Mitochondria: Key Concepts for Quick Review
1. What are Mitochondria?
Imagine tiny, sausage-shaped power plants inside your body's cells (in the cytoplasm [fluid part of the cell]).
Their main job is to make energy, specifically a molecule called ATP [Adenosine Triphosphate], which is like the cell's currency for energy.
They are essential for a process called aerobic respiration [breathing with oxygen] to create this ATP.
When your cell needs energy, it "spends" ATP; energy is released when ATP is broken down (hydrolyzed).
Making ATP is most efficient when oxygen is available, especially in the final steps of aerobic respiration.
Cells that need a lot of energy (like your liver, muscles, and sperm) have many mitochondria; mature red blood cells, which don't need much energy, don't have any.
2. How are Mitochondria Organized?
Mitochondria have a unique structure with two membranes and several compartments:
Outer Mitochondrial Membrane: This is the outer cover, and it's quite leaky, allowing water and small charged particles (ions) to pass through easily.
Inner Mitochondrial Membrane: This membrane is pickier (less permeable), meaning it controls what goes in and out. It's packed with special proteins that transport things and also contains the machinery for making ATP (the respiratory chain and ATP synthase).
Intermembrane Space: This is the narrow space between the outer and inner membranes. It contains various enzymes [proteins that speed up reactions], like creatine kinase, adenylate kinase, and cytochrome c.
Matrix: This is the inner jelly-like space of the mitochondrion. It contains:
Enzymes for the Krebs cycle [a series of reactions that generate energy-carrying molecules like NADH and FADH\text{2}] and fatty acid\text{ }\beta -oxidation [breaking down fats].
Stores calcium (Ca^{\text{2}+}) ions.
Its own DNA (called mitochondrial DNA or mtDNA), ribosomes [cell parts that make proteins], and tRNA [transfer RNA, which helps in making proteins].
Cristae: These are the many folds of the inner membrane that stick into the matrix. These folds greatly increase the surface area, making more room for the machinery that makes ATP, including components of the electron transport chain and ATP synthase.
Types of Inner Membrane Folds: Most cells have plate-like cristae, but cells that make steroid hormones (like in the adrenal gland) have tubular (tube-like) cristae.
3. Special Features of Mitochondria
Own Genetic Material: Mitochondria have their own circular, double-stranded DNA (mtDNA) and ribosomes. This is strong evidence that they originally came from bacteria that were swallowed by early cells (this is called the endosymbiotic origin theory).
Maternal Inheritance: You inherit your mitochondria almost entirely from your mother. The father's sperm contributes very few or no mitochondria to the baby (zygote).
4. How Mitochondria Make Energy (ATP)
The inner membrane is where the magic happens for ATP production, through processes called oxidative phosphorylation and using the ATP synthase enzyme.
Electron Transport Chain (ETC) and ATP Synthesis:
Molecules like NADH and FADH\text{2} (produced from the Krebs cycle) drop off their electrons to the ETC, which is like a series of steps on the inner membrane.
As electrons move down these steps, protons [hydrogen ions] are pumped from the matrix into the intermembrane space.
These protons then flow back into the matrix through a special protein called ATP synthase, which uses this movement to build ATP.
The inner membrane has proteins that do three main things:
Perform oxidation reactions of the ETC.
Synthesize ATP.
Regulate how substances move into and out of the matrix.
The Krebs cycle (also known as the citric acid cycle) takes place in the matrix. It processes nutrients and feeds the NADH and FADH\text{2} molecules to the ETC.
In short, mitochondria take energy from the food you eat and turn it into ATP, which powers all the high-energy needs of your body, especially in tissues like muscles and the brain.
5. Mitochondrial Genetic Material (Genome)
The mitochondrial DNA (mtDNA) is small but mighty:
It's about 1.65\times 10^{\text{4}} bases (or ~16,500 bases) long.
It contains instructions (genes) for making \$13 proteins.
It has \$22 genes for tRNA.
It has \$2 genes for rRNA (12S and 16S types).
This means it's a very compact genome [all genetic material in an organism] that codes for essential RNA and a small set of proteins.
6. Inheritance and Development
As mentioned, mitochondria are inherited only from the mother.
This is important for understanding diseases: if a problem comes from nuclear DNA [DNA in the cell's nucleus], it can be inherited from both parents. But if it's an mtDNA problem, it's always inherited from the mother.
Mitochondria multiply by growing and then splitting in half (a process called fission) to ensure that new daughter cells get their share during cell division (mitosis).
Their resemblance to simple bacteria, with circular DNA and two membranes, points to their ancient evolutionary origin [how they evolved over time].
7. Mitochondrial Disorders (When Things Go Wrong)
Because mitochondria are so important for energy, problems with them often affect parts of the body that need a lot of energy, like muscles and the brain.
These disorders can be caused by mutations [changes] in mtDNA or by defects in the nuclear DNA that affect mitochondrial function.
Often, in mitochondrial diseases, the mitochondria themselves look unusual or damaged.
Examples of Mitochondrial Disorders:
Leber’s Hereditary Optic Neuropathy (LHON):
This is caused by specific mutations in mtDNA that reduce ATP production in the energetic neurons [brain cells] of the eye.
Symptoms: Patients experience progressive degeneration of the optic nerve, leading to blindness. Vision loss usually starts around age 20 and is more common in males.
Myoclonic Epilepsy with Ragged Red Fibers (MERRF):
This is caused by a mutation in a specific tRNA gene in mtDNA (tRNA\text{Lys} mutation).
Symptoms: It leads to muscle jerks (myoclonus), seizures, problems with coordination (ataxia), weakness, and potentially heart and lung failure.
Diagnosis: A muscle biopsy [taking a small sample of muscle tissue] shows characteristic "ragged-red fibers," which are muscle cells with abnormal, clumped mitochondria, visible with a special stain (Gomori trichrome).
The defects in MERRF impair the ETC, leading to reduced ATP production.
8. Where Mitochondria are Abundant (Examples)
Muscle: Muscle cells are very active and have many mitochondria; defects here significantly impact muscle function.
Sperm: Sperm cells need to swim a lot, so their mid-piece contains many mitochondria clustered together to power their movement.
Neurons (Brain Cells): Brain cells have a very high energy demand. Mitochondria are found in important areas like axon terminals and dendritic spines to support communication between neurons (synaptic activity).
Key Takeaways
Mitochondria are organelles with two membranes that create most of the cell's ATP. Their inner folds (cristae) provide a large surface area for this energy production.
The mitochondrial DNA (mtDNA) is small but vital for making parts of the ETC and ATP synthase; it is passed down only from the mother.
Problems with mitochondrial function mainly affect tissues with high energy needs and are the cause of disorders like LHON and MERRF.
Mitochondria are thought to have evolved from bacteria and replicate by splitting; their circular genome is unique and comes from the maternal line.