Classification of cell organelles

Introduction to cytology and cell theory

Cytology is a fundamental biological and medical discipline, the subject of which is the study of the structural features, functioning, and development of cells. In modern science, the cell is recognized as the absolute minimum and elementary structural unit forming any living organism.

Understanding cellular organization is based on the classical cell theory formulated by M. Schleiden and T. Schwann, and later supplemented by R. Virchow. This concept includes several key postulates:

• Absolutely all living organisms, with the exception of viruses, are formed from cells, as well as the products of their specific life activity.
• Cellular structures are homologous. This means that they possess fundamental morphological and functional similarity, which is due to their evolutionary commonality of origin.
• The process of forming new cellular units is carried out exclusively through the division of already existing maternal cells. Any theories of spontaneous generation are completely excluded.
• The general physiological activity of any multicellular organism is not a simple sum, but represents a complex result of the integration and activity of all interacting cells.

In addition, every cell possesses three fundamental properties: the ability to self-preserve its own structure both in space and time; the ability to self-reproduce to maintain the population; and the ability to strictly self-regulate all internal life and metabolic processes.

Main cell systems

Regardless of the complexity of structure, the internal space of the cell is strictly ordered. The entire cellular architecture can be globally divided into three interacting systems:

1. Cell surface apparatus (CSA). This structure performs a crucial barrier function, reliably demarcating the internal contents from the surrounding external environment. The CSA also provides two-way communication and metabolism.
2. Cytoplasm. Represents a complex colloidal system filling the intracellular space. Organelles are located within it, and basic metabolic reactions take place here.
3. Nuclear apparatus. The central informational base of the cell. Its main task consists of the reliable storage, precise reproduction, and subsequent transmission of hereditary information to daughter structures.

Classification of cells by presence of nucleus and organelles

The presence of specific organelles depends directly on the global type of cellular organization. Biology distinguishes three main types:

In prokaryotes, the functions of missing membrane-bound organelles are assumed by mesosomes—specialized folds (invaginations) of the cell membrane. It is on these that the processes of organic matter breakdown and the synthesis of ATP molecules, which provide the cell with energy, take place. Externally, the prokaryotic cell is protected by a dense glycoprotein cell wall, the basis of which is the biopolymer murein.

Mesokaryotes represent a unique evolutionary bridge between the plant and animal worlds. A classical representative is Euglena gracilis. It is distinguished by a mixotrophic type of nutrition: in the presence of light, it is capable of photosynthesis, and in the dark, it switches to the osmotic absorption of nutrients.

Cytological foundations of heredity

In eukaryotic cells possessing a full set of organelles and a formed nucleus, chromosomes act as the carriers of hereditary information. By chemical composition, they represent a complex nucleoprotein complex consisting of DNA molecules and associated proteins (both histone and non-histone).

In order for a giant DNA molecule to fit into a microscopic nucleus, this complex undergoes five successive levels of compaction (packing). As a result, zones with different densities are formed inside the interphase nucleus:

• Heterochromatin — dense, highly spiralized regions.
• Euchromatin — less dense, despiralized regions.

In most cases, chromosomes are indistinguishable outside the period of division, but exceptions exist. In some specialized cells, interphase chromosomes can be visualized even under a regular light microscope. Such unique structures include giant polytene chromosomes, as well as specific lampbrush-like chromosomes.

Class and exam questions

What else to study on this topic

• Levels of compaction of the nucleoprotein complex (from nucleosome to chromosome)
• Structure and functions of the cell surface apparatus (CSA)
• Chemical composition of chromosomes: the role of histone and non-histone proteins
• Features of polytene chromosomes and lampbrush chromosomes
• Mixotrophic type of nutrition using the example of Euglena gracilis

Other topics in the "Additional Topics" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.