Cellular respiration is a fundamental process by which living cells convert nutrients into adenosine triphosphate (ATP), the primary energy currency of the cell. This complex series of metabolic reactions powers virtually all cellular activities, from muscle contraction to the synthesis of new molecules.
The Mitochondrial Matrix
The Krebs cycle, also known as the Citric Acid Cycle or Tricarboxylic Acid (TCA) cycle, takes place within the mitochondrial matrix. Each mitochondrion is a double-membraned organelle.
The outer mitochondrial membrane encloses the organelle, while the inner mitochondrial membrane is highly folded into structures called cristae. These folds increase the surface area for reactions. The space enclosed by the inner membrane is the mitochondrial matrix, a dense solution containing mitochondrial DNA, ribosomes, and enzymes essential for the Krebs cycle.
Role of the Krebs Cycle
The Krebs cycle serves as a central metabolic pathway, oxidizing acetyl-CoA derived from various nutrients. This cycle systematically breaks down the acetyl group, releasing carbon dioxide as a byproduct. During this process, it generates a small amount of ATP directly.
The cycle’s primary output consists of high-energy electron carriers: nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FADH2). These molecules temporarily store the energy released from the breakdown of acetyl-CoA. They are then used in the subsequent stage of cellular respiration to produce a much larger quantity of ATP.
Preparing for the Cycle
Before the Krebs cycle can begin, glucose, a common energy source, undergoes initial processing outside the mitochondrion. This first stage, called glycolysis, occurs in the cytoplasm of the cell. During glycolysis, a six-carbon glucose molecule is broken down into two three-carbon molecules of pyruvate, along with a net production of ATP and NADH.
These pyruvate molecules then enter the mitochondrial matrix, where they undergo a transitional step called pyruvate oxidation. In this process, each pyruvate molecule is converted into a two-carbon molecule called acetyl-CoA, releasing carbon dioxide and generating more NADH. Acetyl-CoA is the molecule that directly enters the Krebs cycle, linking glycolysis to the cycle’s reactions.
Completing Energy Production
Following the Krebs cycle, the majority of ATP is generated through oxidative phosphorylation. This process utilizes the NADH and FADH2 molecules produced during the Krebs cycle and earlier stages. These electron carriers deliver their high-energy electrons to the electron transport chain (ETC), which is embedded in the inner mitochondrial membrane.
As electrons move along the ETC, energy is released and used to pump protons across the inner mitochondrial membrane, creating a proton gradient. This gradient drives the synthesis of a substantial amount of ATP through an enzyme called ATP synthase. This final stage converts the stored energy into a usable form for the cell.