How are simple diffusion and facilitated diffusion similar in passive transport across a cell membrane?

• A. Both are limited to the movement of water molecules and are forms of osmosis.
• B. Both move substances down their concentration gradient without requiring cellular energy (ATP).
• C. Both can move substances against their concentration gradient if enough ATP is available.
• D. Both require an integral membrane transport protein to move substances across the membrane.

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the concept of passive transport. Passive transport is the movement of molecules across a cell membrane without the use of cellular energy (ATP). It relies on the natural kinetic energy of molecules and moves substances down their concentration gradient (from high to low concentration).
2. Step 2: Define simple diffusion. Simple diffusion is the direct movement of small or nonpolar molecules (like oxygen or carbon dioxide) through the phospholipid bilayer of the cell membrane without the assistance of proteins.
3. Step 3: Define facilitated diffusion. Facilitated diffusion also moves substances down their concentration gradient without energy, but it requires specific integral membrane proteins (such as channel or carrier proteins) to help larger or polar molecules cross the membrane.
4. Step 4: Compare the similarities. Both simple diffusion and facilitated diffusion do not require ATP and move substances from areas of higher concentration to lower concentration, which means they are both forms of passive transport.
5. Step 5: Clarify the differences to reinforce understanding. Simple diffusion does not require transport proteins, while facilitated diffusion does. Neither process moves substances against their concentration gradient, and neither is limited only to water molecules (osmosis is a specific type of facilitated diffusion for water).