The cytoplasm fills the interior of a human cell. It consists of the cytosol, a liquid or gel-like substance, the organelles (mitochondria, Golgi apparatus, and others), and the cytoskeleton. Overall, the cytoplasm serves enzymatic biosynthesis and catalysis as well as substance storage and intracellular transport.
What is cytoplasm?
The definition of cytoplasm is not uniform in the literature. Some authors consider the entire bioactive content of the human cell, including the nucleus, to be cytoplasm in its entirety. Other authors do not include the organelles contained in the cell, such as mitochondria and the endoplasmic reticulum, and the nucleus in the cytoplasm, but use the term protoplasm, under which they subsume the entire contents of the living human cell. The nucleus and numerous organelles (up to many thousands) are enclosed in the cytoplasm, and it is traversed by microfilaments, intermediate filaments, and microtubules. These are the cytoskeleton, proteins that give the cell strength and structure and allow intracellular transport of substances – including transport through biomembranes. The fluid or gel-like portion of the cytoplasm is called the cytosol. Changes in consistency within specific regions of the cytosol also transport organelles within the cell. To allow many biochemical reactions to occur in parallel within the cell, spaces called compartments can be formed within the cytoplasm delineated by biomembranes. They allow for the different environmental conditions required in each case.
Anatomy and structure
The cytoplasm contains about 80.5% to 85% water, 10% to 15% proteins, 2% to 4% lipids, and the remainder is distributed among polysaccharides, DNA, RNA, and organic and inorganic molecules and ions. The pH of the cytoplasm is approximately neutral at 7.0 and is kept as stable as possible by buffering. Ion pumps can be used to additionally stabilize or slightly change the PH. The cytoskeleton, which gives the cell its strength and shape and ensures intracellular mass transport, consists of actin filaments (microfilaments), intermediate filaments and microtubules. The cytoskeleton is subject to a dynamic process of assembly and remodeling that allows structural adjustments. Actin filaments are composed of long-chain protein polymers with an extremely thin diameter of about 6 to 9 nanometers. Intermediate filaments are much more complex composed of different structural proteins (keratins), and 5 different types are differentiated. Tubular microtubules, about 24 nanometers in diameter, are composed of tiny globular units of tubulin. Microtubules can reach lengths ranging from fractions of a micrometer to several hundred micrometers. Microtubules can be very short-lived to stably long-lived, depending on the task at hand.
Function and tasks
The individual components of the complex cytoplasm have a wide variety of functions and tasks. Higher-level tasks consist in the storage of certain substances and in enzymatic-catalytic bioactivity, i.e., in the breakdown and degradation of substances that are needed or no longer needed. To perform these higher-level tasks, the cytoplasm or cell has a number of tools at its disposal. Since many conversion processes occur within specific organelles, the cytoplasm can provide intracellular transport of organelles to the optimal “location” within the cell by changing their consistency from gel-like to aqueous and vice versa. Special functions are performed by microtubules, which enable vesicle transport through membranes. Substances for which membranes are not permeable are trapped in vesicles (protrusions of the membranes) and transported through the membranes with the help of microtubules. Microtubules also play a special role in movements within a cell and in the intrinsic movements of certain cell types that move by means of flagella (e.g. sperm). Another special function is performed by microtubules in chromosome assembly during mitosis (normal cell division) after DNA replication.Likewise, microtubules play an important role in stabilizing axons (also known simply as nerves), the nerve processes that serve to transmit nerve impulses from the nerve cell to the target tissue (efferent) or from the sensor to the nerve cell (afferent). The ability of the cytoplasm to form enclosed reaction spaces within the cell by forming membranes enables the cell to allow many biochemical processes, which are controlled enzymatically-catalytically and each require their own reaction environments, to occur simultaneously.
Diseases
The almost unmanageable abundance of functions held by the cytoplasm or certain individual components of the cytoplasm suggest that equally complex and differentiated dysfunctions and ailments related to the cytoplasm may occur. Colchicine, also known as spindle poison, serves as an example of a specific dysfunction. It is an alkaloid of autumn crocus that binds to monomeric tubulin, inactivating it and preventing the formation of spindles for cell division (mitosis). Normal cell division is thus prevented. Vinblastine, a chemotherapeutic agent with a similar spectrum of action, is used specifically in the presence of certain types of cancer to deprive the tumor of the basis for growth. Similarly, toxins that interfere with the cytoplasm’s ability to take ATP from the mitochondria and deliver ADP there can rapidly become life-threatening. So-called tauopathies are due to gene mutations that lead to structural changes in the tau protein. The tau protein is essential for the assembly of microtubules, causing problems especially in the central nervous system (CNS). Diseases such as Pick’s disease, HDDD dementia and several others are causally related to a gene mutation that leads to deposits of tau protein. The best known tauopathy is Alzheimer’s disease.