Final answer:
The creation of Citrate in the first step of the Krebs cycle is achieved by combining Acetyl-CoA with Oxaloacetate to form a six-carbon Citrate molecule.
Step-by-step explanation:
The first step of the Krebs cycle involves the creation of Citrate. This is done by a process called a condensation reaction, in which an Acetyl-CoA molecule combines with a four-carbon molecule, Oxaloacetate (OAA), to form a six-carbon molecule of Citrate. It is important to note that prior to this step, a transitional phase occurs where pyruvate is converted into Acetyl-CoA. This first step is highly exergonic and irreversible, meaning it releases energy and cannot be simply reversed. The rate of this reaction is subject to control by negative feedback depending on the levels of ATP available.