OUR home planet is but one of nine that orbit the sun in our solar system, yet only on earth do conditions seem ideal for the support of life. Two of the planets are so close to the sun that their surface temperatures are too hot for life to exist. Five of the planets are so far from the sun that its feeble rays cannot warm their surfaces enough to sustain life processes. Four of the planets are so large and their gravitational forces so intense, the human body would be crushed. Three are so small their gravitational forces are insufficient to retain an atmosphere.
Here on planet Earth life flourishes. One and one-half million diverse species of living organisms walk, hop, or crawl on the surface; swim, float, or splash in the waters; soar, flap, or buzz in its atmosphere; or remain determinedly rooted to the sub stratum. The study of this bewildering array of living things is full of beauty, fascination, and surprises. In Proverbs 30:18, 19 the wise man exclaims in wonder, "There be three things which are too wonderful for me, yea, four which I know not: the way of an eagle in the air; the way of a serpent upon a rock; the way of a ship in the midst of the sea; and the way of a man with a maid." In the three thousand years since those words were written the wonderment at life and nature has not diminished even though we can now discern a thin thread uniting this bewildering diversity called life. That common denominator is the fact that all living things, from elephants to moss plants, are composed of fundamentally similar sub-units called cells.
Robert Hooke in 1662 set up a demonstration for the members of the Royal Society. Peering into a primitive microscope at a razor-thin section of cork, the astonished dignitaries could see that the cork was made up of compartments. Hooke called these compartments cells. Others helped with much of the groundwork, but it was Schleiden, a German botanist, who, in 1838, convincingly argued, "Higher plants are aggregates of fully individualized, in dependent, separate beings, namely the cells themselves." One year later Schwann, a German zoologist, extended Schleiden's conclusions to include animals as well.
Cell Theory
The cell theory as set forth by Schleiden and Schwann proved to be the key that has opened to biologists' view all life as a cohesive whole. Recognition of structural and functional similarities in the cells of widely diverse organisms has provided a unifying theme in an area that can now be not only beautiful and fascinating but sensible and reasonable as well.
Cells vary in size from the pneumonia bacterium with a diameter of 150 mix (0.000006 of an inch) to an ostrich egg with a diameter averaging 150 mm (6 inches). If both cells were enlarged by the same degree until the bacterium achieved the size of the ostrich egg, the ostrich egg would be more than 94 miles in diameter! Most cells are between 10 and 30 u, in diameter. A cell 15 (A in diameter and an ostrich egg enlarged until the cell is as large as the egg would result in an egg just less than one mile in diameter.
All cells have boundaries formed by selectively permeable membranes that regulate all communication between a cell and its neighbors or with the nonliving environment. The membrane is only 10 mu, thick, but it plays a vital role in regulating, shall we say, "foreign exchange." Fuel, raw materials, and oxygen are given a redcarpet welcome while manufactured products and waste materials and combustion products are unceremoniously dumped. The membrane is very discriminating as to what molecules are permitted to enter or leave the cell. Undesirable aliens (ions) are kept at bay and, if they slip in, are immediately "pumped" out by activities centered in the membrane. Membranes are used to provide a wrapping material for other cell components, as will be seen.
It might help us to think of the cell as a factory. Energy is vital to both cell and factory, and an industry that generates its own electricity would be a good model for comparison. In the cell, rather than one huge central power plant, many tiny power plants (the mitochondria) are scattered about the factory. Sugar, rather than coal, is the fuel, but the combustion is supported by oxygen as in the factory furnace. The cell "electricity" generated is a high-energy molecule called ATP.
Mitochondria have an average diameter of about 0.5 n, but may vary in length from 0.5-7.0 u,. An average mitochondrion, if enlarged until it reached the size of the ostrich egg, would result in an ostrich egg 14 miles across! Mitochondria are wrapped in not one, but two layers of membrane. The inner layer is wonderfully convoluted to provide greater surface area for the generation of cell "electricity" (ATP).
