4.2 Glycolysis

Cellular energy is the lifeblood of living organisms. ATP, the energy currency of cells, powers essential processes. Glycolysis, a universal pathway, breaks down glucose to produce ATP and other crucial molecules.

Glycolysis occurs in nearly all organisms, from bacteria to humans. This ten-step process in the cell's cytoplasm splits glucose into two pyruvate molecules, generating ATP and NADH along the way. It's a key starting point for further energy production.

Cellular Energy and Glycolysis

Function of ATP in cells

• ATP (adenosine triphosphate) serves as the primary energy currency in cells
  • Consists of adenosine, ribose sugar, and three phosphate groups bonded together
• High-energy bonds between phosphate groups store energy for cellular processes
  • Hydrolysis of these bonds releases energy that can be harnessed by the cell
• ATP can be converted to ADP (adenosine diphosphate) by removing one phosphate group (phosphorylation)
  • This reaction releases energy that can be coupled to endergonic reactions (require energy input)
• ADP can be converted back to ATP by adding a phosphate group (dephosphorylation)
  • This reaction requires energy input from exergonic reactions (release energy), such as those in glycolysis (glucose breakdown)

Steps of glycolysis process

• Glycolysis is a ten-step process that breaks down glucose ($C_6H_{12}O_6$) into two pyruvate ($CH_3COCOO^-$) molecules
  • Takes place in the cytoplasm of the cell
• Preparatory phase (steps 1-5):
  1. Glucose is phosphorylated twice, consuming 2 ATP molecules
     • The first phosphorylation is catalyzed by hexokinase
  2. Fructose-1,6-bisphosphate is split into two three-carbon molecules: glyceraldehyde-3-phosphate (G3P) and dihydroxyacetone phosphate (DHAP)
  3. DHAP is converted to G3P, resulting in two G3P molecules
• Payoff phase (steps 6-10):
  4. Each G3P is oxidized and phosphorylated to form 1,3-bisphosphoglycerate (1,3-BPG) 5. 1,3-BPG is converted to 3-phosphoglycerate (3-PG), generating 2 ATP molecules per G3P 6. 3-PG is converted to 2-phosphoglycerate (2-PG) 7. 2-PG is converted to phosphoenolpyruvate (PEP) 8. PEP is converted to pyruvate, generating 2 more ATP molecules per G3P
• Net products of glycolysis:
  • 2 pyruvate molecules
  • 2 ATP molecules (4 produced, 2 consumed)
  • 2 NADH molecules (reduced nicotinamide adenine dinucleotide)

Regulation and Energy Transfer in Glycolysis

• Enzymes play a crucial role in catalyzing each step of glycolysis
• Substrate-level phosphorylation occurs during glycolysis, directly transferring phosphate groups to ADP to form ATP
• Allosteric regulation helps control the rate of glycolysis
  • Phosphofructokinase is a key regulatory enzyme in this process
• Glycolysis can be divided into two phases:
  • Energy investment phase: where ATP is consumed
  • Energy harvesting phase: where ATP is produced

Role of glycolysis across organisms

• Glycolysis is a central metabolic pathway found in nearly all organisms
  • Occurs in both aerobic (oxygen present) and anaerobic (oxygen absent) conditions
• In aerobic respiration, pyruvate enters the citric acid cycle (Krebs cycle) and electron transport chain
  • Generates a large amount of ATP through oxidative phosphorylation (36-38 ATP/glucose)
• In anaerobic conditions, pyruvate is converted to various fermentation products
  • Regenerates NAD+ for continued glycolysis
  • Lactic acid fermentation in muscles and ethanol fermentation in yeast
• Glycolysis is a key source of ATP and metabolic intermediates for biosynthesis
  • Provides precursors for amino acids (alanine), lipids (glycerol), and nucleotides (ribose)
• Some organisms, such as certain protists (Giardia) and bacteria (Streptococcus), rely solely on glycolysis for ATP production
  • Lack mitochondria and the citric acid cycle
• Glycolysis is a highly conserved pathway across species
  • Indicates its evolutionary importance and early origin in the history of life