Which molecules require facilitated diffusion to move across the plasma membrane?

• A. Oxygen
• B. Glucose
• C. Carbon dioxide
• D. Water

Answer: (B)

Verified step by step guidance

1. Understand the concept of facilitated diffusion: Facilitated diffusion is a type of passive transport that allows molecules to cross the plasma membrane with the help of transport proteins. It does not require energy and is used for molecules that cannot easily diffuse through the lipid bilayer.
2. Identify the characteristics of molecules that require facilitated diffusion: Typically, large, polar, or charged molecules cannot pass through the hydrophobic core of the plasma membrane without assistance. These molecules require transport proteins to facilitate their movement.
3. Analyze the given molecules: Oxygen and carbon dioxide are small, nonpolar molecules that can diffuse directly through the lipid bilayer without the need for transport proteins. Water, although polar, can also diffuse through the membrane via osmosis or through aquaporins, which are specific channel proteins for water.
4. Focus on glucose: Glucose is a large, polar molecule that cannot easily pass through the lipid bilayer. It requires a specific transport protein, such as a glucose transporter, to move across the plasma membrane via facilitated diffusion.
5. Conclude that glucose requires facilitated diffusion: Based on the analysis, glucose is the molecule among the options provided that requires facilitated diffusion to cross the plasma membrane, as it cannot diffuse directly through the lipid bilayer due to its size and polarity.