[responsivevoice_button rate=”1″ pitch=”1.2″ volume=”0.8″ voice=”US English Male” buttontext=”Play”]This section of the AP Biology curriculum – 2.7 Facilitated Diffusion – covers many aspects of this passive form of membrane transport. We’ll start by reviewing the differences between active transport and passive transport. Then, we’ll move on to discover the differences between carrier proteins, ion channels, and aquaporins. We’ll see how aquaporins actively select for water molecules and how different types of ion channels can be voltage-gated, ligand-gated, or mechanically-gated. Lastly, we’ll take a look at how active transport creates ion gradients that can be coupled with modes of facilitated diffusion to complete important cellular processes such as sending nerve signals and creating ATP! Check it out…

Video Tutorial

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Resources for this Standard

• Overview & Video Tutorial (This article)
• Quick Test Prep
• Crossword Puzzle

• PowerPoint

ENDURING UNDERSTANDING
ENE-2
Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments.

LEARNING OBJECTIVE
ENE-2.G
Explain how the structure of a molecule affects its ability to pass through the plasma membrane.

ESSENTIAL KNOWLEDGE
ENE-2.G.1
Membrane proteins are required for facilitated diffusion of charged and large polar molecules through a membrane –

1. Large quantities of water pass through aquaporins.
2. Charged ions, including Na⁺ and K⁺, require channel proteins to move through the membrane.
3. Membranes may become polarized by movement of ions across the membrane.

ENE-2.G.2
Membrane proteins are necessary for active transport.

ENE-2.G.3
Metabolic energy (such as from ATP) is required for active transport of molecules and/or ions across the membrane and to establish and maintain concentration gradients.

ENE-2.G.4
The Na⁺/K⁺ ATPase contributes to the maintenance of the membrane potential.

2.7 Facilitated Diffusion Overview

Just to send a signal to your muscles, an entire system of membrane proteins is needed! Without facilitated diffusion, you couldn’t use your nervous systems, muscles, or about any other cell in your body!

Let’s do a quick review of Facilitated Diffusion, a topic we first addressed in unit 2.6. Facilitated diffusion is a type of passive transport. Facilitated diffusion is like normal diffusion, except that it happens through special protein channels and carrier proteins. These integral membrane proteins are necessary because charged particles and large, polar molecules cannot easily make their way through the plasma membrane. However, facilitated diffusion is still a form of passive transport because the molecules are moving from an area of high concentration to an area of low concentration. Let’s take a closer look at the integral membrane proteins that are necessary for facilitated diffusion.

A great example of a carrier protein is glucose importing proteins. In order to keep your blood glucose levels within a specific range, your pancreas releases the hormone insulin when your blood glucose level is high. The insulin hormone binds to a protein receptor on the surface of cells. This protein activates a sequence of events that forces a small vesicle full of glucose carrier proteins to merge with the cell membrane. This effectively adds glucose carrier proteins to the cell membrane and allows them to import glucose!

Like all carrier proteins, glucose carriers operate in a specific way. First, a glucose molecule binds to the active site of the carrier protein. This changes the conformation of the protein by closing off the entrance and opening the exit to the other side of the membrane. When the protein reaches its final shape, the glucose molecule is forced out of the active site and into the cell. Without glucose in the active site, the protein quickly reverts to the original shape to uptake another glucose molecule. Though glucose is a great example, remember that most large, polar molecules have specific carrier proteins that are needed to import or export across the cell membrane.

Since water is a small, polar molecule, it is partially blocked by the plasma membrane. Though small amounts of water can slip through the lipid bilayer, many organisms have a requirement to transfer water much faster between cells. Consider a plant, for example. The plant must uptake large amounts of water through tiny root hairs. This water not only carries nutrients to the leaves, but it is also crucial for the process of photosynthesis. If the plant had to rely on water passing through the cell membrane via osmosis, there is no way it could gather enough water to provide for all its leaves and stems. This is where aquaporins come in.

