Mitochondrial Structure and Function in Cellular Respiration

Mitochondria: Structure and Function

In mitochondria is where cellular respiration occurs (specifically the Krebs cycle, beta-oxidation, electron transport, and oxidative phosphorylation), whose ultimate objective is to obtain energy in the form of ATP. Mitochondria also generate precursors of the major anabolic routes.

The mitochondria are structured as follows: a double membrane formed by an external and an internal membrane, which delimits two different compartments: the intermembrane space and the mitochondrial matrix.

The Outer Mitochondrial Membrane

The outer mitochondrial membrane is in continuous contact with the cytosol. It contains numerous proteins that regulate trade in chemicals with it, including channel proteins (porins) that form large pores that make it highly permeable to small molecules.

The Inner Mitochondrial Membrane

The inner mitochondrial membrane is a continuous membrane that is surrounded by the former. It presents folds oriented toward the mitochondrial light and perpendicular to the axis of this, so-called mitochondrial cristae, which increase the membrane surface. It contains less fat and more protein than the external proteins involved in oxidative phosphorylation and the transport of metabolites between the cytosol and mitochondria. Among the proteins that make up the respiratory chain and the ATPase or ATP synthase, there are two components:

• F1: Overlooking the mitochondrial matrix, it consists of 6 peripheral proteins that are arranged around a central axis that binds to the membrane. It is directly responsible for ATP synthesis coupled with electron transport.
• F0: It is located on the inside of the inner mitochondrial membrane and consists of integral membrane proteins that form a pore on which cross protons (H+) from the intermembrane space, which stimulates the synthesis of ATP by F1.

This membrane is impermeable to most ions and small molecules and maintains the proton gradient that directs oxidative phosphorylation.

Intermembrane Space

It's a small space between the outer mitochondrial membrane and the internal one. Because of the permeability of the outer membrane, its composition is similar to the cytosol.

Mitochondrial Matrix

It is a space defined by the inner mitochondrial membrane, which occupies the center of the mitochondrion, and in which the following are distinguished:

• Mitochondrial DNA (circular DNA): Similar to bacterial DNA, it is responsible for encoding the synthesis of approximately 5% of mitochondrial proteins. The existence of mitochondrial DNA gives some independence from the cell nucleus, but does not make them self-sufficient as they need many proteins encoded by nuclear DNA. It also encodes the two rRNAs found in mitochondrial ribosomes and the 22 tRNAs required for the translation of mitochondrial mRNAs.
• Mitochondrial ribosomes with a sedimentation coefficient of 70 S, like bacterial ribosomes. The proteins of these ribosomes originate from the cytosol.
• Dense granules and calcium ions and phosphate, ADP, ATP, and coenzyme A.
• Major enzymes responsible for catabolic pathways: oxidative decarboxylation of pyruvate, Krebs cycle, and beta-oxidation of fatty acids.