Mitochondria Structure & Function

Mitochondria: Structure and Function

The lecture explains the mitochondrion’s structure and its roles in energy production, metabolism, protein transport, programmed cell death, and limited genetic activity.

Structure

• Outer membrane: Relatively permeable, with channels that allow many substances to pass.
• Inner membrane: Much less permeable and selectively regulates transport into and out of the mitochondrion. It contains proteins involved in the electron transport chain.
• Intermembrane space: The compartment between the two membranes, where protons accumulate during electron transport.
• Cristae: Folds of the inner membrane that increase its surface area for reactions and protein complexes.
• Mitochondrial matrix: The innermost compartment. It contains enzymes, mitochondrial DNA, and ribosomes, and is the site of many metabolic reactions.

Major Functions

1. Importing Proteins

Mitochondria make only a small portion of the proteins they need. Many mitochondrial proteins are encoded by nuclear DNA, made by cytosolic ribosomes, and imported into the organelle.

• Proteins are generally transported in an unfolded form.
• TOM complexes (translocases of the outer membrane) move proteins through the outer membrane.
• Chaperone proteins help guide them through the intermembrane space.
• TIM complexes (translocases of the inner membrane) move them across the inner membrane into the matrix.
• Once inside, proteins can be folded and activated for mitochondrial functions.

2. Transporting Metabolic Materials

Mitochondrial membranes regulate the movement of nutrients, ions, and other molecules needed for mitochondrial reactions. Examples include carbohydrates, fatty acids, and amino acids. Selective transport across the inner membrane is particularly important.

3. Producing ATP

The electron transport chain is located mainly in the inner membrane.

• NADH and FADH₂ deliver high-energy electrons to the chain.
• Electrons pass through a series of protein complexes, releasing energy.
• That energy pumps protons into the intermembrane space, creating a gradient.
• Protons flow back through ATP synthase, which uses the energy to combine ADP and inorganic phosphate into ATP.
• ATP production through this process is called oxidative phosphorylation.
• Some electrons can contribute to the formation of reactive oxygen species, including superoxide and hydrogen peroxide.

4. Supporting Metabolic Reactions

The matrix participates in numerous metabolic pathways, including:

• Conversion of pyruvate to acetyl-CoA
• The Krebs (citric acid) cycle
• Beta-oxidation of fatty acids
• Parts of the urea cycle
• Parts of gluconeogenesis
• Parts of heme synthesis
• Ketogenesis

Some pathways, including the urea cycle, gluconeogenesis, and heme synthesis, take place partly in mitochondria and partly in the cytoplasm. Glycolysis occurs in the cytoplasm; the resulting pyruvate can then enter the mitochondrion.

5. Contributing to Apoptosis

Mitochondria contribute to programmed cell death. When mitochondrial regulation is disrupted, cytochrome c can be released and activate caspases, enzymes that help carry out the cell’s death process. BCL-2-family proteins regulate this release.

6. Using Mitochondrial DNA and Ribosomes

• Mitochondrial DNA can be replicated as mitochondria divide by fission.
• It can also be transcribed into RNA, which mitochondrial ribosomes use to make some mitochondrial proteins.
• Mitochondria have 70S ribosomes, in contrast to the 80S ribosomes found in the eukaryotic cytoplasm.
• These features are connected to the endosymbiotic theory, which proposes that mitochondria originated from bacteria engulfed by early eukaryotic cells.
• Mitochondrial DNA is described as maternally inherited.

Speakers and Sources

• Professor Zach Murphy, presenter and lecturer for Ninja Nerd
• Nucleus Medical Media, source of the mitochondrion animation and its accompanying narration