Mitochondria (OCR A-Level Biology A): Revision Notes

Mitochondria

What are mitochondria?

Mitochondria (singular: mitochondrion) are membrane-bound organelles located in the cytoplasm of eukaryotic cells. These rod-shaped structures typically measure $2-5\,\mu\text{m}$ in length, though some extend beyond this range, and $0.5-1.0\,\mu\text{m}$ in diameter.

The number of mitochondria within a cell correlates strongly with metabolic demand. Highly active cells such as liver hepatocytes and muscle fibres contain substantially more mitochondria than cells with lower energy requirements, such as adipocytes (fat-storage cells).

Mitochondrial structure

The structural organisation of mitochondria reflects their role in aerobic respiration and ATP synthesis. Each mitochondrion possesses several distinct compartments and components that work together to facilitate energy production.

The mitochondrial envelope

Mitochondria are enclosed by an envelope consisting of two separate phospholipid membrane layers. The outer membrane remains smooth, whilst the inner membrane exhibits extensive folding. Both membranes contain channel proteins, carrier molecules and various enzymes that regulate molecular transport and metabolic processes.

Cristae and surface area

The inner mitochondrial membrane folds extensively inward to form structures called cristae. These folds dramatically increase the available surface area, allowing the membrane to accommodate large numbers of electron carrier proteins and ATP synthase enzyme complexes. The greater the surface area, the more sites available for oxidative phosphorylation and ATP production.

The intermembrane space

Between the outer and inner membranes lies the intermembrane space. This compartment plays a vital role in energy generation by maintaining a proton gradient. The space contains a higher concentration of hydrogen ions (H⁺) compared to both the cytosol and the matrix, resulting in a lower pH.

This concentration gradient represents potential energy – stored energy arising from the position of protons rather than their motion. When protons flow back through ATP synthase channels in the inner membrane, this potential energy converts into chemical energy stored in ATP molecules.

The mitochondrial matrix

Within the inner membrane, the matrix forms a gel-like region rich in proteins and lipids. This compartment houses several important components:

• A loop of mitochondrial DNA (mtDNA) – a circular, double-stranded DNA molecule resembling bacterial chromosomes
• 70S ribosomes – smaller ribosomes similar to those found in prokaryotes, distinct from the 80S ribosomes present in the eukaryotic cytoplasm
• Numerous enzyme molecules catalysing the link reaction, Krebs cycle, and contributing to the urea cycle

The matrix serves as the primary site for the link reaction and Krebs cycle during aerobic respiration. The presence of DNA and ribosomes enables the mitochondrion to synthesise some of its own proteins independently of the nuclear genome.

Structure, composition and function

The following table summarises the detailed organisation and roles of mitochondrial components:

Structure	Composition	Function
Outer mitochondrial membrane	Phospholipid bilayer with embedded proteins	Permeable to pyruvate, oxygen ($\text{O}_2$), carbon dioxide ($\text{CO}_2$), ATP and ADP, but impermeable to glucose
Inner mitochondrial membrane (folded into cristae)	Phospholipid bilayer containing protein complexes I–IV of the electron transport chain and ATP synthase	Protein complexes I, III and IV pump protons from the matrix to the intermembrane space; transport proteins facilitate movement of phosphate ions ($\text{P}_i$), pyruvate and ADP from the intermembrane space into the matrix (movement of $\text{P}_i$ and pyruvate couples with inward proton flow); ADP exchanges for ATP; ATP synthase catalyses ATP synthesis from ADP and $\text{P}_i$
Intermembrane space	Region with lower pH than both cytosol and matrix due to elevated proton (hydrogen ion) concentration	Maintains high proton concentration to drive proton movement through ATP synthase channels into the matrix
Matrix	Protein-rich gel containing a DNA loop, 70S ribosomes and numerous enzyme molecules	Site of the link reaction, Krebs cycle and part of the urea cycle; enables protein synthesis using mitochondrial ribosomes
Mitochondrial DNA (mtDNA)	Loop of double-stranded DNA	Codes for 13 proteins used within the mitochondrion; genes undergo transcription to produce mRNA; remaining mitochondrial proteins are encoded by nuclear DNA
70S ribosomes	Composed of ribosomal RNA (rRNA) and proteins	Facilitate translation – the assembly of amino acids into polypeptide chains

Evidence for endosymbiosis

The endosymbiosis theory proposes that mitochondria descended from free-living bacteria that became incorporated into ancestral eukaryotic cells. Several lines of evidence support this hypothesis:

Chloroplasts in plant cells show similar characteristics, indicating they too arose through endosymbiosis.

Adaptation to metabolic demand

Mitochondrial structure adapts to match cellular energy requirements. A trained athlete develops more mitochondria per cell, larger individual mitochondria, and increased numbers of cristae and ATP synthase complexes within each organelle compared to an inactive individual. These adaptations directly enhance the capacity for aerobic respiration and ATP generation.