Cellular respiration is a fundamental biological process where living cells convert chemical energy from nutrients into adenosine triphosphate (ATP), the primary energy currency of the cell. This series of metabolic reactions enables organisms to power nearly all life functions. The process efficiently extracts energy from organic molecules, such as glucose, making it essential for almost all life forms.
Glycolysis
Glycolysis is the initial stage of cellular respiration, involving the breakdown of glucose, a six-carbon sugar. This process occurs in the cytoplasm of both prokaryotic and eukaryotic cells. It cleaves one glucose molecule into two molecules of pyruvate, each containing three carbons.
During glycolysis, a small amount of ATP is directly produced through substrate-level phosphorylation. Two ATP molecules are consumed to initiate the process, but four ATP molecules are generated, resulting in a net gain of two ATP per glucose molecule. Additionally, two molecules of nicotinamide adenine dinucleotide (NADH) are produced. These NADH molecules act as electron carriers, storing energy for later stages of cellular respiration.
Pyruvate Oxidation
Following glycolysis, the two pyruvate molecules undergo a transitional step known as pyruvate oxidation. In eukaryotic cells, this process takes place within the mitochondrial matrix. Pyruvate must first be transported from the cytoplasm into the mitochondria to begin this stage.
Each three-carbon pyruvate molecule is converted into a two-carbon molecule called acetyl-CoA. During this conversion, a carboxyl group is removed from pyruvate and released as carbon dioxide. Concurrently, NAD+ is reduced to NADH, capturing high-energy electrons. This stage serves as an important link, preparing glycolysis products to enter the next major cycle of cellular respiration.
The Krebs Cycle
The Krebs cycle, also known as the Citric Acid Cycle, is a central metabolic pathway occurring in the mitochondrial matrix in eukaryotes. This cyclical series of reactions begins when acetyl-CoA, derived from pyruvate oxidation, combines with a four-carbon molecule, oxaloacetate, to form a six-carbon molecule called citrate. Citrate is then broken down.
For each acetyl-CoA molecule that enters, two molecules of carbon dioxide are released. The cycle also generates energy-carrying molecules: three molecules of NADH and one molecule of flavin adenine dinucleotide (FADH2). A small amount of ATP is directly produced in this cycle via substrate-level phosphorylation. Since two acetyl-CoA molecules are produced per glucose, the Krebs cycle completes two turns, doubling these outputs for each original glucose molecule.
Oxidative Phosphorylation
Oxidative phosphorylation is the final and most efficient stage of cellular respiration, occurring on the inner mitochondrial membrane. This process is composed of two interconnected parts: the electron transport chain (ETC) and chemiosmosis. It harnesses the energy stored in NADH and FADH2 molecules generated during earlier stages.
NADH and FADH2 deliver their high-energy electrons to the ETC, a series of protein complexes embedded in the inner mitochondrial membrane. As electrons move down the chain, they release energy. This energy is used to pump protons (hydrogen ions) from the mitochondrial matrix into the intermembrane space, creating a high concentration gradient across the membrane.
In chemiosmosis, these protons flow back into the mitochondrial matrix through ATP synthase. The movement of protons through ATP synthase drives the synthesis of a large amount of ATP from adenosine diphosphate (ADP) and inorganic phosphate. Oxygen acts as the final electron acceptor at the end of the ETC, combining with electrons and protons to form water, a byproduct of the process. This stage produces the vast majority of ATP, typically yielding about 28 to 32 ATP molecules per glucose.