MODERN BOTANY - P. RAVEN - 1990

SECTION II. ENERGY AND LIVING CELLS

CHAPTER 5. THE FLOW OF ENERGY

Summary

Life on our planet depends on a continuous flow of solar energy. A small fraction of this energy is captured during Photosynthesis and converted into the energy required for the many metabolic reactions that take place in living organisms. As a result of these reactions, living organisms acquire order and Organization.

The thermodynamic relationships between photosynthetic and nonphotosynthetic forms of life are exceptionally complex. In short, during photosynthesis, solar energy is used to form high-energy carbon-carbon and carbon-Hydrogen Bonds; during Respiration, these bonds are broken to yield CThe thermodynamic relationships between photosynthetic and nonphotosynthetic forms of life are exceptionally complex. In short, during photosynthesis, solar energy is used to form high-energy carbon-carbon and carbon-hydrogen bonds; during respiration, these bonds are broken to yield CO₂ and H₂O, releasing energy. A certain portion of usable energy is lost at each step of energy transformation, much like in mechanical engines.

Living systems operate in accordance with the Laws of Thermodynamics. The First Law of thermodynamics states that energy cannot be created or destroyed, but can be transformed from one form to another. The potential energy of the initial state (or starting components) is equal to the potential energy of the final state (or products) plus The energy released during the process or reaction. The Second Law of thermodynamics states that during an energy transformation, the potential energy of the final state is always less than that of the initial state. The energy difference between the initial and final states is termed Free energy and is designated as ∆G. Exergonic reactions (those that release energy) have a negative ∆G value. Factors Determining the magnitude of ∆G include ∆H (The change in heat content) and ∆S (the change in Entropy), which, when multiplied by the absolute Temperature T, serves as a measure of order or disorder:

∆G = ∆Н - T∆S.

Energy transformation in living Cells is carried out through The transfer of electrons from one energy level to another, or from one atom or molecule to others. Reactions Involving the transfer of electrons between atoms or molecules are called oxidation-reduction (redox) reactions. An atom or molecule that loses electrons is oxidized, while the one that accepts them is reduced.

METABOLISM is the sum of all Chemical Reactions occurring within a Cell. The network of reactions leading to the breakdown or degradation of molecules is called Catabolism. Biosynthetic reactions—those leading to The formation of new molecules—are termed anabolic. Metabolic reactions proceed through ordered series of steps called pathways, each playing a specific role within The Cell. Every step in these pathways is controlled by a specific enzyme.

Enzymes serve as catalysts; they greatly accelerate reaction rates without being consumed or altered in the process. Enzymes are massive protein molecules folded in such a way that specific amino acid residues form an Active Site. Reacting molecules—substrates—fit precisely into these active sites. Many enzymes require Cofactors to function, which may simply be ions such as Mg²⁺ or Na⁺, or non-protein organic molecules such as NAD, the latter being referred to as Coenzymes.

Enzyme-catalyzed reactions are under strict cellular control. The Rate of Enzymatic reactions depends on temperature and pH.

ATP supplies energy for the majority of cellular reactions. An ATP molecule consists of a nitrogenous base (adenine), a five-carbon sugar (ribose), and three phosphate groups. Two of the phosphate groups are linked by high-energy bonds, the Cleavage of which releases a relatively large amount of energy. ATP serves as the primary energy source for most reactions occurring in living systems.

Appendix

Е = mс²

One of the oldest and most fundamental concepts in chemistry—the law of conservation of mass—states that matter cannot be created out of nothing nor destroyed without a trace. However, under conditions of extreme temperatures, atomic nuclei fuse to form new elements, and in the process, a portion of their mass is lost. What happens to this mass? This question is answered by the equation that brought Einstein worldwide fame: E = mc² (where E is energy, m is mass, and c is the constant representing the speed of light). This expression demonstrates that under certain extreme and unusual conditions, mass is converted into energy.

The Sun consists primarily of hydrogen nuclei. Deep within its core, at ultrahigh temperatures, these nuclei collide with one another, reaching speeds sufficient to trigger fusion. Through a series of steps, four hydrogen nuclei combine to form a single helium Nucleus. This thermonuclear reaction releases enough energy to sustain the ongoing fusion processes, as well as a vast amount of radiant energy that is dispersed into space. Life on our planet depends entirely on the energy radiated by The Sun as a result of this thermonuclear reaction. This exact same process, as Einstein foresaw, could also be harnessed to create the hydrogen bomb.

A photograph of Albert Einstein in 1905, the year he published his work on The Theory of relativity. At the time, he was 26 years old and working as a third-Class technical expert at the Federal Patent Office in Bern, Switzerland.