[Answer] In the reaction that builds a fat ________ groups react with ________ groups.

Answer: hydroxyl; carboxylFatty acids supply the carboxyl groups; the hydroxyls come from glycerol.

Most relevant text from all around the web:

In the reaction that builds a fat ________ groups react with ________ groups. Alcohols and organic acids join to form esters. The glycerol molecule has three hydroxyl (HO–) groups and each fatty acid has a carboxyl group (–COOH). In triglycerides the hydroxyl groups of the glycerol join the carboxyl groups of the fatty acid to form ester bonds: HOCH 2 CH (OH)CH 2 OH + RCO 2 H + R′CO 2 H + R″CO 2 H → RCO 2 CH 2 CH (O 2 CR′)CH 2 CO 2 R″ + 3H 2 O. Hydrogenation is a chemical reaction between molecular hydrogen (H 2) and another compound or element usually in the presence of a catalyst such as nickel palladium or platinum.The process is commonly employed to reduce or saturate organic compounds.Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule often an alkene. ... Fatty acid metabolism consists of various metabolic processes involving or closely related to fatty acids a family of molecules classified within the lipid macronutrient category. These processes can mainly be divided into catabolic processes that generate energy and anabolic processes that create biologically important molecules such as triglycerides phospholipids second messen...

Disclaimer: 

Our tool is still learning and trying its best to find the correct answer to your question. Now its your turn, "The more we share The more we have". Comment any other details to improve the description, we will update answer while you visit us next time...Kindly check our comments section, Sometimes our tool may wrong but not our users.

Are We Wrong To Think We're Right? Then Give Right Answer Below As Comment