Facilitated Diffusion: Videos & Practice Problems

Facilitated diffusion is a type of passive transport that requires no energy, utilizing integral membrane proteins to move solutes across the membrane. These proteins can be classified as carriers or channels. Carriers undergo conformational changes to transport molecules, while channels form a membrane-spanning tunnel, allowing solutes to pass without alteration. Specific types of channels, like aquaporins, facilitate rapid water movement. Understanding these mechanisms is crucial for grasping how substances traverse cellular membranes efficiently.

Facilitated Diffusion

Facilitated diffusion is a crucial type of passive transport that allows substances to cross cell membranes without the expenditure of energy. This process is distinct from simple diffusion due to the involvement of integral membrane proteins that assist in the transport of solutes. These proteins can be categorized into two main types: carriers (or transporters) and channels (or porins).

Carriers or transporters are integral membrane proteins that facilitate the movement of solutes by undergoing conformational changes. When a solute binds to a transporter, the protein alters its shape to transport the solute across the membrane, after which it returns to its original form to continue the process. This mechanism is essential for transporting molecules that cannot freely diffuse through the lipid bilayer due to their size or polarity.

In contrast, porins and channels form membrane-spanning tunnels that allow specific solutes to pass through without the need for conformational changes. These structures create a direct pathway for molecules to diffuse across the membrane. A notable example of this is aquaporins, specialized channels that facilitate the rapid transport of water molecules. Additionally, ion channels permit the selective passage of ions, contributing to various cellular functions.

Overall, facilitated diffusion plays a vital role in maintaining cellular homeostasis by enabling the selective transport of essential molecules across the membrane, utilizing either conformational changes in carriers or direct pathways through channels. Understanding these mechanisms is fundamental to grasping how cells interact with their environment and regulate internal conditions.

What type of transport does not require the assistance of a membrane protein?

• A. Simple diffusion.
• B. Facilitated diffusion.
• C. Passive transport.
• D. Symport of more than one molecule.
• E. All of the above require a protein.

Answer: (A)

Movement of water across a membrane is facilitated by proteins called _______:

• A. Annexins.
• B. Aquaporins.
• C. Pores.
• D. Hydroporins.
• E. Selectins.
• F. Symporters.

Answer: (B)

Here’s what students ask on this topic:

What is facilitated diffusion and how does it differ from simple diffusion?

Facilitated diffusion is a type of passive transport that moves solutes across a cell membrane without requiring energy. Unlike simple diffusion, which allows solutes to pass directly through the lipid bilayer, facilitated diffusion relies on integral membrane proteins. These proteins can be either carriers, which undergo conformational changes to transport molecules, or channels, which form a membrane-spanning tunnel. This mechanism ensures that specific molecules, such as ions or water, can efficiently traverse the membrane, maintaining cellular function.

What role do carrier proteins play in facilitated diffusion?

Carrier proteins are integral membrane proteins that facilitate the transport of molecules across the cell membrane in facilitated diffusion. They bind to the specific molecule they transport and undergo conformational changes to move the molecule from one side of the membrane to the other. This process does not require energy and allows the molecule to move down its concentration gradient. Carrier proteins are essential for transporting larger or polar molecules that cannot diffuse through the lipid bilayer on their own.

How do channel proteins function in facilitated diffusion?

Channel proteins function by forming a membrane-spanning tunnel that allows specific solutes to pass through the cell membrane without undergoing conformational changes. These channels can be specific for certain ions or molecules, such as aquaporins for water or ion channels for ions. The tunnel created by the channel protein provides a pathway for the solutes to move down their concentration gradient, facilitating efficient and selective transport across the membrane.

What are aquaporins and what is their function in facilitated diffusion?

Aquaporins are a specific type of channel protein that facilitate the rapid transport of water molecules across the cell membrane. They form a pore that allows water to move through the membrane more efficiently than it would by simple diffusion. Aquaporins are crucial for maintaining water balance in cells and tissues, especially in organs like the kidneys, where rapid water transport is essential for filtering blood and forming urine.

Why is facilitated diffusion considered a type of passive transport?

Facilitated diffusion is considered a type of passive transport because it does not require energy input from the cell. Instead, it relies on the concentration gradient of the solute, allowing molecules to move from an area of higher concentration to an area of lower concentration. The process is facilitated by integral membrane proteins, such as carriers and channels, which help specific molecules traverse the membrane efficiently without the need for ATP or other energy sources.