AQA A-Level PE Energy Systems Practice Exam 2026 - Free PE Energy Systems Practice Questions and Study Guide

During the Krebs cycle, acetyl-CoA is oxidized and two decarboxylation steps release carbon dioxide. Specifically, CO2 is released when converting isocitrate to α-ketoglutarate and again when converting α-ketoglutarate to succinyl-CoA. This makes carbon dioxide the gas produced as a byproduct. Energy carriers NADH and FADH2 are formed, while oxygen is not produced in this cycle (it’s used later in the electron transport chain), and hydrogen isn’t released as H2 gas — it’s carried as NADH and FADH2. So carbon dioxide is the byproduct gas.