In Essential Biology, we will probe life all the way down to the submicroscopic scale of molecules such as DNA, the chemical responsible for inheritance. At the other extreme of biological size and complexity, our exploration will take us up to the global scale of the entire biosphere, which consists of all the environments on Earth that support life—including soil; oceans, lakes, and other bodies of water; and the inner atmosphere. To illustrate, let’s start with the biosphere and work our way down to smaller and smaller levels of biological organization. Figure 1.3 begins with a partial view of the biosphere and then zooms to a scene of an African savanna. The savanna is an example of an ecosystem. An ecosystem consists of all organisms living in a particular area, as well as the nonliving, physical components of the environment that affect the organisms, such as water, air, soil, and sunlight. All the organisms in the savanna (zebras, insects, grasses, bacteria, and so on) are collectively called a community. Within communities are various populations, groups of interacting individuals of one species, such as a herd of zebras. Below population in the hierarchy is the organism, an individual living thing, such as a zebra.
Life’s hierarchy continues to unfold within an individual organism. The zebra’s body consists of several organ systems, such as the digestive and circulatory systems. Each organ system consists of organs, such as the heart and blood vessels of the circulatory system. As we continue downward through life’s hierarchy, each organ is made up of several different tissues, each of which consists of a group of similar cells performing a specific function. The cell is the basic unit of life. Finally, we reach the chemical level in the hierarchy. Each cell consists of an enormous number of chemicals that function together to give the cell the properties we recognize as life. At the bottom of Figure 1.3 is a computer graphic of DNA, the chemical of inheritance and the substance of genes. DNA is an example of a molecule, a cluster of even smaller chemical units called atoms. In the tiny segment of the DNA molecule shown, each sphere represents an atom. From the interactions within the biosphere to the molecular machinery within cells, biologists are investigating life at its many levels. Let’s take a closer look here at just two biological levels near opposite ends of the size scale: ecosystems and cells. Life does not exist in a vacuum. Each organism interacts continuously with its environment, which includes other organisms as well as nonliving factors. The roots of a tree, for example, absorb water and minerals from the soil. Leaves take in carbon dioxide gas from the air. Chlorophyll, the green pigment of the leaves, absorbs sunlight, which drives the plant’s production of sugar from carbon dioxide and water. This food production is called photosynthesis. The tree also releases oxygen to the air, and its roots help form soil by breaking up rocks. Both organism and environment are affected by the interactions between them. The tree also interacts with other living things, including microorganisms (microscopic organisms) in the soil that are associated with the plant’s roots and animals that eat its leaves and fruit. We are, of course, among those animals. The dynamics of any ecosystem depend on two main processes (Figure 1.4) . The first major process is the cycling of nutrients. For example, minerals that plants take up from the soil will eventually be recycled to the soil by microorganisms that decompose leaf litter and other organic refuse. The second major process in an ecosystem is the flow of energy from sunlight to producers and then on to consumers and decomposers. Producers are photosynthetic organisms, such as plants; consumers are the organisms, such as animals, that feed on plants, either directly (by eating plants) or indirectly (by eating animals that eat plants); decomposers, such as fungi, recycle the remains of deceased organisms, changing complex dead material into simple mineral nutrients. Thus, energy flows through an ecosystem (entering as sunlight and exiting as heat), whereas nutrients are recycled.
The biosphere is enriched by a great variety of ecosystems. A tropical rain forest in South America is an ecosystem. Very different from tropical forests are the ecosystems of deserts, such as those of the southwestern United States. A coral reef, such as the Great Barrier Reef off the eastern coast of Australia, is an ecosystem, and so is any small pond that may exist on your campus or in your city. Even a woodland patch in New York’s Central Park qualifies as an ecosystem, small and artificial as it is by forest standards and disrupted as it is by human visitors. The fact is, humans are organisms that now have some presence, often disruptive, in all ecosystems. And the collective clout of 6 billion humans and their machines has an impact on the entire biosphere. For example, our fuel-burning, forest-chopping actions are changing the atmosphere and the planet’s climate in ways that we do not yet fully understand, though we already know that this global vandalism jeopardizes the diversity of life on Earth. Let’s downsize now from ecosystems to cells. The cell has a special place in the hierarchy of biological organization: It is the lowest level of structure that can perform all activities required for life, including reproduction. All organisms are composed of cells. They occur singly as a great variety of unicellular organisms, mostly microscopic. And cells are also the subunits that make up the tissues and organs of plants, animals, and other multicellular organisms. In either case, the cell is the organism’s basic unit of structure and function. The ability of cells to divide to form new cells is the basis for all reproduction and for the growth and repair of multicellular organisms, including humans. We can distinguish two major kinds of cells: prokaryotic and eukaryotic (Figure 1.5) . The prokaryotic cell is much simpler and usually much smaller than the eukaryotic cell. The cells of bacteria are prokaryotic. Most other forms of life, including plants, animals, and fungi, are composed of eukaryotic cells. In contrast to the prokaryotic cell, the eukaryotic cell is subdivided by internal membranes into many different functional compartments, or organelles. For example, the nucleus, the largest organelle in most eukaryotic cells, houses DNA, the heritable material that directs the cell’s many activities. Prokaryotic cells also have DNA, but it is not packaged within a nucleus.
Though very different in structural complexity, prokaryotic and eukaryotic cells have much in common at the molecular level. Most importantly, all cells use DNA as the chemical material of genes, the discrete units of hereditary information. Of course, bacteria and humans inherit different genes, but that information is encoded in a chemical language common to all organisms. In fact, the language of life has an alphabet of just four letters. The chemical names of DNA’s four molecular building blocks are abbreviated as A, G, C, and T (Figure 1.6) .
An average-sized gene may be hundreds or thousands of chemical “letters” long. That gene’s meaning to the cell is written in its specific sequence of these letters, just as the message of a sentence is encoded in its arrangement of letters selected from the 26 letters of the English alphabet. One gene may be translated as “Build a blue pigment in a bacterial cell.” Another gene may mean “Make human insulin in this cell.” Insulin is a chemical that helps regulate your body’s use of sugar as a fuel. People who have certain forms of the disease diabetes produce an inadequate supply of insulin. To overcome this shortage, diabetics can inject themselves with insulin produced by genetically engineered bacteria. These bacteria produce insulin because they contain a gene for insulin production transplanted from a human cell . This example of genetic engineering was one of the earliest successes of biotechnology, a field that has transformed the pharmaceutical industry and extended millions of lives (Figure 1.7) . And it is only possible because biological information is written in the universal chemical language of DNA.
The entire “book” of genetic instructions that an organism inherits is called its genome. The nucleus of each human cell packs a genome that is about 3 billion chemical letters long. In recent years, scientists have tabulated virtually the entire sequence of these letters, and the press and world leaders have acclaimed this international achievement as the greatest scientific triumph ever. But unlike past cultural zeniths, such as the landing of Apollo astronauts on the moon, the sequencing of the human genome is more a commencement than a climax. As the quest continues, biologists will learn the functions of thousands of genes and how their activities are coordinated in the development of an organism. Additionally, the genomes of other organisms (such as E. coli bacteria, fruit flies, and dogs) have been sequenced, allowing scientists to compare the genomes of different species. This emerging field of genomics—a branch of biology that studies whole genomes—is a striking example of human curiosity about life at its many levels. |
What is the lowest level of organization that can perform all activities required for life an organisms basic unit of structure and function?
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