"It has taken biologists some 230 years to identify and describe three quarters of a million insects; if there are indeed at least thirty million, as Erwin (Terry Erwin, the Smithsonian Institute) estimates, then, working as they have in the past, insect taxonomists have ten thousand years of employment ahead of them. Ghilean Prance, director of the Botanical Gardens in Kew, estimates that a complete list of plants in the Americas would occupy taxonomists for four centuries, again working at historical rates."-Richard Leakey (1944-)
What do a palm tree, a dolphin, a human, and a mushroom all have in common? Despite the vast differences among these four things, most elementary school students would have no trouble answering the question. All of them are alive.
Biology is the study of living things and the various aspects of life. Biological scientists address many different questions: how do living organisms function; how are they related to one another; how do they depend on one another; how do they survive? These are just a few of the topics of biology.
Beginning in the eighteenth and nineteenth centuries, biologists tried to answer these questions by observing the plants and animals around them. The discovery of microorganisms expanded the field of biology far beyond the organisms observable in our daily lives. Today, the study of life extends down to the interactions of the molecules from which all living things are built. These interactions, occurring constantly inside every living thing, are intimately tied to the big questions that biologists ask.
Are bacteria plants or animals?
Just about anywhere you look, you can find living things. Although there are major differences in the number of organisms you can find in a forest compared to a desert, you can almost always find something alive. You can also classify most of them easily as either plants or animals. But what about the living organisms that you know are there but can’t see? Are bacteria plants or are they animals?
Swedish botanist Carolus Linnaeus developed the first taxonomy, a scientific classification of nature into related categories, in the eighteenth century. He divided the world into three domains: animal, vegetable, and mineral. These classifications remained more or less intact until the middle of the twentieth century. On a large scale, it is usually pretty easy to tell whether something is a plant or an animal. A plant stays in one place and makes its own food; an animal moves around and eats something else. There is the occasional plant that seems to move or animal that seems stuck in place, but it generally works. If you fudge it a bit, you can classify fungi, which don’t make their own food, as plants and everything works out.
Definition
Taxonomy is the science of classification. In biology, living organisms are classified by relationships between species. Early taxonomists used observable characteristics of organisms to arrange living things by their similarities. Modern taxonomic classifications are based on the similarities in the genetic material that define each organism.
On the microscopic scale, though, things don’t fit so cleanly.
There are the algae, which make their own food, but on the cellular level differ more from plants and animals than they differ from one another. Then there are single-celled organisms that make food by photosynthesis but also move around and eat other organisms.
Fast Facts
The human body is filled with bacteria. Recent research indicates that your body may hold 10 times as many bacterial cells as human cells. Although most people associate bacteria with disease, many of the bacteria inside our bodies perform functions that are essential to our survival. For example, bacteria play a large role in the digestion of food inside human intestines.
Bacteria are organisms that have a single cell. With a good microscope, you can find them anywhere from the deepest ocean to the driest desert, and right in your body. In fact, bacteria are so involved with the function of your body that you need them in order to survive. Although they vary in size, you would need to line up about 40,000 of them to make a 1-inch column.
The answer to the original question is that bacteria are neither plants nor animals. Every plant or animal cell has a nucleus, where it manufactures genetic material, DNA. Bacteria, on the other hand, do not have a nucleus. Their DNA floats around inside the cell.
As scientists learned more about microscopic organisms, it became clear that things could not be neatly placed into the plant and animal boxes. A new classification system was adopted in the mid-twentieth century. In that system, all living things were divided into five kingdoms based on the structures of their cells:
"Microorganisms will give you anything you want if you know how to ask them."
♦ Animalia—animals
♦ Plantae—plants
♦ Monera—bacteria and blue-green algae
♦ Protista—protozoans and some algae
♦ Fungi—fungi
While this system is useful, it still classifies very different organisms into broad groups. Other classification systems have been proposed, again based on cellular structure.
As more is learned about an even smaller structure, DNA itself, some scientists have proposed that these classifications do not work at all. They have found some relationships across kingdoms that are much closer than relationships within kingdoms. An entirely new classification system is likely to grow as we learn more about life. But in any case, bacteria are not plants and they are not animals.
Are viruses alive?
Anyone who has had the flu knows that his or her body has been attacked by something. The virus that causes flu symptoms not only gets under your skin, it gets right into your cells and takes over. You can’t treat the disease with antibiotics, though, as you can a bacterial infection. Antibiotics kill bacteria but they have no effect at all on a virus. Are viruses living things like bacteria?
