Facilitated Diffusion

By R. Marsh

Facilitated Diffusion

Facilitated diffusion

Passive movement of molecules across the cell membrane via the aid of a membrane protein.

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Facilitated diffusion

• Movement of molecules that are UNABLE to freely cross the phospholipid bilayer

  • ​Example: large, polar molecules, and ions

How does large, polar molecules, and ions get across?

• The process is mediated (helped) by two types of transport proteins:

• ​Channel proteins

• Carrier proteins

  • (binds to the protein and protein changes conformation)

Channel Proteins:

• Lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other

• Ion-selective and may be gated to regulate the passage of ions in response to certain stimuli

• Only move molecules along a concentration gradient (i.e. are NOT used in active transport)

• Channel proteins have a much faster rate of transport than carrier proteins

​Carrier Proteins:

• Glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane

• Carrier proteins will only bind a specific molecule

• Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)

• Carrier proteins have a much slower rate of transport than channel proteins.

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​Ion Channels:

• Ion channels are pore-forming membrane proteins that allow ions to pass through the channel pore.

  • Ion: ​ atoms that have a charge (Na⁺, K⁺, Ca²⁺, or Cl^–)

• ​ion channels are highly selective because narrow pores in the channel restrict passage to ions of the appropriate size and charge.

• ​Ion Channels are Passive Transport.

​Nerve Cell Transmit Electrical impulses:

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

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​Uses Sodium and Potassium Pumps.

​Sodium-Potassium Pump:

• At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within

• When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

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Multiple Choice

Diffusion & Facilitated Diffusion are both __________ transport.

• Passive

• Active

• Easy

• Free

Multiple Choice

Passive Transport is the movement of particles across the membrane from an area of [ ___ ] to an area of [ ___ ]

• high to low

• low to high

• low to low

• high to high

Multiple Choice

What is the movement of particles with the concentration gradient using protein "helpers"?

• simple diffusion

• facilitated diffusion

• active transport

• enzymatic reactions

Multiple Choice

Passive Transport uses ____ energy

• no

• a lot of

• a little bit of

• some

Multiple Choice

What transports molecules the fastes ?

• Channel proteins

• Carrier proteins

• Sodium and Potassium Channels

Multiple Choice

______________are ion-selective and may be gated to regulate the passage of ions

• Proteins

• Ion Channels

• Simple Diffusion

Multiple Choice

Neuron fires, using what method _______________________

• Channel proteins

• Carrier proteins

• Simple Diffusion

• Sodium and Potassium Channels

Multiple Choice

Facilitated diffusion needs what to get particles from one side of the membrane to the other?

• a membrane-spanning protein

• a surface protein

• a phospholipid

• a carbohydrate