13.3
Facilitated Transport
The chemical and physical properties of plasma membranes cause them to be selectively permeable. Since plasma membranes have both hydrophobic and hydr…
Solutes like charged ions are repelled by the hydrophobic layer of the membrane, thereby halting diffusion. During the process of facilitated transport or facilitated diffusion, molecules can travel across the membrane via channels and carrier proteins that enable diffusion without requiring additional energy.
The first type, channel proteins, form a hydrophilic pore through which charged molecules can pass, thus avoiding the hydrophobic layer of the membrane. These channels are either always open or gated by some mechanism to control flow.
The second type, carriers, bind to a specific solute that changes the protein conformation, enabling the movement of solute down the gradient. For this reason, the rate of transport is not dependent on the concentration gradient, but rather on the number of carrier proteins available.
Even though it is more complex than simple diffusion, facilitated transport enables diffusion to occur at incredible rates, with channel proteins moving tens of millions of molecules a second and carrier proteins moving 1,000 to a million molecules a second.
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Q1: Why can't charged ions simply diffuse across the plasma membrane?
Charged ions are repelled by the hydrophobic layer of the membrane, which prevents them from crossing. Since plasma membranes contain both hydrophobic and hydrophilic regions, charged substances cannot traverse the hydrophobic core without assistance. This is why special membrane transport proteins are required to enable ions to cross successfully.
Q2: What is the difference between channel proteins and carrier proteins in facilitated transport?
Channel proteins form a hydrophilic pore allowing charged molecules to pass through and avoid the hydrophobic layer. Carrier proteins bind to specific solutes, causing a conformational change that enables movement down the concentration gradient. While channel proteins move tens of millions of molecules per second, carrier proteins move 1,000 to a million molecules per second.
Q3: How do gated channels control the movement of ions across membranes?
Gated channels remain closed until a particular ion or substance binds to the channel or another mechanism triggers opening. These channels are found in muscle and nerve cells, where controlled opening and closing regulates ion concentration changes. Without this regulated barrier, muscle contraction would not occur efficiently.
Q4: What determines the rate of transport when using carrier proteins?
The rate of carrier protein transport depends on the number of carrier proteins available, not the concentration gradient. When a carrier protein binds to a specific solute, it undergoes a conformational change enabling movement down the gradient. This means transport speed is limited by protein availability rather than how steep the concentration difference is.
Q5: Why is facilitated transport faster than simple diffusion?
Facilitated transport enables diffusion to occur at incredible rates through specialized membrane proteins. Channel proteins move tens of millions of molecules per second, while carrier proteins move 1,000 to a million molecules per second. This dramatic increase in speed allows cells to transport essential substances efficiently without requiring additional energy.
Q6: What role do aquaporins play in membrane transport?
Aquaporins are channel proteins that specifically facilitate the transport of water through the plasma membrane. As channel proteins, they form a hydrophilic pore allowing water molecules to pass through while avoiding the hydrophobic layer. This selective transport is essential for maintaining proper water balance across cell membranes.
Q7: How does facilitated transport relate to the significance of membrane transport in cells?
Facilitated transport enables cells to move essential charged ions and molecules across membranes without energy expenditure, supporting critical cellular functions. By allowing rapid, selective transport of substances like ions and water, facilitated transport maintains cellular homeostasis and enables processes like muscle contraction. Understanding the significance of membrane transport reveals how cells regulate their internal environment.