IB Biology Topic C1.2.14 Generation of a Proton…

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

Definition

Electron transport chain (ETC)

A series of protein complexes embedded in the inner mitochondrial membrane.

After NADH and FADH₂ donate electrons to the ETC (covered in C1.2.13), those electrons flow through the carriers in sequence.

Tip

Electrons do not release all their energy at once.
The stepwise transfer through multiple carriers lets the cell capture energy at each stage, rather than losing it all as heat.

How Electron Flow Generates the Proton Gradient

The electron transport chain is a sequence of controlled transfers that steadily extracts energy from electrons.

Electrons Enter the Chain
1. Reduced NAD (NADH) donates electrons to the first carrier protein in the ETC.
2. FADH₂ donates electrons to a carrier further down the chain, so it releases less energy and pumps fewer protons.
Electrons Move Through Carriers via Redox Reactions
1. Each transfer is a redox reaction, where one carrier is reduced (gains electrons) then oxidised (passes them on).
2. As electrons pass from carrier to carrier, they move to lower energy levels.
3. Energy is released at each step and captured by the protein complexes.
Released Energy Drives Proton Pumping
1. The protein complexes use this energy to actively pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
2. Protons are pumped against their concentration gradient (from low H⁺ to high H⁺).
3. This is active transport, powered by electron flow rather than by ATP.
The Proton Gradient Forms
1. Continuous pumping creates a concentration difference across the inner mitochondrial membrane.
2. High H⁺ concentration builds up in the intermembrane space.
3. Low H⁺ concentration remains in the matrix.
4. This difference is the proton (electrochemical) gradient.
5. The gradient stores potential energy that is used in the next step of oxidative phosphorylation (C1.2.15).

Common Mistake

The energy for pumping protons does not come from ATP.
It comes from the electrons moving down the chain.
The ETC creates the conditions for ATP synthesis without using ATP.

The Gradient Sits Across the Inner Mitochondrial Membrane

The proton gradient exists across the inner mitochondrial membrane, between the intermembrane space (high H⁺) and the matrix (low H⁺).
The outer membrane is freely permeable to small ions, so it plays no role in holding this gradient.

Active recall

What happens to the energy level of electrons as they move through the carriers?
How do the protein complexes use the energy released during electron transfer?
In which direction are protons pumped, and why is this active transport?
Where exactly does the proton gradient exist in the mitochondrion?

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

Definition

Electron transport chain (ETC)

A series of protein complexes embedded in the inner mitochondrial membrane.

After NADH and FADH₂ donate electrons to the ETC (covered in C1.2.13), those electrons flow through the carriers in sequence.

Tip

Electrons do not release all their energy at once.
The stepwise transfer through multiple carriers lets the cell capture energy at each stage, rather than losing it all as heat.

How Electron Flow Generates the Proton Gradient

The electron transport chain is a sequence of controlled transfers that steadily extracts energy from electrons.

Electrons Enter the Chain
1. Reduced NAD (NADH) donates electrons to the first carrier protein in the ETC.
2. FADH₂ donates electrons to a carrier further down the chain, so it releases less energy and pumps fewer protons.
Electrons Move Through Carriers via Redox Reactions
1. Each transfer is a redox reaction, where one carrier is reduced (gains electrons) then oxidised (passes them on).
2. As electrons pass from carrier to carrier, they move to lower energy levels.
3. Energy is released at each step and captured by the protein complexes.
Released Energy Drives Proton Pumping
1. The protein complexes use this energy to actively pump protons (H⁺) from the mitochondrial matrix into the intermembrane space.
2. Protons are pumped against their concentration gradient (from low H⁺ to high H⁺).
3. This is active transport, powered by electron flow rather than by ATP.
The Proton Gradient Forms
1. Continuous pumping creates a concentration difference across the inner mitochondrial membrane.
2. High H⁺ concentration builds up in the intermembrane space.
3. Low H⁺ concentration remains in the matrix.
4. This difference is the proton (electrochemical) gradient.
5. The gradient stores potential energy that is used in the next step of oxidative phosphorylation (C1.2.15).

Common Mistake

The energy for pumping protons does not come from ATP.
It comes from the electrons moving down the chain.
The ETC creates the conditions for ATP synthesis without using ATP.

The Gradient Sits Across the Inner Mitochondrial Membrane

The proton gradient exists across the inner mitochondrial membrane, between the intermembrane space (high H⁺) and the matrix (low H⁺).
The outer membrane is freely permeable to small ions, so it plays no role in holding this gradient.

Active recall

What happens to the energy level of electrons as they move through the carriers?
How do the protein complexes use the energy released during electron transfer?
In which direction are protons pumped, and why is this active transport?
Where exactly does the proton gradient exist in the mitochondrion?

IB Biology Topic C1.2.14: Generation of a Proton Gradient Notes

IB Biology Topic C1.2.14: Generation of a Proton Gradient Notes

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

How Electron Flow Generates the Proton Gradient

The Gradient Sits Across the Inner Mitochondrial Membrane

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

How Electron Flow Generates the Proton Gradient

The Gradient Sits Across the Inner Mitochondrial Membrane

A - Unity and Diversity

B - Form and Function

C - Interaction and Interdependence

D - Continuity and Change

A - Unity and Diversity

B - Form and Function

C - Interaction and Interdependence

D - Continuity and Change

IB Biology Topic C1.2.14: Generation of a Proton Gradient Notes

IB Biology Topic C1.2.14: Generation of a Proton Gradient Notes

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

How Electron Flow Generates the Proton Gradient

The Gradient Sits Across the Inner Mitochondrial Membrane

Electron Flow Through the Carriers Pumps Protons to Build a Gradient

How Electron Flow Generates the Proton Gradient

The Gradient Sits Across the Inner Mitochondrial Membrane