Question

multiple choice question
which can diffuse directly across the plasma membrane without the help of membrane channels, filtration pores, or intercellular clefts?

glucose
oxygen
proteins
electrolytes

Explanation:

To determine which substance diffuses directly across the plasma membrane, we analyze each option:

• Glucose: Requires membrane transporters (e.g., GLUT proteins) for facilitated diffusion, so it cannot diffuse directly.
• Oxygen: Is a small, nonpolar molecule. Nonpolar molecules can pass through the lipid bilayer of the plasma membrane via simple diffusion, as the lipid bilayer is hydrophobic and allows nonpolar substances to move freely down their concentration gradient.
• Proteins: Are large, polar, and often charged molecules. They are too big and polar to pass through the lipid bilayer directly; they require processes like endocytosis or exocytosis, or specific transport mechanisms (which are not simple diffusion).
• Electrolytes: Are charged ions (e.g., Na⁺, K⁺, Cl⁻). Ions are polar and cannot pass through the nonpolar lipid bilayer directly; they require ion channels or transporters for movement across the membrane.

So, among the options, only oxygen can diffuse directly across the plasma membrane without the help of membrane channels, filtration pores, or intercellular clefts.

Answer:

B. Oxygen