Chapter 5 (Part 2): Transport Proteins and Mechanisms of Transport

Lecture Outline

  • Because learning changes everything.®

  • Chapter 5 Part 2

  • Copyright Information

    • © 2020 McGraw-Hill Education.

    • All rights reserved.

    • Authorized for instructor use only in the classroom.

    • No reproduction or further distribution without prior written consent.

Transport Proteins

  • Definition: Transport proteins are transmembrane proteins that facilitate the movement of ions and hydrophilic molecules across membranes.

  • Classes of Transport Proteins:

    1. Channels

    • Example: Aquaporins and various other types of channels (previously referred to as “pores” in older textbooks).

    1. Transporters

    • Also known as carriers; some referred to as “pumps.”

Channels

  • Function: Channels create an open passageway for the direct diffusion of ions or molecules across the membrane.

    • Examples:

    • Solutes move directly through the channel to reach the other side.

    • Aquaporins: Specialized channels that allow water to rapidly cross the membrane.

  • Characteristics:

    • Many channels are gated, meaning they can open or close in response to stimuli.

    • Control Mechanism: Gated channels can be controlled by the binding of small molecules called ligands.

Aquaporins

  • Discovery: Peter Agre and colleagues discovered that osmosis occurs more quickly in cells with transport proteins that facilitate the diffusion of water.

  • Significance: Water can passively diffuse across plasma membranes; however, some cell types allow it to move significantly faster than anticipated.

  • Key Findings:

    • CHIP28 was identified as a protein abundant in red blood cells, bladder, and kidney cells.

    • A channel-forming protein (CHIP28) was responsible for the rapid movement of water, being renamed aquaporin.

  • Experimental Findings:

    • Frog oocytes expressing CHIP28 swelled and lysed in hypotonic medium due to rapid influx of water through aquaporin channels, compared to control oocytes without aquaporins.

    • Conclusion: CHIP28 protein, now termed aquaporin, allows for the rapid movement of water across cell membranes.

  • Source of Publication: Preston, G. M., et al., "Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein." Science, 1992.

Transporters

  • General Definition: Known as carriers (some referred to as pumps).

    • Mechanism: Conformational change in the transporter moves the solute across the membrane.

  • Process: Solutes bind to a hydrophilic pocket within the transporter, inducing a conformational change, and this switches the exposure of the pocket from one side of the membrane to the other.

  • Key Function: Principal pathway for the uptake of organic molecules including sugars, amino acids, and nucleotides, as well as transport for charged ions.

Types of Transporters

  1. Uniporter

    • Description: Transports a single molecule or ion in one direction.

  2. Symporter (or Cotransporter)

    • Description: Transports two or more ions or molecules in the same direction.

  3. Antiporter

    • Description: Transports two or more ions or molecules in opposite directions.

Active Transport

  • Definition: Movement of a solute across a membrane against its concentration gradient, from a region of low concentration to high concentration.

    • Nature: Energetically unfavorable, requiring an input of energy.

  • Types of Active Transport:

    1. Primary Active Transport

    • Description: Directly uses energy from ATP to transport solutes.

    • Example: Proton pump that moves protons (H+) from areas of low concentration to high concentration.

    1. Secondary Active Transport

    • Description: Utilizes a pre-existing concentration gradient to drive the transport of another solute.

    • Example: Proton/sucrose pump uses the proton gradient to transport sucrose against its concentration gradient.

Ion Electrochemical Gradients

  • Importance: Generated by ATP-driven ion pumps.

    • Example: Sodium-potassium (Na+/K+ -ATPase) pump.

    • Mechanism:

    • Actively transports Na+ and K+ against their concentration gradients using ATP energy.

    • 3 Na+ exported for every 2 K+ imported into the cell.

    • Results in an electrogenic pump that generates one net positive charge and creates an electrical gradient.

Nerve Cell Mechanism

  1. Binding: 3 Na+ ions bind from the cytosol.

  2. Hydrolysis: ATP is hydrolyzed, ADP released, and phosphate (P) is covalently attached, causing a conformational change to E2.

  3. Release: 3 Na+ are released outside the cell.

  4. Binding of K+: 2 K+ ions bind from the extracellular fluid.

  5. Phosphate Release: Phosphate (P) released, returning to E1 conformation, releasing 2 K+ into the cytosol.

Important Functions of Ion Electrochemical Gradients

  • Functionality: Electrochemical gradients are pivotal for various biological processes.

    • Examples:

    • Transport of ions/molecules via symporters and antiporters.

    • Production of energy intermediates in mitochondria and chloroplasts.

    • Regulation of osmotic balance within cells.

    • Na+ and K+ gradients are crucial for neuronal signaling and muscle contraction.

    • H+ gradients drive bacterial flagella rotation.

Exocytosis

  • Definition: A process where material inside the cell is packaged into vesicles and excreted into the extracellular medium.

    • Mechanism: Vesicles derived from the Golgi Apparatus, with a protein coat added during the budding process.

  • Examples of Exocytosis:

    • Secretion of hormones (such as insulin).

    • Release of digestive enzymes from pancreatic exocrine cells.

Endocytosis

  • Definition: The process by which the plasma membrane invaginates (folds inward) to form a vesicle that brings substances into the cell.

  • Types of Endocytosis:

    1. Receptor-mediated Endocytosis: Involves cargo binding to specific receptors.

    2. Pinocytosis: "Cell drinking"; vesicles form around liquid with dissolved solutes.

    3. Phagocytosis: "Cell eating"; vesicles form around solid particles (e.g. macrophages engulfing bacteria).

Receptor-mediated Endocytosis Steps

  1. Binding: Cargo binds to receptors, leading to aggregation.

  2. Invagination: Coat proteins bind to the membrane, initiating vesicle formation.

  3. Vesicle Release: The vesicle is released into the cytosol.

  4. Fusion: The vesicle fuses with an internal organelle, such as a lysosome.

  5. Cargo Release: Cargo is released into the cytosol after shedding the protein coat.