The electron carriers that feed into the ETC are NADH and FADH2. NADH enters the ETC through complex I of the ETC. FADH2 enters the ETC through complex II of the ETC. Solution The electron carriers that feed into the ETC are NADH and FADH2. NADH enters the ETC through complex I of the ETC. FADH2 enters the ETC through complex II of the ETC..