Which of the following statements is correct about passive facilitated diffusion? (select all that apply)
A. It is a process in which molecules move from a region of lower concentration to one of higher concentration (or up a concentration gradient).
B. It requires a transport protein.
C. It requires an expenditure of energy by the cell.
D. It is a process in which molecules move from a region of higher concentration to a region of lower concentration (or down a concentration gradient).
E. It involves movement of molecules up a concentration gradient and requires a transport protein.
F. None
The Correct Answer and Explanation is:
The correct answers are:
B. It requires a transport protein.
D. It is a process in which molecules move from a region of higher concentration to a region of lower concentration (or down a concentration gradient).
Explanation (300+ words):
Passive facilitated diffusion is a type of passive transport that enables certain molecules—usually those that are large, polar, or charged—to move across the cell membrane with the help of transport proteins. Unlike simple diffusion, where substances move freely across the lipid bilayer, facilitated diffusion requires a specific membrane protein, such as a channel protein or carrier protein, to assist in transporting the substance.
A key characteristic of passive processes—including facilitated diffusion—is that no energy (ATP) is required. This is because the movement of molecules occurs down their concentration gradient, which means from an area of higher concentration to an area of lower concentration (Option D). This is in contrast to active transport, where substances move against their concentration gradient and require energy input (ATP).
Let’s examine the provided options:
- A. Incorrect. This describes active transport, not passive facilitated diffusion. Movement from low to high concentration requires energy.
- B. Correct. Facilitated diffusion requires a transport protein. These proteins provide a path for molecules like glucose or ions (e.g., Na⁺, Cl⁻) to pass through the otherwise impermeable phospholipid bilayer.
- C. Incorrect. Energy is not used in passive facilitated diffusion. The cell relies on the natural kinetic energy of molecules moving down their concentration gradient.
- D. Correct. This defines the process of diffusion, including facilitated diffusion—molecules move down the gradient.
- E. Incorrect. This statement confuses active transport with facilitated diffusion. Moving up the gradient requires energy and a protein, which is active transport, not passive.
- F. Incorrect. Because options B and D are correct.
In summary, passive facilitated diffusion involves the movement of molecules down their concentration gradient, requires a transport protein, and does not require energy. It is a vital process that allows cells to efficiently absorb and utilize important nutrients and ions.