{"id":140,"date":"2017-08-16T09:22:53","date_gmt":"2017-08-16T09:22:53","guid":{"rendered":"http:\/\/americanboard.org\/Subjects\/biology\/?page_id=140"},"modified":"2017-08-28T07:10:37","modified_gmt":"2017-08-28T07:10:37","slug":"discovering-cells","status":"publish","type":"page","link":"https:\/\/americanboard.org\/Subjects\/biology\/discovering-cells\/","title":{"rendered":"Discovering Cells"},"content":{"rendered":"
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Workshop Index<\/a>\u00a0Next Lesson \u27a1<\/a><\/p>\n<\/div>\n

<\/p>\n

Discovering Cells<\/h1>\n

Objective<\/h4>\n

In this lesson, you will review cells.<\/p>\n

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Little Animals<\/h3>\n

Antonie van Leeuwenhoek made his living selling cloth, but this 17th-century Dutchman could not resist his interest in making lenses. His obsession with making a perfect lens drastically reduced the income he derived from his neglected business, yet he labored on. In 1675, he produced a single lens that, when mounted in a metal frame, could magnify an object up to 270 times. With his apparatus, an early microscope, Leeuwenhoek was able to see numerous protozoans<\/abbr> in a drop of water. He called these microscopic creatures animalcules, or little animals. His fame grew as he proceeded to identify various bacteria and, most notably, spermatozoa, which he identified from humans and other animals.<\/p>\n

\"Image<\/center><\/p>\n

Image of a Spermatozoan<\/p>\n

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Question<\/h4>\n

Antonie von Leeuwenhoek was the first scientist to identify<\/p>\n

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  1. cell walls<\/li>\n
  2. bacteria<\/li>\n
  3. viruses<\/li>\n
  4. plant cells<\/li>\n<\/ol>\n

    Reveal Answer<\/a><\/p>\n

    B is the correct choice. Choices A and D are incorrect because they were identified by Robert Hooke in 1665. Choice C is incorrect. Martinus Beijerinck identified the first virus (tobacco mosaic) in 1898.<\/p>\n<\/section>\n

    Cell Theory<\/h3>\n

    Earlier, in 1665, Robert Hooke, an English physicist and mathematician, peered into his homemade microscope \u2013 one of the world\u2019s first \u2013 to examine the bark of a cork tree. Hooke saw the tiny boxes that make up cork tissue, which reminded him of monks\u2019 cells. So, Hooke called the minute structures cells.<\/p>\n

    \"Early<\/p>\n

    Though Hooke termed cells, it was not until 1838 that Matthias J. Schleiden, a German botanist, recognized the significance of cells and elucidated the beginnings of cell theory. In 1839, he and Theodore Schwann, a German scientist, stated that cells are \u201cthe elementary particles of organisms,\u201d both plant and animal, and that some organisms are unicellular while others are multicellular. Nine years later, another German botanist, Hugo von Mohl, coined the word protoplasm<\/abbr> to designate a cell\u2019s living material. Cell theory was further advanced by the discovery of the nucleus by Robert Brown in 1833 and the confirmation of its presence in both plant and animal cells.<\/p>\n

    For a decade, scientists attempted to track down the origin of cells. The then widely believed but fanciful \u201cfree cell hypothesis\u201d stipulated that cells arose mysteriously from a formless vaporous substance. Though cell division had been observed among unicellular organisms, it was not until 1855 that anyone presented a cogent idea of cell origin. In that year, Rudolf Virchow, a German biologist, stated cell theory unequivocally: \u201ccells come from cells.\u201d In other words, cells reproduce themselves.<\/p>\n

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    Question<\/h4>\n

    Cell theory states that<\/p>\n

      \n
    1. an embryo forms from both egg and sperm<\/li>\n
    2. all cells contain a membrane-enclosed nucleus<\/li>\n
    3. autotrophs feed by photosynthesis<\/li>\n
    4. cells reproduce to create more cells<\/li>\n<\/ol>\n

      Reveal Answer<\/a><\/p>\n

      The correct choice is D. The union of a sperm and egg to form an embryo is the domain of reproductive biology, so choice A is incorrect. Choice B is incorrect because all cells do not have a nucleus that is enclosed in a membrane (see prokaryotes). The processes and functions of cell metabolism are not part of cell theory; therefore choice C is not correct.<\/p>\n<\/section>\n

      Seeing Cells<\/h3>\n

      So, what is a cell? Biologists today define it as the smallest unit of life that makes up all living things. A single cell, such as a bacterium, may be a complete, independently functioning organism. In multicellular organisms, cells may be adapted to perform specific functions within tissue<\/abbr> and may not be capable of existing independently of that tissue.<\/p>\n

      There are two basic types of unicellular organisms: heterotrophs and autotrophs.<\/p>\n