metabolic pathways

Pathway

Location

Input

Output

Main Purpose

Glycolysis

Cytoplasm

Glucose

Pyruvate, ATP, NADH

Break down glucose

Pyruvate Oxidation

Mitochondria

Pyruvate

Acetyl-CoA, NADH, COâ‚‚

Link glycolysis → Krebs

Krebs Cycle

Mitochondria

Acetyl-CoA

NADH, FADHâ‚‚, ATP, COâ‚‚

Extract electrons

ETC / Ox. Phosphorylation

Inner mitochondrial membrane

NADH, FADHâ‚‚, Oâ‚‚

ATP, Hâ‚‚O

Major ATP production

PPP

Cytoplasm

Glucose-6-phosphate

NADPH, Ribose

Biosynthesis support

β-Oxidation

Mitochondria

Fatty acids

Acetyl-CoA, NADH, FADHâ‚‚

Energy from fat

Gluconeogenesis

Liver

Non-carbs

Glucose

Maintain glucose

Amino Acid Catabolism

Cytoplasm & Mitochondria

Amino acids

Krebs intermediates, NH₃

Energy or glucose source

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Main Central Metabolic Pathways

1.

Glycolysis

  • Location: Cytoplasm

  • What happens: Glucose (a 6-carbon sugar) is broken down into 2 pyruvate molecules.

  • Produces:

    • 2 ATP (net gain)

    • 2 NADH (electron carriers)

  • Purpose: Begins energy extraction from glucose; works with or without oxygen.

2.

Pyruvate Oxidation (Link Reaction)

  • Location: Mitochondrial matrix (in eukaryotes)

  • What happens: Each pyruvate is converted into acetyl-CoA, releasing CO₂ and producing NADH.

  • Purpose: Links glycolysis to the Krebs cycle.

3.

Citric Acid Cycle (Krebs Cycle or TCA Cycle)

  • Location: Mitochondrial matrix

  • What happens: Acetyl-CoA (2C) combines with oxaloacetate (4C) → forms citrate (6C) → broken down to release energy.

  • Produces (per glucose):

    • 2 ATP (or GTP)

    • 6 NADH

    • 2 FADH₂

    • 4 CO₂ (waste)

  • Purpose: Extracts high-energy electrons for use in the next stage.

4.

Electron Transport Chain (ETC) & Oxidative Phosphorylation

  • Location: Inner mitochondrial membrane

  • What happens: NADH and FADH₂ donate electrons → energy used to pump H⁺ ions → forms ATP through ATP synthase.

  • Produces:

    • ~34 ATP per glucose (major energy yield)

    • H₂O as a byproduct

  • Requires: Oxygen (final electron acceptor)

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Other Important Connected Pathways

5. Pentose Phosphate Pathway (PPP)

  • Location: Cytoplasm

  • Produces: NADPH (for biosynthesis) and ribose sugars (for nucleotides).

  • Purpose: Supports anabolic reactions and DNA/RNA synthesis.

6. Gluconeogenesis

  • Reverse of glycolysis — forms glucose from non-carbohydrate sources (like amino acids or lactate).

  • Location: Liver and kidneys.

  • Purpose: Maintains blood sugar during fasting.

7. β-Oxidation of Fatty Acids

  • Location: Mitochondria

  • What happens: Fatty acids are broken into acetyl-CoA units → enter the Krebs cycle.

  • Produces: NADH, FADH₂, and lots of ATP.

8. Amino Acid Catabolism

  • What happens: Amino acids are deaminated (removal of NH₂) → converted to intermediates that enter the Krebs cycle.

  • Purpose: Provides energy when carbs are low or for building glucose (gluconeogenesis).