Plant Physiology - Musienko M.M. 2001

Plants and the Biosphere. Ecological Cycles of Matter and Energy Flow
Energy Flow

The driving force behind the functioning of all living systems is energy, which defines their capacity to perform work. Energy serves as a universal measure of motion and interaction across all forms of matter, and its flow is governed by the Laws of Thermodynamics.

Almost all life forms on Earth obtain the energy they need, directly or indirectly, from the Sun. Each year, approximately 21×10²³ kJ of solar radiation reaches the Earth's surface. Of this amount, about 40%, or 8.4×10²³ kJ, hits land areas and Water bodies with corresponding vegetation. Accounting for all energy losses and the energy yield of Photosynthesis (no more than 2%), the total amount of energy stored in photosynthetic products is approximately 20.9×10²³ kJ. Taking Respiration costs (ranging from 30% to 40%) into account, the planet's plant organisms globally convert a total of about 4.2×10¹⁸ kJ of solar energy annually for respiration and the creation of net primary production.

Living systems are actively integrated into the planetary energy flow. The most complex pathway of solar energy passes through living matter, where it manifests as Chemical Reactions and biological processes. The irreversible conversion of negative Entropy (which is equivalent to Free energy) into positive entropy, in accordance with The Second Law of thermodynamics, is the driving force of life. Green autotrophic plants use sunlight during photosynthesis to convert simple Inorganic Compounds—carbon dioxide, water, and mineral salts—into the complex organic molecules of their Cells and Tissues. Free energy (i.e., energy available to perform useful work) is stored within chemical bonds. In simple molecules, the proportion of free energy is minimal, whereas in complex organic molecules such as CARBOHYDRATES, Proteins, and Lipids, it is several orders of magnitude higher. Furthermore, photosynthesis and respiration generate ATP and other specialized molecules featuring high-energy bonds. The free energy stored by plants in photosynthetic products serves as the energetic foundation for all living systems, including humans. Living systems operate as open thermodynamic systems for matter and energy, avoiding the establishment of thermodynamic equilibrium (ΔG = 0) solely through the continuous degradation of negative entropy. Light quanta continuously supplied by the Sun and absorbed by plants represent the sole natural source of negative entropy available to the biosphere. Sooner or another, through various transformations, the solar energy that arrived on Earth is ultimately radiated back into outer space as heat.

Thus, green plants act as the primary link connecting all living things to solar energy. Plant cover traps photosynthetically active radiation, stabilizes and redistributes it among diverse living organisms, and drives the synthesis of carbohydrates, proteins, and other Organic compounds. It provides valuable industrial raw Materials and maintains the planet's ecological balance by binding carbon dioxide and producing oxygen. Given that there are no grounds to expect the flow of solar radiation to Earth to cease in the foreseeable future, and that organogenic elements are continuously recycled back into the carbon cycle and life processes, we may conclude that, in principle, life on Earth—and consequently human existence—can be eternal. Our only path is to live and work in harmony with the laws of the world around us and to continue moving forward along the evolutionary path. Plant organisms, with their capacity for phototrophic Nutrition, will help us fulfill this mission.