Cellular Energy: Mitochondria and Metabolism
Mitochondria: The Powerhouses of the Cell
Mitochondria are cellular organelles responsible for supplying most of the energy necessary for cell activity. They act as central powerhouses of the cell and synthesize ATP at the expense of metabolic fuels. In the mitochondrial matrix are located enzymes responsible for fatty acid oxidation, amino acids, pyruvic acid, and the citric acid cycle.
Metabolism: Transforming Biomolecules
Metabolism is the set of chemical reactions that occur inside cells and cause the transformation of some biomolecules into others. The chemical reactions of metabolism are called metabolic pathways, and molecules involved are called metabolites. All reactions of metabolism are regulated by enzymes, which are specific for each initial substrate of metabolism.
Metabolism can be considered in 2 phases:
- An organic matter degradation or catabolism which releases energy that is stored in the phosphate bonds of ATP.
- Construction of organic matter or anabolism which is needed for energy supply, provided by the links phosphate of ATP.
Types of Metabolism
Living matter requires the incorporation of all kinds of atoms. The most important are carbon atoms. If the source of carbon is carbon dioxide, it is called autotrophic metabolism, and if the source is organic matter itself, it is called heterotrophic metabolism. If the power source is light, it is called photosynthesis, whereas if it comes to energy released in chemical reactions, it is called chemosynthesis.
The Role of ATP in Metabolism
The nucleotide ATP is extremely important in metabolism. It can serve as a fuel molecule, storing or transferring energy with its two phosphate ester bonds, each capable of storing 7.3 kcal/mol.
ATP synthesis can be accomplished in two ways:
- Substrate-level phosphorylation: Synthesis of ATP using the energy released from a biomolecule to break some of its energy-rich bonds.
- Using enzymes from the group of ATP synthase: The synthesis of ATP by ATPase enzymes present in the ridges of the mitochondria or in the thylakoids of chloroplasts.
ATP is the energy currency of the cell, having a type of stored energy in use soon. In all metabolic reactions, the energy needed for the biosynthesis of the ATP molecule is used. Sometimes other nucleotides are used for the same purpose, such as GPT or PTC.
Catabolic Reactions
- Degradation reactions
- Oxidation
- Energy release
- From many different substrates, almost always the same products are formed.
- Convergence in products
Anabolic Reactions
- Synthesis reactions
- Reduction reactions
- Requires energy
- From a few substrates, many different products can be formed.
- Divergence of products
Catabolism: The Degradative Phase
Catabolism is the degradative phase of metabolism, and its purpose is to obtain energy. Organic molecules are transformed into simpler ones that will be involved in other metabolic reactions to become the end products of catabolism, expelled from the cell, called excretory products. The energy released in catabolism is stored in energy-rich bonds of ATP and subsequently may be used for organic synthesis reactions for cellular activities.
Catabolic reactions are oxidation reactions, i.e., loss of electrons. The manner in which electrons can be lost are:
- Dehydrogenation: Where an organic molecule is oxidized by losing hydrogen atoms.
- Oxygenation: Where an organic molecule is oxidized by adding oxygen atoms.
A single atom can lose electrons (oxidation) if another atom accepts them (reduction). Therefore, these processes are called oxidation-reduction (redox). The hydrogen atoms evolved in the oxidation reactions are captured by molecules called hydrogen carriers, such as NAD+, NADP+, and FAD, and eventually are transferred to the final hydrogen acceptor molecule, which is reduced.
Types of Catabolism
With the nature of the substance to be reduced, there are two types of catabolism: fermentation and respiration.
- In fermentation, the molecule that reduces is always organic.
- In respiration, the reducing molecule is an inorganic compound.
If oxygen is reduced, it is called aerobic respiration, and if a substance other than oxygen is reduced, it is called anaerobic respiration. In aerobic respiration, by reducing oxygen and accepting hydrogen, water is formed. In anaerobic respiration, by reducing nitrate ions, nitrite ions are formed.
Catabolism by Respiration
Catabolic reactions by respiration differ according to the organic substrates to degrade (carbohydrates, lipids, etc.).
In the digestive tract of animals and through the digestive process, the polysaccharides or disaccharides intake of animals are hydrolyzed and converted into their monosaccharide units, such as glucose, fructose, and galactose. Glycogen reserves in muscle tissue can also be hydrolyzed when energy is required for muscular exercise on glucose units.
Catabolism of glucose:
- Respiration: Glycolysis
- Respiration:
- Krebs Cycle
- Respiratory chain
- Fermentation:
- Glycolysis
- Fermentation
In the total degradation of glucose and respiration to complete utilization of all the energy released, there are two phases: glycolysis and respiration. In respiration, there are two processes: the Krebs cycle and electron transport in the respiratory chain.
Glycolysis: The Embden-Meyerhoff Pathway
Glycolysis, also known as the Embden-Meyerhoff pathway, takes place in the cytoplasm. It consists of a series of ten reactions, each catalyzed by a particular enzyme, which transforms a glucose molecule into two molecules of a 3-carbon compound, pyruvic acid.
In the first part, energy is required, which is supplied by two molecules of ATP that will serve to phosphorylate glucose and fructose. At the end of this phase, two molecules of PGAL are obtained, as the molecule becomes DHAP.
In the second phase, both PGAL molecules are affected, and 4 molecules of ATP and two NADH molecules are formed. There is a net gain of two molecules of ATP. At this stage, the energy contained in the molecules of PGAL is removed by redox reactions.
At the end of the process, the glucose molecule is transformed into 2 molecules of pyruvic acid. These are molecules where the biggest part of the energy contained in glucose is now. Glycolysis occurs in most living cells, both prokaryotic and eukaryotic. The final fate of pyruvic acid and NADH produced in glycolysis depends on the type of organism involved in each case and the environment in which it develops, i.e., living in aerobic or anaerobic environments.