Modern Cell Theory

Modern cell theory isn't all that modern when you understand how long ago it originated. With roots in the mid-17th century, multiple scientific scholars and researchers of the day contributed to the tenets of classical cell theory, which postulated that cells represent the basic building blocks of life; all life consists of one or more cells, and the creation of new cells occurs when old cells divide into two.

The first person to observe and discover the cell, Robert Hooke (1635-1703), did so using a crude compound microscope – invented near the end of the 16th century by Zacharias Janssen (1580-1638), a Dutch spectacle-maker, with help from his father – and an illumination system Hooke designed in his role as curator of experiments for the Royal Society of London.

Hooke published his findings in 1665 in his book, "Microphagia," which included hand-sketched drawings of his observations. Hooke discovered plant cells when he examined a thin slice of cork through the lens of his converted compound microscope. He saw a plethora of microscopic compartments that, to him, resembled the same structures found in honeycombs. He called them "cells," and the name stuck.

Dutch scientist Antony van Leeuwenhoek (1632-1705), a tradesman by day and a self-driven biology student, ached to discover the secrets of the world around him, and even though not formally educated, he ended up contributing important discoveries to the field of biology. Leeuwenhoek discovered bacteria, protists, sperm and blood cells, rotifers and microscopic nematodes, and other microscopic organisms.

Leewenhoek's studies brought a new level of awareness of microscopic life to scientists of the day, spurring others on who would, in the end, play a part in contributing to modern cell theory. French physiologist Henri Dutrochet (1776-1847) was the first to claim the cell was the basic unit of biological life, but scholars give credit for the development of modern cell theory to German physiologist Theodor Schwann (1810-1882), German botanist Matthias Jakob Schleiden (1804-1881) and German pathologist Rudolf Virchow (1821-1902). In 1839, Schwann and Schleiden proposed that the cell is the basic unit of life, and Virchow, in 1858, deduced that new cells come from pre-existing cells, completing the main tenets of classical cell theory. (For Schwann, Schleiden and Virchow see https://www.britannica.com/biography/Theodor-Schwann, https://www.britannica.com/biography/Matthias-Jakob-Schleiden, and https://www.britannica.com/biography/Rudolf-Virchow.)

Scientists, biologists, researchers and scholars, though still using the fundamental tenets of cell theory, conclude the following on the modern interpretation of cell theory:

• The cell represents the elementary unit of construction
and function in living organisms.
• All cells come from the division of pre-existing
cells.
• Energy flow – metabolism and biochemistry – happens
within cells.
• Cells contain genetic information in the form of
DNA passed on from cell to cell during division.
• In the organisms of similar species, all cells
are fundamentally the same.
• All living organisms consist of one or more
cells.
• Some cells – unicellular organisms – consist
of only one cell.
• Other living entities are multicellular,
containing multiple cells.  
• The living organism's activities depend upon the
combined actions of individual, independent cells.

Scientists have traced back all life to a single, common unicellular ancestor that lived approximately 3.5 billion years ago, first proposed by evolutionist Charles Darwin more than 150 years ago.

One theory suggests that each of the organisms categorized under biology's three main domains, Archaea, Bacteria and Eukarya, evolved from three separate ancestors, but biochemist Douglas Theobald from Brandeis University in Waltham, Massachusetts, disputes that. In an article on the "National Geographic" website, he says the odds of that happening are astronomical, something like 1 in 10 to the 2,680th power. He came to this conclusion after calculating the odds using statistical processes and computer models. If what he says proves to be true, then the idea held by most all the indigenous people on the planet is correct: everything is related.

People are a jumble of 37.2 trillion cells. But all humans, like every other living entity on the planet, began life as a single-cell organism. After fertilization, the single-celled embryo called a zygote goes into rapid overdrive, beginning the first cell division within 24 to 30 hours after fertilization. The cell continues to divide exponentially during the days the embryo travels from the human fallopian tube to implant itself inside the womb, where it continues to grow and divide.

While there are certainly smaller things inside the body than living cells, the individual cell, like a Lego block, remains a basic unit of structure and function in all living organisms. Some organisms contain only one cell while others are multicellular. In biology, there are two types of cells: prokaryotes and eukaryotes.

