Certainly! Cellular respiration is a series of metabolic processes that convert glucose into energy (ATP), and it occurs in three main stages: Glycolysis, the Citric Acid Cycle (Krebs Cycle), and Oxidative Phosphorylation. Here's a detailed explanation of each stage and how it occurs:
1. Glycolysis
Location: Cytoplasm
Process:
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Glycolysis begins with one molecule of glucose (a six-carbon sugar) and is broken down into two molecules of pyruvate (three-carbon compound).
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The process involves a series of ten enzyme-catalyzed steps and can be divided into two phases: the energy investment phase and the energy payoff phase.
Energy Investment Phase:
- Two ATP molecules are used to phosphorylate glucose and its intermediates, which prepares the molecule for breakdown.
Energy Payoff Phase:
- The six-carbon compound is split into two three-carbon molecules (glyceraldehyde-3-phosphate).
- Each of these molecules is converted into pyruvate, producing four ATP molecules (net gain of two ATP after accounting for the initial investment) and two molecules of NADH (generated through the reduction of NAD+).
2. The Citric Acid Cycle (Krebs Cycle)
Location: Mitochondrial matrix
Process:
- The pyruvate produced in glycolysis is transported into the mitochondria, where it is converted into acetyl-CoA. In this process, one carbon is released as carbon dioxide (CO2), and NAD+ is reduced to NADH.
- Acetyl-CoA enters the citric acid cycle, which consists of a series of enzymatic reactions.
Steps:
- The cycle starts when acetyl-CoA (2 carbons) combines with oxaloacetate (4 carbons) to form citrate (6 carbons).
- Throughout the cycle, citrate undergoes rearrangements and oxidative decarboxylation, resulting in:
- Release of two molecules of CO2
- Reduction of three NAD+ to NADH and one FAD to FADH2
- Production of one GTP (or ATP, depending on the cell type)
- Regeneration of oxaloacetate to continue the cycle.
The citric acid cycle thus completes the breakdown of glucose by oxidizing the carbon skeleton.
3. Oxidative Phosphorylation
Location: Inner mitochondrial membrane
Process:
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This stage consists of two main components: the Electron Transport Chain (ETC) and chemiosmosis.
Electron Transport Chain:
- NADH and FADH2, produced in glycolysis and the citric acid cycle, donate their electrons to the electron transport chain, a series of protein complexes embedded in the inner mitochondrial membrane.
- As electrons pass through the chain via a series of redox reactions, they release energy, which is used to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.
Chemiosmosis:
- The protons flow back into the mitochondrial matrix through ATP synthase (an enzyme that utilizes this proton gradient).
- The flow of protons drives the synthesis of ATP from ADP and inorganic phosphate (Pi) through a process called oxidative phosphorylation.
- At the end of the ETC, electrons combine with molecular oxygen (the final electron acceptor) and protons to form water (H2O), which is essential for maintaining the flow of electrons in the chain.
Summary of ATP Yield
The overall yield of ATP from one molecule of glucose through these processes is approximately 30-32 ATP molecules, depending on the efficiency of the transport systems used to shuttle NADH produced in glycolysis into the mitochondria.
These steps of cellular respiration are interconnected and crucial for the efficient extraction of energy stored in glucose, ultimately sustaining cellular functions.