Follow Us:

A preview of the cell

From the 17th century onwards, European scientists have decoded the basic structure of all life

TAPAN KUMAR MAITRA | Kolkata |

The cell is the basic unit of biology. Every organism either consists of cells or is itself a single cell.

A revolution in biology has brought with it tremendous advances in our understanding of how cells are constructed and how they carry out all the intricate functions necessary for life.

Particularly significant is the dynamic nature of the cell, as evidenced by its capacity to grow, reproduce, and become specialised, its ability to respond to stimuli and adapt to changes in its environment.

Cell biology itself is changing, as scientists from a variety of related disciplines focus their efforts on the common objective of understanding more adequately how cells work.

The convergence of cytology, genetics, and bio-chemistry has made modern cell biology one of the most exciting and dynamic disciplines in contemporary biology.

The story of cell biology started to unfold more than 300 years ago, as European scientists began to focus their crude microscopes on a variety of biological material ranging from tree bark to human sperm.

One such scientist was Robert Hooke, curator of instruments for the Royal Society of London. In 1665, Hooke used a microscope that he had built himself to examine thin slices of cork cut with a penknife. He saw a network of tiny boxlike compartments that reminded him of a honeycomb.

Hooke called these little compartments cellulae, a Latin term meaning “little rooms.” It is from this word that we get our present-day term, cell.

Actually, what Hooke observed were not cells at all but the empty cell walls of dead plant tissue, which is what tree bark really is. However, Hooke would not have thought of his cellulae as dead, because he did not understand that they could be alive!

Although he noticed that cells in other plant tissues were filled with what he called “juices,” he preferred to concentrate on the more prominent cell walls that he had first encountered. One of the limitations inherent in Hooke’s observations was that his microscope could only magnify objects 30-fold, making it difficult to learn much about the internal organisation of cells.

This obstacle was overcome a few years later by Antonie van Leeuwenhoek, a Dutch shopkeeper who devoted much of his spare time to the design of microscopes. Van Leeuwenhoek produced hand-polished lenses that could magnify objects almost 300-fold. Using these superior lenses, he became the first to observe living cells, including blood cells, sperm cells, and single-celled organisms found in pond water.

He reported his observations to the Royal Society in a series of papers during the last quarter of the 17th century. His detailed reports attest to both the high quality of his lenses and his keen powers of observation. Two factors restricted further understanding of the nature of cells.

One was the limited resolution of the microscopes of the day, which even van Leeuwenhoek’s superior instruments could push just so far. The second and probably more fundamental factor was the essentially descriptive nature of 17th century biology.

It was basically an age of observation, with little thought given to explaining the intriguing architectural details of biological materials that were beginning to yield to the probing lens of the microscope. More than a century passed before the combination of improved microscopes and more experimentally minded microscopists resulted in a series of developments that culminated in an understanding of the importance of cells in biological organisation.

By the 1830s, improved lenses led to higher magnification and better resolution, such that structures only one micrometre apart could be resolved. (A micrometer is 107° m, or one-millionth of a metre) Aided by such improved lenses, the English botanist Robert Brown found that every plant cell he looked at contained a rounded structure, which he called a nucleus, a term derived from the Latin word for “kernel.”

In 1838, his German colleague Matthias Schleiden came to the important conclusion that all plant tissues are composed of cells and that an embryonic plant always arises from a single cell. Similar conclusions concerning animal tissue were reported only a year later by Theodor Schwann, thereby laying to rest earlier speculations that plants and animals might not resemble each other structurally.

It is easy to understand how such speculations could have arisen. After all, plant cell walls provide conspicuous boundaries between cells that are readily visible even with a crude microscope, whereas individual animal cells, which lack cell walls, are much harder to distinguish in a tissue sample.

It was only when Schwann examined animal cartilage cells that he became convinced of the fundamental similarity between plant and animal tissue, because cartilage cells, unlike most other animal cells, have boundaries that are well defined by thick deposits of collagen fibers.

Schwann drew all these observations together into a single unified theory of cellular organisation, which has stood the test of time and continues to provide the basis for our own understanding of the importance of cells and cell biology.

As originally postulated by Schwann in 1839, the cell theory had two basic tenets — All organisms consist of one or more cells; and the cell is the basic unit of structure for all organisms.

Less than 20 years later, a third tenet was added. This grew out of Brown’s original description of nuclei, extended by Karl Nageli to include observations on the nature of cell division.

By 1855, Rudolf Virchow, a German physiologist, concluded that cells arose in only one manner – by the division of other, preexisting cells. Virchow encapsulated this conclusion in the now-famous Latin phrase omnis cellula e cellula, which in translation becomes the third tenet of the modern cell theory – All cells arise only from preexisting cells.

Thus, the cell is not only the basic unit of structure for all organisms but also the basic unit of reproduction. In other words, all of life has a cellular basis. No wonder, then, that an understanding of cells and their properties is so fundamental to a proper appreciation of all other aspects of biology.

The writer is associate professor and head, department of botany, Ananda Mohan College, Kolkata.