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
[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
[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
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
[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.