Biology 6th ed. [general intro text] - Raven, Johnson WW.pdf

(55361 KB) Pobierz
Part
I
Defying gravity.
This gecko lizard is able to climb walls and
walk upside down across ceilings. Learning how geckos do this is
a fascinating bit of experimental science.
Unraveling the Mystery of How
Geckos Defy Gravity
Science is most fun when it tickles your imagination. This is
particularly true when you see something your common
sense tells you just
can’t
be true. Imagine, for example, you
are lying on a bed in a tropical hotel room. A little lizard, a
blue gecko about the size of a toothbrush, walks up the wall
beside you and upside down across the ceiling, stopping for
a few moments over your head to look down at you, and
then trots over to the far wall and down.
There is nothing at all unusual in what you have just
imagined. Geckos are famous for strolling up walls in this
fashion. How do geckos perform this gripping feat? Investi-
gators have puzzled over the adhesive properties of geckos
for decades. What force prevents gravity from dropping the
gecko on your nose?
The most reasonable hypothesis seemed suction—
salamanders’ feet form suction cups that let them climb
walls, so maybe geckos’ do too. The way to test this is to see
if the feet adhere in a vacuum, with no air to create suction.
Salamander feet don’t, but gecko feet do. It’s not suction.
How about friction? Cockroaches climb using tiny hooks
that grapple onto irregularities in the surface, much as rock-
climbers use crampons. Geckos, however, happily run up
walls of smooth polished glass that no cockroach can climb.
It’s not friction.
Electrostatic attraction? Clothes in a dryer stick together
because of electrical charges created by their rubbing to-
gether. You can stop this by adding a “static remover” like a
Cling-free sheet that is heavily ionized. But a gecko’s feet
still adhere in ionized air. It’s not electrostatic attraction.
Could it be glue? Many insects use adhesive secretions
from glands in their feet to aid climbing. But there are no
glands cells in the feet of a gecko, no secreted chemicals, no
footprints left behind. It’s not glue.
There is one tantalizing clue, however, the kind that ex-
perimenters love. Gecko feet seem to get stickier on some
surfaces than others. They are less sticky on low-energy
surfaces like Teflon, and more sticky on surfaces made of
polar molecules. This suggests that geckos are tapping
directly into the molecular structure of the surfaces they
walk on!
Tracking down this clue, Kellar Autumn of Lewis &
Clark College in Portland, Oregon, and Robert Full of the
University of California, Berkeley, took a closer look at
gecko feet. Geckos have rows of tiny hairs called setae on
the bottoms of their feet, like the bristles of some trendy
toothbrush. When you look at these hairs under the micro-
scope, the end of each seta is divided into 400 to 1000 fine
projections called spatulae. There are about half a million of
these setae on each foot, each only one-tenth the diameter
of a human hair.
Autumn and Full put together an interdisciplinary team
of scientists and set out to measure the force produced by a
single seta. To do this, they had to overcome two significant
experimental challenges:
Isolating a single seta.
No one had ever isolated a single
seta before. They succeeded in doing this by surgically
plucking a hair from a gecko foot under a microscope and
bonding the hair onto a microprobe. The microprobe
was fitted into a specially designed micromanipulator that
can move the mounted hair in various ways.
Measuring a very small force.
Previous research had
shown that if you pull on a whole gecko, the adhesive
force sticking each of the gecko’s feet to the wall is about
10 Newtons (N), which is like supporting 1 kg. Because
each foot has half a million setae, this predicts that a sin-
gle seta would produce about 20 microNewtons of force.
That’s a very tiny amount to measure. To attempt the
measurement, Autumn and Full recruited a mechanical
engineer from Stanford, Thomas Kenny. Kenny is an ex-
pert at building instruments that can measure forces at
the atomic level.
1
Real People Doing Real Science
The Origin of Living
Things
Begin parallel
pulling
80
60
Force (µN)
40
20
0
-20
Seta pulled
off sensor
0
1
2
Time (s)
3
4
5
The sliding step experiment.
