Inside the Cell 2


Inside the Cell 2

Inside the Cell 2

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  • Eukaryotic cells are constantly taking up fluid and molecules via endocytosis
  • Formation of endocytic vesicles
    • Plasma membrane buds inwards
    • Plasma membrane pinches off to form intracellular endocytic vesicle
    • Can fuse with lysosomes and be recycled to surface
  • Endosomes are membrane-bound organelles that sort and deliver internalised material from the cell surface
  • Exocytosis = counterpart of endocytosis
    • A form of active transport - molecules, e.g. proteins, are transported out of the cell or insert receptors into cell membrane
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  • Function
    • Internalisation of nutrients (e.g. for energy generation)
    • Regulation of cell surface protein expression
    • Uptake and digestion (lysosome) of extracellular debris
  • Can be exploited by pathogens (e.g. Polio and Hepatitis C viruses)
  • Cells require endocytosis because most molecules required for cell function are large, polar molecules that cannot pass through the hydrophobic portion of the plasma cell membrane by passive means
  • Types of endoctosis
    • Pinocytosis, receptor-mediated endocytosis (clathrin/caveolin-mediated endocytosis), phagocytosis
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Pinocytosis and Receptor-Mediated Endocytosis

  • Pinocytosis
    • Small particles are taken in by a cell by splitting off small vesicles from the cell surface
    • Non-specific transport of substances
    • The cell takes in surrounding fluids
    • Active transport (requires some energy)
  • Clathrin-mediated endocytosis
    • Receptor binds ligand
    • Captured by clathrin coated pit
    • Mature pit buds off forming a coated vesicle
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  • A cell engulfs a solid particle to form an internal compartment known as a phagosome
    • Pseudonium (cell protursion)
    • Phagosome (vesicle around a particle)
  • Eliminate debris or pathogens (phagocytes of the immune system) via lysosome
  • The professional phagocytes include white blood cells
    • neutrophils, monocytes, macrophages, mast cells and dendritic cells
      • have receptors to detect (marked) pathogens
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Recognising a Pathogen as a Target

  • Opsonisation of pathogens by antibody; or complement factors
  • Molecular structures: pathogen-associated molecular patterns (PAMPs)
    • Peptidoglycan, found in bacterial cell walls
    • Flagellin, a protein found in bacterial flagella
    • Lipopolysaccharide (LPS) from the outer membrane of gram-negative bacteria
    • Lipopeptides, molecules expressed by most bacteria
    • Nucleic acids such as viral DNA or RNA
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Endoplasmic Reticulum

  • A single compartment extending from the outer layer of nuclear envelope into cytoplasm
  • Sarcoplasmic reticulum: the endoplasmic in striated muscle cells in the heart and skeletal muscle - has specialised function
  • Most extensive membrane system within eukaryotic cells (~50% of total cell membrane)
  • Consists of two functional compartments
    • Rough endoplasmic reticulum - covered in ribosomes
    • Smooth endoplasmic reticulum
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  • Site of protein synthesis (translation)
  • Link amino acids together in the order specified by messenger RNA (mRNA) molecules
  • Two major components: the small ribosomal subunit, which reads the RNA, and the large subunit, which joins amino acids to form a polypeptide chain
  • Amino acids are selected, collected and carried to the ribosome by transfer RNA (tRNA) molecules
  • tRNA enters one part of the ribosome and bind to the messenger RNA chain
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Endoplasmic Reticulum: Function

  • Protein manufacture
    • Protein production
    • Generation of a polypeptide chain
      • Antibodies (Leukocytes)
      • Insulin (pancreatic beta cells)
    • Post-translation modification
      • Glycosylation (carbohydrates attached to proteins)
      • Purpose of glycosylation: folding, stability
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Endoplasmic Reticulum: Function

  • Protein targeting
    • Proteins begin synthesis in cytosol but their destination is varied
      • Secretion
      • Cytosolic
      • Specific organelles
    • Proteins destined for certain organelles or for secretion enter the endoplasmic reticulum first
    • Targeting signals enable the cellular transport machinery to correctly position a protein inside or outside the cell
    • Polypeptide chain or in the folded protein
    • Continuous stretch of amino acid residues = signal peptides
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Smooth Endoplasmic Reticulum Function

  • Steroid/cholesterol production
  • Drug/toxin detoxification
  • Ca2+ storage
  • Storage of proteins and sites of action of many enzymes
    • Membrane (lipid) synthesis
    • Cytochrome P450 enzymes (present in most tissues, but particularly liver) degrade/eliminate drugs
    • Calsequesterin (calcium-binding protein of the sarcoplasmic reticulum)
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Sarcoplasmic Reticulum

  • Further differentiation of smooth endoplasmic reticulum in muscle cells
  • Forms extensive network surrounding muscle sacromeres and supplies them with Ca2+
  • Tubular network
  • Smooth muscle cells are spindle-shaped, with the widest part being ~2-5um and the length ranging up to 500um in visceral muscle and to ~150um in vascular tissues
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Golgi Apparatus

  • Flattened membrane enclosed disc-shaped sacs (cisternae)
  • Located near nucleus
  • Cis (entry) and trans (exit) face
  • Proteins travel from endoplasmic reticulum to golgi and within golgi via transport vesicles
    • incoming transport vesicle (cis)
    • outgoing transport vesicle (trans)
  • The Golgi apparatus tends to be larger and more numerous in cells that synthesise and secrete large amounts of substances: antibody-secreting plasma B cells
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Golgi Apparatus

  • Secretory pathway
    • Exocytic pathway - proteins destined for secretion into extracellular space or plasma membrane
    • Lysosome formation
    • Protein modification and sorting
  • The 'post office' of the cell - Golgi apparatus packages proteins synthesises in the endoplasmic reticulum into membrane-bound vesicles, before the vesicles are sent to their destination
  • Post-translation modification of proteins, e.g. addition of carbohydrates (glycosylation) or phosphorylation of oligosaccharides
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Trans-Golgi Network

  • Proteins are sorted and shipped to their intended destinations by their placement into one of at least three different types of vesicles depending on their signal sequence

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