Carbohydrate metabolism involves the synthesis, breakdown, and transformation of carbohydrates, primarily for energy production.
1. Glycolysis
Location: Cytoplasm of the cells
Process: The conversion of glucose (a six-carbon molecule) into two molecules of pyruvate (three carbons each).
Energy Yield: 2 ATP (net gain) and 2 NADH per molecule of glucose.
Key Points: Glycolysis is an anaerobic process (does not require oxygen) and is the initial step in the utilization of glucose, providing substrates for further metabolic pathways.
2. Pyruvate Decarboxylation
Location: Mitochondria
Process: Conversion of pyruvate into acetyl CoA via the pyruvate dehydrogenase complex.
Outcome: Production of acetyl CoA, which is a critical intermediate that feeds into the citric acid cycle.
Significance: This step links glycolysis to the citric acid cycle and is crucial for aerobic respiration.
3. Citric Acid Cycle (Krebs Cycle or TCA Cycle)
Location: Mitochondrial matrix
Process: Acetyl CoA enters the cycle, undergoing a series of enzyme-catalyzed reactions that produce NADH, FADH2, and GTP/ATP.
Energy Yield: Each acetyl CoA molecule results in 3 NADH, 1 FADH2, and 1 GTP (or ATP), contributing to significant ATP production through oxidative phosphorylation.
Role: Central metabolic pathway that generates high-energy electron carriers for the electron transport chain.
4. Electron Transport Chain (ETC) and Oxidative Phosphorylation
Location: Inner mitochondrial membrane
Process: Utilizes NADH and FADH2 to generate a proton gradient across the membrane, driving the synthesis of ATP through ATP synthase.
Energy Yield: The majority of ATP in cellular respiration is produced here, with approximately 34 ATP molecules generated per glucose molecule.
Mechanism: Electrons from NADH and FADH2 pass through a series of complexes, ultimately reducing oxygen to water.
5. Gluconeogenesis
Location: Mainly in the liver and to some extent in the kidneys
Process: Synthesis of glucose from non-carbohydrate sources, such as lactate, glycerol, and amino acids.
Importance: Essential during fasting or prolonged exercise to maintain blood glucose levels.
6. Glycogenesis and Glycogenolysis
Glycogenesis: The synthesis of glycogen from glucose, primarily occurring in the liver and muscle cells for energy storage.
Glycogenolysis: The breakdown of glycogen back into glucose, providing a rapid source of glucose when needed by the body.
Regulation: These processes are tightly regulated by hormones (e.g., insulin promotes glycogenesis, while glucagon stimulates glycogenolysis).
7. Pentose Phosphate Pathway (PPP)
Location: Cytoplasm
Process: An alternative glucose metabolic pathway that generates NADPH (for biosynthetic reactions and antioxidant defense) and ribose-5-phosphate (a precursor for nucleotide synthesis).
Significance: Provides essential components for fatty acid synthesis, nucleotide biosynthesis, and maintenance of reduced glutathione levels.
Regulation and Hormonal Control
Carbohydrate metabolism is intricately regulated by hormonal signals to maintain glucose homeostasis:
A. Insulin:
Promotes glucose uptake by cells, glycogenesis, and the conversion of excess glucose into fatty acids.
B. Glucagon:
Stimulates glycogenolysis and gluconeogenesis, raising blood glucose levels during fasting.