Protoplasm: History, Characteristics, Components, Functions

The protoplasm is the living cell material. This structure was first identified in 1839 as a distinguishable fluid from the wall. It was considered to be a transparent, viscous and extensible substance. It was interpreted as a structure with no apparent organization and with numerous organelles.

Protoplasm has been considered to be all the part of the cell that is found inside the plasma membrane. However, some authors have included the cell membrane, the nucleus and the cytoplasm within the protoplasm.

Currently, the term protoplasm is not widely used. Instead, scientists have preferred to refer directly to cellular components.

History

The term protoplasm is attributed to the Swedish anatomist Jan Purkyne in 1839. It was used to refer to the formative material of animal embryos.

However, as early as 1835 the zoologist Felix Dujardin described the substance inside the rhizopods. It gives it the name sarcoda and indicates that it has physical and chemical properties.

Later, in 1846 the German botanist Hugo von Mohl reintroduced the term protoplasm to refer to the substance present inside plant cells.

In 1850 the botanist Ferdinand Cohn unifies the terms, indicating that both plants and animals have protoplasm. The researcher points out that in both organisms, the substance that fills the cells is similar.

In 1872, Beale introduced the term bioplasm . In 1880, Hanstein proposed the word protoplast , a new term to refer to the entire cell, excluding the cell wall. This term was used by some authors to replace the cell.

In 1965, Lardy introduced the term cytosol , which was later used to name the fluid within the cell.

Protoplasmic theory

Anatomist Max Schultze proposed at the end of the 19th century that the fundamental basis of life is protoplasm. Schultze suggested that protoplasm is the substance that regulates the vital activities of tissues in living things.

Schultze’s works are considered to be the starting point of protoplasmic theory. This theory was supported by the proposals of Thomas Huxley in 1868 and by other scientists of the time.

The protoplasmic theory stated that protoplasm was the physical basis of life. In such a way that the study of this substance would allow us to understand the functioning of living beings, including the mechanisms of inheritance.

With the better understanding of cell function and structure, the protoplasmic theory has lost its validity.

General characteristics

Protoplasm is made up of various organic and inorganic compounds. The most abundant substance is water, which constitutes almost 70% of its total weight and functions as a carrier, solvent, thermoregulator, lubricant and structural element.

In addition, 26% of the protoplasm is made up of generally organic macromolecules. These are large molecules formed by polymerization of smaller subunits.

Among these we find carbohydrates, macromolecules composed of carbon, hydrogen and oxygen, which store energy for the cell. They are used in the various metabolic and structural functions of the protoplasm.

Likewise, there are various types of lipids (neutral fats, cholesterol and phospholipids), which also serve as a source of energy for the cell. In addition, they are a constituent part of the membranes that regulate the different protoplasmic functions.

Proteins make up almost 15% of the composition of protoplasm. Among these we have structural proteins. These proteins form the protoplasmic scaffold, contributing to its organization and cellular transport.

Other proteins present in protoplasm are enzymes. They act as catalysts (substances that modify the speed of a chemical reaction) of all metabolic processes.

Likewise, various inorganic ions are present that only correspond to 1% of its composition (potassium, magnesium, phosphorus, sulfur, sodium and chlorine). These contribute to maintaining the pH of the protoplasm.

Components

Protoplasm is made up of the plasma membrane, the cytoplasm, and the nucleoplasm. However, today, thanks to advances in electron microscopy, it is known that the cell structure is even more complicated.

There are, in addition, a large number of subcellular compartments, and structurally very complex cellular contents. In addition to the organelles, which are included here as part of the cytoplasm.

Plasma membrane

The plasma membrane or plasmalemma is made up of approximately 60% proteins and 40% lipids. Its structural arrangement is explained by the fluid mosaic model. In this, the membrane presents a phospholipid bilayer where proteins are embedded.

All cell membranes are considered to have this same structure. However, the plasmalemma is the thickest membrane of the cell.

The plasmalemma cannot be seen with the light microscope. It was not until the late 50s of the 20th century that its structure could be detailed.

Cytoplasm

The cytoplasm is defined as all the cell material that is found within the plasmalemma, not including the nucleus. The cytoplasm includes all organelles (cellular structures with defined form and function). Likewise the substance in which the different cellular components are immersed.

