5.10
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 do not readily cross the hydrophobic core of the lipid bilayer by simple diffusion.
During the process of facilitated transport, or facilitated diffusion, solutes move down their concentration gradient through channel and carrier proteins as a form of passive transport, without requiring additional energy.
The first type, channel proteins, form a hydrophilic pore through which charged molecules can pass, avoiding the hydrophobic layer of the membrane.
These channels are either always open or gated by some mechanism to control flow.
The second type, carrier proteins, bind to specific solutes, and this changes their protein conformation, enabling the movement of solutes down the gradient.
As a result, the rate of transport depends on both the concentration gradient and the number of available carrier proteins.
Even though it is more complex than simple diffusion, facilitated transport allows solutes to move efficiently across the membrane through channel and carrier proteins.
Q1: Why can't charged ions cross the lipid bilayer through simple diffusion?
Charged ions cannot cross the lipid bilayer's hydrophobic core through simple diffusion because the hydrophobic region repels charged molecules. The membrane's selective permeability requires these solutes to use special transport proteins that provide hydrophilic pathways, allowing ions to bypass the hydrophobic layer and move across the membrane efficiently.
Q2: What is the difference between channel proteins and carrier proteins in facilitated transport?
Channel proteins form hydrophilic pores allowing charged molecules to pass through directly, while carrier proteins bind to specific solutes and undergo conformational changes to enable movement. Channel proteins move tens of millions of molecules per second, whereas carrier proteins transport one thousand to one million molecules per second, making channels significantly faster.
Q3: How do gated channels control the movement of ions across the membrane?
Gated channels remain closed until a specific ion or substance binds to the channel or another mechanism triggers opening. This controlled regulation is essential in muscle and nerve cells, where opening or closing gates changes ion concentrations across the membrane, enabling efficient muscle contractions and cellular signaling.
Q4: What determines the rate of transport through carrier proteins?
The rate of carrier protein transport depends on the number of available carrier proteins rather than the concentration gradient alone. When carrier proteins bind to solutes and change shape, they enable movement down the gradient, but transport speed is limited by how many carrier proteins are present in the membrane.
Q5: Why is facilitated transport considered a form of passive transport?
Facilitated transport is passive because solutes move down their concentration gradient without requiring additional energy input. Both channel and carrier proteins enable this movement by providing pathways through the membrane, allowing molecules to flow from high to low concentration naturally, similar to simple diffusion but through protein channels.
Q6: What role do aquaporins play in cellular transport?
Aquaporins are channel proteins that specifically facilitate the transport of water molecules across the plasma membrane. These highly selective proteins form hydrophilic pores, allowing water to pass through the membrane efficiently while excluding other substances, making them essential for maintaining cellular water balance.
Q7: How does facilitated transport compare to primary active transport in terms of energy requirements?
Facilitated transport requires no additional energy because solutes move down their concentration gradient passively. In contrast, primary active transport uses ATP energy to move solutes against their gradient. Facilitated transport is therefore more efficient for moving substances down gradients, while active transport is necessary when moving substances uphill.