One property of lipids that you’re likely to encounter has to do with how the carbon and hydrogen atoms in the molecule bond together. When neighboring atoms share a pair of electrons, the bond is said to be saturated; if two pairs, the bond is unsaturated. Animal lipids, which are saturated, are generally insoluble in water, in contrast to unsaturated plant lipids like olive oil. Many health problems in America arise from overconsumption of saturated animal fats, which are more likely to lead to harmful deposits on the walls of your blood vessels.
THE CHEMICAL FACTORIES OF LIFE
Cells are the basic unit of life. Many living things, in fact, consist of only one cell. Others are multicelled—your body, for example, contains about 10 trillion cells. Cells come in a wide variety of shapes and sizes. The largest cell (the yolk of an ostrich egg) is bigger than most animals, and the smallest (certain bacteria) can barely be seen with the strongest light microscope. Most cells are about a ten-thousandth of an inch across, a bit smaller than the particles of smoke that make the sky hazy after a fire.
Cells have two primary functions: to provide a framework that supports the complex chemical reactions required to sustain life, and to produce exact copies of themselves so that the organism of which they are a part can go on living even after those cells die. In this chapter we focus on the cell as a chemical factory; in the next, on how cells reproduce.
Like any factory, each cell has several essential systems. It must have a front office, a place to store information and issue instructions to the factory floor to guide the work in progress. It must have bricks and mortar—a building with walls and partitions where the actual work goes on. Its production system must include the various machines that produce finished goods as well as the transportation network that moves raw materials and finished products from place to place. And finally, there must be an energy plant to power the machinery.
The Front Office
In each cell of your body a nucleus acts as the front office. Separated from the rest of the cell by a double membrane, the nucleus keeps the nucleic acid DNA on file. DNA is something like an instruction manual. The DNA manual itself can’t do the work, but it contains information that programs the cell to carry out its functions.
Primitive cells do not have nuclei, but carry their DNA instructions loose inside their cell wall. Such cells are called prokaryotes (before the nucleus). More advanced cells, including all those in multicelled organisms like human beings, isolate the DNA in a nucleus. Such cells are called eukaryotes (true nucleus).
Bricks and Mortar
Cellular factories consist of walls, partitions, and loading docks. Cell membranes provide all interior and exterior walls. A typical cell membrane is made of lipid molecules arranged in a double layer, with water-repelling ends head-to-head on the inside and water-attracting ends on the outside.
Here and there on the membrane surface are large protein and carbohydrate molecules, which have complex three-dimensional shapes and act like specially designed loading docks. These receptors lock onto only one specific kind of molecule in the outside environment. When a receptor “recognizes” its specific molecule (perhaps a sugar or an amino acid), it binds to it. A common outcome of this binding is for the external object to be sucked into the cell’s interior, where it is surrounded by a small bit of membrane called a vesicle. The vesicle then becomes the vehicle in which the material is transported around the cell. The reverse process, in which a vesicle approaches the cell wall from the inside, joins to it, and dumps its contents outside the wall, is the primary mechanism by which a cell returns materials to the environment.
A typical cell membrane consists of a double layer of lipid molecules, interspersed with large protein molecules that act as receptors.
Your cell’s loading dock system can sometimes be fooled, with tragic consequences. The virus that causes AIDS, for example, happens to fit the receptors normally found in the membrane of the human white blood cell. Thus, once taken into the body, the virus readily enters those cells and kills them, destroying the person’s immune system in the process.
The enfolding of external objects by a cell membrane is thought to explain the existence of the nucleus as well as some other structures inside the cell that you’ll meet shortly. Scientists think that early in life’s history all cells were prokaryotes—none had nuclei. At some point one cell swallowed another, forming a new symbiotic system. Double membranes that now surround the nucleus and many of the other parts of your cells provide a telltale indicator of this process. Any engulfed cell carries its own membrane as well as the part of the original cell membrane involved in the engulfing process. Thus, many biologists think of eukaryotic cells as evolved colonies of simpler cells, with each part contributing to the whole.
The Production System
Production areas that carry out the chemical work are themselves separated from the rest of the cell by membranes. These structures, and any other organized body within the cell, are called organelles.
Each organelle performs a specific chemical function. Some supply the cell with energy, some digest the molecules that serve as food, some produce proteins necessary for the operation of the cell, some put finishing touches on molecules that have been created elsewhere in the cell, and some serve as stable platforms on which the process of protein assembly takes place. At any given moment, all of these functions operate at different places within the cell.
