Endocytosis & Exocytosis

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40 Terms

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Exocytosis

The process by which intercellular secretory vesicles release their contents to the outside of the cell (the extra cellular space[ECS])

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Endocytosis

The process by which cells internalize contents from the ECS

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How does exocytosis releases intracellular molecules to the outside of the cell?

Fusion via the plasma membrane

  • Macromolecules contained in an intracellular vesicle are released to the extracellular space after the vesicle faces with the plasma membrane.

  • Animal cells secrete hormones neurotransmitters, mucus, milk proteins, & digestive enzyme.

  • Plant & fungal cells secrete enzyme and structural proteins for building the cell wall

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What is the first type of Exocytosis?

“Trafficking” of a lipid- or protein-bearing intracellular vesicle to the plasma membrane (cell surface)

  • Active (energy-dependent) process

    • ATP- & GTP-dependent proteins

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What is the second step of Exocytosis?

“Fusion” of vesicular and plasma membranes (share a common lipid bilayer)

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What is the third step of Exocytosis?

“Opening/rupture” of the intracellular vesicle & start of membrane integration

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What is the fourth step of Exocytosis?

“Release” of macromolecular contents in the EC space & completion of membrane integration

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What is the orientation of a vesicular membrane?

  • The luminal (inner) membrane of the vesicle becomes part of the outer surface of the plasma membrane

  • —Glycolipids & glycoproteins (sugar-containing macromolecules) formed in the ER and Golgi lumens will face the extracellular space

    • Hydrophilic (water-loving sugars)

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<p>What are the mechanisms of exocytosis?</p>

What are the mechanisms of exocytosis?

  • Mechanism(s) of exocytotic vesicle movement to the cell surface are not entirely clear…

    • But, the cytoskeleton plays an important role by “laying the tracks” for vesicle transport

      • intracellular “highway systems”

    • Microtubules use “adaptor proteins” & ATP-dependent ”molecular motors” to bind and move vesicles

  • Evidence: vesicle movement is blocked when cells are treated with colchicine, a microtubule assembly inhibitor

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What are the different types of exocytosis?

  1. All cells need to routinely move material (lipids, proteins) to the PM:

    • e.g., building/expanding before cell division, adding receptors, replacing the sugar coat of the ECM.

    • Constitutive exocytosis

    • “need to survive”

  2. Cells also need to respond to environmental signals in a stimulus & time-dependent fashion

    • Regulated exocytosis

    • “need to adapt”

<ol><li><p>All cells need to routinely move material (lipids, proteins) to the PM:</p><ul><li><p>e.g., building/expanding before cell division, adding receptors, replacing the sugar coat of the ECM.</p></li><li><p><span style="font-family: Wingdings 2; color: red"><strong>Constitutive exocytosis</strong></span></p></li><li><p><span style="font-family: Wingdings 2"><em>“need to survive”</em></span></p></li></ul></li><li><p>Cells also need to respond to environmental signals in a stimulus &amp; time-dependent fashion</p><ul><li><p><span style="color: red"><strong>Regulated exocytosis</strong></span></p></li><li><p><em>“need to adapt”</em></p></li></ul></li></ol><p></p>
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What is the definition of endocytosis?

Pathways bring extracellular or PM-associated materials into the cells

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What’s the first step of Endocytosis?

Plasma membrane “invaginates” to capture the to-be transported cellular material

  • like exocytosis, an active process

  • direction: EC > cytoplasm

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What’s the second step of Endocytosis?

Plasma membrane pinches to enclose the material

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What’s the third step of Endocytosis?

Plasma membrane forms a cytoplasmic vesicle

  • “budding off”

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What’s the fourth step of Endocytosis?

The new vesicle “separates” from the plasma membrane as is ready for cystolic use

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<p>What type of endocytosis is this?</p>

What type of endocytosis is this?

Phagocytosis

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<p>What type of endocytosis is this?</p>

What type of endocytosis is this?

Pinocytosis

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<p>What type of endocytosis is this?</p>

What type of endocytosis is this?

Receptor-mediated endocytosis

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What is transported during pinocytosis?

Small particulate matter + extracellular fluid (ECF)

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Examples of Pinocytosis

  1. Adipose tissue: fat droplet absorption

  2. Intestinal Microvilli: absorb nutrients from the GI tract

  3. Kidney Ducts: formation of urine

  4. Reproductive Eggs: absorb nutrients prior to fertilization

  5. In unicellular organisms (amoeba): water/nutrient uptake

  6. Plasma membrane: Recycling of lipid bilayer components

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What molecules are seen in receptor-mediated Endocytosis?

