Which of the following transports substances across cell membranes without energy input?

The correct answer is "Passive Transport," as this process refers to the movement of substances across a cell membrane without the requirement of energy. Passive transport utilizes natural concentration gradients, allowing molecules to move from areas of higher concentration to areas of lower concentration until equilibrium is reached. This process is vital for maintaining cellular homeostasis and involves mechanisms such as diffusion, osmosis, and facilitated diffusion.

Facilitated diffusion, while also a form of passive transport, specifically involves the use of protein channels or carriers to assist molecules that cannot easily pass through the lipid bilayer. Although it does not require energy, it is a subtype of passive transport.

Endocytosis and active transport, on the other hand, do require energy. Endocytosis involves the cell membrane engulfing substances to bring them into the cell, which requires energy. Active transport directly moves substances against their concentration gradient, requiring energy input, typically from ATP.