The adhesive force of a single seta
was measured. An initial push perpendicularly put the seta in
contact with the sensor. Then, with parallel pulling, the force
continued to increase over time to a value of 60 microNewtons
(after this, the seta began to slide and pulled off the sensor). In a
large number of similar experiments, adhesion forces typically
approach 200 microNewtons.
Closeup look at a gecko’s foot.
The setae on a gecko’s foot are
arranged in rows, and point backwards, away from the toenail.
Each seta branches into several hundred spatulae (inset photo).
The Experiment
Once this team had isolated a seta and placed it in Kenny’s
device, “We had a real nasty surprise,” says Autumn. For
two months, pushing individual seta against a surface, they
couldn’t get the isolated hair to stick at all!
This forced the research team to stand back and think a
bit. Finally it hit them. Geckos don’t walk by pushing their
feet down, like we do. Instead, when a gecko takes a step, it
pushes the palm of the foot into the surface, then uncurls
its toes, sliding them backwards onto the surface. This
shoves the forest of tips
sideways
against the surface.
Going back to their instruments, they repeated their ex-
periment, but this time they oriented the seta to approach
the surface from the side rather than head-on. This had the
effect of bringing the many spatulae on the tip of the seta
into direct contact with the surface.
To measure these forces on the seta from the side, as well
as the perpendicular forces they had already been measur-
ing, the researchers constructed a micro-electromechanical
cantilever. The apparatus consisted of two piezoresistive
layers deposited on a silicon cantilever to detect force in
both parallel and perpendicular angles.
The Results
With the seta oriented properly, the experiment yielded re-
sults. Fantastic results. The attachment force measured by
the machine went up 600-fold from what the team had
been measuring before. A single seta produced not the 20
microNewtons of force predicted by the whole-foot mea-
surements, but up to an astonishing 200 microNewtons
(see graph above)! Measuring many individual seta, adhe-
sive forces averaged 194+25 microNewtons.
Two hundred microNewtons is a tiny force, but stupen-
dous for a single hair only 100 microns long. Enough to hold
up an ant. A million hairs could support a small child. A little
gecko, ceiling walking with 2 million of them (see photos
above), could theoretically carry a 90-pound backpack—talk
about being over-engineered.
If a gecko’s feet stick
that
good, how do geckos ever
become unstuck? The research team experimented with
unattaching individual seta; they used yet another micro-
instrument, this one designed by engineer Ronald Fearing
also from U.C. Berkeley, to twist the hair in various ways.
They found that tipped past a critical angle, 30 degrees,
the attractive forces between hair and surface atoms
weaken to nothing. The trick is to tip a foot hair until its
projections let go. Geckos release their feet by curling up
each toe and peeling it off, just the way we remove tape.
What is the source of the powerful adhesion of gecko feet?
The experiments do not reveal exactly what the attractive
force is, but it seems almost certain to involve interactions at
the atomic level. For a gecko’s foot to stick, the hundreds of
spatulae at the tip of each seta must butt up squarely against
the surface, so the individual atoms of each spatula can come
into play. When two atoms approach each other very
closely—closer than the diameter of an atom—a subtle nu-
clear attraction called Van der Waals forces comes into play.
These forces are individually very weak, but when lots of
them add their little bits, the sum can add up to quite a lot.
Might robots be devised with feet tipped with artificial
setae, able to walk up walls? Autumn and Full are working
with a robotics company to find out. Sometimes science is
not only fun, but can lead to surprising advances.
To explore this experiment further,
go to the Virtual Lab at
www.mhhe.com/raven6/vlab1.mhtml
1
The Science of Biology
Concept Outline
1.1 Biology is the science of life.
Properties of Life.
Biology is the science that studies
living organisms and how they interact with one another and
their environment.
1.2 Scientists form generalizations from observations.
The Nature of Science.
Science employs both deductive
reasoning and inductive reasoning.
