Mitochondria: Structure and Function

Mitochondria are double-membraned organelles that generate most of the cell's ATP by oxidative phosphorylation. Their inner membrane is folded into cristae that house the electron-transport chain, and they retain their own genome, reflecting their endosymbiotic bacterial ancestry. Beyond energy supply they are hubs of metabolism, calcium handling, and the regulation of cell death.

Definition

Mitochondria are double-membrane-bounded organelles, with a cristae-folded inner membrane and a protein-rich matrix, that produce ATP by oxidative phosphorylation, carry their own DNA, and integrate metabolism, calcium signalling and the control of programmed cell death.

Scope

The entry covers mitochondrial ultrastructure (outer and inner membranes, cristae, matrix), oxidative phosphorylation and the chemiosmotic mechanism, the mitochondrial genome and protein import, the dynamics of fission, fusion and quality control, and the organelle's role in apoptosis. It is a cell-biology and histology reference topic, not clinical guidance.

Core questions

• How does the inner membrane couple electron transport to ATP synthesis?
• Why do mitochondria have two membranes and their own genome?
• How are mitochondrial proteins imported and how do the organelles divide and fuse?
• How do mitochondria participate in calcium handling and in triggering cell death?

Key concepts

• Outer and inner mitochondrial membranes
• Cristae and the matrix
• Electron-transport chain and oxidative phosphorylation
• ATP synthase and the proton-motive force
• Mitochondrial DNA and maternal inheritance
• Mitochondrial fission and fusion dynamics
• Mitochondrial control of apoptosis

Key theories

Chemiosmotic theory

Electron transport pumps protons across the inner mitochondrial membrane, creating an electrochemical gradient whose energy is used by ATP synthase to phosphorylate ADP—coupling respiration to ATP production through a transmembrane proton-motive force.

Endosymbiotic origin

Mitochondria are descended from an engulfed alpha-proteobacterium, accounting for their double membrane, their own circular genome and their dedicated protein-import machinery.

Mechanisms

Substrate oxidation in the matrix feeds electrons into the inner-membrane electron-transport chain, which pumps protons into the intermembrane space; the resulting electrochemical gradient drives ATP synthase to make ATP, the chemiosmotic mechanism. The inner membrane's cristae greatly expand the surface available for this machinery, while the matrix houses the citric-acid cycle and the mitochondrial genome. Most mitochondrial proteins are encoded in the nucleus, synthesised in the cytosol, and imported through dedicated translocases in the two membranes. Mitochondria continually divide and fuse, balancing biogenesis against the removal of damaged organelles, and they integrate calcium signals. By releasing intermembrane-space proteins, mitochondria also serve as a decision point in apoptosis.

Clinical relevance

Inherited and acquired mitochondrial dysfunction is linked to a spectrum of disorders affecting high-energy tissues, and mitochondrial control of cell death is central to many disease processes and to ageing. This entry describes the normal structure and function involved and is not a basis for individual diagnostic or treatment decisions.

History

Mitochondria were described microscopically in the late nineteenth century and linked to respiration by mid-twentieth-century biochemistry. Peter Mitchell's chemiosmotic hypothesis, initially controversial, explained how respiration is coupled to ATP synthesis and earned the 1978 Nobel Prize in Chemistry. Lynn Margulis revived and popularised the endosymbiotic theory of mitochondrial origin, now supported by genomic evidence.

Key figures

• Peter Mitchell
• Lynn Margulis
• Albert Lehninger
• David Chan
• Nikolaus Pfanner

Seminal works

• saraste1999
• nunnari2012
• chan2006

Frequently asked questions

Why do mitochondria have two membranes?

The two membranes reflect their endosymbiotic bacterial origin; the inner membrane is folded into cristae that hold the electron-transport chain, while the outer membrane separates the organelle from the cytosol.

How do mitochondria make ATP?

Electron transport across the inner membrane pumps protons to build an electrochemical gradient, and ATP synthase uses the energy of protons flowing back down that gradient to phosphorylate ADP—the chemiosmotic mechanism.