Simple and Facilitated Diffusion: Videos & Practice Problems

Simple and facilitated diffusion are forms of passive transport, requiring no energy as molecules move down their concentration gradients. Simple diffusion allows small uncharged molecules to pass directly through the cell membrane, while facilitated diffusion uses transport proteins, such as porins and carriers, to transport charged molecules. Porins create membrane-spanning channels for ions and water, while carriers undergo conformational changes to move substances across the membrane. Both processes are essential for maintaining cellular homeostasis and involve the movement of solutes without energy input.

Simple and Facilitated Diffusion

Simple diffusion and facilitated diffusion are both forms of passive transport, meaning they occur without any energy input. This process relies on the natural movement of molecules from areas of high concentration to areas of low concentration, which is why no energy is required.

Simple diffusion involves the direct movement of small, uncharged molecules through the cell membrane. These molecules can squeeze between the phospholipids that make up the membrane, allowing them to passively diffuse from one side to the other along their concentration gradient.

In contrast, facilitated diffusion also occurs without energy but involves the assistance of transport proteins. This method is primarily used for charged molecules, or ions, which cannot pass through the membrane as easily as uncharged molecules. The transport proteins create channels or tunnels in the membrane, enabling these ions to diffuse down their concentration gradients from high to low concentration.

Both types of diffusion are crucial for maintaining cellular homeostasis, allowing cells to regulate their internal environments effectively. Understanding the differences between simple and facilitated diffusion is essential for grasping how substances move across cell membranes and the role of concentration gradients in these processes.

Which of the following processes includes all of the others?

• A. Osmosis
• B. Facilitated diffusion
• C. Passive transport
• D. Transport of an ion down its electrochemical gradient

Answer: (C)

The difference between simple and facilitated diffusion is that facilitated diffusion:

• A. Requires a protein transporter.
• B. Moves molecules against their concentration gradient.
• C. Requires energy.
• D. Freely diffuses molecules against their concentration gradient.

Answer: (A)

Transport Proteins of Facilitated Diffusion

Facilitated diffusion is a vital process that allows molecules to cross cell membranes with the help of specific transport proteins. There are two primary types of transport proteins involved in this process: porins (or channels) and transporters (or carriers).

Porins and channels create membrane-spanning tunnels that facilitate the movement of molecules across the membrane. These structures allow substances to pass through easily, similar to a tunnel. A notable example of porins is aquaporins, which specifically transport water molecules, enhancing the rate of osmosis—the diffusion of water across a semipermeable membrane. This mechanism is crucial for maintaining cellular hydration and homeostasis.

In contrast, transporters or carriers do not form continuous tunnels. Instead, they are only open to one side of the membrane at any given time. To transport molecules, these proteins undergo conformational changes, allowing them to switch their orientation and release the molecule on the opposite side of the membrane. This process is essential for moving substances against their concentration gradients, although it still occurs passively without the need for energy input.

Both porins and transporters play significant roles in facilitated diffusion, enabling the passive transport of various molecules down their concentration gradients. Understanding these mechanisms is fundamental for grasping how cells regulate their internal environments and respond to changes in their surroundings.

Which of the following does not accurately describe a channel or a carrier?

• A. Channel – open to both sides of the membrane simultaneously.
• B. Carrier – open to one side of the membrane at a time.
• C. Carrier – requires a conformation change to complete function.
• D. Channel – not selective for molecules that move through it.

Answer: (D)

Which type(s) of molecules cannot enter/exit the cell via simple diffusion and require facilitated diffusion?

• A. Nonpolar oxygen gas molecules.
• B. Charged Ca²⁺ ions
• C. Nonpolar carbon dioxide gas molecules.
• D. Nonpolar water molecules
• E. Charged Na⁺ ions
• F. b and e only
• G. a, c, and d only

Answer: (F)

Here’s what students ask on this topic:

What is the difference between simple diffusion and facilitated diffusion?

Simple diffusion and facilitated diffusion are both forms of passive transport, meaning they do not require energy. Simple diffusion involves the direct movement of small, uncharged molecules through the cell membrane, moving from an area of high concentration to an area of low concentration. Facilitated diffusion, on the other hand, requires the help of transport proteins to move charged molecules or larger molecules across the membrane. These transport proteins can be channels, like porins, or carriers that undergo conformational changes to transport substances. Both processes move molecules down their concentration gradients without energy input.

How do transport proteins facilitate diffusion?

Transport proteins facilitate diffusion by providing a pathway for molecules to move across the cell membrane. There are two main types of transport proteins: channels (porins) and carriers. Channels create a tunnel through the membrane, allowing ions and water molecules to pass through. Aquaporins, a type of channel, specifically facilitate the movement of water. Carriers, on the other hand, bind to the molecule they transport and undergo a conformational change to move the molecule from one side of the membrane to the other. Both types of transport proteins enable the passive movement of molecules down their concentration gradients without the need for energy.

Why does facilitated diffusion not require energy?

Facilitated diffusion does not require energy because it involves the movement of molecules down their concentration gradients, from areas of high concentration to areas of low concentration. This process is driven by the natural tendency of molecules to spread out and achieve equilibrium. Since the movement is along the concentration gradient, no additional energy input is needed. Transport proteins, such as channels and carriers, assist in this process by providing a pathway for molecules that cannot directly pass through the cell membrane, but they do not require energy to function.

What types of molecules are transported by simple diffusion?

Simple diffusion transports small, uncharged molecules directly through the cell membrane. These molecules can easily pass between the phospholipids of the membrane without the need for transport proteins. Examples of molecules that typically undergo simple diffusion include oxygen (O₂), carbon dioxide (CO₂), and small lipophilic molecules. Because these molecules are small and uncharged, they can move freely down their concentration gradients from areas of high concentration to areas of low concentration without requiring energy.

What role do aquaporins play in facilitated diffusion?

Aquaporins are a specific type of channel protein involved in facilitated diffusion, primarily responsible for the transport of water molecules across the cell membrane. They create a pore that allows water to move more efficiently through the membrane, facilitating the process of osmosis. By providing a dedicated pathway for water, aquaporins enable cells to regulate their water content more effectively, which is crucial for maintaining cellular homeostasis. This process occurs without the need for energy, as water moves down its concentration gradient through the aquaporins.