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.
Barker was born on November 29, 1907 in Oakland, California. He moved with his family to Palo Alto, California when he was 11 years old. He spent a year in Germany following high school, learning the German language and absorbing its culture. He attended Stanford University, graduating in 1929 with a bachelor's degree in physical science, and was awarded a Ph.D. in chemistry in 1933.
After graduating from Stanford, he performed a two-year postgraduate fellowship at the Hopkins Marine Station under the supervision of microbiologist C. B. van Niel, who fostered Barker's interest in botany and taught him techniques for isolating microorganisms. He then spent a year at the Delft Microbiology Laboratory in the Netherlands under Albert Kluyver.
Barker was hired in 1936 by the University of California, Berkeley to teach soil microbiology. He was part of a team that developed the use of Carbon-14 as a radioactive tracer, using the technique in 1944 to show how sucrose is synthesized in living cells by enzymes.