Believe it or not, this is a question that does not have a clear answer. Viruses are particles, consisting of a DNA or RNA molecule surrounded by a protective protein coat. Viruses reproduce by transferring their DNA to a living cell. Essentially, they hijack the cell, turning it into a factory to produce new viruses. The host cell uses all its energy producing hundreds or thousands of new viruses and then bursts apart, sending them out into the world.
Viruses attack the cells only of specific types of hosts. Although they can attack more than one species—for example, the flu virus can attack both humans and birds-different species may be affected very differently. Also, the viruses that invade plant cells are not able to affect animals. Many viruses prey on bacteria. None of these viruses can have any effect on plant or animal cells. Because of the similarities of DNA in the virus and the host, some biologists propose that viruses are more closely related to their hosts than to other viruses.
Determining whether viruses are alive or not depends on first defining life. It turns out that this is not as simple as it may seem. Since the middle of the nineteenth century, the cell theory has defined life on the basis of cells. Cells are surrounded by a membrane in which the functions of life—such as production of energy, growth, and reproduction—take place. Because viruses do not have cell membranes, do not grow, and do not produce or digest food, viruses are clearly not living things under this definition.
Because they can both cause disease, many people think viruses and bacteria are similar and use the term "bug" to describe both. In fact, viruses differ more from bacteria than bacteria differ from humans. Even the smallest bacteria are thousands of times larger than a virus.
On the other hand, some biologists define life as the ability to pass genetic information from generation to generation. In this sense, viruses are definitely alive. And, although they do not grow once they have been manufactured by a cell, viruses do grow as the cell builds them and they use the mechanism of the cell to convert food into new viruses.
While scientists, and humans in general, have a need to classify things to better understand them, viruses do not fit neatly into either class—living or nonliving.
Why don’t evergreens lose their needles in winter?
In temperate areas, many trees enter fall with a flashy show of color and then lose all their leaves. Others, such as pine, spruce, and fir trees stay green year-round. Why don’t these trees lose their needles in the same way that oaks and maples drop their leaves?
The key to answering this question is an understanding of how growing plants take advantage of solar energy. Leaves absorb radiation as light and use this energy to produce sugars that fuel growth and reproduction, and all the functions that support these processes. Near the equator, in tropical areas, the amount of sunlight does not vary much during the year. Plants tend to have large leaves in order to absorb as much light as possible and they tend to grow rapidly, staying green year-round. Leaves tend to stay on the plant until they become so shaded by the leaves above them that they do not produce sugars well.
In temperate regions, the amount of available light varies significantly throughout the year. Deciduous trees—maples, for example—are covered leaves whose shapes resemble those of tropical plants. They collect energy all summer and the plant grows rapidly. In winter, though, sunlight is not strong enough to power these sugar factories.
Then the large leaves can be a problem. Their surface area allows them to lose moisture which may be difficult to replace from frozen soil. Their broad shapes allow them to collect snow and ice whose weight can snap even large branches and trunks. Plants that drop their leaves have a better chance of surviving the cold winds and heavy ice of winter.
But what about the colder regions closer to the poles? Here, the difference between summer and winter is even greater. During summer, days are long and sunlight can last for many more hours each day than in the temperate zones. In the winter, there is very little light available. The trees, however, do not lose their leaves. Why not?
It takes a lot of energy to grow leaves every year. Because the season is shorter, it is hard for deciduous trees to start fresh each year and still absorb enough energy during the short growing season. Trees that are ready to grow as soon as spring begins have an advantage here. That’s why the far northern forests of Europe, Asia, and North America are home to evergreen trees.
Fast Facts
By any measure, the largest living things are cone-bearing evergreen trees, known as conifers. The tallest known tree is the redwood; the tree with the greatest diameter and the greatest total bulk is the sequoia. Both of these species are native to the western United States.
These trees are not like the tropical evergreens, however. Instead of broad leaves, these trees, such as pines and firs, have small needles. The needles have waxy coats that retain moisture and their small, rounded shapes do not hold snow the way broad leaves do. Also, the trees have the classic "Christmas tree" shape that allows them to shed snow easily. The trees that thrive in an area are generally the ones whose energy needs balance what is available. In many temperate areas, deciduous and evergreen trees grow side by side, both competing effectively for resources.
Why are most plants green?
If you surveyed a number of people, asking "What color are plants?", you would generally receive the answer "Green." Even though some plants have leaves that are not green, or combine green with other colors, it is true that most plants are green. Why? as photosynthesis. Chlorophyll uses red and blue wavelengths of light most efficiently and tends to reflect the green wavelengths, so the plant appears to be green.
