5: The Cell
Life takes many forms, from giant redwood trees towering hundreds of feet in the air to the tiniest known microbes, which measure only a few billionths of a meter. Humans have long pondered life’s origins and debated the defining characteristics of life, but our understanding of these concepts has changed radically since the invention of the microscope. In the 17th century, observations of microscopic life led to the development of the cell theory: the idea that the fundamental unit of life is the cell, that all organisms contain at least one cell, and that cells only come from other cells.
Despite sharing certain characteristics, cells may vary significantly. The two main types of cells are prokaryotic cells (lacking a nucleus) and eukaryotic cells (containing a well-organized, membrane-bound nucleus). Each type of cell exhibits remarkable variety in structure, function, and metabolic activity. This chapter will focus on the historical discoveries that have shaped our current understanding of microbes, including their origins and their role in human disease. We will then explore the distinguishing structures found in prokaryotic and eukaryotic cells.
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- 5.1: Unique Characteristics of Eukaryotic Cells
- Eukaryotic cells are defined by the presence of a nucleus containing the DNA genome and bound by a nuclear membrane (or nuclear envelope) composed of two lipid bilayers that regulate transport of materials into and out of the nucleus through nuclear pores. Eukaryotic cell morphologies vary greatly and may be maintained by various structures, including the cytoskeleton, the cell membrane, and/or the cell wall. The nucleolus in the nucleus of eukaryotic cells is the site of ribosomal synthesis.
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- 5.2: Eukaryotic Cell Anatomy
- The cell is the basic unit of life. Based on the organization of their cellular structures, all living cells can be divided into two groups: prokaryotic and eukaryotic (also spelled procaryotic and eucaryotic). Animals, plants, fungi, protozoans, and algae all possess eukaryotic cell types. Only bacteria have prokaryotic cell types.
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- 5.3: The Cytoplasmic Membrane
- The cytoplasmic membrane (also called the plasma or cell membrane) of eukaryotic cells is a fluid phospholipid bilayer embedded with proteins and glycoproteins. It contains glycolipids as well as complex lipids called sterols. The cytoplasmic membrane is a semipermeable membrane that determines what goes in and out of the cell. Substances may cross the cytoplasmic membrane of eukaryotic cells by simple diffusion, osmosis, passive transport, active transport, endocytosis and exocytosis.
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- 5.6: Other Internal Membrane-Bound Organelles
- Because of their larger size, Eukaryotic cells require a variety of specialized internal membrane-bound organelles to carry out metabolism, provide energy, and transport chemicals throughout the cell. Eukaryotic cells contain a variety of internal membrane-bound organelles that are not a part of the endomembrane system. These include mitochondria, chloroplasts, lysosomes, peroxisomes, vacuoles, and vesicles. We will now look at the various membrane-bound organelles.
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- 5.7: Ribosomes
- Ribosomes are composed of rRNA and protein and consist of 2 subunits. In eukaryotic cells, the subunits have densities of 60S and 40S. The ribosomes are both attached to the endoplasmic reticulum and free in the cytoplasm. They serve as a workbench for protein synthesis, that is, they receive and translate genetic instructions for the formation of specific proteins or polypeptides.
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- 5.8: The Cytoskeleton
- The cytoskeleton is a network of microfilaments, intermediate filaments, and microtubules. The cytoskeleton has a variety functions including, giving shape to cells lacking a cell wall, allowing for cell movement, enabling movement of organelles within the cell, endocytosis, and cell division.
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- 5.9: Flagella and Cilia
- Flagella are long and few in number whereas cilia are short and numerous. Both flagella and cilia consist of 9 fused pairs of protein microtubules with side arms of the motor molecule dynein that originate from a centriole. These form a ring around an inner central pair of microtubules that arise from a plate near the cell surface. This complex of microtubules is surrounded by a sheath continuous with the cytoplasmic membrane. Flagella and cilia function in locomotion.
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- 5.E: The Eukaryotic Cell (Exercises)
- These are homework exercises to accompany Kaiser's "Microbiology" TextMap. Microbiology is the study of microorganisms, which are defined as any microscopic organism that comprises either a single cell (unicellular), cell clusters or no cell at all (acellular). This includes eukaryotes, such as fungi and protists, and prokaryotes. Viruses and prions, though not strictly classed as living organisms, are also studied.
Thumbnail: A 3D rendering of an animal cell cut in half. (CC -BY-SA 4.0; Zaldua I., Equisoain J.J., Zabalza A., Gonzalez E.M., Marzo A., Public University of Navarre).