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Amino acids are the small molecule building blocks that make up proteins. Each amino acid contains a carboxylic acid and an amine group. These groups are both polar entities. The phospholipid bilayer that makes up a cell membrane has a highly hydrophobic (non polar) interior, thus that makes it difficult for charged or polar species to pass through the bilayer naturally, including amino acids. But active transport (a means of traversing a membrane via ATP powered transmembrane proteins) does allow for these types of polar species to pass through. These transmembrane proteins are designed to allow specific chemicals through the membrane, and certain ones can allow amino acids through.
The situation is the same with glycerol. It is a small molecule with three polar hydroxyl groups that is the base for lipid molecules. But glycerol on its own is polar and requires active transport to cross a membrane.
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