At first glance, the petal of a flower or the skin on the back of a human hand may seem smooth and seamless, as if they were composed of a single, indistinct substance. In reality, however, many tiny individual units called cells make up these objects and almost all other components of plants and animals. The average human body contains over 75 trillion cells, but many life forms exist as single cells that perform all the functions necessary for independent existence. Most cells are far too small to be seen with the naked eye and require the use of high-power optical and electron microscopes for careful examination.
The relative scale of biological organisms as well as the useful range of several different detection devices are illustrated in Figure 1. The most basic image sensor, the eye, was the only means humans had of visually observing the world around them for thousands of years. Though excellent for viewing a wide variety of objects, the power of the eye has its limits, anything smaller than the width of a single human hair being able to pass unnoticed by the organ. Therefore, when light microscopes of sufficient magnifying capability were developed in the late 1600s, a whole new world of tiny wonders was discovered. Electron microscopes, invented in the mid-twentieth century, made it possible to detect even tinier objects than light microscopes, including smaller molecules, viruses, and DNA. The detection power of most electron microscopes used today, however, stops just short of being able to visualize such incredibly small structures as the electron orbital systems of individual atoms. Atoms are considered the smallest units of an element that have the characteristics of that element, but cells are the smallest structural units of an organism capable of functioning independently.
Yet, until the mid-seventeenth century, scientists were unaware that cells even existed. It wasn't until 1665 that biologist Robert Hooke observed through his microscope that plant tissues were divided into tiny compartments, which he termed "cellulae" or cells. It took another 175 years, however, before scientists began to understand the true importance of cells. In their studies of plant and animal cells during the early nineteenth century, German botanist Matthias Jakob Schleiden and German zoologist Theodor Schwann recognized the fundamental similarities between the two cell types. In 1839, they proposed that all living things are made up of cells, the theory that gave rise to modern biology.
Since that time, biologists have learned a great deal about the cell and its parts; what it is made of, how it functions, how it grows, and how it reproduces. The lingering question that is still being actively investigated is how cells evolved, i.e., how living cells originated from nonliving chemicals.
Numerous scientific disciplines—physics, geology, chemistry, and evolutionary biology—are being used to explore the question of cellular evolution. One theory speculates that substances vented into the air by volcanic eruptions were bombarded by lightning and ultraviolet radiation, producing larger, more stable molecules such as amino acids and nucleic acids. Rain carried these molecules to the Earth's surface where they formed a primordial soup of cellular building blocks.
A second theory proposes that cellular building blocks were formed in deep-water hydrothermal vents rather than in puddles or lakes on the Earth's surface. A third theory speculates that these key chemicals fell to earth on meteorites from outer space.
Given the basic building blocks and the right conditions, it would seem to be just a matter of time before cells begin to form. In the laboratory, lipid (fat) molecules have been observed joining together to produce spheres that are similar to a cell's plasma membrane. Over millions of years, perhaps it is inevitable that random collisions of lipid spheres with simple nucleic acids, such as RNA, would result in the first primitive cells capable of self-replication.
For all that has been learned about cells in over 300 years, hardly the least of which is the discovery of genetic inheritance and DNA, cell biology is still an exciting field of investigation. One recent addition is the study of how physical forces within the cell interact to form a stable biomechanical architecture. This is called "tensegrity" (a contraction of "tensional integrity"), a concept and word originally coined by Buckminster Fuller. The word refers to structures that are mechanically stable because stresses are distributed and balanced throughout the entire structure, not because the individual components have great strength.
In the realm of living cells, tensegrity is helping to explain how cells withstand physical stresses, how they are affected by the movements of organelles, and how a change in the cytoskeleton initiates biochemical reactions or even influences the action of genes. Some day, tensegrity may even explain the mechanical rules that caused molecules to assemble themselves into the first cells.
Posted in: Microbiology
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