Which process is a form of passive transport?

Which process is a form of passive transport?

• A. Diffusion
• B. Active transport
• C. Endocytosis
• D. Exocytosis

Answer: (A)

Understanding whether transport across a cell membrane happens with or without energy is the key idea. Diffusion is the form of passive transport because it moves particles from higher to lower concentration without supplying energy. Molecules spread down their concentration gradient simply due to random motion, and this can occur directly through the lipid bilayer for small nonpolar substances or via channels for certain polar ones, all without ATP. In contrast, processes that require energy move substances against their gradient or involve membrane remodeling and vesicles: active transport uses energy to push particles uphill; endocytosis and exocytosis are bulk transport mechanisms that depend on energy to form and fuse vesicles. So diffusion fits the definition of passive transport because it proceeds spontaneously without cellular energy input.

Understanding whether transport across a cell membrane happens with or without energy is the key idea. Diffusion is the form of passive transport because it moves particles from higher to lower concentration without supplying energy. Molecules spread down their concentration gradient simply due to random motion, and this can occur directly through the lipid bilayer for small nonpolar substances or via channels for certain polar ones, all without ATP. In contrast, processes that require energy move substances against their gradient or involve membrane remodeling and vesicles: active transport uses energy to push particles uphill; endocytosis and exocytosis are bulk transport mechanisms that depend on energy to form and fuse vesicles. So diffusion fits the definition of passive transport because it proceeds spontaneously without cellular energy input.