An efficient and well-informed management is, of course, essential to both cell and factory. The nucleus is the cell's administrative office. Two layers of membrane separate the nucleus from the rest of the cell. We enter a tiny pore through the double membrane and discover ourselves in a super filing and records office. Fortunately, security is relaxed and most of the "top secret" signs have been taken down. The essentials of the recently declassified in formation of cell management are as follows: Precise detail concerning each step in the total cell process is reduced to twisted strands of coded DNA molecules (the master template). The DNA is folded and put into a file folder (the gene). Many file folders, each representing individual steps, are crammed into a file drawer (the chromosome). The number of chromosomes is usually constant and characteristic for any given species. For instance, an onion has 16 chromosomes, a man has 46 chromosomes, and a crayfish has 200! Let us assume a specific step is to be carried out. The coded DNA for that step unravels (gene activation). Copies of the linearly coded gene message are made and the messages themselves become the messengers (messenger RNA)! Each messenger hurries from the office (nucleus) to the factory floor (the cytoplasm) where the assembly lines (the endoplasmic reticulum) are located.
The messenger is immediately beset by a crowd of eager decoders (Transfer-RNA) and enthusiastic workers (ribosomes). There are twenty different kinds of decoders, each firmly attached to one of twenty different amino acids. Once every code position (codon) of the messenger RNA has been "recognized" by a decoder, the ribosomes begin their work. Ribosomes are rather tiny. They measure a mere 20 mμ,. Using our previous scale of comparison with the ostrich egg: by the time our enlarged ribosome reaches the size of an ostrich egg, the egg would exceed 700 miles in diameter! Using the messenger RNA as a track, the ribosomes attach themselves and begin moving. Each decoder is bumped off the track after it hands its amino acid to the ribosome. When the ribosome has lurched past, another identical decoder with its amino acid takes the position just vacated. By the time each ribosome has slipped off the end of the track it has acquired a long string of amino acids all in the exact order specified by the genetic code. The amino acid chain is called a polypeptide. With some twisting and bending the polypeptide becomes a protein, and proteins are the very stuff of life. Proteins constitute not only important structural components of the cell but may also act as catalysts that regulate biochemical reactions. Many hundreds of these protein catalysts, called enzymes, serve on assembly lines where, at each step, changes are brought about in molecules that are being transformed into end products, such as hormones, that will be exported and have far-reaching effects on the organism.
The growth of an organism results mainly from an increase in cell number. There is no good analogy to cell division in our factory model so you will have to let your imagination run riot! Picture first a stealthy accumulation of genetic code letters (nucleotides) followed by a duplication of every single gene message in the records office. It does not stop there. There is a doubling of file folders (genes) and even a doubling of file drawers (chromosomes). When all is in readiness, the barriers (nuclear membranes) delimiting the administrative office (nucleus) disappear and a precise division of the replicated file drawers takes place (mitosis). Each complete set of chromosomes is covered with new membrane and there is an ordered pandemonium in the cell cytoplasm between the two nuclei as a separation of cell contents (cytokinesis) takes place. The completion of this process replaces one cell with two daughter cells. No one ever heard of a factory doubling in this manner!
Any well-run factory would have to devote some of its efforts to housekeeping activities. The shipping, custodial, and maintenance divisions of the factory have their cell counterparts in the Golgi Assembly. Manufactured products are packed by the assembly into tiny membrane sacs and moved to the cell perimeter. There the membrane of the sacs deftly merges with the cell-limiting membrane and suddenly the product has been exported!
Thousands of similar cells may dump their products into a tiny duct, where the accumulated material may be moved by the blood stream to other parts of the organism where it exerts an important regulatory effect. Waste products may be excreted from the cell in a similar manner. In plant cells, build-up of a rigid plant wall outside the cell membrane is probably largely the function of the Golgi.
Some cells do not renew themselves by cell division but appear headed for senility and decrepitude. They will never make it! There is an elaborate "self-destruct" mechanism in such cells. Tiny "suicide bags" (lysosomes) packed with powerful enzymes prowl the cytoplasm. Once again it is the membrane of the little bags that keeps the enzymes from harming healthy cells. When the lysosomes are triggered the enzymes pounce upon the elaborately complex cell machinery and take it apart, nut by nut and bolt by bolt. Spare parts are readily transported and admitted by growing cells where they may be reassembled into shiny new machines. It is better to contemplate the cell's way than to think of the heaps of discarded machinery, rusting and overgrown with weeds, one often sees in a factory yard.
