Facilitated diffusion across a biological membrane requires:

• A. movement of molecules against their concentration gradient
• B. a transport protein
• C. input of cellular energy (ATP)
• D. vesicle formation

Answer: (B)

Verified step by step guidance

1. Understand the concept of facilitated diffusion: Facilitated diffusion is a type of passive transport where molecules move across a biological membrane with the help of specific transport proteins. This process does not require energy (ATP) because it occurs along the concentration gradient, from a region of higher concentration to a region of lower concentration.
2. Identify the role of transport proteins: In facilitated diffusion, transport proteins are essential as they provide a pathway for molecules that cannot directly pass through the lipid bilayer of the membrane. These proteins can be channel proteins or carrier proteins, depending on the type of molecule being transported.
3. Eliminate incorrect options: Facilitated diffusion does not involve movement against the concentration gradient (this would require active transport), does not require ATP input (it is a passive process), and does not involve vesicle formation (which is associated with processes like endocytosis or exocytosis).
4. Focus on the correct answer: The correct answer is 'a transport protein,' as it is the key component that facilitates the movement of molecules across the membrane in this process.
5. Summarize the process: Facilitated diffusion is a passive transport mechanism that relies on transport proteins to move molecules across the membrane along their concentration gradient, without the need for cellular energy or vesicle formation.