Cytosol

The cytosol is the fluid phase of the cytoplasm. It is an almost liquid gel that contains more than 20% of the cell’s proteins. Most of these are enzymes.

Cytoskeleton

The cytoskeleton constitutes a protein framework that forms the cellular framework. It is made up of microfilaments and microtubules. Microfilaments are made primarily of actin, although there are other proteins.

These filaments have different chemical composition in different types of cells. Microtubules are tubular structures basically made of tubulin.

Organelles

Organelles are cellular structures that fulfill a specific function. Each is bounded by membranes. Some organelles have only one membrane (vacuole, dicthyosomes), while others are bounded by two membranes (mitochondria, chloroplasts).

The organelle membranes have the same structure as the plasmalemma. They are thinner and their chemical composition is different depending on the function they fulfill.

Within organelles, various chemical reactions catalyzed by specific enzymes occur. On the other hand, they are able to move in the aqueous phase of the cytoplasm.

In the organelles there are different reactions of great importance for the functioning of the cell. In them, the secretion of substances, photosynthesis and aerobic respiration occurs, among others.

Nucleoplasm

The nucleus is the cellular organelle that contains the genetic information of the cell. In it, the processes of cell division occur.

Three components of the nucleus are recognized: nuclear envelope, nucleoplasm, and nucleolus. The nuclear envelope separates the nucleus from the cytoplasm and is made up of two membrane units. 

The nucleoplasm is the internal substance that is internally bounded by the nuclear envelope. It constitutes an aqueous phase that contains a large number of proteins. Mainly they are enzymes that regulate the metabolism of nucleic acids.

Chromatin (DNA in its dispersed phase) is contained in the nucleoplasm. In addition, the nucleolus is presented, which is a structure formed by proteins and RNA.

Features

All the processes that occur in the cell are associated with protoplasm, through its various components.

The plasma membrane is a selective structural barrier that controls the relationship between a cell and the environment that surrounds it. Lipids prevent the passage of hydrophilic substances. Proteins control the substances that can cross the membrane, regulating their entry and exit into the cell.

Various chemical reactions occur in the cytosol, such as glycolysis. This is directly involved in changes in cell viscosity, amoeboid movement and cyclosis. Likewise, it is of great importance in the formation of the mitotic spindle during cell division.

In the cytoskeleton, microfilaments are associated with cell movement and contraction. While microtubules are involved in cell transport and help to shape the cell. They also participate in the formation of centrioles, cilia and flagella.

Intracellular transport, as well as transformation, assembly, and secretion of substances, is the responsibility of the endoplasmic reticulum and dictyosomes.

The processes of transformation and accumulation of energy occur in photosynthetic organisms that have chloroplasts. Obtaining ATP through cellular respiration occurs in the mitochondria.

Physiological properties

Three physiological properties associated with protoplasm have been described. These are metabolism, reproduction, and irritability.

In the protoplasm all the metabolic processes of the cell occur. Some processes are anabolic and are related to the synthesis of protoplasm. Others are catabolic, and are involved in its disintegration. Metabolism includes processes such as digestion, respiration, absorption, and excretion.

All the processes associated with reproduction by cell division, as well as the coding for the synthesis of proteins required in all cellular reactions, occur in the nucleus of the cell, contained within the protoplasm.

Irritability is the protoplasm’s response to an external stimulus. This is capable of triggering a physiological response that allows the cell to adapt to the environment that surrounds it.

References

  1. Liu D (2017) The cell and protoplasm as container, object and substance: 1835-1861. Journal of the History of Biology 50: 889-925.
  2. Paniagua R, M Nistal, P Sesma, M Álvarez-Uría, B Fraile, R Anadón, FJ Sáez and M Miguel (1997) Plant and animal cytology and histology. Biology of animal and plant cells and tissues. Second edition. McGraw Hill-Interamericana from Spain. Madrid Spain. 960 p.
  3. Welch GR and J Clegg (2010) From protoplasmic theory to cellular systems biology: a 150-year reflection. Am. J. Physiol. Cell Physiol. 298: 1280-1290.
  4. Welch GR and J Clegg (2012) Cell versus protoplasm: revisionist history. Cell Biol. Int. 36: 643-647.

Add a Comment

Your email address will not be published. Required fields are marked *