Facilitated diffusion is a crucial process that allows molecules to move across cell membranes with the help of transport proteins. Unlike simple diffusion, it enables larger and polar molecules to cross the membrane barrier, playing a vital role in cellular functions.

Channel proteins are key players in facilitated diffusion, forming pores that allow specific molecules to pass through. These proteins can be gated or non-gated, regulating the flow of ions and other molecules essential for various cellular processes like nerve impulse transmission and metabolism.

Facilitated Diffusion and Channel Proteins

Facilitated vs simple diffusion

• Facilitated diffusion is a type of passive transport that allows molecules to move across a membrane with the help of transport proteins
  • Occurs down the concentration gradient from high to low concentration without requiring energy input from ATP
• Simple diffusion involves molecules moving directly through the phospholipid bilayer without the assistance of proteins
  • Limited to small, nonpolar molecules (O2, CO2, hydrophobic molecules)
  • Facilitated diffusion requires specific transport proteins to move larger, polar molecules (glucose, amino acids) across the membrane

Structure of channel proteins

• Channel proteins are integral membrane proteins that form hydrophilic pores through the membrane allowing specific molecules (usually ions) to pass through based on size and charge
• Composed of multiple subunits that span the entire membrane and arrange to form a central aqueous pore
  • Pore lined with specific amino acid residues that interact with the passing molecules
• Facilitate the rapid movement of specific ions (Na+, K+, Ca2+, Cl-) across the membrane
  • Maintain the electrochemical gradient and membrane potential crucial for various cellular processes (nerve impulse transmission, muscle contraction, cell signaling)

Gated vs non-gated channels

• Non-gated channel proteins are always open and allow continuous passage of specific molecules like aquaporins, which facilitate the movement of water molecules across the membrane
• Gated channel proteins can be opened or closed in response to specific stimuli
  • Voltage-gated channels open or close in response to changes in membrane potential (voltage-gated Na+ and K+ channels in neurons)
  • Ligand-gated channels open or close when specific molecules (ligands) bind to the channel (acetylcholine receptor in neuromuscular junctions)
  • Mechanically-gated channels open or close in response to mechanical stimuli (stretch-activated channels in sensory cells)
• Regulation of gated channels occurs through:
  1. Phosphorylation or dephosphorylation by enzymes (protein kinases, phosphatases)
  2. Binding of intracellular or extracellular ligands (ions, neurotransmitters, hormones)
  3. Conformational changes induced by voltage or mechanical stimuli

Examples of facilitated diffusion

• Glucose transported by glucose transporter (GLUT) proteins is essential for cellular energy production and metabolism
• Amino acids transported by amino acid transporter proteins are required for protein synthesis and cell growth
• Ions transported by various ion channels:
  • Na+, K+ are crucial for maintaining the membrane potential and cell excitability
  • Ca2+ is important for cell signaling, muscle contraction, and neurotransmitter release
  • Cl- is involved in maintaining cell volume and pH balance
• Nucleosides transported by nucleoside transporter proteins are necessary for DNA and RNA synthesis
• Neurotransmitters transported by neurotransmitter transporter proteins are involved in synaptic transmission and signaling between neurons