By Boyce Rensberger, Oxford University Press, 1998, 0-19-512500-2
[p8] Individual celss of Dictyostelium roam the forest floor as free-living amoebas, but if the food supplies turn scarce, they congregate in huge masses and assemble themselves by the thousands into a much larger multicelled organism that looks and acts like a slug. The slug creeps about for a while, then settles down and metamorphoses into something that looks like a fungus. The blob of celss sprouts a thin stalk that projects upward perhaps a quarter of an inch. Then other cells of the slug climb up the stalk and organize themselves into a ball balanced on top. The outer celss harden into a shell; the inner celss shrivel into dustlike spores. Eventually the shell cracks open and the spores are scattered to the wind. Spores that land in wet places change inot amoebas and slither off to begin the cycle anew.
[p13] Millions of [killer cells] roam the body, searching for other cells that have turned cancerous. Once the killer cell finds its prey, it presses close and exudes a substance that kills the cell. This newly emerging understanding of natural killer cells, incidentally, is leading to new ideas on the prevention, cause , and cure of cancer. Some researchers suspect cells frequently turn cancerous but are usually killed before they can proliferate into tumors. One reason tumors arise, then, may be defective killer cells, so some researchers are looking for ways to cure cancer by boosting the number of killer cells in the body.
[p15] The phenomenon, sometimes called apoptosis, is in fact the way “natural killer cells” destroy cells that have turned cancerous. It also plays a key role in embryonic development, performing the cellular equivalent of removing the scaffolding after construction is complete. For example, during the fifth week of human embryonic life, the hands are flat paddles with no distinct fingers.
[p16] The main thing cultured cells need is to be bathed in a fluid that contains some food – mainly sugar, mineral salts, some vitamins, and some amino acids, which are the building blocks of proteins. The fluid should both suply oxygen and take up the waste product carbon dioxide.
[p16] Periodically the cells cease their travels, let go of all attachments to the surface, pull themselves into a round ball, and undergo perhaps the most dramatic of life’s many astonishing phenomena – cell division. The cell breaks down som of its old internal structures, recycling the components to assemble new ones. The cells’ genetic endowment, made fo the chemical DNA, is copied into a duplicate set of genes.
[p16] Many cells living in culture remember their old lives as parts of tissues in larger organisms. Skin cells, for example, will will multiply until they form a sheet covering the bottom of their container, just as they used to make skin in their native habitat. Under certain conditions, the layer of skin cells will even develop into proper, multilayered skin – human tissue assembling itself right in a dish.
[p20] What happens is that the water outside the cells freezes before the water inside the cells. This is because all the molecules normally present inside the cell lower the freezing point. THe antifreeze keeps the water crystals outside the cell from becoming large enough to pierce the cell membranes. As the outside water slowly crystallizes, however, the liquid water inside the cell is drawn out, difussing through pores in the cell’s membrane and joining hte ice forming outseide the cell. The cell shrinks, literally deflated by the loss of water. If conditions are right, virtually all fo the water will have left the cell before the inside becomes cold enough that it would freeze water into crystals big enough to rupture internal membranes.
[p24] Molecules of a given shpae and composition possess the power not only to link themselves into larger structures but to act on entirely different molecules, causing them to break apart in specific ways or to combine with sill other molecules in predictible ways. The molecules that act upon other molecules, for example, are intracellular mechanics called enzymes.
[p25] Mono’s book was deeply disturbing to many because it asserted that no event in the life of a cell or, indeed, in the life of a whole human body, was the result of any supernatural guiding hand.
[p30] But unlike glassware, most membranes completely seal the volume within them, allowing passage in or out only to certain specific molecules and then usually only the molecules that pass inspection by gatekeeper molecules embedded in the membrane. The gatekeepers – called receptors or, sometimes, docking proteins – peer out fo the membrane at the passing scene, waiting for just the right molecule to come along. Then the receptor grabs the molecule. Or you can think of the receptor as a docking site shaped so that only one particular kind of molecule can come along and fit into the receptor. Once the two molecules embrace, drawn together by physical forces between one another, the receptor changes its own shape. THe part of it that sticks inside the organelle then triggers some specific process. In some receptors, this process leads to the arriving molecule being taken isnside the organelle, or even into the cell as a whole because similar receptors exist on the outer surfaces of cells. In other case, the molecule stays outside but simply triggers some process within the organelle or cell.
[p31] Receptors are ofthen thought of as locks that can be opened only by the right keys.
[p31] Organelles also send out substances to be taken up by some other organelle in the cell. Most of this shipping is containerized; that is, the molecules are transported inside tiny bubbles of membrane, called besicles. This process, too is mediated by receptors.
[p34] Szent-Gyorgyi didn’t know it at the time, but the hot-water extract contained one of the most crucial kinds of molecules in cells, adenosine triphosphate, or ATP. It is a molecule manufactured in all cells that, somewhat like a battery, stores and releases energy to power most of life’s activities.
[p38] As it happens, kinesin moves freight only in the outbound direction – from the nerve cell’s main body through the axon to a distant structure called the synapse, which is where one nerve cell r3elays its segnal to another. (Inside the vesicles are signal molecules, called neruotransmitters, that one nerve will release to act upon antoehr.) Different vesicles, however, also move inbound along the same microtubules, propelled by different motor molecules.
[p39] Dynein [outbound motors] had been known for years for creating a special kind of motion inside the hairlike projections, called cilia and flagella, that many cells possess.
[p41] As some cell biologists imagine it, the process probably works like an imaginary postal system in which letters have addresses but are dispatched randomly in trucks and planes. At every post office, somebody checks the address. If the parcell happens to be at its destination, it is accepted. IF not, it is tossed back and sent elsewhere.
Once the vesicle arrives at its intended destination, its membrane fuses with that of the organelle and the carge is automatically dumped inside. Vesicle fusion, of course, must itself be a strictly controlled process, for if it were not, vesicles throught the cell would fuse into one big vesicle, destroying the orderliness of the compartmentalization.
[p44] This creature was a macrophage, one of the free-roaming types of white (actually colorless) blood cells that functions as part of the immune system and one off the most fascinating types of cells in the human body. Macrophages inhabit the bloodstream but can also slip through tiny pores in the walls of blood vessels and wander in other tissues of the body. Macrophage is Latin for “big eater.” As it happens, macrophages maintain the ancient ways of their protozoan ancestors by creeping about their habitat and feeding on other organisms. In the human body macrophages swallow invading bacteria and viruses whole the way an amoeba does, by oozing around them and engulfing them. Macrophages are also the boy’s garbage collectors, and sometimes even its undertakers, eating aging or fatally damaged cells of other types.
[p48] Myosin pulls on a different kind of filament called actin. It is now well established that in muscle cells both actin molecules and myosin molecules braided into separate filaments that lie parallel to one another. Myosin can pull because it has little projecting “heads” that act like kinesin, grabbing the actin and pulling on it.