5.2 Facilitated diffusion and channel proteins

Facilitated Diffusion and Channel Proteins

Facilitated diffusion solves a problem for the cell: how do large or polar molecules cross a hydrophobic membrane without energy input? The answer is transport proteins, which provide a path through the lipid bilayer so these molecules can still move down their concentration gradient passively.

Facilitated vs Simple Diffusion

Both facilitated and simple diffusion are forms of passive transport, meaning they move molecules down the concentration gradient (high to low) with no ATP required. The difference is how molecules get across.

• Simple diffusion happens directly through the phospholipid bilayer. Only small, nonpolar molecules can do this, such as $O_2$, $CO_2$, and other hydrophobic molecules. The bilayer's hydrophobic core doesn't block them.
• Facilitated diffusion uses transport proteins (channels or carriers) embedded in the membrane. This is how larger or polar molecules like glucose and amino acids cross, since they can't slip through the hydrophobic interior on their own.

Both types share a key feature: net movement stops once concentrations equalize on both sides (equilibrium). Neither can move molecules against their gradient.

Structure of Channel Proteins

Channel proteins are integral membrane proteins, meaning they span the entire lipid bilayer. Multiple protein subunits arrange together to form a central hydrophilic pore, creating a water-filled tunnel through the membrane.

• The pore is lined with specific amino acid residues that interact with passing molecules, selecting for size and charge. This is why a given channel typically allows only certain ions through and excludes others.
• Channels facilitate the rapid movement of ions like $Na^+$, $K^+$, $Ca^{2+}$, and $Cl^-$ across the membrane. "Rapid" is the key word here: channel proteins move ions much faster than carrier proteins because ions flow straight through the pore rather than waiting for a protein to change shape.

This selective ion flow is what maintains the electrochemical gradient and membrane potential, which cells depend on for nerve impulse transmission, muscle contraction, and cell signaling.

Gated vs Non-Gated Channels

Not all channels are open all the time. This distinction matters for how cells control what crosses the membrane and when.

Non-gated (leak) channels are always open, providing a constant, unregulated path for specific molecules. The best-known example is aquaporins, which allow water molecules to move rapidly across the membrane at all times.

Gated channels open or close in response to specific stimuli. There are three main types:

• Voltage-gated channels respond to changes in membrane potential. Voltage-gated $Na^+$ and $K^+$ channels in neurons are the classic example; they open and close in sequence to propagate an action potential.
• Ligand-gated channels open when a specific molecule (the ligand) binds to the channel. The acetylcholine receptor at neuromuscular junctions is a ligand-gated channel: when acetylcholine binds, the channel opens and allows ions to flow, triggering muscle contraction.
• Mechanically-gated channels respond to physical forces like pressure or stretch. Sensory cells in your inner ear use these to convert sound vibrations into electrical signals.

Gated channels can also be regulated by additional mechanisms:

1. Phosphorylation/dephosphorylation by enzymes (protein kinases and phosphatases), which alter the channel's shape
2. Binding of intracellular or extracellular ligands such as ions, neurotransmitters, or hormones
3. Conformational changes triggered directly by voltage shifts or mechanical force

Examples of Facilitated Diffusion

Facilitated diffusion isn't limited to ion channels. Carrier proteins also mediate facilitated diffusion for larger polar molecules. Here are the major examples:

• Glucose is transported by GLUT (glucose transporter) proteins. GLUT proteins are carriers, not channels: glucose binds, the protein changes shape, and glucose is released on the other side. This is how most cells take up glucose for energy production.
• Amino acids cross via amino acid transporter proteins, supplying the building blocks for protein synthesis and cell growth.
• Ions move through the channel proteins described above:
  • $Na^+$ and $K^+$ maintain membrane potential and cell excitability
  • $Ca^{2+}$ acts as a signaling molecule and is critical for muscle contraction and neurotransmitter release
  • $Cl^-$ helps regulate cell volume and pH balance
• Nucleosides use nucleoside transporter proteins, providing raw materials for DNA and RNA synthesis.
• Neurotransmitters are taken back up from the synaptic cleft by neurotransmitter transporter proteins, which is how signaling between neurons is terminated and recycled.

One thing to keep straight: GLUT proteins and amino acid transporters are carrier proteins (they change shape to move the molecule), while ion channels are channel proteins (they form an open pore). Both count as facilitated diffusion because both use membrane proteins and move molecules down their concentration gradient without ATP.