The first and second laws of thermodynamics in biology

The first law of thermodynamics, also known as the law of energy conservation, is a fundamental principle in physics and applies to the energy flow in biology as well. It states that energy cannot be created or destroyed in an isolated system but can only be converted from one form to another or transferred between different parts of the system.

Energy enters biological systems primarily in the form of sunlight or chemical energy from inorganic matters.

Photoautotrophic organisms, such as cyanobacteria, algae and plants, produce organic matter through photosynthesis. They use pigments to convert the energy from sunlight into chemical energy in high-energy compounds, such as NADPH and ATP, and release oxygen as a by-product. These high-energy compounds are then converted into glucose via the Calvin cycle.

Chemoautotrophs derive their energy from the chemical reactions they perform. These organisms utilize a process called chemosynthesis to convert inorganic substances into organic compounds. They typically obtain energy by oxidizing inorganic molecules, such as hydrogen sulfide (H₂S), ammonia (NH₃), iron (Fe). The energy released from these reactions is used to perform metabolic processes. They are primarily found in environments where sunlight is scarce or absent, such as deep-sea hydrothermal vents, caves, and certain soil habitats.

Heterotrophic organisms cannot make organic matter on their own and can only eat other organisms for energy. For example, the cellulose in grass is broken down by cattle into glucose, which is then converted by aerobic respiration into water, carbon dioxide, the high-energy compound ATP and heat. Once energy is acquired, it undergoes various transformations within the biological system. Energy is used for essential processes like metabolism, growth and reproduction. The direct energy consumed by cattle for their life activities comes from ATP, and the energy in ATP comes from the solar energy converted by plants. Some other nutrients in the grass, such as amino acids, excess lipids and saccharides, are used to synthesize proteins, lipids and glycogen in cattle. Another group of heterotrophs, such as bacteria and fungi, decompose plant and animal carcasses and excreta to obtain energy. The initial source of these energy is the solar energy captured by photosynthesis in plants.

The second law of thermodynamics states that the total entropy of an isolated system always increases or remains constant. Entropy is a measure of the disorder degree or randomness in a system. The law implies that natural processes tend to move toward greater states of disorder or randomness. Another equivalent way to describe this is that a portion of the energy is dissipated as heat during energy conversion, or energy cannot be spontaneously absorbed from a lower temperature body. It describes the concept of entropy, the efficiency of energy conversion, and the direction of spontaneous energy flow.

This law must also be followed in biology. For example, during the metabolism of a cell, energy is obtained from the breakdown of organic molecules. However, not all of this energy can be used for work. Instead, a large portion is dissipated as heat to increase the entropy of the system. Only 40% energy is transferred to ATP during glucose oxidation, and 60% is dissipated as heat energy.

The second law also influences ecological systems and energy transfer among different trophic levels. As energy flows through a food chain or food web, each level of consumption results in a loss of energy. This loss occurs due to metabolic processes and heat dissipation. Consequently, there is a decrease in available energy as we move up the trophic levels, limiting the length and complexity of food chains.

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