What is the main driving force for diffusion of molecules across a cell membrane?

• A. Pressure gradient
• B. Temperature gradient
• C. Concentration gradient
• D. Electrical gradient

Answer: (C)

The main driving force for diffusion of molecules across a cell membrane is the concentration gradient. A concentration gradient exists when there is a difference in the concentration of a substance in two regions. Molecules tend to move from an area of higher concentration to an area of lower concentration in order to reach equilibrium, which is a state where the concentration is uniform across the space. This process is a passive transport method that does not require energy, relying solely on the inherent kinetic energy of the molecules.

Molecules will naturally travel down their concentration gradient until the concentrations on both sides of the membrane are equal. This principle applies to various substances, such as oxygen, carbon dioxide, and small nonpolar molecules, which can diffuse freely across the lipid bilayer of the cell membrane.

Understanding diffusion through the concentration gradient is crucial in biological systems, as it helps explain how essential molecules enter and exit cells, maintaining vital functions like respiration and nutrient uptake.