Cellular respiration is a cornerstone of life, enabling organisms to convert nutrients into energy. Among its critical components are the Tricarboxylic Acid (TCA) cycle and the Electron Transport System (ETS). Let’s explore these processes, their significance, and the roles they play in energy generation.
The Tricarboxylic Acid Cycle (TCA cycle), also known as the Citric Acid Cycle or Krebs Cycle, is a sequence of chemical reactions occurring in the mitochondria of aerobic organisms. This cycle is pivotal in cellular respiration, breaking down Acetyl CoA—a derivative of carbohydrates, fats, and proteins—to release stored energy in the form of ATP (adenosine triphosphate).
Key Features of the TCA Cycle
The cycle was elucidated by Sir Hans Adolf Krebs, a British biochemist, earning it the moniker “Krebs Cycle.” His groundbreaking work explained many of the reactions within this cycle, forming the basis of modern bioenergetics. Coenzyme A, discovered by Fritz Lipmann and Nathan Kaplan, further completed our understanding of the cycle.
The TCA cycle is an eight-step enzymatic process that systematically converts Acetyl CoA into carbon dioxide and energy. Each step is facilitated by specific soluble enzymes.
Acetyl CoA (two carbons) combines with oxaloacetate (four carbons) to form citrate (six carbons), catalyzed by citrate synthase. CoA is released.
Citrate is converted into its isomer, isocitrate, through the removal and addition of a water molecule, facilitated by the enzyme aconitase.
Isocitrate undergoes oxidative decarboxylation, releasing one molecule of CO2 and forming α-ketoglutarate (five carbons). NAD+ is reduced to NADH in this step.
α-Ketoglutarate undergoes another oxidative decarboxylation to form succinyl CoA (four carbons), releasing CO2 and reducing another NAD+ to NADH.
The phosphate group replaces CoA in succinyl CoA, generating succinate and one molecule of ATP (or GTP) through substrate-level phosphorylation.
Succinate is oxidized to fumarate, and FAD is reduced to FADH2. This reaction occurs with the help of succinate dehydrogenase, an enzyme embedded in the mitochondrial membrane.
Water is added to fumarate, converting it into malate, catalyzed by fumarase.
Malate is oxidized to regenerate oxaloacetate, reducing NAD+ to NADH in the process.
For each cycle, the following products are generated:
Since one glucose molecule produces two Acetyl CoA molecules, these outputs are effectively doubled per glucose molecule.
The Electron Transport System is the final stage of cellular respiration, where the energy-rich molecules NADH and FADH2 donate electrons to produce ATP. This process occurs in the inner mitochondrial membrane and involves a chain of proteins, lipids, and enzymes.
Key Steps in ETS
Products of ETS
The ETS produces approximately 34 ATP molecules per glucose molecule, making it the most energy-efficient stage of cellular respiration.
Summary of Cellular Respiration
Combining glycolysis, the TCA cycle, and the ETS:
The TCA cycle and ETS are central to energy production in aerobic organisms. Together, they ensure the efficient utilization of nutrients to sustain life. Understanding these processes not only highlights the elegance of cellular machinery but also underscores the intricate ways in which life harnesses energy.
Although ATP produced directly in one TCA cycle is very small (2 ATP molecules per cycle), it participates in the release of many ATP atoms indirectly with the help of NADH and FADH2 produced in a cycle. Both of these are electron carriers and attach their electrons to the electron transport chain (ETC) to drive ATP molecules through oxidative phosphorylation. The TCA cycle acts as the last oxidative mechanism of degradation of carbohydrates, proteins, lipids, amino acids, via Acetyl CoA, or other intermediate cycles.
The amphibolic pathway is the most active as a catabolic as well as the anabolic pathway. In the TCA cycle, Coenzyme A Reaction with anabolic citrate is an anabolic pathway and additional steps follow the catabolic pathway. The TCA cycle is therefore called the Amphibolic pathway.