Levels of Organization of Living Things
Overview
Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 1.15), which contains the instructions for the structure and functioning of all living organisms.
Introduction
Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by one or more chemical bonds. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 1.15), which contains the instructions for the structure and functioning of all living organisms.
Watch this video that animates the three-dimensional structure of the DNA molecule shown in Figure 1.15.
Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. (This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack the reproductive mechanism of a living cell; only then can they obtain the materials they need to reproduce.) Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic. Prokaryotes are single-celled or colonial organisms that do not have membrane-bound nuclei; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.
In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.
All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many pine trees. All of these pine trees represent the population of pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the sum of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure 1.16), the biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.
Exercises
1. What is the level of organization represented by each of the following?
a) the heart or the brain
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b) all the organisms living in the desert
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2. Which of the following statements is false?
a) Tissues exist within organs which exist within organ systems.
b) Communities exist within populations which exist within ecosystems.
c) Organelles exist within cells which exist within tissues.
d) Communities exist within ecosystems which exist in the biosphere.
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The Human Project of Categorizing Life
We humans are compelled to organize and categorize the world around us. We keep food in the kitchen and socks in the dresser drawer, and would be very disturbed to find a can of beans nestled among the dress socks. This compunction to organize and categorize also applies to our understanding of what constitutes life on earth.
Thousands of years ago when people lived as hunter-gatherers, herders or farmers, they divided their world into plants, such as the trees and grasses, and animals such as the sheep, dogs and fish. The plants were green, grew with rain and sunshine, and provided food for the animals. The animals moved, ate plants or other animals and were rarely green. In this binary world, only the kingdom of plants and the kingdom of animals existed.
Ancient categories of plants and animals worked well except for mushrooms. Although they appear to grow like plants, they are not green like plants and are not dependent on sunlight. Therefore a new kingdom needed to be established: the fungi. Subsequent studies have shown that fungi have chitin in the cell call rather than the cellulose of plants. Later, it was determined that yeast also belong in the fingi kingdom. Yeast has long served civilization in making bread and femented beverages, both of which are present in most of the world’s societies. The kingdom of fungi includes many different varieties with different llife cycles and mophologies. They are present throughout the world: in water on land and even as pathogens such as the fungus genus that causes athlete’s foot, Trycophyton.
The invention of the microscope changed the way that people looked at the natural world. Zaccharias Jansen, a Dutch spectacle maker, is credited with inventing the first compound microscope in the 1590’s. It was picked up by another Dutchman, Anton van Leeuwenhoek, who used it to discover the world of single celled organisms seventy years later. This discovery was met with derision and rejection by the Royal Society of National Science for several years, but van Leeuwenhoek persisted and his observations were eventually accepted and, even celebrated by the end of his life.
Among van Leeuwenhoek’s contributions of over 500 papers to the Royal Society, were studies of the single-celled organisms living in fresh water ponds and streams including the Euglena.Euglena can both move like an animal and photosynthesize like a plant. Clearly, they do not belong in one of the three kingdoms already in existence; a new kingdom was needed. Originally called the protozoa (“early animals”), the Kingdom Protista, include a large variety of single-celled aquatic organisms. Some of them are photosynthetic, such as the Euglena or Stentor; others are heterotrophs, eating food to provide them with energy. The Kingdom Protista also include most algae, or seaweed. Even the gigantic kelp of the Pacific Northwest, which grows to over ten feet tall, is made of individual protists.
During the eighteenth and nineteenth centuries, many people used microscopes to study the cells of different organisms. They found that all the plants, animals, fungi and protists had several cell structures in common. For example, all the organisms had a large nucleus that was easily stained. There were also other structures within the cells, some circular and some ovoid. Although early biologists weren’t able to determine their functions, they all agreed that all their known cells had many internal structures in common. From the perspective of the twenty first century, we know that the animals, plants, fungi and protists were all eukaryotes. These are organisms whose cells contain many internal structures called organelles. Eukaryotes divide the work of being a cell among these organelles, which increases efficiency. While eukaryotes can be single-celled, such as the protists and some fungi, only eukaryotes can be multicellular such as trees and elephants and spiders.
Although van Leeuwenhoek was credited with seeing and drawing the first large bacteria, this discovery was largely ignored until around 1850, when the idea of invisible factors could be disease-causing agents started to become accepted. This germ-theory idea made sense through the work of many people, including Dr. Lister (of “Listerine” antiseptic) who advocated surgeons wash their hands between surgeries and use clean instruments, and Dr. John Snow who thought that a “miasma” or cloud was a cause of the cholera epidemic in London in the 1850’s. By this time, microscope technology had advanced so that organisms could be magnified one thousand times or more. This made microscopes powerful enough for people to see the small organisms often associated with disease. Furthermore, advances in staining cells allowed for these small organisms to be seen and differentiated from each other. This was the beginning of microbiology, the study of micro-organisms.
These micro-organisms or bacteria as they were called, had a very different internal structure than the cells previously studied. Bacteria had a much simpler internal structure with no nucleus or other organelles. A new category of organism was needed. The Domain Bacteria (later Eubacteria) was introduced. These single-celled organisms had the plasma membrane, DNA, ribosomes and cytosol of all other cells, but the DNA was circular and condensed in the center of the cell, not sequestered in the nucleus as in all the other cells. Furthermore, there were none of the internal organelles present in the other cells. Clearly this was a different way of organizing life. After some discussion, the scientific community established the two major organizations of living cells: the prokaryotes or “early cells” which were the bacteria, and the eukaryotes or “true nuclei” which were the kingdoms of the protists, fungi, plants and animals, with their more complex cellular organization of organelles including nuclei containing the DNA.
The division between eukaryotes and prokaryotes existed for more than fifty years, until scientists began to analyze the prokaryotes found in hot springs and salt lakes. At first, they were considered a sub-category of bacteria, called Archaeobacteria, after the Greek word for “ancient ones.” Like bacteria, these organisms did not have membrane-bound organelles. However, they often had linear DNA similar to the linear chromosomal DNA of eukaryotes, so were given their own domain: Archaea. It was originally thought that Archaea only lived under extreme conditions, such as deep in the ocean or in hot springs of places like Yellowstone Park. Today, Archaea have been found throughout the world, including in the guts of many animals. They are thought to be important contributors to both the carbon cycle and the nitrogen cycle.