I have been watching a learning program available from Great Courses about science.  There are sixty lectures in all covering subjects from Astronomy  to Zoology, and taught by a magnificent, articulate and learned professor from Amherst University.  The lectures are 30 minutes in length and he teaches without using a teleprompter or reference to notes.  All of lectures are informative, but I found the one about cells the most intriguing.  Cells make up all organisms and are the chemical factories that make life possible.  They are the structural and functional units of all life forms except viruses (which may not deserve the label of “life”) and are the smallest living biological structure. The word cell comes from the Latin cellula, meaning a small room.   

History

The discovery of cells was made by the British physicist Robert Hooke (1635-1702), who was renowned for ingenious mechanical inventions and demonstrations.   In a 1665 book (Micrographia),  Robert Hooke used this name for cork cells that he saw through his microscope because they reminded him of the small rooms that house monks in a monastery.  In 1838, the German botanist Matthais Schleden proposed that all plants are made of cells, and in the following year, his countryman, zoologist Theodur Schwann, extended this idea to animals by proposing three tenets of cell theory, namely that (1) all living things are composed of cells, (2) the cell is the fundamental unit of life, and (3) that all cells arise from previous cells.

Description

The human body comprises 10 to 100 trillion cells (not counting bacterial residents), depending on whom you believe. The number varies greatly from person to person. A typical cell is 10 millionths of a meter in size, with a mass of one billionth of a gram. Some 50 million of the cells in your body will have died and been replaced while you have been reading this sentence. Humans shed and regrow all outer skin cells about every four weeks, replacing a half-million per hour, resulting in almost 1,000 new skins in a typical lifetime. Intestinal cells last just a few days, urinary cells two months, red blood cells four months, and some brain cells, years.

Composition

Inside the human cell are found numerous separate structures called organelles.  Each organelle is itself surrounded by a cell membrane, and each performs a separate function in the cell.  The nucleus is the largest organelle.  It contains the cell’s genetic blueprint—the instructions for all of the chemical operations the cell needs to build copies of itself.  Organelles also play the central role in energy production in the cell. One of them, mitochondria, are the miniature power plants of all human cells.  They convert energy-rich carbohydrates such as glucose into the compact battery-like molecule  adenosine triphosphate (ATP).  ATP molecules literally plug into other cellular molecules to provide the energy for work.  Mitochondria are most abundant in cells that do mechanical work, such as muscles, where a steady supply of energy is required.  Each molecule of ATP is a chemical factory processing more than 700 different chemical substances in long, interweaving assembly lines along the surface of its intricately folded membranes. Many fuel sources are utilized in the production of ATP, which powers almost every activity of the cell and organism. ATP is used to build complex molecules, contract muscles, and generate electric pulses in nerve cells, among a host of other tasks. Some amino acids are metabolized also, all but one of which are put to work in the mitochondrial matrices. ATP has another important role as a signaling molecule that allows cells and tissues throughout the body to communicate with one another. Thus the universal fuel serves as a common language as well.

All organelles in the cell are suspended within a gelatinous matrix, the cytoplasm, which is contained within the cell membrane. In addition to the nucleus and mitochondria, the main organelles are as follows : § cell membrane § endoplasmic reticulum § Golgi apparatus § lysosomes §  perioxisomes § microfilaments and microtubules.  One of the few cells in the human body that lack almost all organelles are the red blood cells.  They transport oxygen from the lungs to other cells, and are the simplest and most abundant cells in the body. Several types of white blood cells combat infections, consume cellular debris in the blood stream and produce antibodies that attack foreign cells and viruses.

Final thoughts

The  most remarkable aspect of cellular differentiation is that every multicellular organism, including us, begins as a single cell—a fertilized egg.  Contained within that egg is something even smaller and more fundamental that the cell in controlling life’s chemistry—the unit of genetic information—the gene.