In passive transport, how does $C_0{O}_2$ most commonly cross the cell membrane?

• A. By simple diffusion directly through the phospholipid bilayer down its concentration gradient
• B. By vesicular transport (endocytosis) into the cytoplasm
• C. By active transport through a membrane pump that hydrolyzes ATP
• D. By facilitated diffusion through aquaporin channels

Answer: (A)

Verified step by step guidance

1. Understand that passive transport involves the movement of molecules across the cell membrane without the use of cellular energy (ATP).
2. Recognize that oxygen (O\_2) is a small, nonpolar molecule, which influences how it crosses the membrane.
3. Recall that the phospholipid bilayer of the cell membrane is hydrophobic in its interior, allowing nonpolar molecules to pass through easily by simple diffusion.
4. Identify that simple diffusion moves molecules down their concentration gradient, from an area of higher concentration to an area of lower concentration, without assistance from proteins or energy input.
5. Conclude that oxygen most commonly crosses the cell membrane by simple diffusion directly through the phospholipid bilayer down its concentration gradient.