Cell transport physiology: passive diffusion across biological membranes In cellular physiology, solute molecules cross the lipid bilayer by passive diffusion primarily as a result of which driving force?

Introduction / Context:
Passive diffusion is a fundamental transport process in cell biology. Without using metabolic energy, small nonpolar molecules (for example, O2, CO2) move across the phospholipid bilayer. Understanding what actually drives this movement clarifies how cells exchange gases and small solutes and why some substances require channels or carriers.

Given Data / Assumptions:

• No transporter proteins or pumps are involved (that would be facilitated diffusion or active transport).
• The solute is moving directly through the lipid bilayer or an always-open pore without gating or coupling to ATP.
• The membrane is intact and at constant temperature and pressure for both sides of the bilayer in typical laboratory/physiological settings.

Concept / Approach:

Passive diffusion is driven by a gradient in chemical potential. For uncharged solutes, the chemical potential gradient is essentially the concentration difference across the membrane. Ions can be influenced by electrical potential as well, but simple passive permeation of ions through pure lipid is negligible, which is why cells require channels. Therefore, when we speak about passive diffusion across the bilayer, the operative driving force is the concentration difference of a permeant solute.

Step-by-Step Solution:

Verification / Alternative check:

Fick’s first law states: J = -D * (dC/dx). For a thin membrane, this scales with the difference in concentrations across the bilayer, consistent with observed oxygen and carbon dioxide fluxes in cells and tissues.

Why Other Options Are Wrong:

• Pressure difference: Drives bulk flow/filtration through porous barriers, not molecular diffusion through lipid.
• Ionic difference: Electrical gradients matter for ions through channels; bare bilayers are effectively impermeable to most ions.
• All of these: Overbroad; only concentration difference is correct for passive diffusion across lipid bilayer.
• Temperature difference: Affects diffusion coefficient slightly but is not the directional driving force in typical physiology.

Common Pitfalls:

• Confusing passive diffusion with osmosis/filtration, which involve water movement or pressure gradients.
• Assuming ions diffuse freely through lipid; in reality they require channels due to charge and hydration shell.

Final Answer:

concentration difference