BIOLOGY Lecture Notes - Golden Pages 2003

1. THE CELL

Foundations of Cell Theory

The Discovery of the cellular Structure of organisms is directly linked to the invention of the Microscope. Robert Hooke, who first observed a Cell in 1665 using a homemade microscope, noticed empty compartments resembling a honeycomb. In essence, he did not see actual Cells, but rather only their cell walls. Thus began The history of cell doctrine. Robert Hooke coined the term "cell" (from the Greek "kytos" — a hollow vessel). Later, Grew and Malpighi replicated Hooke's observations across various plants and discovered tiny cavities amidst a homogeneous mass, which they called "vesicles." Although the cellular theory of animal and plant structure is commonly associated with Schleiden (1838) and Schwann (1839), it had actually been proposed even earlier by A number of researchers. For instance, Mirbel (1808–1809) concluded that plants are formed from membranous cellular tissue. In 1809, Lamarck asserted that "no body can contain life if its constituent parts do not form cellular tissue." Similar ideas were expressed by Dutrochet (1824), Turpin (1826), Meyen (1830), and von Mohl (1831), in whose works the Cell Theory is clearly articulated.

Despite the findings of these scientists, the authors of many biology textbooks consider Schleiden, a professor of botany in Jena, to be the founder of cell theory. Schleiden's data on Water/32.html">Plant Cell Structure were confirmed for animals by Schwann. He conducted a thorough investigation of animal Tissues and was the first to use the term "cell theory," arguing that "cells are organisms, and animals, like plants, are an aggregate of these organisms arranged according to definite laws." Schwann's findings served as a solid foundation for cell theory. He articulated distinct views not only regarding the morphological but also the physiological significance of cells. According to Schwann, cellular phenomena can be divided into two groups: "plastic phenomena," i.e., the combination of molecules forming The Cell, which in modern terminology corresponds to cell Morphology, and physiological phenomena, which result from "chemical changes either in the particles making up The Cell as such, or in the surrounding Cytoplasm." He defined these processes as metabolic phenomena. Thus, Schwann formulated our modern concepts. For this reason, he can be called the "father" of modern Cytology.

Cell theory quickly spread to unicellular organisms as well: Protozoans began to be viewed as animals consisting of a single cell (von Siebold, 1845), and Haeckel divided the animal kingdom into two major groups — Protozoa and Metazoa. Albert Kölliker, the famous Swiss anatomist, applied the data of cell theory to Embryology. Virchow extended this theory to pathology.

In the early 19th century, researchers focused their attention on the cell contents, which various authors described as a "jelly-like" or mucous, sticky sap. Within plant cell sap, Robert Brown discovered The Nucleus in 1831, which is one of the most vital and permanent Components of the cell. Max Schultze concluded in 1861 that There is a fundamental similarity between the protoplasm of animal and plant cells, thereby formulating the theory that O. Hertwig later, in 1892, named the protoplasm theory. According to this theory, which is broader in concept than cell theory, the cell is an aggregation of living matter, or protoplasm, clearly bounded in space and containing a nucleus and a cell membrane. Thus, the original Concept of the cell evolved. With The Emergence of these foundational theories, histological research began to develop rapidly. Thus, the phenomena of direct Cell Division — amitosis (Remak) — and indirect cell division (Schneider, Strasburger) were discovered. The latter form of cell division is also known as karyokinesis (Schleicher) or mitosis (Flemming). It was established that the core feature of mitosis is The formation of nuclear threads or Chromosomes (Waldeyer). Within the cytoplasm, the cell center (van Beneden, Boveri) and the reticular apparatus (Golgi) were discovered. Concurrently with tissue research, attention increasingly focused on the cell as the fundamental structural unit of living organisms. In 1892, O. Hertwig published his monograph "The Cell and the Tissues," in which he summarized biological phenomena based on the Characteristic Features of the cell, its structure, and function. The author demonstrated in this book that solutions to various biological problems can be found in the processes occurring within cells, thereby establishing cytology as a modern branch of science.

The unit of life is the cell. It contains everything necessary to sustain life and ensure its continuity. If a cell is damaged and the damage cannot be repaired, all its activity ceases within a short time and its components disintegrate. Cells vary greatly in size, shape, and nature of activity. Let us examine only the minimum set of components without which no cell can exist. Such a "minimal" cell must include:

1) a system of membranes that surround the cell, partition its interior, govern chemical processes, and bear a number of essential catalysts;

2) an "apparatus" for producing exact copies of the cell by replicating its principal structures;

3) an "apparatus" that supplies various cellular Functions with energy derived from oxidative processes.

Although the type of equipment required for any cell is easy to state, it is by no means simple to determine what this equipment actually is and how it is arranged within the cell. In Multicellular Organisms, there is a functional division based on structural differentiation. Thus, in differentiated cells of higher organisms, there is variation in the number of cell Organelles (sometimes differences in their fine structure, as well as in their intracellular distribution). Cells can be specialized: they may exhibit, for example, contractility, photosensitivity, or secretory activity. Some cells may be distinguished by the presence of specialized molecules — tools for performing specific functions; Hemoglobin in red Blood Cells serves to transport oxygen, rhodopsin in retinal cells for light perception, and Actin and Myosin in Muscle cells for contraction. In specialized cells, The Plasma Membrane possesses certain specific features. For instance, it may perform functions related to Digestion AND ABSORPTION (microvilli of the intestinal epithelium), transmission of nerve impulses (Nerve Cells), or secretion and selective uptake (cells lining the proximal tubules of the Kidney).

To maintain basic cellular functions, certain structures known as cell organelles emerged during the course of evolution. They ensure the coordinated and regulated progression of fundamental processes necessary for the constant manifestation of vital functions. The following organelles are essential for the existence of a living Organism: the nucleus, Mitochondria, Endoplasmic reticulum, Ribosomes, Golgi apparatus, Lysosomes, and Microbodies. The size and shape of organelles vary greatly depending on the cell type. Thus, mitochondria in Liver cells are spherical, in kidney cells — cylindrical, and in fibroblasts — thread-like. Depending on the metabolic state of the cell, the shape and volume of mitochondria can undergo rapid changes. Mitochondria are often located within the cell in close proximity to structures that consume energy. In actively working muscle cells, they are arranged in regular rows along the myofibrils; in epithelial cells performing a secretory function, they often align with the direction of secretion movement, which requires energy. Frequently, they are positioned along lipid droplets in the cytoplasm, which serve as a fatty-acid fuel source for oxidation processes. In liver cells, mitochondria are capable of moving freely within the cytoplasm, whereas in muscle cells their position is fixed.