Facilitated Diffusion: The Role of Channel Proteins
- While the phospholipid bilayer forms a barrier to most polar molecules and ions, specialized proteins called channel proteins enable the movement of these substances across the membrane.
- This process, known as facilitated diffusion, allows cells to control the exchange of essential substances without expending energy.
- Note that facilitated diffusion is still a form of passive transport, not active.
- But it uses channels to facilitate the diffusion.
Channel Proteins Are Integral Transmembrane Proteins
Definition
Channel protein
A specialized protein embedded in the membrane, allowing specific ions or molecules to diffuse across.
- Channel proteins have a pore that connects the cell's cytoplasm to the aqueous environment outside.
- Selective Pores: Each channel is highly specific, allowing only one type of particle to pass. For example:
- Sodium ions (Na⁺)
- Potassium ions (K⁺)
- Selective Pores: Each channel is highly specific, allowing only one type of particle to pass. For example:
- Its Determinants of Selectivity include:
- Pore Diameter: Ensures only appropriately sized molecules can pass.
- Chemical Properties: Attract specific ions while excluding others.
Facilitated diffusion through channel proteins is faster than simple diffusion because the hydrophilic interior of the channel reduces resistance to the movement of particles.
Facilitated Diffusion is A Passive Process
- This means no energy (ATP) is required.
- Molecules move down their concentration gradient (from high to low concentration).
- While molecules can move in either direction, the net movement is always toward the lower concentration.
- Channel proteins then act as pathways, enabling polar or charged particles to bypass the hydrophobic core of the phospholipid bilayer.
Cells Can Regulate Channel Permeability
- Cells can regulate permeability by controlling the synthesis and placement of channel proteins in the membrane.
- Opening and Closing: Some channels are gated, meaning they can open or close in response to:
- Signals (e.g., voltage or chemical gradients).
- Environmental Changes (e.g., ion concentrations).
- Adaptability: This regulation allows cells to adjust their permeability based on metabolic needs.
Nerve cells use gated sodium and potassium channels to propagate electrical signals during nerve impulses.
Self reviewHow do channel proteins achieve their selectivity?
