In which direction does passive transport move substances?

• A. Against their concentration gradient
• B. With their concentration gradient
• C. In a random direction
• D. In one set direction

Answer: (B)

Passive transport is a biological process that facilitates the movement of substances across cell membranes without the expenditure of energy. This process relies on the natural tendency of molecules to move from an area of higher concentration to an area of lower concentration, which is referred to as following their concentration gradient.

When substances move with their concentration gradient, they are seeking to equalize concentrations on both sides of the membrane. This process can involve various mechanisms, such as simple diffusion, facilitated diffusion, or osmosis, but all share the common characteristic of not requiring energy input from the cell.

For example, in the case of simple diffusion, small nonpolar molecules like oxygen and carbon dioxide passively move through the lipid bilayer of the cell membrane, while facilitated diffusion involves specific transport proteins that help polar or charged molecules (like ions or glucose) to cross the membrane without using energy.

The other options describe scenarios that don't apply to passive transport. Moving against the concentration gradient would require energy, and therefore, is a characteristic of active transport. Movement in a random direction does not reflect a systematic process like passive transport, which is directed by concentration differences. Lastly, movement in one set direction does not accurately portray the essence of passive transport, which is based on the relative concentrations on either