Plants are green because the cells in leaves (and sometimes stems) contain chlorophyll, a large molecule that absorbs certain colors of light. This light provides the energy needed to convert carbon dioxide and water into food for the plant, a process known Photosynthesis is the process used by green plants, as well as some bacteria and algae, to produce glucose from carbon dioxide and water. The term photosynthesis, literally "building with light," is used because the energy for the chemical reaction comes from sunlight.
Why would chlorophyll absorb green more than other colors? This might be a result of the evolution of plants. The light from the sun is not evenly distributed across the spectrum. Certain wavelengths, especially in the red and blue regions, are stronger than others. It may be that chlorophyll dominates because it absorbs energy most efficiently where it is strongest. Plants that developed other mechanisms for absorbing light may have lost out because chlorophyll was more efficient.
Chlorophyll is not the only compound in leaves that gives them color, but it is the most visible. In temperate regions, we can see some of the other pigments in the fall. As photosynthesis becomes less efficient when light and heat drop off, the plants quit making chlorophyll. Then the reds, yellows, and purples of other pigments in the leaves make their mark.
How do chameleons change color?
Chameleons are African lizards that can change color, an ability that is unusual among animals. Each chameleon can display a range of colors—brown, black, blue, green, red, yellow, or white. How can a chameleon change colors so easily while most animals can be identified by their constant color?
Its ability to change color lies where you would expect—right in its skin. A chameleon’s skin has several layers. The protective outer layer is transparent and colorless. Just below it lies another layer of skin, made of cells called chromatophores, which contain
A chameleon does not change its color to match the background. In fact, many of the shades that a chameleon takes when it is not agitated or excited provide camouflage in its natural environment. Generally, chameleons going about their daily business undisturbed will be various shades of green, blue-green, or brown.
Red and yellow pigments. The third layer contains chromatophores rich in melanin, the same pigment that accounts for the different shades of human skin. The melanin layer creates black and brown colors and it also reflects blue light. Delving even deeper, you find the fourth, and final, layer of skin. This layer reflects the light that reaches it.
The color of the chameleon’s skin changes as the chromatophores expand and contract. The temperature and the brightness of light can affect the size of the chromatophores and, with them, the color of the animal. Most of the control, however, comes from nerve impulses from the brain and changes in hormone levels in the cells.
When a chameleon is cold, the red and yellow cells shrink and the brown/black cells expand, giving the skin a dark color which absorbs more energy from light. When it is defending its territory or interested in mating, it produces bright, showy colors. The meaning of a particular color or pattern of colors varies with the species of chameleon.
What is the oldest living thing on Earth?
The oldest person whose age could be confirmed lived 122 years. There are a few animals that live longer. The oldest giant tortoise whose age has been confirmed lived 188 years. You won’t find the oldest living things among the animals, though. What is the oldest living thing?
The age of a tree can be determined by counting its growth rings. Many large trees are very old. Counts of the ring growth in core samples have shown that some redwoods and sequoias have been growing for more than 2,000 years. Until recently, the oldest known organism was a bristlecone pine in California, which has been named "Methuselah." A count of its growth rings from a core sample taken in 1957 showed that the tree was 4,723 years old.
But that is not the end of the story. Botanists have found an even older plant in southern California. Creosote bushes do not have the grand size of a sequoia, or the gnarled, ageless appearance of the old bristlecone pine. In fact, an old creosote bush looks like a ring of scruffy shrubs with a distinctive odor.
The development of genetic testing has shown that the shrubs in a ring actually comprise one plant. The original stem of a creosote bush dies after a few decades, leaving several new stems to take its place. Although the stems appear to be separate bushes, they share an interconnected root system. As these stems are replaced and die, the ring moves outward. The plant develops into separate bushes, but each of them has a legitimate claim as being the original plant. All of the bushes are genetically identical. One ring of creosote bushes has been measured by carbon dating to be more than 11,000 years old, twice the age of the oldest known tree.
Fast Facts
Although each ring represents one year of growth, tree rings are not all the same size. During dry years trees have less growth than during moist years, so the ring is narrower. By studying the patterns of ring size, scientists can determine the climate of a region, tracing periods of rain and drought over hundreds of years. Comparisons of the patterns can even be used to date logs of trees that died or were cut many centuries ago.
Even more recently, another discovery threatens to make the creosote bushes rank as toddlers in the old-age contest. Bacteria have been recovered from ancient ice samples far below ground in Siberia. These cells appear to have been alive and functioning at a very slow rate for more than 600,000 years. For now that is the record, but who knows what may be found next?
How can grasshoppers jump so far?
Picture an athlete in the Olympics, preparing to make a world-record long jump. He takes a long run, getting to sprint speed, and launches at the line. A really good jump carries him almost 9 meters, about four-and-a-half times his height.
