Cellular Respiration Overview and Glycolysis

Cellular Respiration Overview

• Cellular respiration is a series of metabolic pathways that release energy stored in glucose.
• The overall reaction is: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + 25ATP (or glucose + oxygen → carbon dioxide + water + ATP)
• Photosynthesis converts light energy into chemical energy (glucose). Cellular respiration then extracts that energy.

Glycolysis

• Also known as the Embden-Meyerhof-Parnas Pathway
• Occurs in the cytoplasm
• Anaerobic process (doesn't require oxygen)
• Breaks down one glucose molecule (6C) into two pyruvate molecules (3C)
• Produces ATP and NADH

Glycolysis Steps

• Step 1: Hexokinase phosphorylates glucose to glucose-6-phosphate.
• Step 2: Isomerase converts glucose-6-phosphate to fructose-6-phosphate.
• Step 3: Phosphofructokinase phosphorylates fructose-6-phosphate to fructose-1,6-bisphosphate using ATP.
• Step 4: Aldolase cleaves fructose-1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate.
• Step 5: Isomerase converts dihydroxyacetone phosphate to glyceraldehyde-3-phosphate.
• Step 6: Glyceraldehyde-3-phosphate dehydrogenase oxidizes glyceraldehyde-3-phosphate, producing NADH and 1,3-bisphosphoglycerate.
• Step 7: Phosphoglycerate kinase transfers a phosphate from 1,3-bisphosphoglycerate to ADP, forming ATP and 3-phosphoglycerate.
• Step 8: Phosphoglyceromutase converts 3-phosphoglycerate to 2-phosphoglycerate.
• Step 9: Enolase removes water from 2-phosphoglycerate, producing phosphoenolpyruvate (PEP).
• Step 10: Pyruvate kinase transfers a phosphate from PEP to ADP, forming ATP and pyruvate.

Fate of Glycolysis Products

• Pyruvate, NADH, and ATP from glycolysis are used in the next stages of aerobic respiration (Krebs Cycle).
• Pyruvate is oxidized to Acetyl CoA.

Krebs Cycle (Citric Acid Cycle, TCA Cycle)

• Acetyl CoA enters the Krebs cycle.
• It combines with oxaloacetate to form citrate.
• The cycle releases CO₂, produces NADH, FADH₂, and GTP (which is converted to ATP)
• Cycle regenerates oxaloacetate to continue.
• Key steps are listed below, with specific enzymes/products mentioned.

Krebs Cycle Steps:

• Step 1: Acetyl CoA + oxaloacetate → citrate
• Step 2: Citrate → isocitrate
• Step 3: Isocitrate → α-ketoglutarate, releasing CO₂, and producing NADH; catalyzed by isocitrate dehydrogenase (rate-limiting).
• Step 4: α-ketoglutarate → succinyl CoA, releasing CO₂ and producing NADH
• Step 5: Succinyl CoA → succinate, producing GTP (converted to ATP)
• Step 6: Succinate → fumarate, producing FADH₂
• Step 7: Fumarate → malate
• Step 8: Malate → oxaloacetate, producing NADH