Cells can make proteins that need to be transported out of the cell and moved to another part of the body. Cells also make carbon dioxide as a waste product of cellular respiration. Proteins are large molecules; carbon dioxide is a small molecule. Which best describes how the proteins and carbon dioxide are going to exit the cell?

Proteins are large molecules, so they leave the cell in vesicles that fuse with the cell membrane, a process called exocytosis. Carbon dioxide is small and can pass directly through the membrane by diffusion, moving from where it is more concentrated to where it is less concentrated. Thus, the correct choice is B: proteins exit by exocytosis, while carbon dioxide diffuses through the membrane.

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