Pi: Describe the Carbohydrate Metabolism


Metabolism = the chemical ∆ in a cell when there is breakdown & synthesis of stored E

Carbohydrate Metabolism

  • 40 – 50% E requirements from oxidation of carbs
  • Glucose = main product of carb digestion & absorption
  • Glucose is absorbed via Portal System & either:

a. Utilised for E

b. Stored as Glycogen

  • Glucose is metabolised by:
      1. Glycolysis
      2. Kreb’s Cycle
      3. Electron Transport Chain (ETC)


  • Sequence of reactions which convert GIucose → Pyruvate
  • With concomitant production small amount ATP
  • Occurs in cytoplasm

Glucose (6C) + 2Pi + 2ADP + 2NAD+ → 2 Pyruvate (3C) + 2ATP + 2NADH + 2H+ + 2H2O

  • Under aerobic conditions, pyruvate enters mitochondria & is oxidised in Kreb’s cycle
  • To enter Kreb’s; pyruvate → Acetyl CoA
  • Occurs inside mitochondria
  • Regulated by [NADH]
  • ↑[NADH] inhibits Pyruvate Dehydrogenase

Citric Acid Cycle

  • Final common pathway for oxidation of fuel molecules (amino acids, carbs, fats)
  • Fuel enters Krebs as Acetyl CoA
  • Takes place in matrix of mitochondria
  • Main purpose of Krebs is to produce NADH & FADH2 which then enters ETC

Acetyl CoA + 3NAD+ + FAD + GDP + Pi + 2H2O → 2CO2 + 3NADH + FADH2 + GTP + 2H+ + CoA

  • Although O2 doesn’t participate directly in Krebs, it can only operate in aerobic conditions because NAD+ & FAD can be regenerated in mitochondria only by transfer of e to molecular O2
  • Krebs also provides one GTP per glucose, which can convert ADP → ATP


  • NADH & FADH2 formed from Krebs is E rich
  • Each contains pair e with high E
  • When these e are donated to O2, a large amount of free E is liberated
  • Electrons flow from NADH & FADH2 to molecular O2 through protein complexes on inner mitochondria membrane
  • Causes H+ to be pumped out of mitochondria matrix, generating a H+ motive force
  • ATP is synthesised when H+ flow back into mitochondrial matrix by ATPase
  • Overall 1 glucose will generate 32ATP

C6H12O6 + O2 → 6H2O + 6CO2 + 23ATP

ATP Balance Sheet

Glycolysis 2

Krebs 2

ETC 34

  • ATP = a nucleic acid. Each ATP has 3 phosphate groups which yield high E when broken ∴when a cell needs E, it removes Pi from ATP by hydrolysis
  • Respiration = process of making ATP

Glycogen Synthesis

  • When glucose enters a cell it can be used immediately to release E
  • OR stored as glycogen
  • Glycogen = large glucose polymer
  • All cells can store glycogen but main stores are liver & skeletal m.
  • Glucagon, adrenaline → stimulate glycogenolysis