Per1 Deficiency Impairs Daily Mitochondrial Morphology and Function in the Liver and Intestine. A) The production rate of mitochondrial ATP in fresh livers (left) and intestine samples (right) of WT (top) and Per1−/− (bottom) mice (6–8 weeks old) collected at ZT1 and ZT13, respectively (n = 4, 6–8 weeks old, **p < 0.01, ***p < 0.001, night versus day group). B) The OCR of mitochondria in fresh livers (left) and intestine samples (right) of WT (top) and Per1−/− (bottom) mice (6–8 weeks old) collected at ZT1 and ZT13, respectively (n = 4, 6–8 weeks old, *p < 0.05, **p < 0.01, night versus day group). State 2 indicates respiration in the absence of ADP; state 3, ADP (1 mmol/L)-stimulated respiration; state 4, oligomycin (1 g/mL)-inhibited respiration. C, D) Representative TEM images of liver samples (C) and proximal intestine sections (D) from WT and Per1−/− mice (n = 3, 8 weeks old) at ZT1 and ZT13, respectively, Scale bar, 0.5 μm. E, F) The percentage of mitochondrial less than 500 nm in length (left) and total mitochondrial lengths (right) calculated from EM images of liver samples (E) and proximal intestine sections (F) from WT and Per1−/− mice at ZT1 and ZT13, respectively. Throughout, all data were expressed as the mean ± SEM, **p < 0.01, Per1−/− versus WT group. Analyses were performed using two-way ANOVA for A, B, E, F. Male mice for all experiments were maintained on standard chow.