Electron Transport Chain (ETC)

🧬 The Electron Transport Chain – What Is It?

A series of protein complexes in the inner mitochondrial membrane that:

• Accepts electrons from NADH and FADH₂

• Uses their energy to pump H⁺ (protons)

• Builds an electrochemical gradient

• Ends by making ATP and water

🧱 ETC Protein Complexes:

✅ Complex I – NADH Dehydrogenase

• Accepts 2 electrons from NADH

• Passes them to Ubiquinone (Q) (an electron shuttle)

• Pumps 1 proton (H⁺) into the intermembrane space

✅ Complex II – Succinate Dehydrogenase

• Accepts 2 electrons from FADH₂

• Also sends them to Ubiquinone

• ❌ Does NOT pump protons

✅ Ubiquinone (Q)

• A hydrophobic shuttle (moves in membrane)

• Carries electrons from Complex I or II to Complex III

Complex III – Cytochrome B-C1

• Accepts electrons from Ubiquinone

• Passes electrons to Cytochrome C (another shuttle)

• Pumps 1 proton (H⁺)

Cytochrome C

• A mobile protein shuttle

• Carries electrons to Complex IV

🔁 Summary of Proton Pumping:

omplex	Accepts from	Pumps H⁺
Complex I	NADH	✅ (yes)
Complex II	FADH₂	❌ (no)
Complex III	Ubiquinone	✅
Complex IV	Cytochrome C	✅

✅ The H⁺ are pumped into the intermembrane space, building a proton gradient.

💥 Energy from Electrons

• As electrons move down the ETC, they release energy (exergonic reaction)

• That energy is used to actively pump H⁺ into the intermembrane space

🧠 Analogy:
Think of a marble rolling down a spiral ramp — the further it rolls, the more energy it loses. That energy is used to pump H⁺ uphill.

💧 ATP Synthesis: Oxidative Phosphorylation

Once there’s a buildup of H⁺ outside the matrix:

1. H⁺ flows back into the matrix through ATP synthase

2. ATP synthase spins and makes ATP (ADP + Pi → ATP)

   • ATP is made using the energy from electrons that move through the Electron Transport Chain, and the H⁺ gradient created by that movement (called the electrochemical gradient), through a process called chemiosmosis.

This process is called oxidative phosphorylation

🧮 ATP Yield:

Electron Carrier	Enters at	ATP Made
NADH	Complex I	3 ATP (because 3 H⁺ pumps)
FADH₂	Complex II	2 ATP (skips Complex I, fewer pumps)

🎯 Final Products of ETC:

Reactants	Products
NADH, FADH₂, O₂	ATP, H₂O, NAD⁺, FAD

🧠 Master Summary – Step-by-Step Flow:

• NADH gives electrons to Complex I → pumps H⁺

• FADH₂ gives electrons to Complex II → no H⁺ pumped

• Ubiquinone (Q) shuttles electrons to Complex III → pumps H⁺

• Cytochrome C brings electrons to Complex IV → pumps H⁺

• Oxygen grabs the electrons and H⁺ → makes water

• All H⁺ flow back into matrix through ATP synthase

• ATP is made!