Definition of Cellular Respiration
Cellular respiration is a biochemical process through which cells convert nutrients, primarily glucose, into energy in the form of adenosine triphosphate (ATP). This process involves a series of metabolic pathways that utilize oxygen and produce carbon dioxide and water as byproducts, particularly in aerobic conditions.
Three Stages of Cellular Respiration
- Glycolysis: The breakdown of glucose into pyruvate, occurring in the cytoplasm, which produces a small amount of ATP and NADH.
- Krebs Cycle (Citric Acid Cycle): A series of chemical reactions that occur in the mitochondria, where pyruvate is further broken down, generating more NADH, FADH2, and some ATP.
- Electron Transport Chain (ETC): A series of protein complexes located in the inner mitochondrial membrane that drive the synthesis of ATP through oxidative phosphorylation, utilizing electrons from NADH and FADH2.
Structure of the Mitochondrion
The mitochondrion is often described as the "powerhouse of the cell" and has a unique double membrane structure:
- Outer Membrane: Smooth and permeable to small molecules and ions.
- Inner Membrane: Highly folded into structures called cristae, which increase the surface area for chemical reactions.
- Intermembrane Space: The space between the outer and inner membranes.
- Matrix: The innermost compartment, containing enzymes for the Krebs cycle, mitochondrial DNA, and ribosomes.
The structure of the mitochondrion is crucial for cellular respiration because the inner membrane hosts the electron transport chain and ATP synthase, while the matrix is where the Krebs cycle occurs. The folding of the inner membrane maximizes the area available for these critical reactions.
Glycolysis
Glycolysis is the first stage of cellular respiration in which glucose (a six-carbon sugar) is enzymatically split into two molecules of pyruvate (three-carbons each). This process takes place in the cytoplasm and occurs in ten steps, involving both energy investment and energy payoff phases.
During glycolysis:
- ATP Production: A net gain of 2 ATP molecules (4 are produced, but 2 are used in the process).
- NADH Production: 2 NADH molecules are generated when glucose is oxidized.
Krebs Cycle
The Krebs cycle, also known as the citric acid cycle or TCA cycle, is the second stage of cellular respiration that occurs in the mitochondrial matrix. It involves a series of enzymatic reactions that further metabolize the pyruvate derived from glycolysis.
Products of the Krebs Cycle per cycle:
- 3 NADH
- 1 FADH2
- 1 ATP (or GTP, depending on the cell type)
- 2 CO2 (as waste products)
Since each glucose molecule generates two pyruvate molecules, the cycle runs twice for each glucose molecule, effectively doubling the products from the cycle.
Overall Task of Stage III of Aerobic Respiration
The overall task of Stage III (Electron Transport Chain) is to utilize the high-energy electrons carried by NADH and FADH2 produced in previous stages to create a proton gradient across the inner mitochondrial membrane. This gradient drives ATP synthesis through the enzyme ATP synthase as protons flow back into the mitochondrial matrix. Additionally, oxygen acts as the final electron acceptor in the electron transport chain, forming water as a byproduct.
Maximum Number of ATP Molecules in the Electron Transport Stage
The maximum yield of ATP molecules during the electron transport chain from one molecule of glucose is approximately 28 to 34 ATP molecules. When combined with the ATP produced in glycolysis (2) and the Krebs cycle (2), the total ATP yield from one molecule of glucose during aerobic respiration can be between 30 and 38 ATP in ideal conditions, depending on the shuttle systems used for transporting electrons into the mitochondria.
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