phytoglobin is a protein that consists of a single globin sub-unit with one heme group. this globin sub-unit has a similar amino acid sequence as the alpha- and beta-subunits of human hb. by binding up the free o2 in a plant cell, it protects organelles from oxygen damage. which statement best describes the o2-dissociation curve of phytoglobin compared to that of hemoglobin?