Bio Macro

Overview of Membrane Transport

  • Continuation of previous lectures focusing on mechanisms of membrane transport.

  • Differentiation between micro transfer (small molecules) and macro transfer (larger molecules).

Micro vs. Macro Transfer

  • Micro Transfer: Covered previously; involves small molecules and ions.

  • Macro Transfer: Involves larger structures (e.g., viruses, bacteria).

Endocytosis and Exocytosis

  • Endocytosis: Process of bringing materials into the cell.

  • Exocytosis: Process of releasing materials from the cell into the external environment.

    • Incorporates membrane proteins into the membrane.

  • Both processes utilize vesicles (membrane-bound vesicles surrounded by phospholipids).

Exocytosis

  • Vesicles transport from the Golgi apparatus through the cytoplasm to the cell membrane.

  • Two outcomes:

    1. Contents of the vesicle released outside the cell.

    2. Membrane proteins incorporated into the cell membrane.

  • Types of Exocytosis:

    • Constitutive Exocytosis: Continuous process where vesicles are added to the membrane.

    • Regulated Exocytosis: Materials stored in vesicles and released under certain conditions (e.g., insulin release).

  • Example - Insulin Release:

    • High glucose levels stimulate release of insulin from pancreatic beta cells.

    • Influx of potassium ions causes depolarization.

    • Depolarization opens calcium channels, causing an influx of calcium.

    • Calcium influences vesicle trafficking, leading to insulin release into the bloodstream.

Mechanisms in Neuronal Cells

  • Neurons use similar calcium-dependent mechanisms for releasing neurotransmitters.

  • Involves depolarization caused by action potentials in the nerve cells.

  • translocation of a vesicle over a significant distance

Vesicle Transport Mechanisms

  • Long-distance transport occurs along microtubules.

  • Short-distance transport occurs along microfilaments (actin filaments).

  • Protein interactions determine the transport system used by vesicles.

Steps in Membrane Fusion

  1. Tethering: Anchoring vesicles to the membrane.

    1. snare is present

    2. bridging complex is formed

  2. Docking: Bringing the vesicle closer to the membrane.

  3. Priming: Biochemical preparation of the membranes for fusion.

  4. Fusion: Merging vesicle membrane with the cell membrane to release contents.

    1. meging is driven by the snare protiens

SNARE Proteins

  • Essential for vesicle docking and fusion:

    • v-SNAREs: Found on vesicle membranes.

    • t-SNAREs: Found on cell membranes.

  • Form a complex that helps bring membranes to close proximity.

Phagocytosis and Endocytosis Mechanisms

  • Phagocytosis: Engulfing large particles like bacteria by white blood cells.

  • Process:

    • Antibodies bind to bacteria and interact with receptors on phagocytic cells.

    • this makes an isolating vacuole and secondary lysosomes are now involved

    • Formation of a phagosome, which digests the bacteria using various enzymes (proteases, lipases, nucleases).

    • Exocytosis: Waste materials may be expelled after digestion.

Fluid-phase Endocytosis

  • Involves small-scale uptake of macromolecules and large proteins.

  • Receptor-mediated Endocytosis: Cell selectively takes up specific materials:

    • Utilizes integral membrane proteins with an extracellular binding domain.

    • Clathrin protein assists in vesicle formation at the membrane (coated pit).

  • LDL uptake as an example:

    • LDL binds to its receptor, initiating membrane invagination and vesicle formation.

Macrophages and Projections

  • Macrophages can engulf significant volumes (up to 25% of their own volume).

  • Characterized by extension of membrane projections for increased phagocytic activity.

Balance of Endocytosis and Exocytosis

  • Endocytosis and exocytosis processes must balance to maintain cellular function.