Each organelle has its own complement of receptors. Materials are moved across the boundaries of organelles, and from one place to another within the cell, in vesicles of the type described above. Inside the cell, connecting all its parts, a complex web of fine filaments serves as the streets and highways of the cell’s transportation system. At any instant, thousands of cargo-laden vesicles, carrying raw materials and finished products, are whizzing around on these filaments from one part of the cell to another. Organic molecules in the membrane of the vesicles fit into receptor molecules in cell membranes, guaranteeing that the load gets delivered to the right place every time.
Every living thing is composed of one or more cells, each of which has a complex anatomy. A “generic” cell contains many structures and organelles—tiny chemical factories.
The Power Plant
Cells, like any factory, need energy to operate. Two different kinds of cellular power plants have evolved. Some cells absorb energy directly from the sun, while others gather energy by eating other organisms that have stored it. Plants use the first strategy, animals the second, and the structure of their cells reflects this difference.
Plants acquire energy directly from sunlight through the process of photosynthesis. In this process, molecules of chlorophyll or related pigments absorb photons from the sun. The photons’ energy is converted into chemical energy that the plant can use to grow and reproduce. In the course of this rather complicated chemical process, carbon dioxide and water from the cell’s surroundings are converted into glucose (or other carbohydrates) plus oxygen. The net effect of photosynthesis, then, is to remove carbon dioxide from the air, produce energy-rich sugar molecules for the cell, and give off oxygen as a waste product.
Animals, unlike plants, cannot convert the sun’s energy directly to energy-rich molecules and therefore must get their food by eating plants or by eating animals that eat plants. The food you eat contains energy in the form of the bonds that hold its molecules together. After the food has been broken down, it is taken into the cells where its energy is released by a process called respiration. Think of respiration as a slow burning. It allows molecules like glucose to combine with oxygen, releasing the energy tied up in the molecular bonds in the process. Its waste product is carbon dioxide, which you breathe out.
Photosynthesis and respiration are complementary. The carbon dioxide you breathe out is used by plants to create glucose; the oxygen that plants give off in turn serves as the raw material for respiration. This cycle between plants and animals is a key feature of ecosystems on our planet.
The simplest single-celled organisms use fermentation, a more primitive and less efficient method of burning their fuel that does not require the presence of oxygen. The so-called anaerobic bacteria responsible for turning a pile of garbage into humus operate in this way, as does the yeast that ferments grape juice into wine. Presumably all life generated energy by fermentation in the early Earth, when there was no oxygen in the atmosphere.
More advanced cells with nuclei often retain the ability to produce energy by fermentation as a backup to the more efficient process of respiration. Strenuously working your muscles, for example, may deprive them of oxygen, causing your cells to fall back on fermentation, which in humans produces lactic acid as a by-product.
The cell’s energy is generated in tiny sausage-shaped bodies called mitochondria. Partially digested carbohydrates, fats, and proteins from food are ferried to the mitochondria, where they are “burned” to produce energy to run the cell. A typical cell has hundreds of mitochondria at work.
Energy extracted from glucose or other fuel in a mitochondrion is usually not used immediately, so the cell must have a way to transport the energy from the place where it is produced to the place where it is to be used. To carry out this job, the cell uses a variety of molecules that serve as energy carriers. They play the same role in the cell that cash plays in an economy, allowing energy created in one place and time to be “cashed in” at another place and time, thereby making the working of the cell possible.
The “energy coin” molecules all work in the same way. The cell hooks electrons or groups of atoms to the molecule in one place, which requires energy. The molecules then move to another place where the electron or group of atoms is unhooked, releasing the stored energy. Almost all cells use a molecule called ATP (adenosine triphosphate) for relatively small amounts of energy, and a variety of other molecules to transport larger amounts when necessary.
Thus the cell is a dynamic, bustling place involving thousands of individual parts, each carrying out its own specialized chemical function to support the effort of the whole.
THE ORGANIZATION OF LIFE
When you studied biology in school, chances are you spent a lot of time learning the names of organisms and their parts. This book doesn’t do that, because detailed nomenclature, while essential for describing individual beasts and flowers, reveals little about general biological principles and is unlikely to be the subject of public debate or news reports. Taxonomy—giving things names—is important to science but is not essential to achieving scientific literacy.
Living things may consist of one cell only, or they may be multi-cellular. In a one-celled organism, the process of energy acquisition and the production of chemicals must all go on within each cell. In more complex organisms, like humans, cells specialize and the work is divided. The cells are organized into organs, and the organs into organ systems. Your digestive system, for example, contains a trillion cells, each its own complex chemical factory, each designed to do its bit in the system that takes in food and converts it into raw materials for use in the rest of your body. A combination of organ systems makes a complete organism.
