Gravitational biology

Plant tropisms are directional movements of a plant with respect to a directional stimulus. One such tropism is gravitropism, or the growth or movement of a plant with respect to gravity. Plant roots grow towards the pull of gravity and away from sunlight, and shoots and stems grow against the pull of gravity and towards sunlight.

Gravity has had an effect on the development of animal life since the first single-celled organism. The size of single biological cells is inversely proportional to the strength of the gravitational field exerted on the cell. That is, in stronger gravitational fields the size of cells decreases, and in weaker gravitational fields the size of cells increases. Gravity is thus a limiting factor in the growth of individual cells.

Cells which were naturally larger than the size that gravity alone would allow for had to develop means to protect against internal sedimentation. Several of these methods are based upon protoplasmic motion, thin and elongated shape of the cell body, increased cytoplasmic viscosity, and a reduced range of specific gravity of cell components relative to the ground-plasma.[3]

The effects of gravity on multicellular organisms is considerably more drastic. During the period when animals first evolved to survive on land some method of directed locomotion and thus a form of inner skeleton or outer skeleton would have been required to cope with the increase in the apparent force of gravity due to the weakened upward force of buoyancy. Prior to this point, most lifeforms were small and had a worm- or jellyfish-like appearance, and without this evolutionary step would not have been able to maintain their form or move on land.

In larger terrestrial vertebrates gravitational forces influence musculoskeletal systems, fluid distribution, and hydrodynamics of the circulation.

Recent experiments have proven that alterations in metabolism, immune cell function, cell division, and cell attachment all occur in the hypogravity of space. For example, after a matter of days in microgravity (< 10⁻³ g), human immune cells were unable to differentiate into mature cells. One of the large implications of this is that if certain cells cannot differentiate in space, organisms may not be able to reproduce successfully after exposure to zero gravity.

Scientists believe that the stress associated with space flight is responsible for the inability of some cells to differentiate. These stresses can alter metabolic activities and can disturb the chemical processes in living organisms. A specific example would be that of bone cell growth. Microgravity impedes the development of bone cells. Bone cells must attach themselves to something shortly after development and will die if they cannot. Without the downward pull of a gravitational force on these bone cells, they float around randomly and eventually die off. This suggests that the direction of gravity may give the cells clues as to where to attach themselves.