Final answer:
The student's statement is true; substances can diffuse across membranes without additional cellular energy, which is called passive transport. Diffusion and facilitated diffusion are types of passive transport, contrasting active transport that requires energy.
Step-by-step explanation:
The statement that substances diffuse across membranes without requiring the cell to use additional energy is true. This type of transport is known as passive transport, which includes the process of diffusion. Through diffusion, substances naturally move from an area of higher concentration to an area of lower concentration, following the concentration gradient. No cellular energy in the form of adenosine triphosphate (ATP) is needed for this process, in contrast to active transport, which does require energy to move substances against their concentration gradient.
Examples of passive transport are the movement of oxygen (O₂) into cells and carbon dioxide (CO₂) out of cells because they move down their concentration gradients. Additionally, other forms of passive transport include the diffusion of small nonpolar molecules and facilitated diffusion, which involves carrier proteins and channel proteins for substances that cannot easily pass through the lipid bilayer due to their size, polarity, or charge.