IB Biology Topic B2.2.4: Adaptations of the Mitochondrion for Production of ATP by Aerobic Cell Respiration Notes

The Mitochondrion: Where Aerobic Respiration Happens

1. The mitochondrion is a double-membrane organelle found in nearly all eukaryotic cells.
2. It is the site of aerobic cell respiration, the reactions that transfer energy from organic molecules to ATP.
   1. Every structural feature of the mitochondrion relates directly to this function.
3. Mitochondria are typically sausage-shaped, roughly 1 to 5 µm long and 0.5 to 1 µm wide.
   1. Cells with high energy demands, such as muscle fibres, active-transport epithelia, and sperm cells, contain thousands of mitochondria, while less active cells have fewer.

Outer Membrane: Controlling Entry

1. The outer membrane is smooth and contains large channel proteins called porins.
   1. Porins let small molecules (pyruvate, oxygen, CO2, ADP, ATP) pass freely between the cytoplasm and the intermembrane space.
   2. Larger molecules and proteins cannot cross without specific transport.

Inner Membrane and Cristae: The Site of Oxidative Phosphorylation

1. The inner membrane is folded into structures called cristae (singular: crista).
   1. These folds massively increase the surface area for the electron transport chain (ETC) and ATP synthase.
   2. More cristae means more surface area, which means more ATP production.
   3. Cells with very high energy demands, like cardiac muscle cells, have mitochondria with densely packed cristae.
2. The inner membrane is selectively permeable and, unlike the outer membrane, has no porins.
   1. This lets it maintain a difference in hydrogen ion (H+) concentration between the intermembrane space and the matrix.
   2. That difference is the proton gradient, which stores the potential energy that drives ATP synthesis.
3. As electrons pass along the ETC in the inner membrane, their energy pumps H+ ions from the matrix into the intermembrane space.
   1. This builds a high H+ concentration in the intermembrane space relative to the matrix.
   2. The H+ ions then flow back into the matrix through ATP synthase, and the energy of that flow drives the phosphorylation of ADP to ATP.
   3. This process is called chemiosmosis.

The Matrix: Site of the Link Reaction and Krebs Cycle

1. The matrix is the fluid-filled interior of the mitochondrion, enclosed by the inner membrane.
   1. It contains the enzymes for the link reaction (which converts pyruvate to acetyl CoA) and the Krebs cycle (which oxidises acetyl CoA to release CO2, reduced NAD, and reduced FAD).
   2. These reduced coenzymes carry electrons to the ETC on the inner membrane.
2. The matrix also contains 70S ribosomes (smaller than the 80S ribosomes in the cytoplasm) and a small circular DNA molecule.
   1. These let the mitochondrion make some of its own proteins independently of the nucleus.

Mitochondrial DNA and the Endosymbiotic Theory

1. Mitochondria have their own circular DNA and 70S ribosomes, and they replicate by binary fission independently of the cell cycle.
   1. These features are strikingly similar to those of bacteria.
2. The endosymbiotic theory explains this: an ancestral eukaryotic cell engulfed an aerobic bacterium.
   1. Instead of being digested, the bacterium survived inside the host and supplied it with ATP from aerobic respiration.
   2. Over time the bacterium lost many genes to the host nucleus, and the host came to depend on its ATP, producing the mitochondrion we know today.
3. Evidence includes the double membrane, with the inner membrane from the original bacterium and the outer from the host's engulfing vesicle.
   1. Further evidence is the circular DNA, the 70S ribosomes, and the fact that mitochondria are roughly the same size as bacteria.
   2. Mitochondrial DNA is inherited maternally, because the egg contributes all the cytoplasm, and therefore all the mitochondria, to the zygote.

Mitochondrial Disease: When Mitochondrial DNA Mutates

1. Because mitochondria have their own DNA, mutations in mitochondrial genes can cause disease.
   1. Mitochondrial diseases mainly affect tissues with the highest energy demands: the brain, heart, skeletal muscles, and liver.
   2. Symptoms range from muscle weakness and fatigue to seizures and organ failure.
2. Since mitochondrial DNA is maternally inherited, these diseases pass from mother to all her children.
   1. Affected fathers do not pass them on, so the pattern differs from autosomal or sex-linked inheritance.