Mitochondria

Key Terms

Reduction: Addition of an electron (e⁻).

Oxidation: Removal of an electron (e⁻).

Outer Membrane - Barrier containing porins for the exchange on ions and small molecules

Cristea - Highly folded inner lipid bilayer membrane, with increased permeability to ions, site of the electron transport chain + ATP synthesis

Matrix - The inner fluid filled space, containing enzymes, mitochondrial DNA and ribosomes.

Cardiolipin - Increases inner membrane impermeability and stability.

Mitochondria Basics

First discovered in the 19th century.

Size

  • Cylinders

  • Diameter - 0.5 – 1 µm

  • Length - 7 µm

Nature

  • They can change shape

  • move around the cell

  • fuse together and separate again

  • Travel on microtubules

Abundance

  • Depends on the energy requirements of the cell

  • Average Cell - 200

  • Liver Cell - 1000,2000

  • Skeletal muscle - 4000

  • Adipocytes & Skin - few

Structure

  • Double membrane structure

Outer Membrane - Barrier containing porins for the exchange on ions and small molecules

Cristea - Highly folded inner lipid bilayer membrane, with increased permeability to ions, site of the electron transport chain + ATP synthesis

  • Contains Cardiolipin which further Increases inner membrane impermeability.

Matrix - The inner fluid filled space, containing enzymes, mitochondrial DNA and ribosomes.

  • Site of the Krebs Cycle

  • Enzymes that metabolize pyruvate and fatty acids to produce acetyl CoA.

Redox Reactions

OIL RIG

Reduction: Addition of an electron (e⁻).

Oxidation: Removal of an electron (e⁻).

HYDROGEN = ELECTRON +PROTON

H = e⁻ + H⁺

Key Redox Reactions

  • FAD + 2H → FADH₂

  • NAD⁺ + 2H → NADH + H⁺.

Be careful with proton involvement:

  • Protons (H⁺) neither reduce nor oxidize.

Glucose Metabolism

Glycolysis - This is the process of oxidising Glucose into Pyruvate.

  • Occurs in the cytosol

Net production under aerobic conditions.

  • 2 ATP

  • 2 Pyruvate

  • 2 NADH

  1. Start with glucose, from diets which we use to produce energy 

  2. Glycolysis - net production of 2 ATP, it is not very efficient 

Citric Acid Cycle

The citric acid cycle occurs in the mitochondrial matrix

  • Produces reduced electron carriers (e.g., NADH) for the electron transport chain.

Inputs come from food, which is broken down in digestion 

  • these are then converted into NADH and FADH 2 these have a lot of energy in them from their electrons. 

  • the energy from food is in the electron 

  • this energy is then transferred into ATP.

Electron Transport Chain

Where it happens

  • Inner mitochondrial membrane

  • Proteins are arranged in a line

What the ETC actually does

  • Take the energy from NADH & FADH₂ and convert it into ATP without wasting any.

  • It does this by passing electrons through a series of proteins that perform redox reactions.

Complex I — NADH Dehydrogenase

  • Accepts electrons from NADH

  • Pumps H⁺ into intermembrane space

  • Passes electrons to CoQ (ubiquinone)

Complex II — Succinate Dehydrogenase

  • Accepts electrons from FADH₂

  • Does NOT pump H⁺

  • Also passes electrons to CoQ

Complex III — Cytochrome b‑c₁

  • Accepts electrons from CoQ (Ubiquinone)

  • Pumps H⁺

  • Passes electrons to cytochrome c

Complex IV — Cytochrome Oxidase

  • Accepts electrons from cytochrome c

  • Pumps H⁺

  • Transfers electrons to O₂ → forming H₂O

Oxygen is the final electron acceptor.
Without oxygen, the chain stops.

ATP Synthase

How it works:

  • ATP synthase utilizes the proton gradient created by the electron transport chain to synthesize ATP. The potential energy stored in the proton gradient is used to drive ATP synthesis.

Structure:

  • ATP synthase is a multi-subunit protein complex with a mass of over 500,000 Da responsible for catalyzing the formation of ATP from ADP and inorganic phosphate (Pi).

Efficiency

Overall Reaction:

  • Glucose + O₂ → CO₂ + H₂O + 686 kcal/mol.

  • The synthesis of ATP from ADP + Pi releases 7.3 kcal/mol.

ATP Output: In respiration, 1 glucose yields 38 ATP molecules.

Energy Calculation:

  • Energy from glucose = 38 × 7.3 kcal/mol = 277 kcal/mol.

  • Efficiency = 277 / 686 = 40%. (Remainder lost as heat.)

Mitochondrial uncoupling proteins (UCPs)

UCPs - transport proteins present in the inner mitochondrial membrane found in mammals and plants.

Function: They allow for energy from substrate oxidation to dissipate as heat rather than being captured as ATP.

Prioritization in Energy Usage

  • Priority:

    • Heat generation over energy conservation.

    • Mitochondrial membrane can be proton-leaky or non-leaky, affecting respiration and ATP production.

  • Types:

    • Uncoupled (reduced ATP synthesis) versus well-coupled systems (efficient ATP synthesis).

    • This mechanism is crucial in thermogenic cells like brown adipocytes, which help maintain body temperature.