Aquaporins are a type of channel protein that is specific to water. When an aquaporin is open, water can freely flow through the hollow center of the protein at a much faster rate than it can travel through the cell membrane. Plus, the aquaporin has a number of amino acids exposed on the inside of the tube that create a series of charged surfaces to actively select for water. Water can pass easily, but if an ion or large molecule tries to enter the aquaporin it will quickly be rejected. These aquaporins are present in plants, animals, fungi, and even bacterial cells to allow a fast passage for water in various cell types.

Similar to how aquaporins only allow water through, there are a large number of Ion Channels that only allow specific ions through the cell membrane. There are several different types of ion channels that allow for facilitated diffusion of ions under different conditions.

Some ion channels are always open. These channels allow for the passage of ions in either direction, all the time. Typically, this form of ion channel is used when a concentration gradient needs to be constantly relieved due to the buildup of ions from biochemical reactions elsewhere in the cell.

Some ion channels are ligand-gated. A ligand is a molecule that can bind to a receptor, and neurotransmitters are a good example. Ligand-gated ion channels are present on neurons. When a neurotransmitter binds to the channel, it immediately opens. This releases a flood of ions into the cell, changing the electrical gradient and polarizing the cell membrane. This creates an electrical signal which can flow through the cell.

Further on down the nerve cell are voltage-gated ion channels. When the electrical impulse travels down the nerve and hits these channels, they open. This flow of ions reestablishes the electrical signal and keeps it traveling down the cell membrane.

Certain ion channels are mechanically-gated, meaning they open when the plasma membrane physically moves. For example, hairs in your inner ear wiggle when they are hit by sound waves and move nearby nerve cells. This mechanical motion opens up mechanically-gated ion channels, creating a nervous signal that travels to your brain. Your brain interprets this signal as a sound, which is essentially how hearing works!

We’ve seen how facilitated diffusion can alleviate a concentration gradient. So that begs the question: how are these ion gradients created to begin with? The answer is active transport!

For example, consider the enzyme that creates ATP – ATP synthase. ATP synthase relies on a gradient of hydrogen ions to operate and create new ATP molecules. ATP synthase is essentially extracting energy from this hydrogen ion gradient by allowing the facilitated diffusion of hydrogen ions. As the molecules pass back through ATP synthase, the molecule turns and catalyzes the formation of ATP molecules.

This gradient is built by three other molecules that are actively pumping hydrogen ions to one side of the membrane. We’ll cover this specific further in a future section, though it should be noted that these active transport systems can be powered in different ways. For instance, electron-carriers like NADH can give energy to the protein, allowing it to pump hydrogen ions against the gradient. Other proteins rely on the energy they can extract from passing electrons, while others rely on the excess energy given off by the formation of water molecules. Together, these proteins are constantly restocking the hydrogen ion gradient so that ATP synthase can function!

The Sodium/Potassium ATPase is one of the most important proteins in your cells because it carries out several important tasks. First off, it establishes a resting membrane potential on your nerve and muscle cells. This slight electrical charge is caused by the export of 3 positive charges on sodium ions with the corresponding import of 2 positively-charged potassium ions. So, with every cycle of the sodium/potassium pump, there is a net of 1 positive charge added to the outside of the cell. This creates an electrical potential across the cell membrane.

Nerves use this resting electrical potential to quickly send signals.

At the start of a neuron, ligand-gated sodium channels are opened when a neurotransmitter binds to them. This causes a massive influx of sodium ions back into the cell. The influx creates an action potential as the membrane becomes depolarized. This change in voltage quickly opens voltage-gated sodium channels all along the neuron, sending the electrical signal down the length of the neuron’s cell membrane.

Then, the cell must repolarize the cell membrane in order to prepare for the next signal. Potassium ion channels are opened, slightly repolarizing the membrane. The sodium/potassium ATPase finished the process by using ATP to pump all of the sodium out and all of the potassium back in – reestablishing the resting electrical potential! This is an extremely energy-demanding process, using up to 75% of the total ATP a nerve cell makes!

However, this sodium/potassium pump is not only found in nerve cells. It also helps regulate the ion balance in almost every cell of your body, and the ion gradients this protein can establish are used in many, many cellular processes that use secondary active transport to move different substances against their gradients!