Why do some animals lay eggs? 96 – Science Mysteries Explained

Q:
A:
life science
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
82
What makes spider silk so amazingly strong
and light?
Spider silk has such amazing properties of strength, lightness, and flexibility it makes
human engineers jealous. So why is this stu so incredible?
Spider silk is made of protein and has what engineers call “exceptional mechanical properties.” It’s not just
strong, it’s also very stretchy. The secret? Special glands in the spider that “assemble” the silk.
As humans gradually learned how to smelt metals
and dress stone, building stronger and stronger
structures to protect us from the elements, little
did we know that the humble spider was spinning a
material that, to this day, outperforms almost all of
our most sophisticated creations.
If you walk through a really big web, you might
get a sense of the strength of spider silk. Even
though this structure made by a tiny arachnid is
barely visible, you actually need to exert quite a bit
of force to push through it. What’s more, a web
normally breaks where it’s anchored to plants
or objects—it wraps around your face and you
have to pull it o. Usually while shrieking.
Spiders make dierent types of web to do
dierent jobs, from the famous sticky fibers
to catch insects (capture-spiral silk), to
amazingly strong “guy ropes” to hold the web
up (major-ampullate or dragline silk), and
a super-tough version for wrapping up prey
(aciniform silk). They can even make incredibly
thin strands of gossamer that baby spiders use
to fly to new hunting grounds in a process called
ballooning.
We talk about spiders “spinning” silk be-
cause it does really look like they are spinning
the silk from their bodies—sometimes they even
gather the silk with their back legs similar to a
human working a spindle. But in fact, spider silk
LIFE SCIENCE
83
is made in a process called “pultrusion,” where the force of
pulling the silk material out of a gland full of pre-silk goop
forms it into a thin strand. Spider silk is unique because
almost all other biological fibers are made by smooshing
material together, whether it be keratin (like in our hair)
or even poop. Spiders can also eat and reuse their silk.
Think that’s cool? Once mechanical engineers started
analyzing spider silk in detail, things hit a whole new
level ….
These tiny, often transparent strands have our tough-
est materials beaten hands-down. By weight, spider silk is
five times stronger than steel, and ten times tougher than
Kevlar—which is used to make bulletproof vests! It can
stretch to five times its length before breaking. It can hold
that strength between -40°F and 428°F; and if you put it in
water, it contracts by 50 percent.
All these properties make it ideal for human uses. But
we don’t fully understand how it’s made, and attempts to
produce artificial silk, while improving, still have a long
way to go.
So why not just farm spiders? We can certainly “milk”
individual spiders for silk. But there’s a problem: unlike
silkworms, if you put a whole bunch of spiders together
they usually just kill each other. They are, after all, territo-
rial predators.
We’re not giving up, though. Spider silk, or an artificial
fiber derived from it, would change the face of human
engineering. It’s a prize worth working for.
Q:
A:
life science
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
84
Why can’t animals make energy from sunlight like
plants?
Being able to get a little extra energy from sunlight sounds like it would be a good idea for
animals, especially through tough times. So why don’t any animals do it?
Sunlight actually provides very little energy, and carrying around the ability to photosynthesize just isn’t
worth it for animals. Though that hasn’t stopped some species trying ….
Ever been hungry and looked at a plant and
thought, that guy just gets all his food for free from
the Sun—I wish I could lay back, soak up some rays,
and feel refreshed and re-energized?
Photosynthesis, the ability to extract ener-
gy from sunlight, is an amazing adaptation that
solves a big survival challenge for plants: how to
get enough food when you’re stuck in one place for
your whole life.
But it turns out photosynthesis isn’t that
great. You need to grow lots of leaves so you can
have a massive surface area to catch the most
rays. And even then the Sun doesn’t provide you
with much energy at all—at least, not compared
to the sheer bulk of calories consumed by an
animal every day.
Plants don’t move around because they just
don’t get the energy for it from the Sun. In terms
of calories, a plant gets by with far less energy
than you do—even a plant that weighs the same
as you.
