A living entity survives by respiration. The oxygen inhaled is transmitted all through the body to sustain the normal functioning of all organs within. But have you ever wondered, how this process of respiration occurs at cellular level. Before understanding that, one must know which cell organelle is responsible for cellular respiration. It is mitochondria, which is also referred to as the powerhouse of cells. Every cell of our body contains this organelle.
Like all other cell organelles, mitochondria is also a membrane enclosed cell organelle, contained in the cytosol (intracellular fluid) of eukaryotic cells (cells that contain a nucleus). The structure is constituted of the following parts:
This is constituted of a semi permeable phospholipid bilayer, made of porins (protein structures). This layer is permeable to ions, ATP (adenosine triphosphate), ADP (adenosine diphosphate) and nutrient molecules.
This is a complex but permeable membrane made of complex molecules of electron transport system, ATP synthetase complex and transport proteins. This layer allows oxygen, water and carbon dioxide.
Cristae are shelf like folds in inner membrane. They help in expansion of the inner cell membrane structure when there is a need for more space to accommodate more molecules of mitochondrial DNA.
This is the space between outer membrane and inner membrane. Intermembrane space is primarily responsible for oxidative phosphorylation.
Cytoplasmic matrix contains the DNA molecules (responsible for cellular respiration), enzymes (responsible for citric acid cycle reactions), dissolved gases (like oxygen, carbon dioxide), recyclable intermediates (serve as energy shuttles) and water.
Functions of the Mitochondria
Now that we know the parts of mitochondria, let's understand the important functions of this organelle. One of the major mitochondrial functions in a cell is cellular respiration. So what is cellular respiration after all? It is a chemical process of releasing energy stored in glucose. The energy utilized in breaking down glucose is supplied by ATP molecules. And ATP molecules are produced by this cell organelle. The entire process of aerobic cellular respiration is a 3 step processes. The steps are listed below:
The term glycolysis itself means "Splitting sugar". This is the first stage of cellular respiration. Glucose is a six carbon sugar. The enzymes in the cytoplasmic matrix initiate glycolysis in which a glucose molecule is oxidized to 2 molecules of three carbon sugar. Products of glycolysis are two molecules of ATP, two molecules of pyruvic acid and two NADH (Nicotinamide adenine dinucleotide) molecules (which are electron carrying molecules).
Citric Acid Cycle
This is the second phase of cellular respiration. Citric Acid cycle is also known as Krebs Cycle. The three carbon molecules which have been produced as a result of glycolysis are converted into acetyl compounds. However, the intermediary reactions of this process yields ATP molecules of energy and NAD and FAD (flavin adenine dinucleotide) molecules too. NAD and FAD molecules are further reduced in the Kerb's cycle to high energy electrons.
The electron transport chain is constituted of a series of electron carriers generated in the membrane of the mitochondria, from Krebs's cycle. The ATP molecules are further produced by the chemical reactions of these electron carrier molecules. A eukaryotic cell produces about 36 ATP molecules after cellular respiration.
Other functions include:
- Cell signaling for neurons
- Managing apoptosis
- Controlling cell cycle
- Monitoring cell differentiation, growth and development