Narrow Results

All organisms on earth have evolved at unit gravity and thus are probably adapted to function optimally at 1 g. However, with the advent of space exploration, it has been shown that organisms are capable of surviving at much less than 1 g, as well as at greater than 1 g. Organisms subjected to increased g levels exhibit alterations in physiological processes that compensate for novel environmental stresses, such as increased weight and density-driven sedimentation. Weight drives many chemical, biological, and ecological processes on earth. Altering weight changes these processes. The most important physiological changes caused by microgravity include bone demineralization, skeletal muscle atrophy, vestibular problems causing space motion sickness, cardiovascular deconditioning, etc. Manned missions into space and significant concerns in developmental and evolutionary biology in zero and low gravity conditions demand a concentrated research effort in space-medicine, physiology and on a larger scale — gravitational biophysics. Space exploration is a new frontier with long-term missions to the moon and Mars not far away. Research in these areas would also provide us with fascinating insights into how gravity has shaped our evolution on this planet and how it still governs some of the basic life processes. Understanding the physiological changes caused by long-duration microgravity remains a daunting challenge. The present concise review deals with the effects of altered gravity on the biological processes at the cellular, organic and systemic level which will be helpful for the researchers aspiring to venture in this area. The effects observed in plants and animals are presented under the classifications such as cells, plants, invertebrates, vertebrates and humans.

One of the major characteristics of living organisms is metabolic rate — the amount of energy produced per unit of time. When the mass of organisms increases, the metabolic rate also increases (as a power function of mass), but usually slower than mass. This effect is called metabolic allometric scaling. Its causes are considered unknown. The effect has important implications for individual and population organismal development. It was shown in the first part of this study, presented in a separate paper, that in the case of multicellular organisms, this effect is a consequence of natural selection and optimization of nutrient distribution between the species of a food chain, sharing resources of a common habitat. Here, in the second part that studies unicellular organisms, we discover that the same principle of natural selection guided by optimization of nutrient distribution between the species of a food chain defines also metabolic allometric scaling of unicellular organisms. To find that, we consider the metabolic properties of Amoeba proteus, fission yeast Schizosaccharomyces pombe, Escherichia coli, Bacillus subtilis, Staphylococcus. The sharing of nutrients is optimized in such a way that bigger microorganisms have progressively bigger nutrient influx per unit of surface. This evolutionary arrangement secures the stability of a food chain by providing certain metabolic advantages for bigger organisms. Accounting for this regular increase of nutrient influx with mass increase, we obtained allometric exponents and their ranges close to experimental values, thus proving that metabolic allometric scaling of both multicellular and unicellular organisms is defined by the same fundamental evolutionary principle of optimized sharing of nutrients between the species of a food chain.