Horace Albert "Nook" Barker (November 29, 1907 – December 24, 2000) was an American biochemist and microbiologist who studied the operation of biological and chemical processes in plants, humans and other animals, including using radioactive tracers to determine the role enzymes play in synthesizing sucrose.
He was recognized with the National Medal of Science for his role in identifying an active form of Vitamin B12.