Cambridge A-Level Biology 9700 – Respiration: Mitochondria Structure & Function
Respiration – The Role of Mitochondria
Cellular respiration is the set of metabolic pathways that convert biochemical energy from nutrients into adenosine‑triphosphate (ATP), and release waste products. In eukaryotic cells the majority of ATP is produced in the mitochondria through oxidative phosphorylation.
Outer membrane – smooth, contains porins that allow diffusion of small molecules.
Intermembrane space (IMS) – site of proton accumulation during electron transport.
Inner membrane (IMM) – highly folded into cristae; houses the electron transport chain (ETC) complexes and ATP synthase.
Cristae – increase the surface area of the IMM, maximising the number of ETC complexes.
Matrix – contains enzymes of the citric acid (Krebs) cycle, mitochondrial DNA, ribosomes, and enzymes for fatty‑acid oxidation.
Structure ↔ Function Relationship
The specialised architecture of the mitochondrion directly supports each stage of aerobic respiration:
Glycolysis (cytosol) – produces pyruvate and NADH, which are transported into the mitochondrion.
Pyruvate oxidation (matrix) – pyruvate enters the matrix via the mitochondrial carrier proteins and is converted to acetyl‑CoA.
Citric‑acid cycle (matrix) – enzymes are soluble in the matrix, allowing efficient diffusion of substrates and products.
Electron transport chain (inner membrane) – the IMM’s extensive cristae provide a large surface area for the four ETC complexes and ATP synthase, facilitating high rates of electron flow and proton pumping.
Oxidative phosphorylation (intermembrane space & matrix) – protons pumped into the IMS create an electrochemical gradient; ATP synthase uses this gradient to synthesize ATP as protons flow back into the matrix.
Summary Table: Structural Features vs. Functional Roles
Structural Feature
Functional Role in Respiration
Outer membrane with porins
Permits free diffusion of ADP, ATP, inorganic phosphate and metabolites between cytosol and intermembrane space.
Intermembrane space
Holds protons pumped by ETC complexes, establishing the proton‑motive force.
Highly folded inner membrane (cristae)
Maximises surface area for embedding ETC complexes (I–IV) and ATP synthase, increasing ATP output.
Stabilises protein complexes of the ETC and optimises membrane fluidity for electron transfer.
Matrix enzymes
Host the citric‑acid cycle, β‑oxidation, and mitochondrial DNA replication, all occurring in a soluble environment.
Electron Micrograph Description
An electron micrograph of a typical animal cell mitochondrion shows a double‑membrane envelope. The outer membrane appears as a smooth, continuous line, while the inner membrane is seen as a series of densely packed, finger‑like projections (cristae) extending into the matrix. The matrix appears electron‑dense, indicating the presence of enzymes and mitochondrial DNA. The intermembrane space is visible as a narrow, lighter band separating the two membranes.
Suggested diagram: Cross‑sectional schematic of a mitochondrion highlighting the outer membrane, intermembrane space, inner membrane with cristae, and matrix. Include labels for each compartment and indicate the location of the electron transport chain complexes and ATP synthase.
Key Points to Remember
The inner membrane’s extensive cristae are essential for providing the surface area required for high‑capacity oxidative phosphorylation.
Compartmentalisation (matrix vs. intermembrane space) creates distinct chemical environments that drive proton gradients.
Porins in the outer membrane allow rapid exchange of metabolites, linking cytosolic glycolysis to mitochondrial respiration.
Cardiolipin, a unique phospholipid of the inner membrane, is crucial for the structural integrity of the ETC complexes.
Potential Examination Questions
Explain how the structure of the inner mitochondrial membrane enhances ATP production.
Describe the role of the intermembrane space in establishing the proton‑motive force.
Using a labelled diagram, illustrate the flow of electrons through the ETC and the synthesis of ATP by ATP synthase.