Photosynthesis
Perhaps the most wonderful cell process of all is photosynthesis, which takes place in green plant cells. These cells actually convert light energy into sugar and starch. The organelle that performs this amazing feat is the chloroplast. Little sacs called thylakoids are the basic components of the chloroplast. Thylakoids are shaped a little like lollipops. Imagine the round flat candy parts of several lollipops piled one on top of the other. This pile is similar to the way thylakoids are piled up to produce what is known as a granum. The green and yellow photosynthetic pigments of the thylakoids are organized into sub-units called quantasomes that act like energy traps. The energy of eight little bundles (photons) of light is required to energize the light traps to the point where enough energy can accumulate to split a molecule of water (H2<O). When this energy is used to pry the hydrogens away from oxygen, the oxygens pair and bubble off as a gas (02). This by-product of photosynthesis is of sufficient magnitude to completely replace the oxygen in the atmosphere once every two thousand years. The hydrogens and carbon dioxide embark upon a complex series of reactions that ultimately result in the production of sugar. The sugar, in a sense then, stores the light energy used to split water. Excess sugar is made by cell factories into starch. Starchrich wheat cells are made into bread. When a person eats that bread, the starch is digested into sugar and the sugar enters the body cells as fuel. The mitochondrial power houses complete the breaking up of the sugar into CO2 and hydrogen. In a series of leaps the hydrogen is reunited with oxygen by electron transport to form water (H2O). It is in these final steps that the energy provided by light to split water is released. Frugal ATP molecules hoard this energy and make it available for all reactions like protein synthesis that require energy.
So far we have been stressing the similarities in cells of diverse organisms. Now, let us consider the diversity of cells within the same organism. A newborn baby has about 20 billion cells. Schleiden would perhaps think of the baby as an aggregation of 20 billion "fully individualized, independent separate beings" and he would be correct, yet there is also a remarkable division of labor among the cells for the good of the organism. All cells in an individual are derived from a single cell the zygote by a process of cell division (mitosis) that results in precise replication of coded genetic in formation. What does a cell in your big toe do with a gene coding for blue eyes? The answer is obvious. As cells specialize, genes carrying unneeded information are simply shut down.
Consider for one moment the vast array of skills required of cells in a single individual. Some specialized cells produce hard supporting material like bone. Other cells have the capacity to contract on demand and relax again. Large numbers of these cells may constitute a muscle that makes movement possible. Specialized cells may sense light, still others may sense heat, cold, odor, taste, or pressure. Highly specialized nerve cells convert the response of the sensing cell to stimuli into a steady train of electrical impulses that move along an intricate pathway of conducting cells to a precise center in the brain where a sense impression is made and the organism be comes "aware" of light, heat, cold, odor, taste, or pressure. The brain, constantly monitoring this steady stream of sensations, may decide to take action that may constitute a response of the whole organism to a sense stimulus. The response may involve hundreds of billions of cells each "doing its thing," true, but also contributing its share to the whole organism.
The Psalmist's Wonderment
The psalmist in his day certainly had but a fragmentary knowledge of embryology, yet recognized the mystery and complexity of the process of how a cell becomes a baby. Here is how he expressed his wonderment: "Thou it was who didst fashion my inward parts; thou didst knit me together in my mother's womb. . . . Thou knowest me through and through: my body is no mystery to thee, how I was secretly kneaded into shape and patterned in the depths of the earth. Thou didst see my limbs unformed in the womb, and in thy book they are all recorded; day by day they were fashioned, and not one of them was late in growing" (Ps. 139:13-16, N.E.B.).*
Perhaps the most fitting conclusion for us, having contemplated the wonders of the cell and the organism of which it is a part, is to repeat with David the next two verses: "How deep I found thy thoughts, O God, how inexhaustible their themes! Can I count them? They outnumber the grains of sand; to finish the count, my years must equal thine" (verses 17, 18, N.E.B.).
* Texts credited to N.E.B. are from The New English Bible. © The Delegates of the Oxford University Press and the Syndics of the Cambridge University Press 1970. Reprinted by permission.
