Mitochondria, Cell Powerhouses: Structure & Function

Mitochondria are double-membraned organelles found in eukaryotic cells. They are widespread in all eukaryotic cells except for certain cells such as human red blood cells. They play a crucial role in cellular energy production through a process called cellular respiration. Approximately 95% of the energy (ATP) required for various cellular activities comes from mitochondria, earning them the nickname "powerhouses of the cell."

The abundance of mitochondria reflects the metabolic activity of cells.

The richness of mitochondria varies among different types of cells. Animal cells have more mitochondria compared to plant cells because plants can partially substitute the functions of mitochondria with chloroplasts, and they do not require as much energy for movement as animals do. Within the same organism, different cells contain varying numbers of mitochondria. The active cells, such as liver cells, cardiac muscle cells, and pancreatic cells, have more mitochondria, while less active cells have fewer mitochondria.

Even within the same cell, mitochondria are not evenly distributed. In general, mitochondria tend to cluster in regions with high energy demand. They are often found near the Golgi apparatus, rough endoplasmic reticulum, and the cell nucleus, where substantial protein and nucleic acid synthesis consume a lot of energy. Mitochondria are also abundant near the actin filaments to provide energy for muscle cell contraction.

Structure of Mitochondria: Outer membrane, Inner membrane, Intermembrane space, Matrix

Mitochondria are granular or rod-shaped, ranging in length from 0.1 μm to 1 μm. Their size and morphology can change depending on physiological conditions. Mitochondria exhibit high plasticity, allowing them to adjust their shape and size to meet the metabolic needs of the cell. The internal structure of mitochondria cannot be resolved under an optical microscope and can only be clearly observed using an electron microscope.

Outer membrane: It separates mitochondria from the surrounding cytoplasm to form an independent compartment. The outer membrane contains channel proteins that selectively exchange substances between the cytoplasm and the interior of mitochondria, maintaining a relatively stable internal environment.

Intermembrane space: The space between the outer membrane and the inner membrane is called the intermembrane space. It contains enzymes involved in various metabolic reactions.

Inner membrane: It has low permeability and only allows the passage of uncharged small molecules. Larger molecules or ions entering the matrix require the assistance of specific carrier proteins. The inner membrane is folded into the central cavity of mitochondria, and these protrusions are called cristae, significantly increasing the surface area for biochemical reactions. The inner membrane is embedded with numerous enzymes related to respiration, making up about 80% of the total membrane protein content. These proteins include: ① oxidative enzymes in the electron transport chain; ② ATP synthase; ③ transport proteins that control the movement of metabolites in and out of the matrix; ④ DNA replication, transcription and translation enzymes.

Matrix: It is the gel-like compartment enclosed by the inner mitochondrial membrane. The citric acid cycle, β-oxidation of fatty acids, and deamination of amino acids occur in the mitochondrial matrix. Some processes such as the urea cycle and fatty acid synthesis also take place partially in the matrix. Therefore, the matrix contains enzymes required for various biochemical reactions. It also contains DNA and ribosomes for protein synthesis.

Frequently Asked Questions

Are all proteins in mitochondria self-synthesized?

The mitochondrial matrix contains circular DNA molecules that can self-replicate and transcribe to form RNA. Proteins synthesized from these RNAs are directly utilized by mitochondria. However, the proteins encoded by the mitochondrial genome are only a small fraction. The majority of proteins are encoded by nuclear DNA, synthesized by cytoplasmic ribosomes, and then transported into mitochondria. Therefore, mitochondria are considered semi-autonomous organelles.

Does aerobic respiration in cells occur in mitochondria?

Cellular aerobic respiration primarily occurs in mitochondria, not in all cellular compartments. Glycolysis, which is part of aerobic respiration, takes place in the cytoplasm. The products of glycolysis enter the mitochondria for oxidation and further breakdown.