Now watch a grasshopper jump. From a standing start, the 4-centimeter-long insect springs forward, landing more than a meter away, about 40 times its body length. And that’s an average jump, not an Olympic record. Why can a grasshopper jump so much farther than a human compared to its size?
There are several reasons why an insect can jump proportionally farther than a person. The two organisms have different body designs and different methods for getting oxygen to muscles. The most important factor, though, has to do with changes in scale as size increases. The force that a muscle can exert increases as the cross section of the muscle increases. The cross section is an area measurement, a square of length. If the grasshopper doubled in length, keeping the same body proportions, its muscle cross section would increase by four. If it tripled in length, its cross section would increase by nine, and so on. A 6-foot grasshopper would have about 2,500 times as much jumping power in its huge rear legs.
"Because all of biology is connected, one can often make a breakthrough with an organism that exaggerates a particular phenomenon, and later explore the generality."
But here is where the scaling problem comes in Area is squared as length increases, but volume is cubed. As the length of the grasshopper increased by a factor of 50, its volume increases by a factor of 125,000. That means its mass also increases by that factor. Although the muscles are stronger, they must propel 50 times as much mass per cross-sectional area. A grasshopper the size of an Olympic jumper might not even be able to jump as far as the human.
Why are moths attracted to light?
If you sit outside on a dark night with the porch light turned on, you are almost certain to see many moths fluttering around the light. It seems strange that a nocturnal insect would want to be close to a source of light because there is not any natural source of nighttime light that would be useful to moths. So why are the moths attracted to light?
This question is an example of an observation that can be misleading. Because many moths gather around the light, we assume that they are attracted to the light. There is no evidence that the moths approach light because they are attracted to it. In fact, many species of moths are repelled by light and fly away from it.
Fast Facts
While moths may not be attracted by streetlights, bats are. Although bats rely mostly on their ears to hunt insects, they are not blind. Bats learn that the best place to seek out food is near the light, where moths are blundering about. They learn to head for the lights the same way that people head for the lighted signs of a fast-food restaurant.
There are several theories about the interaction between moths and light. Most scientists who study moths think that the moths approach the light as a result of confusion, not attraction. It may be that moths, which have good eyesight, normally use natural light sources—the moon and the stars—to orient themselves so that they fly in a straight line. Consider the effect on the insect’s navigation, if this theory is correct. A moth would fly toward the light, not because it is trying to approach it (a moth cannot approach the moon) but because it sees a reference point. Unlike the natural light sources that the moth evolved to use for reference, the light gets closer and closer, so the reference becomes confusing and the moth flies in circles.
Another possible explanation is that the moths are looking for food. Some moths are attracted to night-blooming flowers. Many of these flowers are large and white, reflecting as much light as possible. If this is the case, it might explain why some types of light seem to be more attractive to moths than others. For example, ultraviolet and blue wavelengths seem to be more attractive than yellow wavelengths. Flowers may not reflect all wavelengths equally.
Yet another explanation could be that the moths come upon the light by accident. Their eyes have many facets that collect light. As the moth approaches the bright light, it is temporarily blinded and its eyes need to adjust, just as your eyes must adjust to a sudden bright illumination of a dark theater. After adjusting to the light, the moth is then unable to see in the dark as it leaves the light and it blunders back to the light.
The ultraviolet light traps that "zap" bugs in many backyards are not very effective at preventing bug bites. Mosquitoes do not show any great tendency to approach them.
Does a camel really store water in its hump?
It’s easy to understand why people think the big hump on a camel’s back is a water container. The animal can travel many days in the hot, dry desert without drinking any water and then gulp down 20 gallons all at once. Is this water actually stored in the hump?
Although the camel can store a lot of water in its body, the hump is not a stock of water. The camel’s hump is a large fat deposit—a store of food rather than water. The fat in the hump allows the camel to go for long periods without eating. When food is short, the fat is metabolized for energy, in the same way that a store of fat carries a bear through a winter without eating. Camels can go for up to two weeks without eating by relying on this reserve. It is uncertain why the camel stores fat on its back, but one possibility is that the fat also acts as insulation, protecting the animal from the hot sun.
Even though water is not stored in the hump, a camel can tolerate long periods without water. For most mammals, drinking the equivalent of the camel’s 20-gallon chug would dilute body fluids, perhaps fatally. The camel, however, can adjust to the swing in water concentration in its body. In addition, its body temperature can range from 94°F to 105°F. This allows the camel to adapt to the heat of the day and not start sweating until long after most mammals would have needed to expend moisture to cool their bodies.