Evolution isn’t just about “survival of
the fittest.” It’s also about finding the most
energy-ecient way to keep an organism alive.
Adding photosynthesis to an animal’s ability to
extract energy from food just wasn’t ecient.
The amounts of energy are so small, you would
have to stand in the Sun for weeks just to get as
many calories as eating a big steak.
LIFE SCIENCE
85
By weight, plants use a lot more water than animals—
they can be as much as 95 percent water (humans are
about 60 percent water). And plants have the luxury of
being able to absorb water slowly and constantly through
roots and dew. We have to drink.
Animals don’t photosynthesize because there’s never
been an evolutionary reason for them to “eat” sunlight.
Though of course, since this is nature we’re talking about,
there are some exceptions … kind of.
There are some groups of invertebrates that use sun-
light to make food. Well—it’s a bit trickier than that. What
they do is encourage algae (tiny green plants) to grow
inside their tissues. The algae gets a safe place to live, and
the animal gets to steal some of the energy the algae makes
from sunlight.
The most famous animals to use this system are the
corals. Contrary to common belief, the algae in coral is
brown. The amazing colors come from proteins made by
the coral itself. If water conditions are poor, the coral may
stimulate more algae to grow, causing “browning.” It’s the
opposite of coral “bleaching,” where the animals expel the
algae from their tissues, again in a response to poor water
quality.
Giant clams also grow algae in their flesh to get a little
extra boost of energy. But both giant clams and the corals
have something else in common—they don’t move around.
Anchored to the seafloor, the extra energy provided by the
Sun is worth the trouble of managing all that algae.
Within a hundred years, or maybe even less, it’s likely
that human technology will emerge as the best photo-
synthesizer on Earth. Our solar panels can extract huge
amounts of solar energy, putting plants to shame. And we
turn it directly into electricity—no messing about with
sugars!
Water
Humans
are about
60% water
Plants can be
up to 95% water
Humans use
far more
calories than
plants
Q:
A:
life science
IDIOT’S GUIDES: SCIENCE MYSTERIES EXPLAINED
86
Doesn’t higher CO
2
in the atmosphere make plants
healthier?
Every grade-school student knows plants take in carbon dioxide and release oxygen. So it
seems common sense to think that if theres more CO
2
in the atmosphere, it will help our
crops grow. But is that really true?
Yes, plants benefit from more CO
2
. But the equation is more complex than that, because the way in which
plants respond to more CO
2
isn’t always good for us ….
There are a bunch of standard arguments used by
people who want to believe that pumping lots of
carbon dioxide into the atmosphere isn’t necessar-
ily a bad thing. One of them is that plants need CO
2
and will grow more vigorously and healthily in a
CO
2
-enriched environment.
The simple answer to this is yes, plants do ben-
efit from higher CO
2
. And in prehistoric times, CO
2
levels were much higher than they are today. But
the issue of climate change isn’t primarily about
how other life forms will be aected—it’s about how
humans will be aected.
Plants will benefit from higher CO
2
. But will we
benefit from the changes that occur in those plants?
Not necessarily ….
Photosynthetic organisms use CO
2
, water,
and energy from the Sun to drive a chemical
reaction that makes sugar. Plants then use this
sugar for energy. Change the amount of CO
2
,
water, or sunlight, and the amount of energy
changes.
When plants have lots of energy, they grow
vigorously. But we don’t necessarily want
plants—especially crops—to grow willy-nilly.
Humans mostly eat the reproductive organs
of plants: the seeds and fruits. When we do eat
leaves, we prefer young, juicy leaves. We can’t
digest wood, and we don’t much like big, thick
stalks with lots of fibers in them.
Unfortunately, extra CO
2
gives plants the
energy they need to grow exactly the parts we
don’t want. Experiments with high CO
2
see
plants grow bushier, putting out more leaves
and stems, but they don’t necessarily make
more seeds.