Final answer:
In the first step of the Krebs cycle, Acetyl CoA combines with oxaloacetate to form citrate, producing NADH and CO2 as a byproduct.
Step-by-step explanation:
In Step 1 of the Krebs cycle, also known as the citric acid cycle, the substrate that "goes in" is Acetyl Coenzyme A (Acetyl CoA). This is produced when pyruvate, which is made from glucose during glycolysis, is oxidized. During this step, Acetyl CoA combines with oxaloacetate to form citrate. This process also results in the release of carbon dioxide and the reduction of NAD+ to NADH. So, summarizing the inputs and outputs: Acetyl CoA "goes in" along with oxaloacetate, and citrate "comes out" along with NADH and CO2 as a waste product. The NADH produced during the Krebs cycle is then utilized by the electron transport chain in the mitochondria to generate ATP, which is the main energy currency of the cell.