Before it is possible to discuss a living thing, it is necessary to name it. In other words, every organism needs an identity. Thousands of years ago, for example, a Chinese emperor studied, tasted, and named hundreds of herbs used to make medicine. These names made communication about the medicinal plants possible.
Later, a Greek philosopher organized animals into two large groups, those with and without red blood. Today, those groups represent the vertebrates and invertebrates.
As technology improved, scientists gathered more information, which influenced the classification systems they devised. Today, scientists generally agree upon three broad domains of life. These domains are based on cell types, and they include the Archaea, or Archaebacteria, Bacteria, and Eukarya. The last group, the Eukarya, are divided into four kingdoms-Protista, Fungi, Plantae, and Animalia.
The greatest number of living organisms on Earth are also the smallest. Trillions and trillions of different kinds of bacteria populate the planet. These organisms are microbes, meaning they are too small to be seen without the help of a microscope. Most microbes are less than 4/1,000 inch (0.1 millimeter) across.
Although you cannot see microbes, you may be familiar with some of their effects. Some bacteria, for example, cause decay. Some bacteria and viruses cause disease. Others microbes are essential ingredients in making alcohol, bread, and cheese. Yeast is a microbe that causes bread dough to rise.
Microbiology is the study of microbes. Biotechnology is the use of microbes to make useful materials for human consumption.
Archaea, which were once called archaebacteria, are single-celled organisms that are thought to have been the first organisms to have lived on Earth. They thrive in extremely harsh conditions, such as hot springs or in waters with high salt content. Archaea are also found in oxygen-free and highly acidic environments, such as the digestive tracts of animals. Bacteria
Bacteria grow well in warm, moist places, but they also inhabit harsh environments, such as deserts, the arctic tundra, and ocean waters. They also live inside the digestive tracts of humans and other animals. Some bacteria are valuable decomposers. They break down the bodies of once-living things, adding nutrients to the soil that plants need to thrive, or grow. Decomposers also help prevent the accumulation of dead matter on Earth’s surface and in its soil and water.
Bacteria can cause food to spoil, a process that can be slowed by freezing, boiling, or drying food. Some bacteria cause disease, such as strep throat and tuberculosis. Bacterial diseases can be tr~ated with antibiotics, such as penicillin, an antibacterial substance produced by mold, a kind of fungus. Three basic shapes help identify bacteria. They are bacilli (rod-shaped), cocci (round-shaped), and spirilla (spiral-shaped).
Eukarya include all organisms with eukaryotic cells, or cells containing membrane-bound organelles, or cell structures. Organisms in Kingdom Protista, or the protists, are single-celled eukaryotes. Some have plant-like traits. Others, like Euglena, have traits of both plants and animals. Euglena contain chlorophyll and make their own food. They also swim and capture food.
Slime molds are also protists. When food is scarce, the cells of one kind of slime mold join to create colonies that look like blobs of petroleum jelly. These blobs migrate to reproduce elsewhere.
Viruses are microbes, but not cells. Many scientists hesitate to call them living things because they do not grow or reproduce. They have no cytoplasm, no nucleus, and no cell membrane. They do, however, have a core of genetic material surrounded by a protein shell. Viruses depend on living cells for their survival. Once they invade a living cell, they control the cell and use it to produce more viruses. They are responsible for a host of diseases including flu, the common cold, chicken pox, mumps, measles, and AIDS. AIDS is believed to be caused by the HIV (human immunodeficiency virus). This virus harms the body’s natural defenses so that other viruses or bacteria can strike. Unable to fend off disease-causing organisms that take advantage of an already weakened system, the body weakens further, and the victim dies.
All organisms begin life as a single cell, but some do not stay single-celled. Cells in multicellular organisms specialize. Their shapes tell you what special functions they perform.
Tissues are groups of specialized cells. Different kinds of tissues may form an organ, a part of the body that performs one or more functions. For example, the heart contains muscle tissue, blood tissue, and nerve tissue.
Groups of organs that work together to perform a life process are known as an organ system. For example, the tongue, teeth, stomach, and intestines are all part of the digestive system. They all work together to remove nutrients from food. Examples of simple multi-celled organisms include fungi and simple plants, such as algae, mosses, and ferns.
Mold, mildew, yeast, and mushrooms are all fungi. With the exception of yeast, fungi are multicellular organisms. They live in environments that are continuously moist or wet.
Like plants, fungi are rooted in one place. Their cytoplasm is at least partially enclosed by a rigid cell wall. However, fungi cannot make their own food because they do not contain chlorophyll. They “feed” by releasing a chemical that digests the organic matter around them. Fungi feed on plants and animals that are dead or alive. When fungi attach themselves to living organisms, they can cause a variety of diseases, one of which is athlete’s foot.
Some fungi reproduce by growing a new cell, called a bud. Others reproduce by releasing tiny reproductive cells, called spores.
Algae are the simplest non-flowering plants (a category that also includes mosses and ferns). Although all algae contain chlorophyll, not all algae are green. Some are red or brown. Algae cannot survive out of water. Because algae live in water and absorb their food from the water, they do not need true roots. The water supports the algae, so they do not need true stems.
Mosses live in wet places on land. Like algae, mosses do not have a group of specialized cells to transport water or to carry food. Instead, mosses depend on diffusion to move water into their cells. Because diffusion is a slow process, mosses do not grow to be very large.
Mosses have two stages in their life cycle. The first stage produces both male and female cells. These cells meet to produce the second stage, a spore-bearing plant. Spores are tiny reproductive cells. Upon ripening, the capsules that contain spores open and scatter the spores. Spores may be scattered by the wind or by animals brushing up against the ripened capsules. If the spores land on fertile ground, they develop into new plants.
Some plants have true roots, stems, and leaves but no true seeds. Instead, these plants reproduce by forming spores on the undersides of their leaves. The most familiar plant in this category is the fern. Most ferns live in damp, shady places. Instead of stretching up to the Sun, fern stems grow horizontally underground.
Years ago seedless plants, such as ferns, were abundant. As they died, they were buried, and the earth pressed them. Today, we dig up this decayed ancient plant matter and burn it as coal.