular) endoplasmic reticulum. When free ribosomes predominate in a cell, the synthesized proteins are primarily used for the cell's own internal needs. Conversely, when bound ribosomes are more abundant, proteins are synthesized predominantly for export.

Microtubules appear as unbranched hollow cylinders with an outer diameter of approximately 24 nm. Their wall consists of subunits formed by the globular protein tubulin. They function as part of the Cytoskeleton, participate in the Formation of the mitotic spindle, centrioles, cilia, and flagella, and influence the direction of intracellular transport and directed cell movement.

Microfilaments have a smaller diameter than microtubules (approximately 5 nm). They participate in forming the cell surface complex and contain contractile proteins that affect cell movement and the formation of Cleavage furrows that divide daughter cells during mitosis.

Centrioles appear as small cylinders 0.2 µm in width and 0.3–0.5 µm in length, around the periphery of which nine microtubule triplets and A number of additional structures are arranged.

In addition to the aforementioned general-purpose organelles characteristic of most cells, cellular elements also include so-called special-purpose organelles that perform specialized functions, namely:

- tonofibrils, characteristic of certain types of epithelial and neuroglial cells;

- myofibrils, located within Muscle cells or fibers;

- neurofibrils, found within Nerve Cells;

- cilia and the brush border, found in epithelial cells;

- flagella, found in spermatozoa;

- synaptic vesicles, located in the terminal sections of neurites (the longest processes of nerve cells).

Some special-purpose organelles, such as myofibrils and neurofibrils, are highly developed versions of general-purpose organelles. Specific details regarding the Structure and Functional Significance of special-purpose organelles will be examined in The Study of the Tissues to which cells possessing such organelles belong.

3. Inclusions are non-permanent structural Components of the cytoplasm whose presence or absence is determined by the metabolic CHARACTERISTICS OF THE cell. Inclusions are divided into exogenous inclusions, which arise from the uptake of Materials into the cell from the outside (e.g., during phagocytosis), and endogenous inclusions, which are formed within the cell itself. Endogenous inclusions include:

- trophic inclusions, represented by lipid droplets, Glycogen granules (a glucose biopolymer), or protein granules;

- secretory inclusions, formed by the accumulation of cellular secretion products;

- excretory inclusions, containing waste products destined for removal from the cell;

- pigmentary — which may have a different composition and are capable of temporarily or permanently changing the color of a tissue or organ.