What defines the mechanism of diffusion across a biological membrane?

• A. Active movement of molecules requiring energy
• B. Passive movement from an area of lower to higher concentration
• C. Passive movement from an area of higher concentration to an area of lower concentration
• D. Movement that occurs only through protein channels

Answer: (C)

Diffusion across a biological membrane is defined by the passive movement of molecules from an area of higher concentration to an area of lower concentration. This process occurs spontaneously and does not require energy, as it relies on the natural tendency of particles to move towards equilibrium.

In a biological context, when a substance is in higher concentration in one compartment (like outside a cell) compared to another (like inside the cell), molecules of that substance will naturally move into the area of lower concentration (inside the cell) until the concentrations are equal. This movement is driven purely by the concentration gradient and is a fundamental concept in cellular biology for processes like gas exchange and nutrient uptake.

It's important to note that while some forms of movement across membranes can occur through protein channels, this is not a defining characteristic of diffusion. Additionally, diffusion does not involve energy input, which distinguishes it from active transport processes that require ATP to move substances against their concentration gradient.