“ligands” (peptides, small molecules, and lipoproteins)

  • Cell express receptor proteins on the plasma membrane that bind to & concentrate certain molecules needed for/n the cell to function properly.

  • Concentrate ligands in the cell by a factor of ≥ 1,000-fold

  • Selectiveness between ligand and receptor

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What is the first step in receptor-mediated endocytosis?

Ligand-Receptor binding simulates an intracellular signal to initiate the process of receptor endocytosis.

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What is the second step in receptor-mediated endocytosis?

Accumulation of ligand-receptor complexes start to form a “pit” coated with the protein cathrin

  • Cathrin-coated pit: plasma membrane-ligand-receptor complex

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What is the third step in receptor-mediated endocytosis?

Triggers mobilization of cystolic “adaptor” proteins, including dynamin

  • Further induces Pit curvature > scission

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What is the fourth step in receptor-mediated endocytosis?

Pit completely pinches off, creating a coated vesicle

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What is the fifth step in receptor-mediated endocytosis?

Cathrin coat is released, leaving an uncoated vesicle

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What is the fifth step in receptor-mediated endocytosis?

Coat proteins & dynamin are recycled to the plasma membrane & the uncoated vesicle fuses with an early endosome

  • process is relatively quick (seconds to minutes)

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What is the sixth step in receptor-mediated endocytosis?

Endosome-sequestered receptors are often recycled back to the PM due to the acidified (low pH) early endosome

  • The pH gradually lowers as the endosome matures, facilitated by an ATP-dependent proton pump

  • The lower pH dissociates ligand and receptors, allowing receptors to be returned to the membrane

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What is the seventh(a) step in receptor-mediated endocytosis?

Some receptor complexes are carried to a lysosome via “late endosomes'“ to be degraded

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What is the seventh(b) step in receptor-mediated endocytosis?

Some complexes are recycled back to the plasma membrane to restore receptor activity

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What is the seventh(c) step in receptor-mediated endocytosis?

Some complexes can travel via “transport vesicles” to a distinct region of the plasma membrane (often the opposite side of the cell), where they are secreted (transcytosis)

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What is the name of the name for vesicles involving protein and lipid transport?

Coated Vesicles

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What are the types of coated vesicles?

  • Clathrin (PM/EE)

  • COPI (GN><GN, GN>ER)

  • COPII (ER>GN)

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What are the three main function of coated vesicles?

  1. Determine destination

  2. Induce curature needed for vesicle formation

  3. Prevent non-specific fusion with other membranes

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What are Caveolin coated vesicles?

  • Common non-Clathrin-dependent mechanism

  • Type of a ‘lipid raft’, a PM region rich in cholesterol & sphingolipids

  • May be involved in cholesterol uptake & maintaining the stability of receptor complexes

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What are Clathrin-coated vesicles?

  • surrounded by coats of two multimeric proteins, clathrin and adaptor protein-2 (AP2).

  • shape & assembly of clathrin provides induces a flat membrane to form a spherical vesicle

    • AP binding requires ENERGY (i.e., active/ATP-dependent process)

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Exocytosis/Endocytosis are…

active (energy-dependent), eukaryotic cell mechanisms to transport molecules (mainly proteins & lipids) out of (exo-) and into (endo-) cells.

  • —Cells use vesicular transport & the cytoskeleton (esp. microtubules) to accomplish these processes

  • —Exo/endocytosis can occur in an environmental signal-independent (constitutive) & -dependent (regulated) fashion

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3 types of endocytosis:

—phagocytosis (cellular “eating”), pinocytosis (cellular “drinking”), & receptor-mediated endocytosis (as I personally call it… cellular “sleeping”)

  • —Phagocytosis & pinocytosis can be distinguished based primarily on the size of the material they are internalizing into the cell (phagocytosis involves BIG materials, whereas pinocytosis involves small materials)

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Receptor-mediated endocytosis requires…

Clathrin-Coated Pit formation & adaptor proteins (e.g., dynamin, AP) to internalize cell surface receptors

  • —Receptors have 3 main fates: (1) degradation (2) recycling to the cell surface & (3) transcytotic release

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Vesicle transport can…

be defined by the Coat Proteins that surround them

  • —Common Coat Proteins are: Clathrin, COPI, COPII, & Caveolin

    —Coat Proteins determine the vesicle’s (1) destination (2) formation & (3) target specificity