Life thrives on organization, a state maintained by energy, the fundamental currency of all living organisms. When we think of energy, food often comes to mind. However, at the cellular level, energy is synonymous with ATP (adenosine triphosphate). Cellular respiration is the fascinating process that cells employ to transform the food we consume, like sugars, into ATP, the usable energy currency for cellular activities.
Cellular respiration is essentially the breakdown of energy-rich molecules, such as glucose, to liberate energy. This process is universal, occurring in both plants and animals. The energy harvested from glucose catabolism is meticulously stored within cells in the form of ATP. The overall process of cellular respiration can be represented by the following equation:
Figure 1. A simplified equation representing cellular respiration.
Cellular respiration unfolds in three interconnected stages:
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Glycolysis: This initial stage involves the splitting of a glucose molecule into two pyruvate molecules. In this process, a net gain of 2 ATP molecules is achieved per glucose molecule, along with the generation of 2 NADH molecules. Glycolysis occurs in the cytoplasm and remarkably, does not require oxygen.
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Krebs Cycle (Citric Acid Cycle): The pyruvate molecules produced in glycolysis are further processed in the Krebs cycle. This cycle yields another 2 ATP molecules and generates a wealth of electron carriers, FADH2 and NADH, crucial for the subsequent electron transport chain. The Krebs cycle takes place in the mitochondrial matrix and is an oxygen-dependent process.
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Electron Transport Chain (ETC): Leveraging the NADH and FADH2 molecules synthesized during glycolysis and the Krebs cycle, the electron transport chain orchestrates the creation of a proton gradient. This gradient is the driving force behind the production of approximately 32 ATP molecules. The ETC is located in the inner mitochondrial membrane and, like the Krebs cycle, requires oxygen.
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Figure 2. An overview of the three stages of cellular respiration: glycolysis, the Krebs cycle, and the electron transport chain.
Key Players in the Cellular Respiration Symphony
Glucose: A simple six-carbon sugar, glucose is the body’s preferred primary energy source, readily available and efficiently metabolized.
ATP (Adenosine Triphosphate): ATP is the cell’s principal energy currency. This high-energy molecule is designed for energy storage and transport within the cellular environment, powering numerous cellular processes.
NADH (Nicotinamide Adenine Dinucleotide): A high-energy electron carrier, NADH plays a critical role in shuttling electrons derived from glycolysis and the Krebs cycle to the electron transport chain, fueling ATP production.
FADH2 (Flavin Adenine Dinucleotide): Similar to NADH, FADH2 is another high-energy electron carrier. It also transports electrons from glycolysis and the Krebs cycle to the electron transport chain, contributing to the generation of ATP.
Glycolysis: The Sugar Splitting Pathway
Glycolysis, the first act in cellular respiration, initiates the breakdown of glucose to produce ATP. In this pathway, a six-carbon glucose molecule is cleaved into two molecules of three-carbon sugars. These three-carbon sugars then undergo oxidation, and their atoms are rearranged to form two pyruvate molecules.
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Figure 3. A visual representation of the glycolysis process, showing glucose being split into pyruvate.
Glycolysis is characterized by two distinct phases:
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Preparatory Phase (Energy Investment Phase): The inherent energy within glucose is not immediately accessible. This phase requires an initial investment of energy in the form of 2 ATP molecules. These ATP molecules are used to phosphorylate glucose, resulting in a glucose molecule with two phosphate groups. This phosphorylation destabilizes glucose, making it more reactive and primed for breakdown.
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Payoff Phase (Energy Harvesting Phase): The energy invested in the preparatory phase yields significant returns in this phase. A net of 4 ATP molecules and 2 NADH molecules are generated, alongside the two molecules of pyruvate. This phase marks the net energy gain from glycolysis.
Krebs Cycle: Unlocking Energy in the Mitochondria
The pyruvate produced at the end of glycolysis is actively transported into the mitochondria, the cell’s powerhouses. Within the mitochondria, pyruvate undergoes a transformation into acetyl coenzyme A, or acetyl CoA. This conversion step also results in the transfer of electrons to NAD+, storing energy in the form of NADH.
The Krebs Cycle, also known as the Citric Acid Cycle, is a cyclic series of enzyme-catalyzed reactions, traditionally divided into three key stages:
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Citrate Formation: Acetyl CoA, a two-carbon molecule, combines with a four-carbon molecule called oxaloacetate to form a six-carbon molecule, citrate. This initiates the cycle.
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Carbon Dioxide Release: In subsequent steps, two carbon atoms are removed from the citrate molecule, released as carbon dioxide (CO2), a waste product of cellular respiration.
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Oxaloacetate Regeneration: The cycle concludes with the regeneration of the four-carbon starting molecule, oxaloacetate. This regeneration is crucial for the cycle to continue, allowing for the processing of more acetyl CoA.
The Krebs Cycle is a significant contributor to cellular energy production. It generates ATP and, more importantly, produces a substantial amount of energized electrons carried by FADH2 and NADH, which are essential for the electron transport chain.
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Figure 4. An illustration of the Krebs cycle, highlighting the cyclic nature and key inputs and outputs.
Electron Transport Chain: The ATP Powerhouse
The mitochondrial electron transport chain (ETC) shares functional similarities with the electron transport chain found in chloroplasts for photosynthesis, but operates in reverse, generating energy instead of using it to fix carbon. NADH and FADH2 molecules, carrying high-energy electrons from glycolysis and the Krebs cycle, deliver their cargo to the ETC. The electron transport chain harnesses the energy from these electrons to establish a proton gradient across the inner mitochondrial membrane. This proton gradient serves as a reservoir of potential energy that is ultimately tapped to produce a large quantity of ATP.
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Figure 5. A diagram of the electron transport chain, showing electron flow and proton pumping across the mitochondrial membrane.
The electron transport chain’s action can be summarized in a series of steps:
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Electron Delivery: Electrons, carried by NADH and FADH2 harvested from glycolysis and the Krebs cycle, are transferred to the electron transport chain.
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Proton Pumping: As electrons move through the ETC, their energy is utilized to pump protons (H+) from the mitochondrial matrix into the intermembrane space. This active transport creates a steep electrochemical gradient of protons across the inner mitochondrial membrane.
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Water Formation: At the end of the ETC, oxygen acts as the final electron acceptor. It combines with electrons and protons to form water (H2O), a byproduct of cellular respiration. This step is crucial for maintaining the flow of electrons through the chain.
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ATP Synthesis: Protons, driven by their concentration gradient, flow back down into the mitochondrial matrix through a protein complex called ATP synthase. This flow of protons provides the energy to ATP synthase to catalyze the synthesis of ATP from ADP and inorganic phosphate. This process is known as chemiosmosis.
The electron transport chain is responsible for generating the vast majority of ATP produced during cellular respiration, making it the most prolific ATP-producing stage.
Cellular respiration, encompassing glycolysis, the Krebs cycle, and the electron transport chain, is a marvel of biological engineering. This intricate process efficiently extracts energy from food molecules, converting it into the readily usable form of ATP, powering life’s diverse activities.