Prokaryotes represents cells without a nucleus and membrane-enclosed organelles, though they do have DNA and ribosomes. Genetic material in a prokaryote exists inside the membrane walls of the cell along with other microscopic elements. Eukaryotes on the other hand, have a nucleus inside the cell and bound within a separate membrane, as well as membrane-enclosed organelles. Eukaryotic cells also have something prokaryotic cells do not: organized chromosomes for retaining genetic material.

Cells give birth to other cells by a pre-existing cell dividing into two daughter cells. Scholars call this process mitosis – cell division – because one cell produces two new genetically identical daughter cells. While mitosis occurs after sexual reproduction as the embryo develops and grows, it also occurs throughout the lifespan of a living organisms to replace old cells with new cells.

Classically divided into five distinct phases, the cell cycle in mitosis includes prophase, prometaphase, metaphase, anaphase and telophase. In the break between cell division, interphase represents part of the cell-cycle phase where a cell pauses and take a break. This allows the cell to develop and double its internal genetic material as it gets ready for mitosis.

Multiple biochemical reactions happen inside the cell. When combined, these reactions make up the cell's metabolism. During this process, some chemical bonds in the reactive molecules get broken, and the cell takes in energy. When new chemical bonds develop to make products, this releases energy in the cell. Exergonic reactions occur when the cell releases energy to its surroundings, forming stronger bonds than the ones broken. In endergonic reactions, energy comes into the cell from its surroundings, creating weaker chemical bonds than the ones broken.

To reproduce, a cell must have some form of deoxyribonucleic acid, the self-replicating substance present in all living organisms as the essential elements of chromosomes. As DNA is the carrier of genetic data, the information stored in the original cell's DNA duplicates in daughter cells. The DNA provides a blueprint for the final development of the cell, or in the case of eukaryotic cells in the plant and animal kingdoms, for example, the blueprint for the multicellular life form.

The reason biologists classify and categorize all lifeforms is to understand their positions in the hierarchy of all life on the planet. They use the Linnaean taxonomy system to rank all living creatures by domain, kingdom, phylum, class, order, family, genus and species. By doing this, biologists learned that in organisms of similar species, individual cells contain basically the same chemical composition.

All prokaryotic cells are basically unicellular, but there is evidence that many of these unicellular cells join to form a colony to divide the labor. Some scientists consider this colony as multicellular, but the individual cells don't require the colony to live and function. Living organisms categorized under the Bacteria and Archaea domains are all single-celled organisms. Protozoa and some forms of algae and fungi, cells with a distinct and separate nucleus, are also single-celled organisms organized under the Eukarya domain.

All living cells in the Bacteria and Archaea domains consist of single-celled organisms. Under the Eukarya domain, living organisms in the Protista kingdom are single-celled organisms with a separately identified nucleus. Protists include protozoa, slime molds and unicellular algae. Other kingdoms under the domain Eukarya include Fungi, Plantae and Animalia. Yeast, in the Fungi kingdom, are single-celled entities, but other fungi, plants and animals are multicellular complex organisms.

The activities within a single cell cause it to move, take in or release energy, reproduce and thrive. In multicellular organisms, like the human being, cells develop differently, each with their individual and independent tasks. Some cells group together to become the brain, the central nervous system, the bones, muscles, ligaments and tendons, major body organs and more. Each of the individual cell actions work together for the good of the whole body to allow it to function and live. Blood cells, for example, function on many levels, carrying oxygen to needed parts of the body; fighting pathogens, bacterial infections and viruses; and releasing carbon dioxide through the lungs. Disease occurs when one or more of these functions break down.

Scientists, biologists and virologists all don't agree on the nature of viruses because some experts consider them as living organisms, yet they do not contain any cells whatsoever. While they mimic many features found in living organisms, by the definitions cited in modern cell theory, they are not living organisms.

Viruses are the zombies of the biological world. Living in a no-man's land in a gray area between life and death, when outside the cells, viruses exist as a capsid encased in a protein shell or as a simple protein coat sometimes enclosed inside a membrane. The capsid encloses and stores either RNA or DNA material, which contains codes of the virus.

Once a virus enters a living organism, it finds a cellular host in which to inject its genetic material. When it does this, it recodes the host cell's DNA, taking over the cell's function. Infected cells then begin to produce more viral protein and reproduce the viruses' genetic material as it spreads the disease throughout the living organism. Some viruses can remain asleep inside host cells for a long time, causing no obvious change in the host cell called the lysogenic phase. But once stimulated, the virus enters the lytic phase where new viruses replicate and self-assemble before killing the host cell as the virus bursts out to infect other cells.