We lay the chemical foundation for our study of life in the next three chapters, after which we will turn to cells and the processes by which they live, reproduce, age, and die.
Some organisms are unicellular, consisting of a single cell that carries out all the functions of life (Figure 1.1A–C). Others are multicellular, made up of many cells that are specialized for different functions (Figure 1.1D–G). Viruses are acellular, although they depend on cellular organisms. The discovery of cells was made possible by the invention of the microscope in the 1590s by the Dutch spectacle makers Hans and Zaccharias Janssen (father and son). In the mid- to late 1600s, Antony van Leeuwenhoek of Holland and Robert Hooke of England both made improvements on the Janssens’ technology and used it to study living organisms. Van Leeuwenhoek discovered that drops of pond water teemed with single-celled organisms, and he made many other discoveries as he progressively improved his microscopes over a long lifetime of research.
Hooke put pieces of plants under his microscope and observed that they were made up of repeated units he called cells (Figure 1.2). In 1676, Hooke wrote that van Leeuwenhoek had observed “a vast number of small animals in his Excrements which were most abounding when he was troubled with a Loosenesse and very few or none when he was well.” This simple observation represents the discovery of bacteria—and makes one wonder why scientists do some of the things they do. More than a hundred years passed before studies of cells advanced significantly. As they were dining together one evening in 1838, Matthias Schleiden, a German biologist, and Theodor Schwann, from Belgium, discussed their work on plant and animal tissues, respectively. They were struck by the similarities in their observations and came to the conclusion that the basic structural elements of plants and animals were essentially the same. They formulated their conclusion as the cell theory, which states that:
• Cells are the basic structural and physiological units of all living organisms.• Cells are both distinct entities and building blocks of more complex organisms.
But Schleiden and Schwann also believed (wrongly) that cells emerged by the self-assembly of nonliving materials, much as crystals form in a solution of salt. This conclusion was in accordance with the prevailing view of the day, which was that life can arise from non-life by spontaneous generation—mice from dirty clothes, maggots from dead meat, or insects from
pond water. The debate continued until 1859, when the French Academy of Sciences sponsored a contest for the best experiment to prove or disprove spontaneous generation. The prize was won by the great French scientist Louis Pasteur, who demonstrated that sterile broth directly exposed to the dirt and dust in air developed a culture of microorganisms, but a similar container of broth not directly exposed to air remained sterile (see Figure 4.7). Pasteur’s experiment did not prove that it was microorganisms in the air that caused the broth to become infected, but it did uphold the conclusion that life must be present in order for new life to be generated.
Today scientists accept the fact that all cells come from preexisting cells and that the functional properties of organisms derive from the properties of their cells. Since cells of all kinds share both essential mechanisms and a common ancestry that goes back billions of years, modern cell theory has additional elements:• All cells come from preexisting cells.• All cells are similar in chemical composition.• Most of the chemical reactions of life occur in aqueous solution within cells.• Complete sets of genetic information are replicated and passed on during cell division.• Viruses lack cellular structure but remain dependent on cellular organisms.
At the same time Schleiden and Schwann were building the foundation for the cell theory, Charles Darwin was beginning to understand how organisms undergo evolutionary change.
Some organisms are unicellular, consisting of a single cell that carries out all the functions of life (Figure 1.1A–C). Others are multicellular, made up of many cells that are specialized for different functions (Figure 1.1D–G). Viruses are acellular, although they depend on cellular organisms. The discovery of cells was made possible by the invention of the microscope in the 1590s by the Dutch spectacle makers Hans and Zaccharias Janssen (father and son). In the mid- to late 1600s, Antony van Leeuwenhoek of Holland and Robert Hooke of England both made improvements on the Janssens’ technology and used it to study living organisms. Van Leeuwenhoek discovered that drops of pond water teemed with single-celled organisms, and he made many other discoveries as he progressively improved his microscopes over a long lifetime of research.
Hooke put pieces of plants under his microscope and observed that they were made up of repeated units he called cells (Figure 1.2). In 1676, Hooke wrote that van Leeuwenhoek had observed “a vast number of small animals in his Excrements which were most abounding when he was troubled with a Loosenesse and very few or none when he was well.” This simple observation represents the discovery of bacteria—and makes one wonder why scientists do some of the things they do. More than a hundred years passed before studies of cells advanced significantly. As they were dining together one evening in 1838, Matthias Schleiden, a German biologist, and Theodor Schwann, from Belgium, discussed their work on plant and animal tissues, respectively. They were struck by the similarities in their observations and came to the conclusion that the basic structural elements of plants and animals were essentially the same. They formulated their conclusion as the cell theory, which states that:
• Cells are the basic structural and physiological units of all living organisms.• Cells are both distinct entities and building blocks of more complex organisms.
But Schleiden and Schwann also believed (wrongly) that cells emerged by the self-assembly of nonliving materials, much as crystals form in a solution of salt. This conclusion was in accordance with the prevailing view of the day, which was that life can arise from non-life by spontaneous generation—mice from dirty clothes, maggots from dead meat, or insects from
pond water. The debate continued until 1859, when the French Academy of Sciences sponsored a contest for the best experiment to prove or disprove spontaneous generation. The prize was won by the great French scientist Louis Pasteur, who demonstrated that sterile broth directly exposed to the dirt and dust in air developed a culture of microorganisms, but a similar container of broth not directly exposed to air remained sterile (see Figure 4.7). Pasteur’s experiment did not prove that it was microorganisms in the air that caused the broth to become infected, but it did uphold the conclusion that life must be present in order for new life to be generated.
Today scientists accept the fact that all cells come from preexisting cells and that the functional properties of organisms derive from the properties of their cells. Since cells of all kinds share both essential mechanisms and a common ancestry that goes back billions of years, modern cell theory has additional elements:• All cells come from preexisting cells.• All cells are similar in chemical composition.• Most of the chemical reactions of life occur in aqueous solution within cells.• Complete sets of genetic information are replicated and passed on during cell division.• Viruses lack cellular structure but remain dependent on cellular organisms.
At the same time Schleiden and Schwann were building the foundation for the cell theory, Charles Darwin was beginning to understand how organisms undergo evolutionary change.
Ref: Life The Science of Biology NINTH EDITION
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