How Science Is Done.
Scientists construct hypotheses
from systematically collected objective data. They then
perform experiments designed to disprove the hypotheses.
1.3 Darwin’s theory of evolution illustrates how science
works.
Darwin’s Theory of Evolution.
On a round-the-world
voyage Darwin made observations that eventually led him to
formulate the hypothesis of evolution by natural selection.
Darwin’s Evidence. The fossil and geographic patterns of
life he observed convinced Darwin that a process of evolution
had occurred.
Inventing the Theory of Natural Selection.
The
Malthus idea that populations cannot grow unchecked led
Darwin, and another naturalist named Wallace, to propose
the hypothesis of natural selection.
Evolution After Darwin: More Evidence.
In the century
since Darwin, a mass of experimental evidence has supported
his theory of evolution, now accepted by practically all prac-
ticing biologists.
FIGURE 1.1
A replica of the
Beagle,
off the southern coast of South
America.
The famous English naturalist, Charles Darwin,
set forth on H.M.S.
Beagle
in 1831, at the age of 22.
1.4 This book is organized to help you learn biology.
Core Principles of Biology.
The first half of this text is
devoted to general principles that apply to all organisms, the
second half to an examination of particular organisms.
ou are about to embark on a journey—a journey of
discovery about the nature of life. Nearly 180 years
ago, a young English naturalist named Charles Darwin set
sail on a similar journey on board H.M.S. Beagle (figure
1.1 shows a replica of the
Beagle).
What Darwin learned on
his five-year voyage led directly to his development of the
theory of evolution by natural selection, a theory that has
become the core of the science of biology. Darwin’s voyage
seems a fitting place to begin our exploration of biology,
the scientific study of living organisms and how they have
evolved. Before we begin, however, let’s take a moment to
think about what biology is and why it’s important.
Y
3
1.1
Biology is the science of life.
WITHIN CELLS
Properties of Life
In its broadest sense,
biology is the study of living things—the
science of life.
Living things come in an astounding variety of
shapes and forms, and biologists study life in many differ-
ent ways. They live with gorillas, collect fossils, and listen
to whales. They isolate viruses, grow mushrooms, and ex-
amine the structure of fruit flies. They read the messages
encoded in the long molecules of heredity and count how
many times a hummingbird’s wings beat each second.
What makes something “alive”? Anyone could deduce
that a galloping horse is alive and a car is not, but
why?
We
cannot say, “If it moves, it’s alive,” because a car can move,
and gelatin can wiggle in a bowl. They certainly are not
alive. What characteristics
do
define life? All living organ-
isms share five basic characteristics:
1. Order.
All organisms consist of one or more cells
with highly ordered structures: atoms make up mole-
cules, which construct cellular organelles, which are
contained within cells. This hierarchical organization
continues at higher levels in multicellular organisms
and among organisms (figure 1.2).
2. Sensitivity.
All organisms respond to stimuli. Plants
grow toward a source of light, and your pupils dilate
when you walk into a dark room.
3. Growth, development, and reproduction.
All or-
ganisms are capable of growing and reproducing, and
they all possess hereditary molecules that are passed to
their offspring, ensuring that the offspring are of the
same species. Although crystals also “grow,” their
growth does not involve hereditary molecules.
4. Regulation.
All organisms have regulatory mecha-
nisms that coordinate the organism’s internal func-
tions. These functions include supplying cells with nu-
trients, transporting substances through the organism,
and many others.
5. Homeostasis.
All organisms maintain relatively
constant internal conditions, different from their envi-
ronment, a process called homeostasis.
All living things share certain key characteristics: order,
sensitivity, growth, development and reproduction,
regulation, and homeostasis.
Cell
Organelle
Macromolecule
FIGURE 1.2
Hierarchical organization of living things.
Life is highly orga-
nized—from small and simple to large and complex, within cells,
within multicellular organisms, and among organisms.
Molecule
4
Part I
The Origin of Living Things

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin