What is Catabolism, Breakdown pathway？
Catabolism is the pathway in organisms that involves breaking down complex molecules or subcellular structures into simpler molecules. For example, some aging or damaged organelles, cells, and misassembled proteins are broken down by lysosomal hydrolases. Alternatively, some energy storing substance is decomposed by enzymes into smaller units. For instance, carbohydrates like starch or glycogen are broken down into glucose, proteins are broken down into amino acids, and triglycerides are broken down into glycerol and fatty acids.
Catabolism is not only responsible for waste recycling and elimination; its most important function is energy production. Original chemical bonds are broken and transformed into more available forms to provide energy for various cellular functions, such as the cell's main energy currency, adenosine triphosphate (ATP). Cellular aerobic respiration is the primary way most organisms obtain energy. Other pathways include anaerobic cellular respiration, glycolysis, and fermentation.
Cellular Respiration
Cellular respiration is closely related to gas exchange. In single-celled organism, the primary way to obtain oxygen is through free diffusion across the cell membrane. In multicellular organisms, gas exchange occurs in lungs, gills, or skin. Then, oxygen enters each cell through circulatory system. Some primitive invertebrates, like sponges and jellyfish, do not even have a true circulatory system, and still relies on free diffusion of oxygen. Glucose and fatty acids are common energy sources for cellular respiration that are oxidized into water and carbon dioxide to produce ATP in mitochondria or prokaryotic cells. Carbon dioxide is then expelled via circulatory system or free simple diffusion.
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP
C₁₇H₃₅COOH + 26O₂ → 18CO₂ + 18H₂O + ATP
The energy from fatty acids is about twice of glucose. There is approximately 167 kJ in 10g glucose. An adult performing intense exercise for about 15 minutes will consume this amount of energy. Each ATP molecule contains about 1% energy in a glucose molecule. So, does each oxidized glucose releases 100 ATP molecules? Does this mean the efficiency of energy production is 100%? No, cellular respiration can only utilize about 40% energy stored in glucose. In addition, molecules shuttling across biological membranes consume some ATP, and in reality, only about 30% energy is stored in ATP. Compared to heat engines, this is a high efficiency. For example, the thermal efficiency of a steam engine is only about 8%; internal combustion engines and transmission systems can only convert about 25% gasoline energy into kinetic energy for cars. Even compared to fermentation and anaerobic respiration, aerobic respiration is highly efficient. For example, even the most efficient nitrate respiration achieves only half or less efficiency; yeast fermentation in an anoxic environment only exploits about 2% energy in glucose..
In general, maintaining basal metabolism consumes 70% energy from the diet. For example, heart beating, lungs breathing, body temperature maintenance, and digestion of food in human body, while thinking and physical activity consume only a small portion. Therefore, human cells are constantly synthesizing ATP, whether during sleep or intense exercise. It is estimated that an adult requires about 2,000 to 2,500 kcal per day under normal activity conditions.
Cells obtain energy from organic matter through redox reactions
Energy in organic matter is stored in chemical bonds (potential energy of electrons), especially in carbon-hydrogen bonds. Since carbon and hydrogen have similar electronegativity, the electrons are not too close to positively charged atomic nucleus. Electrons are like satellites orbiting the Earth: the farther they are, the greater potential energy. Aerobic respiration leads to recombination of chemical bonds: carbon and hydrogen are oxidized into carbon dioxide and water respectively. The very electronegative oxygen attracts electrons from hydrogen or carbon to its nucleus, like a satellite, and the closer it is, the less potential energy it has. Therefore, some reduced potential energy is stored in ATP, and some dissipated as heat. Generally speaking, organic compounds with many carbon-hydrogen bonds contain more chemical energy, which is why fats have more calories than sugars.
These organic compounds exist stably at room temperature. Electrons do not spontaneously release energy by rushing into a lower energy state, because they must overcome an activation energy barrier. Lowering activation energy or providing very high energy are both ways to break through this barrier. Sugar and fat can be ignited by fire, and the final products are both water and carbon dioxide. In fact, the overall chemical equation for combustion and cellular respiration is the same; combustion is so fast that it emits heat and light. In contrast, enzymes lower the activation energy to allow organic matter to be decomposed slowly in a series of steps. Cellular respiration is a mild biochemical reaction carried out at room temperature in an aqueous environment.