Aerobic respiration is a complex, multi-stage process that efficiently produces ATP, the primary energy currency for cells. Respiration is a fundamental process that occurs in cells that extracts energy from organic molecules. While respiration can occur with or without oxygen, aerobic respiration specifically requires oxygen. Here is the definition of aerobic respiration, its significance, the organisms that rely on it, and the stages involved.
Aerobic Respiration Definition
Aerobic respiration is a cellular process in the cell uses oxygen to metabolize glucose and produce energy in the form of adenosine triphosphate (ATP). It is the most efficient form of cellular respiration and is utilized by most eukaryotic organisms.
Importance of Aerobic Respiration
Aerobic respiration is crucial for several reasons:
- Energy Production: It provides a high yield of ATP, which is the primary energy currency of cells.
- Efficiency: Compared to anaerobic respiration, aerobic respiration extracts more energy from each glucose molecule.
- Waste Products: Carbon dioxide and water, the waste products of aerobic respiration, are less toxic than the lactic acid or ethanol produced in anaerobic respiration.
Which Organisms Use Aerobic Respiration
Most eukaryotic organisms, including plants, animals, and fungi, use aerobic respiration. Some prokaryotes, like certain bacteria, also utilize this process. However, certain organisms, especially those in oxygen-deprived environments, rely on anaerobic respiration or fermentation.
While the core process of aerobic respiration is similar in both plants and animals, there differ in how they obtain glucose:
- Plants: Plants first produce glucose through photosynthesis. This glucose is then used in aerobic respiration to produce energy.
- Animals: Animals obtain glucose from the food they consume. Proteins, fats, and carbohydrates are all potential sources of glucose. This glucose is then metabolized during aerobic respiration.
Overall Chemical Equation for Aerobic Respiration
The process of aerobic respiration requires several steps, but the overall reaction is that one glucose molecule requires six oxygen molecules for a reaction that yields six carbon dioxide molecules, six water molecules, and up to 38 ATP molecules.
C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + Energy (ATP)
Steps of Aerobic Respiration
The four main steps of aerobic respiration are glycolysis, pyruvate decarboxylation (link reaction), the Krebs cycle (Citric Acid Cycle or Tricarboxylic Acid Cycle), and the electron transport chain with oxidative phosphorylation.
- Glycolysis
- Location: Cytoplasm
- Consumed: Glucose, 2 NAD+, 2 ADP + 2 Pi
- Produced: 2 Pyruvate, 2 NADH, 2 ATP
- Reaction: C6H12O6 + 2 NAD+ + 2 ADP + 2 Pi → 2 C3H4O3 + 2 NADH + 2A TP
- Pyruvate Decarboxylation (Link Reaction)
- Location: Mitochondrial matrix
- Consumed: 2 Pyruvate, 2 NAD+
- Produced: 2 Acetyl-CoA, 2 NADH, 2 CO2
- Reaction: 2 C3H4O3 + 2 NAD+ → 2 C2H3O−CoA + 2 NADH + 2 CO2
- Krebs Cycle (Citric Acid Cycle)
- Location: Mitochondrial matrix
- Consumed: 2 Acetyl-CoA, 6 NAD+, 2 FAD, 2 ADP + 2 Pi
- Produced: 4 CO2, 6 NADH, 2 FADH2, 2 ATP
- Reaction: For each Acetyl-CoA: C2H3O−CoA + 3 NAD+ + FAD + ADP + Pi → 2 CO2 + 3 NADH + FADH2 + ATP
- Electron Transport Chain (ETC) and Oxidative Phosphorylation
- Location: Inner mitochondrial membrane
- Consumed: 10 NADH, 2 FADH2, 6 O2, 32-34 ADP + 32-34 Pi
- Produced: 10 NAD+, 2 FAD, 6 H2O, 32-34 ATP
- Reaction: Electrons from NADH and FADH2 are passed through protein complexes, pumping protons into the intermembrane space. Oxygen acts as the final electron acceptor, forming water. The proton gradient drives ATP synthesis.
