Mitochondria are organelles that help eukaryotic cells convert energy from food into ATP, the main energy currency used for cellular work. They are especially abundant in cells with high energy demands, such as muscle cells, nerve cells, and liver cells. Their folded internal structure gives them a large surface area for the reactions that release usable energy.
Understanding mitochondria connects cell biology, chemistry, evolution, and human health.
A mitochondrion has an outer membrane, an inner membrane, folded cristae, and a fluid-filled matrix. The citric acid cycle occurs in the matrix, while the electron transport chain and ATP synthase are embedded in the inner membrane. As electrons move through the chain, protons are pumped into the intermembrane space, creating a gradient that powers ATP production.
Mitochondria also contain their own DNA and ribosomes, supporting the idea that they evolved from ancient bacteria living inside larger cells.
Key Facts
- ATP is the main energy molecule made by mitochondria: ADP + Pi + energy = ATP.
- The outer membrane surrounds the mitochondrion and helps control which substances enter and leave.
- The inner membrane contains the electron transport chain and ATP synthase.
- Cristae are folds of the inner membrane that increase surface area for ATP production.
- The matrix contains enzymes for the citric acid cycle, mitochondrial DNA, and mitochondrial ribosomes.
- Overall aerobic respiration can be summarized as C6H12O6 + 6O2 = 6CO2 + 6H2O + ATP.
Vocabulary
- Mitochondrion
- A membrane-bound organelle in eukaryotic cells that produces much of the cell's ATP through aerobic respiration.
- Cristae
- Folds of the inner mitochondrial membrane that increase surface area for energy-producing reactions.
- Matrix
- The fluid-filled space inside the inner membrane where the citric acid cycle and other reactions occur.
- ATP
- Adenosine triphosphate, the molecule cells use to store and transfer usable energy.
- Endosymbiosis
- The evolutionary process in which one cell lived inside another and eventually became an organelle such as the mitochondrion.
Common Mistakes to Avoid
- Saying mitochondria make energy from nothing is wrong because they convert chemical energy in food molecules into ATP.
- Confusing the inner membrane with the outer membrane is wrong because the inner membrane holds the electron transport chain and forms cristae, while the outer membrane is the external boundary.
- Thinking all mitochondrial reactions happen in the matrix is wrong because key ATP-making steps occur on the inner membrane.
- Ignoring mitochondrial DNA is wrong because mitochondria have their own DNA and ribosomes, which is important evidence for their endosymbiotic origin.
Practice Questions
- 1 A muscle cell contains 1,500 mitochondria, while a skin cell contains 300 mitochondria. How many times more mitochondria does the muscle cell have?
- 2 If one mitochondrion produces 2.0 x 10^6 ATP molecules per minute, how many ATP molecules are produced by 400 mitochondria in one minute?
- 3 A poison blocks ATP synthase in the inner mitochondrial membrane. Explain how this would affect the proton gradient, ATP production, and the cell's ability to do work.