A researcher finds a large amount of an unusual sugar in the cells of a recently discovered species. Which of the following are potential mechanisms the cell is using to import the sugar?

I. Passive diffusion

II. Receptor-mediated endocytosis

III. Symport

Leucine is a hydrophobic amino acid. In which of the following portions of a transmembrane protein would a leucine residue most likely be located?

What type membrane protein changes its shape when allowing substances to cross the membrane?