Www.svsd.net



Kingdoms of Living ThingsLiving things or organisms are found almost everywhere on Earth: from deep in the oceans to high in the air, from the ice-covered poles to the tropical forests of the equator.?Biologists, scientists who study organisms, have identified about 2 million species, or kinds, and estimate that as many as 100 million now exist. To help make sense of this abundance of life, scientists place all living things within large groups of related organisms called kingdoms.Classifying Living ThingsSince early times, people have classified living things. In ancient Greece, the philosopher and scientist Aristotle developed the first all-encompassing classification system. He divided the living world into two kingdoms--animals and plants. For more than 2,000 years, Aristotle's system endured.As scientists learned more, classification systems were improved. In classifying organisms, scientists have long considered factors such as anatomy, nutrition, reproduction, and growth. They now consider chemical and genetic differences, too.In the late 1950's American biologist Robert H. Whittaker proposed a system that divided living things into five kingdoms: animals, plants, fungi, protists, and monerans. This system won broad support and is still popular. More recent proposals emphasize subtle chemical differences between organisms that have similar appearances. For example, in the 1980's American biologist Carl Woese suggested a six-kingdom system. It eliminated monerans and added two new kingdoms. One is called Eubacteria. It includes bacteria we often refer to as germs. The other, Archaebacteria, includes organisms that thrive in harsh environments and are considered more ancient.Systems with as many as 13 kingdoms have been proposed. Because scientists keep learning more about living things, no system is final.AnimalsAnimals make up the largest kingdom of living things, Kingdom Animalia. Biologists have identified more than 1 million species of animals. The many kingdom members exhibit great variety of shape, size, and structure. But however different animals such as elephants, mice, whales, pigeons, snakes, fish, sponges, insects, lobsters, worms, jellyfish, and even humans might appear, they share important similarities.Animals are complex, multicelled organisms. Each cell in an animal's body is bounded by a delicate skinlike covering called the cell membrane. This protects the inner parts of the cell and gives the cell its shape. Within the cell are various structures, each of which has certain functions. Perhaps the most important structure is the nucleus, which is the cell's command center. It directs the cell's activities.The cells of most animals are arranged in groups or layers called tissues. All the cells in a tissue look alike and do the same job. For example, muscle tissue is made up of muscle cells and makes movement of the body or of certain parts of the body possible. Nervous tissue is built of nerve cells and is responsible for carrying nerve signals throughout the body. An organ, such as the heart, stomach, or brain, is formed when different kinds of tissue are combined. An organ performs special tasks for the body.An animal's life cycle includes a distinctive early stage of rapid growth as a tiny, multicellular form called an embryo. As an animal grows, it follows a definite growth pattern. When it reaches a certain size, it stops growing. Its shape is like that of all animals of its kind. For example, all eels have a slithery snakelike form, all spiders have eight legs, all bats have two wings.In order to live, animals must take in food. Some animals, such as deer, eat only plants. Other animals, such as tigers, eat meat. Still others, such as humans, eat both plant and animal matter. Animals digest, or break down, the food they eat to provide the energy to power life-sustaining body functions. They take in oxygen to help them obtain energy from the food. And they excrete, or release, carbon dioxide and other body wastes.Most animals can move. They swim, walk, crawl, or fly from one place to another, sometimes covering great distances. This ability allows them to find food, escape enemies, or search for mates. Even animals that remain in one place are able to move parts of their bodies. For example, a sea anemone swims only during an early stage in its life. Then it settles onto the seafloor, where it spends the rest of its life anchored to a rock or shell. It waves its tentacles back and forth in the water, capturing prey and dragging it into its mouth.PlantsKingdom Plantae contains more than 350,000 known species. These include ferns, mosses, liverworts, trees, grasses, and flowering herbs. Plants are multicelled organisms, as are animals, with cells organized into tissues. But a plant cell has an additional structure. Outside the cell membrane is a thick cell wall composed of a material called cellulose.Plant tissues also are different from animal tissues. Plants do not have muscle, nerve, skin, or blood tissues. But their specialized tissues accomplish many of the same kinds of tasks--transport tissue carries water, minerals, and other materials to various parts of the plant; growth tissue produces new parts as well as increases in the plant's size; and protective tissue covers parts of the plant to keep the insides safe from injury.The life cycle of a plant, which includes an embryo stage, ranges from days to centuries. Unlike most other living things, plants grow throughout their lives. They do not stop when they reach a certain size. Some sequoias and bristlecone pines are thousands of years old and still growing! Most growth takes place at the ends of roots and stems, producing new branches, leaves, roots, and other parts.Plants occupy a unique role in nature. Most plants make their own food in a process known as photosynthesis. A plant's food-making factories are cellular structures called chloroplasts, which are usually located in the cells of a plant's leaves. Inside the chloroplasts is the substance chlorophyll, a green light-absorbing pigment. Through the action of chlorophyll, carbon