Inside the Cell 2

Inside the Cell 2

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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

• 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)

• 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

• 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

• 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

• 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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