Lecture 5 - Endomembrane System and Bulk Transport Processes PDF

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The endomembrane system includes:  Nuclear envelope  Endoplasmic reticulum (smooth and rough)  Golgi apparatus  Vesicles  Lysosomes  Vacuoles  Plasma membrane A membrane system interconnected by direct physical contact or transfer by vesicles Endoplasmic Reticulum (ER)  Composed of two regions  Smooth ER (sER) and rough ER (rER) o They look different and have different functions Functions of the smooth endoplasmic reticulum (sER)  Metabolism of carbohydrates  Lipid synthesis for membranes  Detoxification of drugs and poisons o Filters out food coloring from foods as they are not nutritious  Storage of calcium ions (used as a signal in the cell) o Muscle cells stores more calcium to suit its function  Extensive sER in cells active in these processes  The amount of sER can be increased or decreased to meet demand Functions of the rough endoplasmic reticulum (rER)  Rough appearance due to ribosomes  Involved in protein synthesis  Secreted and membrane bound proteins enter the lumen (interior) of the rER  And are processed by the rER and the rest of the endomembrane system for release from the cell or retention on the cell membrane Synthesis of cytoplasmic proteins occurs on free ribosomes Function of the Golgi complex  Series of membrane sacs and associated vesicles  Receives, modifies, sorts and ships proteins arriving from the rER  Has polarity - cis and trans face  Vesicles from endoplasmic reticulum arrive at the cis face  Processed vesicles leave at the trans face Proteins are modified and then sent to the right place in the cells, however the process is not entirely understood  Glycosylation o Addition (or modification) or carbohydrates to proteins o Important for secreted or cell surface proteins o Golgi also produces many polysaccharides from which may also be secreted from the cell  Sorting proteins o Adds molecular markers to direct proteins to the correct vesicles before "budding" from the trans face  For example a special phosphorylated sugar (mannose 6-phosphate) identifies proteins that will become lysosomal enzymes

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Directing vesicle trafficking o Adds molecular "tags" to vesicles leaving the trans face to direct them to the correct targets o Such tags are often short proteins exposed on the vesicle surface o Act as docking sites when they reach their target o Some tags direct vesicles to the lysosome o Others direct to secretory pathways o Important for release and surface expression

Bulk transport across the plasma membrane  Exocytosis o Transports material (glycoproteins) out of the cell or delivers it to the cell surface o Constitutive exocytosis releases extracellular matrix proteins o Regulated exocytosis releases hormones and neurotransmitters  Endocytosis o The cell takes in molecules and particulate matter at the plasma membrane  Phagocytosis o cell “eating” o uptake of “food” particles o forms a phagocytic vacuole which is “digested” by the lysosomes o in humans this occurs in macrophages.  Pinocytosis o Cell "drinking" o Uptake of extracellular fluid containing various solutes such as protein and sugars o Uptake vesicle is formed with the aid of a coat protein o Uptake is non-selective  Receptor-mediated endocytosis o Specialized form of pinocytosis o Allows the cell to take up bulk quantities of specific substances which may be present at only low concentrations in the extracellular fluid o Receptor proteins are used to selectively capture the required solute Lysosomes  Phagocytic vacuoles fuse with lysosomes  Which are membrane bound organelles made by the rER and Golgi body containing hydrolytic enzymes  The interior of a lysosome is acidic which is required for the enzymes to be active  They degrade proteins, lipids, carbohydrates and nucleic acids and release breakdown products into the cell  Lysosomes digest and recycle unwanted cellular materials  This process is called autophagy and is important for cell health  Lysosomal direction is also important in programmed cell death in which whole cells "intentionally" die.  Defects in lysosomal enzymes can result in lysosomal storage diseases Vacuoles  Large vesicles derived from the rER and Golgi  Food vacuoles involved in phagocytosis  Vacuoles are important in plants o Can perform lysosome-like functions

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Small vacuoles store organic compounds (proteins, pigments, poisons) Large central vacuole absorbs water allowing plant cells to grow without a large increase in cytoplasm...