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Unit 3: The Cellular Basis of Life

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    • 3.1: Animal Cells
      The idealized animal cell contains many structures.
    • 3.2: Cell Membranes
      One universal feature of all cells is an outer limiting membrane called the plasma membrane. In addition, all eukaryotic cells contain elaborate systems of internal membranes which set up various membrane-enclosed compartments within the cell. Cell membranes are built from lipids and proteins.
    • 3.3: The Nucleus
      The nucleus is the hallmark of eukaryotic cells; the very term eukaryotic means having a "true nucleus".
    • 3.4: Ribosomes
      Ribosomes are the protein-synthesizing machines of the cell. They translate the information encoded in messenger RNA (mRNA) into a polypeptide.
    • 3.5: Endoplasmic Reticulum
      The endoplasmic reticulum (ER) is a system of membrane-enclosed sacs and tubules in the cell. Their lumens are probably all interconnected, and their membranes are continuous with the outer membrane of the nuclear envelope. All the materials within the system are separated from the cytosol by a membrane.
    • 3.6: Golgi Apparatus
      The Golgi apparatus is a cell structure mainly devoted to processing the proteins synthesized in the endoplasmic reticulum (ER). Some of these will eventually end up as integral membrane proteins embedded in the plasma membrane. Other proteins moving through the Golgi will end up in lysosomes or be secreted by exocytosis (e.g., digestive enzymes).
    • 3.7: Centrosomes and Centrioles
      Centrioles are built from a cylindrical array of 9 microtubules, each of which has attached to it 2 partial microtubules. Centrioles are a key feature of eukaryotic cells and presumably arose with the first eukaryotes.
    • 3.8: Lysosomes and Peroxisomes
      Lysosomes are roughly spherical bodies enclosed by a single membrane. They are manufactured by the Golgi apparatus  and contain over 50 different kinds of hydrolytic enzymes including proteases, lipases, nucleases, and polysaccharidases. The pH within the lysosome is about pH 5, substantially less than that of the cytosol (~pH 7.2). All the enzymes in the lysosome work best at an acid pH.
    • 3.9: Protein Kinesis
      All proteins are synthesized by ribosomes using the information encoded in molecules of messenger RNA (mRNA). This process is called translation and is described in Gene Translation: RNA -> Protein. Our task here is to explore the ways that these proteins are delivered to their proper destinations.
    • 3.10: The Proteasome
      Lysosomes and proteasomes are two major intracellular devices in which damaged or unneeded proteins are broken down. Protein degradation is as essential to the cell as protein synthesis. For example, to supply amino acids for fresh protein synthesis, to remove excess enzymes, and to remove transcription factors that are no longer needed.
    • 3.11: The Cytoskeleton
      The cytoskeleton is made up of three kinds of protein filaments: Actin filaments (also called microfilaments), Intermediate filaments, and Microtubules.  Cells contain elaborate arrays of protein fibers that serve such functions as establishing cell shape, providing mechanical strength, and locomotion. These fibers participate in chromosome separation in mitosis and meiosis and intracellular transport of organelles.
    • 3.12: Cilia
      These whiplike appendages extend from the surface of many types of eukaryotic cells. If there are many of them, they are called cilia. If only one, or a few, they are flagella. Flagella also tend to be longer than cilia but are otherwise similar in construction.
    • 3.13: Animal Tissues
      Cells contain elaborate arrays of protein fibers that serve such functions as establishing cell shape, providing mechanical strength, and locomotion. These fibers participate in chromosome separation in mitosis and meiosis and intracellular transport of organelles.
    • 3.14: Adipose Tissue
      Two kinds of adipose tissue are found in mammals: white adipose tissue (WAT) and brown adipose tissue (BAT). White adipose tissue is the most common and is the fat that so many of us complain of acquiring. Brown adipose tissue is present in small mammals (e.g., mice) and in newborn humans. Most of it disappears in adult humans.The cells in both types of fat are called adipocytes although they differ in origin, structure, and function in the two types of tissue.
    • 3.15: Junctions between Cells
      In many animal tissues (e.g., connective tissue), each cell is separated from the next by an extracellular coating or matrix. However, in some tissues (e.g., epithelia), the plasma membranes of adjacent cells are pressed together. Four kinds of junctions occur in vertebrates: Tight junctions Adherens junctions Gap junctions Desmosomes In many plant tissues, it turns out that the plasma membrane of each cell is continuous with that of the adjacent cells.
    • 3.16: Plant Cells
      Plant cells are eukaryotic and have many of the structures found in animal cells.  Plant cells differ from animal cells as they lack centioles and intermediate filament; they also do not have plastids and a cell wall and large vacuoles.
    • 3.17: Chloroplasts
      The chloroplast is made up of 3 types of membrane: A smooth outer membrane which is freely permeable to molecules. A smooth inner membrane which contains many transporters: integral membrane proteins that regulate the passage in an out of the chloroplast of small molecules like sugars proteins synthesized in the cytoplasm of the cell but used within the chloroplast A system of thylakoid membranes
    • 3.18: Chlorophylls and Carotenoids
      Two types of chlorophyll are found in plants and the green algae: chlorophyll a and chlorophyll b.
    • 3.19: Plant Tissues
      A mature vascular plant (any plant other than mosses and liverworts), contains several types of differentiated cells. These are grouped together in tissues. Some tissues contain only one type of cell. Others consist of several cells.
    • 3.20: Apoptosis
      Apoptosis is a process of programmed cell death that occurs in multicellular organisms. There are two ways in which cells die: They are killed by injurious agents or they are induced to commit suicide.
    • 3.21: Collagens
      Collagens are insoluble, extracellular glycoproteins found in all animals. They are the most abundant proteins in the human body and are essential structural components of all connective tissues such as cartilage, bone, tendons, ligaments, fascia, skin. Gelatin is solubilized collagen. 29 types of collagens have been found in humans.
    • 3.22: Chromatophores
      Chromatophores are irregularly shaped, pigment-containing cells. If the pigment is melanin, they are called melanophores. Chromatophores are common in crustaceans, cephalopod mollusks, lizards and amphibians, and some fishes.
    • 3.23: Diffusion, Active Transport and Membrane Channels
      All cells acquire the molecules and ions they need from their surrounding extracellular fluid (ECF). There is an unceasing traffic of molecules and ions in and out of the cell through its plasma membrane (Examples: glucose, sodium, and calcium ions). In eukaryotic cells, there is also transport in and out of membrane-bounded intracellular compartments such as the nucleus, endoplasmic reticulum, and mitochondria.
    • 3.24: Endocytosis
      In endocytosis, the cell engulfs some of its extracellular fluid (ECF) including material dissolved or suspended in it. A portion of the plasma membrane is invaginated, coated with molecules of the protein clathrin, and pinched off forming a membrane-bounded vesicle called an endosome.
    • 3.25: Exocytosis
      Exocytosis is the reverse of endocytosis and that is just as well. In 30 minutes an active cell like a macrophage can endocytose an amount of plasma membrane equal to its complete plasma membrane. So the cell must have a mechanism to restore the normal amount of plasma membrane. Exocytosis is that mechanism.

    Thumbnail: A diagram of a typical prokaryotic cell. (Public Domain; LadyofHats).

    This page titled Unit 3: The Cellular Basis of Life is shared under a CC BY 3.0 license and was authored, remixed, and/or curated by John W. Kimball via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.

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