A camel is perfectly adapted to the desert. It has three eyelids to protect its eyes from the sand and dust of a desert sandstorm and it can close its nose to protect its breathing passages. A camel’s broad, leathery feet can find traction in rocky soil and soft sand.
Why are fossil shark teeth common but not shark skeletons?
We know that even the great white shark would be dwarfed by sharks of the past, especially the giant megalodon, which is estimated to have grown to more than 50 feet in length. Size estimates for this giant fish are based on fossil teeth. No one has ever found a fossil megalodon skeleton, even though dinosaur skeletons many times older are common. Why is it that there are no fossils of ancient shark skeletons but fossilized shark teeth are very common?
Sharks are amazingly agile fish, able to make a tighter turn than other fish the same size. One reason for the shark’s maneuverability is its skeleton. Unlike most other vertebrates, the shark does not have a skeleton made of bone. Instead, it is made of cartilage, the strong, lightweight material that forms the shape of your nose.
Cartilage is much lighter than bone, which helps keep the shark from sinking and is more flexible for turning efficiency. In addition, the shark’s swimming muscles are not connected to the skeleton, but instead they connect directly to its tough skin, increasing the efficiency of the muscles. However, because the cartilage is not hard like bone, it does not tend to form fossils. Very few fossil shark skeletons have been discovered.
The belief that sharks do not get cancer has led to a market for shark cartilage as a cancer treatment. In fact, sharks do get cancer, although their immune systems seem to be particularly good cancer fighters. Scientists are doing research to determine whether sharks produce compounds that would be effective in treating human cancers. However, there has never been a scientific study that showed that ground-up shark cartilage has any effect as a treatment.
The shark’s teeth are hard, like your own, and easily form fossils. In addition, sharks have lots of teeth and they make new teeth throughout their lives. Sharks do not have one row of teeth like humans. Instead, they have several rows of teeth attached to the jaw along with 5 to 15 rows of spares. As teeth become worn or damaged, they fall out. When a tooth is lost, a new one takes its place, generally within a day. A single shark can produce more than a thousand teeth every year. That makes a lot of material for fossils.
Although the shape and size of sharks’ teeth vary from one species to another and even from one part of the jaw to another, the size of the teeth can be used as a measure of the size of the fish. We know that the megalodon was a truly huge shark because it had huge teeth. While the teeth from a great white shark may measure about 2′/2 inches long, some megalodon teeth exceed 7 inches.
Why do animals migrate?
An aerial photograph of a massive migration, whether it is caribou in North America or wildebeest in Africa, is an impressive sight. Thousands of animals move together as one large herd. Why do animals migrate and how do they know where to go?
Some animals migrate extremely great distances, traveling thousands of miles annually. Migrations consume large amounts of energy, so they are generally undertaken for important reasons: reproduction, food supplies, or warmer climates. Antelope travel across the plains, looking for greener pastures. Geese fly from the abundant summer food supplies of the northern tundra as days shorten and the temperature drops, only to return in the spring when the days lengthen. Salmon fight the currents of swift-flowing streams to return to the place of their birth, laying the eggs of the next generation.
Migrating lemmings do not commit mass suicide by flinging themselves off cliffs into the sea. Lemmings do migrate from one area to another when their population exceeds the food supply. During the migration, they swim across rivers and some of them drown. This may have been the origin of the suicide myth. The most famous migration of lemmings over a cliff occurred in a 1958 movie. The film was made far from the sea and the "suicidal" lemmings were actually tossed over a ledge by the movie’s producers.
Although it is not too difficult to figure out why a particular migration takes place, the how can be a bit tougher. How can a monarch butterfly travel from Canada to Mexico, or a Pacific trout swim a thousand miles to the mouth of the stream in which it was spawned?
Fast Facts
The phenomenon of migration is not limited to animals. Human migrations have led to populating the entire planet. Like animal migrations, human migrations occur when people seek escape from threatening conditions, such as war, or search for places with better resources, such as the migration across North America in the nineteenth and twentieth centuries. There are even some seasonal human migrations. For example, migrant farm workers follow the harvest seasons in many countries.
Researchers have studied the physical characteristics of migratory animals and the characteristics of their movement to come to conclusions about how animals navigate. Many migrating birds, including ducks, use the skies to orient themselves, observing the positions of the sun or the stars. Some animals, including loggerhead turtles, have sensory organs that detect Earth’s magnetic field, a built-in compass. Salmon detect their home stream after traveling vast distances in the ocean by its smell. It is likely that many migratory animals use several environmental cues, including these to find their way around. Only humans use printed maps, but the animals don’t seem to miss them.
