Cellular Metabolism: Mitochondria, Redox Reactions, and ATP
Cellular Metabolism and Mitochondria
The cellular metabolism occurs within the mitochondria, often called the “powerhouses” of cells. Here, molecules like glucose are oxidized, breaking down into smaller pieces. During this process, hydrogen atoms (each with an electron and a proton) are released. These electrons are transferred to oxygen, forming water molecules.
Oxidation-Reduction Reactions
Chemical reactions involve energy transformations. Energy stored in chemical bonds is used to form new molecules. The transfer of electrons between atoms or molecules is called oxidation. The loss of electrons is oxidation, while the gain of electrons is reduction. These reactions are known as oxidation-reduction reactions or redox. Often, a proton accompanies the electron transfer. In such cases, oxidation involves the loss of hydrogen atoms, not just electrons, and reduction involves the gain of hydrogen atoms. The main function of mitochondria is to carry out these reactions: nutrients + O2 → CO2 + H2O + energy (ATP).
Oxidative Phosphorylation
This process is called oxidative phosphorylation. It requires ADP, phosphate, and oxygen. The output includes water, carbon dioxide, and ATP (adenine + ribose + 3 phosphates). ATP is a high-energy molecule synthesized within the mitochondria. The energy stored in ATP is used for cellular processes. To release energy, ATP is broken down into ADP + phosphate. Cells requiring large amounts of energy, like sperm cells, contain many mitochondria. Mitochondria move to areas where energy is needed. They combine with oxygen to produce CO2, making them responsible for cellular respiration. This process involves many steps, each regulated by specific enzymes.
Energy Transduction
In summary, mitochondria are energy transducers. The energy from food is stored in chemical bonds. This energy is released gradually through degradation processes, culminating in ATP production. ATP is the common link for all energy-requiring processes.
Metabolic Processes
All cells require energy for their functions. Food provides energy and materials for building cellular components. Digestion breaks down large macromolecules into smaller pieces. These substances are transported to cells, where they provide energy and materials. These reactions are collectively known as metabolism.
Catabolism and Anabolism
Metabolism includes all reactions that obtain energy and materials from digested substances. The breakdown of large macromolecules into smaller ones is called catabolism. The synthesis of new molecules using energy is called anabolism. Catabolism releases energy, while anabolism consumes energy. The energy released during catabolism is used for anabolism. ATP is the common link between these processes.
How is energy produced and used? About 50% of the energy is lost as heat, while the rest is captured by the cell through ATP. ATP stores energy in its bonds. When ATP is broken down into ADP + phosphate, energy is released. ADP can also break down into AMP + phosphate, releasing more energy. This process is crucial for anabolism. Therefore, ATP is the molecule that links catabolism and anabolism. ATP has a dual function: it traps energy and releases it when needed. Cells do not store large amounts of ATP. It is constantly synthesized and degraded, with a daily turnover equivalent to its weight.
Catabolism
Catabolism involves the breakdown of food molecules into smaller ones, releasing energy. Most catabolic reactions occur in the mitochondria, which contain enzymes that facilitate these reactions. The breakdown occurs in a step-by-step manner. For example, glucose degradation requires about 30 steps, each with specific enzymes. Nutrients + O2 → CO2 + H2O + energy (ATP).
Cellular Respiration
Cellular respiration involves oxidation reactions. The chemical energy from food is transformed into energy for the cell. Oxidation of organic molecules is the main way cells obtain energy. There are two types of cellular respiration:
- Aerobic: The complete degradation of organic molecules into inorganic molecules, using oxygen. This process yields more energy than anaerobic respiration.
- Anaerobic: Energy is obtained without oxygen. The degradation is incomplete, resulting in intermediate compounds. There are two subtypes: anaerobic respiration and fermentation. This occurs in anaerobic organisms, including amoebas.
Aerobic respiration: glucose → CO2 + H2O + energy
Anaerobic respiration: glucose → CO2, ethyl alcohol, energy
Fermentation: glucose → CO2, lactic acid, energy
Glucides provide the most energy. In the mitochondria, glucides are enzymatically degraded, releasing energy to synthesize ATP. Glucose catabolism is fundamental for vertebrates. The overall catabolic reaction is: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy. About 686 kcal per mole of glucose are obtained. When glucose is oxidized, it loses hydrogen atoms and gains oxygen, forming water and releasing energy to synthesize ATP. There are three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation. Glycolysis and the Krebs cycle release hydrogen atoms to form CO2. In oxidative phosphorylation, hydrogen reacts with oxygen to form water, while ATP is synthesized (ADP + P).
Glycolysis occurs in the cytoplasm, while the rest of cellular respiration occurs in the mitochondria.
Glycolysis
Glycolysis is a set of reactions where one glucose molecule is converted into two pyruvic acid molecules (C3H4O3). C6H12O6 → 2 C3H4O3. This occurs in the cytoplasm under anaerobic conditions. It is a universal process for all cells. Glycolysis produces 2 ATP molecules. It involves about 9 stages, each with specific enzymes. From pyruvic acid, the process can follow an anaerobic or aerobic route. The aerobic route includes the Krebs cycle and oxidative phosphorylation.
In the presence of oxygen, pyruvic acid crosses the mitochondrial membrane and undergoes complete degradation in a step-by-step process. There are two stages: the Krebs cycle (or citric acid cycle), which occurs inside the mitochondria with specific enzymes, and oxidative phosphorylation. 2 C3H4O3 → 6CO2 + 6H2O + energy (38 ATPs)
Krebs Cycle
Krebs cycle: Pyruvic acid is oxidized, losing a carbon atom and converting into acetic acid (2C). Acetic acid (2C) combines with oxaloacetic acid (4C) to form citric acid (6C). Then, it loses one carbon (5C), then another (4C), becoming oxaloacetic acid (4C) again, restarting the cycle.