11 Membrane Proteins Channels & Enzymes

Membrane Proteins Overview

  • Evaluation of membrane proteins that act as channels and carriers.

Channel Proteins

  • Function as pathways for ions, which are small and polar.

  • Types:

    • Leakage Channels: Always open.

    • Gated Channels: Closed unless opened by a messenger (ligand).

      • Example: Ligand-gated channels requiring a binding structure to open.

    • Types of Ligands: Hormones in bloodstream or neurotransmitters in synaptic cleft.

Carrier Proteins

  • Similar to channels but require a shape change to transport substances.

    • Function: "Change shape" to release substances on the other side of the membrane.

Transport Mechanisms

  • Movement from high to low concentration is called diffusion.

  • Example: If ions are abundant outside the membrane, they will diffuse inward when a channel opens, allowed by passive transport.

Types of Gated Channels

  • Ligand-gated: Open when a ligand binds.

  • Mechanically-gated: Open in response to mechanical forces (e.g., touch receptors).

  • Voltage-gated: Open due to changes in membrane potential (e.g., important in muscles and nerves).

Resting Membrane Potential

  • Characterized by a negative charge inside the membrane and a positive charge outside due to ion distribution.

  • Key ions involved:

    • High Sodium (Na+) outside (extracellular fluid).

    • High Potassium (K+) and large negative anions inside (intracellular fluid).

  • This establishes an electrochemical gradient necessary for cell signaling.

Neurotransmitters

  • Example: Acetylcholine (ACh)

    • Functions as a messenger between neurons and muscle cells.

    • When ACh binds to its receptor, it opens gated channels allowing ions to flow in, impacting charge and membrane potential.

Membrane Proteins Functions

  • Receptors: Interact with ligands to initiate cellular responses.

  • Channel/Carrier Proteins: Facilitate transport of ions and molecules.

  • Cell Recognition and Adhesion: Help cells adhere to each other and identify self.

  • Cytoskeleton Anchors: Connect cells to the cytoskeleton providing structural support.

  • Enzymatic Activity: Some membrane proteins function as enzymes, catalyzing reactions on the cell surface.

Microvilli in the Small Intestine

  • Structure: Extensions of the cell membrane, increasing surface area for absorption.

  • Function: Enzymes attached to microvilli help in breaking down food particles in the lumen.

    • Examples of brush border enzymes: Maltase, Sucrase, Lactase.

  • Important for digestion and nutrient absorption.