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2: Cellular Level of Organization

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  • The concept of a cell started with microscopic observations of dead cork tissue by scientist Robert Hooke in 1665. Without realizing their function or importance, Hook coined the term “cell” based on the resemblance of the small subdivisions in the cork to the rooms that monks inhabited, called cells. About ten years later, Antonie van Leeuwenhoek became the first person to observe living and moving cells under a microscope. In the century that followed, the theory that cells represented the basic unit of life would develop. These tiny fluid-filled sacs house components responsible for the thousands of biochemical reactions necessary for an organism to grow and survive. In this chapter, you will learn about the major components and functions of a generalized human cell, sometimes referred to as a prototypical cell, and discover some of the different types of cells found in the human body.

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    • 2.1: Introduction to the Cellular Level of Organization
      The human body consists of trillions of cells, all derived from a single fertilized egg cell.  Through the process of mitosis and differentiation, this single cell becomes many specialized cells that all work together to carry out the functions of human life.
    • 2.2: The Cell Membrane
      All living cells in multicellular organisms have a surrounding cell membrane. This cell membrane provides a protective barrier around the cell and regulates which materials can pass in or out.
    • 2.3: The Cytoplasm and Cellular Organelles
      The intracellular components of a cell, that is the materials contained inside the cell membrane, can vary drastically from cell to cell depending on their function. A prototypical cell can be studied to learn about all of the internal components, and their functions, that may be found in a human body cell.
    • 2.4: The Nucleus
      The nucleus is the largest and most prominent of a cell’s organelles. The nucleus is generally considered the control center of the cell because it stores all of the genetic instructions.
    • 2.5: Cell Growth and Division
      Human body cells must constantly replace themselves when they become worn out (e.g. red blood cells), worn off (e.g. skin cells), or damaged. However, this process of replacement must be closely monitored, to avoid the replacement of too many cells too fast (which leads to cancer). The cell cycle is the sequence of events that a cell goes through in its lifetime, that ultimately leads to cell division.
    • 2.6: Cellular Differentiation
      How does a complex organism such as a human develop from a single cell—a fertilized egg—into the vast array of cell types such as nerve cells, muscle cells, and epithelial cells that characterize the adult? Throughout development and adulthood, the process of cellular differentiation leads cells to assume their final morphology and physiology. Differentiation is the process by which unspecialized cells become specialized to carry out distinct functions.