A Closer Look at the Steps
Glycolysis
Glycolysis is the initial step of both aerobic and anaerobic respiration and the only step that occurs in the cytoplasm of the cell. It involves the breakdown of one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). The process consists of ten enzyme-catalyzed reactions. These reactions consume two ATP molecules, but since four ATP molecules are produced, there is a net gain of two ATP. Additionally, the reaction generates two molecules of NADH, which find use in the later stages of aerobic respiration.
Pyruvate Decarboxylation (Link Reaction)
Once inside the mitochondrial matrix, each pyruvate molecule undergoes a decarboxylation reaction. The enzyme pyruvate dehydrogenase facilitates the reaction. The reaction removes one carbon atom pyruvate in the form of carbon dioxide. The remaining two-carbon compound attaches to coenzyme A, forming acetyl-CoA. The yield is one molecule of NADH for each pyruvate.
Krebs Cycle (Citric Acid Cycle)
The Krebs Cycle, also known as the citric acid cycle, is a series of chemical reactions that produce energy through the oxidation of acetyl-CoA. Like pyruvate decarboxylation, it occurs in the mitochondrial matrix. Each acetyl-CoA molecule combines with a four-carbon molecule, oxaloacetate, and forms a six-carbon molecule, citrate. As citrate undergoes a series of transformations, two molecules of CO2 are released, and the original four-carbon oxaloacetate is regenerated.
Since one glucose molecule produces two pyruvate molecules, and each pyruvate leads to one acetyl-CoA, the Krebs Cycle runs twice for each glucose molecule.
Each acetyl-CoA that enters the Krebs Cycle produces:
- Three molecules of NADH
- One molecule of FADH2
- One molecule of ATP (or GTP, in some organisms) through substrate-level phosphorylation
- Two molecules of CO2
Each glucose molecule (which gives rise to two acetyl-CoA molecules) produces:
- Six molecules of NADH
- Two molecules of FADH2
- Two molecules of ATP (or GTP)
- Four molecules of CO2
Electron Transport Chain (ETC) and Oxidative Phosphorylation
The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. NADH and FADH2, produced in earlier stages, donate their electrons to these complexes. As electrons move through the chain, they release energy. This energy pumps protons (H+ ions) across the inner mitochondrial membrane, creating a proton gradient. This gradient drives the synthesis of ATP via an enzyme called ATP synthase. Oxygen acts as the final electron acceptor, combining with electrons and protons to form water. This step is crucial, as it prevents the backup of electrons in the ETC, allowing the continued flow and production of ATP.
Key Points
- Oxygen Requirement: Aerobic respiration requires oxygen to act as the final electron acceptor in the ETC.
- Stages: Comprises four main stages – Glycolysis, Pyruvate Decarboxylation, Krebs Cycle, and Electron Transport Chain. Some of the stages have different names.
- ATP Production: Ideally, aerobic respiration produces a net gain of approximately 36-38 ATP molecules per glucose molecule, making it highly efficient. However, in reality the gain is only 30-32 ATP/glucose. There are a variety of reasons, but ultimately the stoichiometry is a bit more complicated during oxidative phosphorylation.
- Location: While glycolysis occurs in the cytoplasm, the remaining stages take place in the mitochondria.
- By-products: Carbon dioxide and water are the primary waste products.
- NADH and FADH2: These are electron carriers produced during various stages, crucial for the ETC.
- Proton Gradient: The ETC creates a proton gradient, which is essential for ATP synthesis during oxidative phosphorylation.
- Versatility: While the core process remains consistent, different organisms have slight variations in the process or its efficiency.
References
- Reece, Jane B.; Urry, Lisa Al; et al. (2010). Campbell Biology (9th ed.). Benjamin Cummings. ISBN: 9780321558237.
- Stryer, Lubert (1995). Biochemistry (4th ed.). New York: W. H. Freeman and Company. ISBN 978-0716720096.
- Watt, Ian N.; Montgomery, Martin G.; Runswick, Michael J.; Leslie, Andrew G. W.; Walker, John E. (2010). “Bioenergetic Cost of Making an Adenosine Triphosphate Molecule in Animal Mitochondria”. Proc. Natl. Acad. Sci. USA. 107 (39): 16823–16827. doi:10.1073/pnas.1011099107