Cellular Respiration

The process by which organisms combine oxygen with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding waste products.

Overview

Cellular respiration is the cornerstone of metabolism for aerobic organisms. It's a set of metabolic reactions that convert biochemical energy from nutrients into adenosine triphosphate (ATP), the universal energy currency of the cell. This process is crucial for powering cellular activities, from muscle contraction to DNA replication. The overall, simplified chemical equation for aerobic respiration is:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

The process is typically broken down into four main stages: Glycolysis, Pyruvate Oxidation, the Krebs Cycle, and Oxidative Phosphorylation.

Glycolysis

Glycolysis, meaning "sugar splitting," is the initial pathway for glucose breakdown. It is an anaerobic process, meaning it does not require oxygen, and it occurs in the cytoplasm of the cell. It serves as the metabolic foundation for both aerobic and anaerobic respiration.

Inputs & Outputs

For every one molecule of glucose that enters glycolysis, a net total of two pyruvate molecules, two ATP molecules, and two NADH molecules are produced.

The ATP is produced via substrate-level phosphorylation, and the NADH, a crucial electron carrier, will transport energy to the final stage, the Electron Transport Chain.

Pyruvate Oxidation

Before the Krebs Cycle can begin, the pyruvate from glycolysis must be transported into the mitochondrial matrix. Here, it is converted into a two-carbon molecule called an acetyl group, which is then attached to Coenzyme A (CoA), forming acetyl-CoA. This reaction is a critical link between glycolysis and the rest of aerobic respiration. During this conversion, one molecule of CO₂ is released and one molecule of NADH is formed per pyruvate.

The Krebs Cycle

Also known as the Citric Acid Cycle, this is the central hub of cellular metabolism. The acetyl-CoA produced during Pyruvate Oxidation enters the cycle by combining with a four-carbon molecule, oxaloacetate, to form citrate. Through a series of eight enzyme-catalyzed steps, the citrate is systematically broken down, releasing CO₂, and regenerating the oxaloacetate to continue the cycle.

Key Products

The primary purpose of the Krebs Cycle is not to produce large amounts of ATP directly. Instead, its main function is to generate a rich supply of electron carriers. For each turn of the cycle (one acetyl-CoA), the products are:

  • 3 molecules of NADH
  • 1 molecule of FADH₂
  • 1 molecule of ATP (or GTP)
  • 2 molecules of CO₂

These electron carriers, NADH and FADH₂, are vital for the final and most productive stage: Oxidative Phosphorylation.

Oxidative Phosphorylation

This is the grand finale of cellular respiration, where the majority of ATP is produced. It consists of two tightly coupled components: the Electron Transport Chain and Chemiosmosis. It takes place on the inner mitochondrial membrane.

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. The electron carriers (NADH and FADH₂) produced in earlier stages donate their high-energy electrons to the chain. As electrons are passed from one complex to the next, they release energy. This energy is used to pump protons (H⁺) from the mitochondrial matrix into the intermembrane space, creating a steep electrochemical gradient.

Chemiosmosis

The proton gradient established by the ETC represents a form of stored energy. These protons flow back down their gradient, into the matrix, through a remarkable enzyme channel called ATP synthase. The flow of protons causes ATP synthase to spin, harnessing the kinetic energy to catalyze the phosphorylation of ADP to ATP. This process, linking the chemical reaction of phosphorylation to the transport process, is called Chemiosmosis and is responsible for producing the vast majority of the cell's ATP.

ATP Yield Summary

While the exact number can vary depending on cellular conditions, the theoretical maximum yield of ATP from one molecule of glucose is approximately 38 ATP molecules. However, the more commonly accepted and realistic figure is around 30-32 ATP due to energy costs associated with transporting molecules across mitochondrial membranes.