Finding a way to order and catalog the wonderful diversity of living things was one of the main tasks of biology in the eighteenth and nineteenth centuries. The general scheme used today grows out of the classification first proposed by the great Swedish botanist Carl Linnaeus (1707–78). Think of every living thing as being situated somewhere on the branches of a great tree. The Linnaean classification locates a specific organism by specifying the main branch, then the side branch, then a smaller side branch, and so on until we arrive at the final twig on which the organism is found.
Aside from occasional regroupings (particularly of fossils), the job of classifying living things is no longer considered to be a major research area in biology. Apart from medical research, neither is the study of complete organisms and their internal organs a major focus of modern biological sciences. For the most part, the study of cells, their constituents, and the molecules that compose them is where the action is today.
The Five Kingdoms
The classification of living things has undergone something of a revolution in the past quarter of a century. Many biologists (and biology textbooks) continue to adopt a taxonomy based on the physical appearances of organisms. This approach leads to a division of all life into five kingdoms, including plants, animals, fungi, single-celled organisms with a nucleus (or protista), and single-celled organisms without a nucleus (or monera).
Each kingdom is further divided into increasingly specialized branches called phylum, class, order, family, genus, and species. The human being, for example, is a member of the animal kingdom, phylum of chordates (animals with spinal cords), sub-phylum of vertebrates (chordates with backbones), class of mammals (vertebrates with hair who suckle their young), order of primates (mammals with opposable thumbs and big brains), and family of hominids (primates that walk erect and have other distinctive skeletal features). All members of this family except for genus Homo, species sapiens, are extinct. Human beings, therefore, are referred to as Homo sapiens in the Linnaean scheme. The further we move back toward the main trunk in this scheme, the more inclusive the categories become—rabbits are mammals but not primates, fish are vertebrates but not mammals, and so on.
The end product of this classification scheme is the species, which biologists define to be a single interbreeding population. All human beings can interbreed, for example, so we are all members of the same species. Polar bears and black bears, on the other hand, both members of the genus Ursus, do not interbreed, and so are different species. We commonly describe organisms by two Latin words—like the dinosaur Tyrannosaurus rex or man’s best friend Canis familiaris—that represent the genus and species, and the entire classification scheme is implied by those names. Note that in this scheme two kingdoms are devoted to single-celled organisms. Here’s how several familiar one-celled creatures fit into this picture.
Bacteria, a phylum of monera, are usually named according to their shape: cocci (spheres) or bacilli (rods). They cause a number of diseases, like syphilis, tuberculosis, and cholera, but also are instrumental in producing many antibiotics. Anaerobic bacteria get their energy from fermentation and can survive without oxygen. They decompose much of the world’s organic garbage.
Plankton include any small organism that floats on the surface of water. The most abundant plankton are bacteria, referred to loosely as blue-green algae, which produce much of the world’s oxygen supply.
The familiar amoeba is not just a free-form blob, but a single-celled organism with a nucleus and a complex internal structure. It moves around, engulfs its food, and is a favorite creature in high school biology labs.
Three Domains of Life
The classic five-kingdom taxonomy, which relies principally on the physical appearance of organisms, is an eminently logical approach in a world where amoebas, mushrooms, maples, and humans appear so different from each other. However, as biologists discover more about the underlying chemistry of life, it appears that these disparate life-forms are remarkably similar to each other in terms of their biochemistry. By contrast, tiny cells without nuclei may appear relatively simple and similar to each other in a microscope, but their underlying chemistries often turn out to be wildly different from one another.
These biochemical discoveries have led to an alternate division of life based entirely on chemical differences among organisms. This new view of life was introduced by University of Illinois microbiologist Carl Woese, who employed the techniques of molecular genetics (see Chapter 16) to compare different cells. In the process he discovered a large group of simple one-celled organisms, collectively known as Archaea, that often thrive in such extreme environments as acidic hot springs, arctic ice, and in solid rock kilometers below the surface. These cells have remarkably distinctive and varied biochemical processes that set them apart from all other cellular life. These profound differences, w
hich are reflected in the unique genetic makeup of Archaea, led Woese to propose that life can be divided into three distinct domains.
According to this new and now widely accepted view, the old kingdom of monera should be split into Archaea and Bacteria, which are chemically distinct domains of single-celled life without nuclei. The third domain in Woese’s new scheme, Eucarya, encompasses all life based on cells with nuclei, including the multicellular kingdoms of plants, animals, fungi, and the single-celled protista. This new three-domain classification scheme reflects the fact that fungi, plants, animals, and amoebas are chemically and genetically extremely similar to each other, at least compared to Archaea and Bacteria.
Having introduced these two separate schemes of cataloguing living things, we have to add that there is no one “correct” classification scheme. How you want to classify living things depends on what you want to do. The traditional species-based system would be appropriate for someone studying the ecology of a forest, while the RNA-based scheme might be appropriate for someone studying microbes in a hot pool.
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