Unit 5: Carbohydrate Metabolism

Carbohydrate metabolism is the set of biochemical processes responsible for the synthesis, breakdown, and interconversion of carbohydrates in living organisms.

1. Glycolysis (Embden-Meyerhof Pathway)

Glycolysis is a 10-step metabolic pathway that converts one molecule of Glucose (6C) into two molecules of Pyruvate (3C).

• Location: Cytosol of all cells.
• Phases:
  1. Preparatory Phase (Energy Investment): Steps 1-5. Consumes 2 ATP to "activate" the glucose.
  2. Payoff Phase (Energy Generation): Steps 6-10. Produces 4 ATP and 2 NADH.

Net Yield of Glycolysis (per 1 Glucose):

• 2 Pyruvate
• 2 ATP (4 produced - 2 consumed)
• 2 NADH

2. Fate of Pyruvate

The fate of the pyruvate produced in glycolysis depends on the presence or absence of oxygen.

a) Aerobic Conditions (Oxygen present)

Pyruvate is transported into the mitochondria. It is then converted to Acetyl-CoA by the Pyruvate Dehydrogenase Complex. This Acetyl-CoA then enters the TCA cycle for complete oxidation.

Pyruvate + CoA + NAD⁺ → Acetyl-CoA + CO₂ + NADH

b) Anaerobic Conditions (Oxygen absent)

The cell needs to regenerate NAD⁺ from NADH to allow glycolysis to continue. This is done through fermentation.

• Lactic Acid Fermentation (in muscle cells, bacteria): Pyruvate is reduced to Lactate by the enzyme lactate dehydrogenase, oxidizing NADH to NAD⁺.

Pyruvate + NADH + H⁺ → Lactate + NAD⁺

• Alcoholic Fermentation (in yeast): Pyruvate is first converted to acetaldehyde, which is then reduced to Ethanol, regenerating NAD⁺.

Pyruvate → Acetaldehyde + CO₂
Acetaldehyde + NADH + H⁺ → Ethanol + NAD⁺

3. Pentose Phosphate Pathway (PPP)

The Pentose Phosphate Pathway (PPP), or HMP shunt, is an alternative pathway for glucose oxidation.

• Location: Cytosol.
• Primary Functions (NOT ATP production):
  1. To produce NADPH: This is a key reducing agent used in anabolic (biosynthetic) pathways (like fatty acid synthesis) and for antioxidant defense.
  2. To produce Ribose-5-Phosphate: This is the pentose sugar used to build nucleotides (for DNA, RNA, ATP).

4. Gluconeogenesis

Gluconeogenesis is the synthesis of new glucose from non-carbohydrate precursors. It is essentially the "reverse" of glycolysis.

• Location: Primarily in the liver (and kidney).
• Precursors: Lactate, Pyruvate, Glycerol (from fats), and certain amino acids.
• Key Point: It is not a simple reversal. The three irreversible steps of glycolysis must be bypassed using a different set of enzymes.

5. Glycogenolysis

Glycogenolysis is the breakdown of stored glycogen into glucose-1-phosphate, which is then converted to glucose-6-phosphate.

• Location: Liver (to release glucose into the blood) and Muscles (to provide glucose for its own use).
• Key Enzyme: Glycogen Phosphorylase, which cleaves the α-1,4 glycosidic bonds.

6. TCA Cycle (Krebs Cycle / Citric Acid Cycle)

The TCA Cycle is the final common pathway for the oxidation of carbohydrates, fats, and proteins. It completely oxidizes Acetyl-CoA to CO₂.

• Location: Mitochondrial Matrix.
• Process: The cycle begins when Acetyl-CoA (2C) combines with Oxaloacetate (4C) to form Citrate (6C). In a series of 8 steps, the cycle regenerates Oxaloacetate and releases energy.

Net Yield of TCA Cycle (per 1 Acetyl-CoA):

• 2 CO₂
• 3 NADH
• 1 FADH₂
• 1 GTP (or ATP)

7. Electron Transport Chain (ETC)

The ETC (or Oxidative Phosphorylation) is the process that uses the high-energy electrons from NADH and FADH₂ (produced in glycolysis and the TCA cycle) to generate the vast majority of ATP in the cell.

• Location: Inner Mitochondrial Membrane.
• Components: A series of protein complexes (Complex I, II, III, IV) and mobile carriers (Coenzyme Q, Cytochrome c).

The Process (Chemiosmosis)

1. Electron Transport: Electrons from NADH and FADH₂ are passed down the chain from one complex to the next.
2. Proton Pumping: As electrons move, Complexes I, III, and IV use the energy to pump protons (H⁺) from the matrix into the intermembrane space.
3. Proton-Motive Force: This creates a strong electrochemical gradient (a "proton-motive force").
4. Final Electron Acceptor: The electrons at the end of the chain are transferred to Oxygen (O₂), which combines with H⁺ to form Water (H₂O).
5. ATP Synthesis: The H⁺ ions flow back into the matrix down their gradient, passing through a channel in the ATP Synthase enzyme. This flow drives the "motor" of ATP synthase, which phosphorylates ADP to make ATP.

This process is highly efficient, producing ~28-30 ATP per molecule of glucose, compared to just 2 ATP from glycolysis alone.