dioxide, water, and light energy are combined to produce a plant's food. Some of the food is used immediately by the plant for activities such as growth. Any excess is transformed into other food products or stored for future use. Plants are essential to the survival of most other living things because they provide food for animals and other organisms.Even though plants do not move about, they are capable of some movement. A geranium turns its leaves toward light. Sweet peas wrap tendrils around fence posts. The leaves of a mimosa tree fold up when touched.FungiScientists have identified more than 100,000 species of fungi. Members of the Kingdom Fungi, which are mostly land dwellers, include mushrooms, molds, rusts, mildews, truffles, puffballs, and yeasts. A yeast consists of only one cell. But the majority of fungi are multicelled.Each fungal cell has a cell wall, which gets its stiffness from cellulose or a material called chitin or both. The body, or mycelium, of a fungus consists of a mass of slender branching tubes called hyphae. In some species, a hypha consists of one long cell. In other species, each hypha is divided into many cells by walls called septa (plural of septum). Each cell contains at least one nucleus, but in some species, cells have two or more nuclei.Unlike animals and plants, fungi do not have an embryo stage. A fungus begins life as a spore that develops into a hypha. The hypha, which grows at its tip, branches and eventually forms the mycelium. Hyphae grow mostly underground or within the object on which the fungus feeds. Thus, the mycelium of a fungus usually is invisible. The part of a mushroom or other fungus commonly seen by people is the reproductive structure. It is a dense, tangled mass of special hyphae.The fungal cell lacks chlorophyll, so a fungus cannot make its own food. Instead, fungi secrete powerful chemicals that break down food into molecules that can be absorbed. Some fungi feed on living organisms. For example, corn smut grows on the ears of corn. Other fungi feed on dead organisms or on products of organisms, including their wastes. For example, bread mold gets its nutrients from bread, which is made from plant grains. Still other fungi absorb nutrients from a variety of foods. Honey mushrooms usually feed on dead tree branches. But under certain conditions they will feed on living trees.A fungus spends its entire life in one spot. It does not move about, but it can react to changes in its environment. For example, when a hypha of one fungal species touches hyphae of another species, it will turn away and grow in a different direction.ProtistsKingdom Protista contains more than 100,000 known species, including most kinds of algae and one-celled organisms commonly called protozoans, meaning "first animals." These include paramecia (plural of paramecium), amoebas, and many other tiny organisms, most of which can be seen only when viewed through a microscope.Most protists consist of a single cell, with a cell membrane. Some also have a cell wall. Within its one-celled body, the organism carries out the same basic life processes that are carried out by multicelled organisms. The cell has many specialized structures, including at least one nucleus.There is great variety among these organisms that can be found wherever there is water or moisture. Some contain chloroplasts and can make their own food, while others must find food. Some protists live in one spot, but many have parts that help them move and gather food. A paramecium, for example, has many hairlike extensions called cilia. As the cilia beat back and forth, the paramecium is propelled through the water. A euglena swims using a long, whiplike flagellum (singular of flagella) that it swings back and forth as it moves.Many species of protists form colonies. The colonies have a definite shape. For example,?Volvox?cells join together in a ball-shaped colony, while a spirogyra colony is threadlike, consisting of cells joined end to end. These colonies may be large enough to be seen with the unaided eye.MoneransThe Kingdom Monera includes all the one-celled organisms known as bacteria. Scientists have identified more than 10,000 species of bacteria. They are very ancient organisms. In fact, they were probably among the first living organisms.Bacteria are extremely tiny organisms. It would take as many as 50,000 average-sized bacteria, placed end-to-end in a row, to equal a distance of 1 inch (2.5 centimeters). While bacteria are single-celled creatures, some kinds of cyanobacteria (also known as blue-green algae) form colonies. The colonies are clumps of cells inside a blob of slime or cells arranged end to end, in threads.A cell wall, usually surrounded by a layer of slime, encloses the cell. But monerans differ from all other living things in that their cells do not have nuclei or other specialized structures. Most bacteria do not contain chlorophyll or other chemicals that allow them to make their own food. They must obtain their food from other organisms or from nonliving matter. The only bacteria that contain chlorophyll and can carry out photosynthesis are the cyanobacteria. Cyanobacteria also can absorb food from their surroundings.Bacteria are simple creatures that live in some of the harshest environments. They can survive freezing temperatures for years. Some species thrive in hot springs and others in very hot acids. Most scientists now classify many of these extreme species as archaea, a branch of life separate from bacteria.Many kinds of bacteria make their home in the human body. Some are harmful, others are not. They cover the skin, pack the digestive tract, and live in the mouth. For example, more than one hundred species of bacteria may live in your mouth, with their total number greater than the number of people who have ever lived!Most bacteria cannot move from place to place on their own. A few species move by twisting the entire cell. Species that live in water or other liquids often have flagella. The number of flagella depends on the species.Jenny TesarAuthor ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download