High priority has been given to studies of weightlessness. Gravity is
one of the most fundamental forces that acts on living organisms, and
all life on Earth except the smallest appears to be oriented with
respect to gravity, although certain organisms are more responsive to it
than others. The gravity force on Earth is 1 g, but this force may be
experimentally varied from zero g, or weightlessness, to many thousands
of g’s.
Zero gravity or decreased gravity occurs during freefall, in parabolic
trajectory, or during orbit around the Earth. Gravitational force
decreases by the square of the distance away from the Earth’s center. It
is reduced about 5 percent at about 200 nautical miles’ altitude.
Gravitational force greater than 1 g can be obtained by acceleration,
deceleration, or impact. It also can be increased by using a centrifuge
which adds a radial acceleration vector to the 1 g of Earth.
On the ground, the biological effects of gravity have been studied at 1
g, and experimentally, forces of many g have been produced. In addition,
modifications of the effects of the 1-g force have been induced by
suspension of the organism in water or by horizontal immobilization of
an erect animal such as man. The biological effects of such modification
have been of significant value in understanding some of the possible
consequences of human exposure to the zero-g environment of space.
Weightlessness in an Earth-orbiting satellite occurs when the continuous
acceleration of Earth’s gravity is exactly counterbalanced by the
continuous radial acceleration of the satellite. In such a weightless
state, organisms are liberated from their natural and continuous
exertion against 1 g, but this liberation may carry with it certain
serious physical penalties.
Some of the physical processes which probably have the greatest
biological effects are (1) convective flow of fluid, e.g., protoplasmic
streaming, transport of nutrient materials, oxygen, waste products, and
CO₂ from the immediate environment of the cell, and (2) sedimentation
occurring within cells; substances of higher density sediment in a
gravitational field, and those of lighter density rise. A separation of
particles of different densities probably occurs. The removal of gravity
would change a distribution of particles like mitochondria by 10 percent
([ref.64]).
Gravity has effects on the physical processes involved in mitosis and
meiosis. Study under weightlessness might contribute to our
understanding of the general cellular information-relay process.
A gravitational effect is known in the embryonic development of the frog
_Rana sylvatica_. After fertilization, the eggs rotate in the
gravitational field so that the black animal hemisphere is uppermost.
Development becomes abnormal if this position is disturbed. If the egg
is inverted following the first cleavage and held in this position, two
abnormal animals result, united like Siamese twins. This phenomenon
appears to be related to the gravitational separation of low- and
high-density components of the egg. The size of the egg is about 1 to 2
mm and is suspended in water of about the same density. This system is
very sensitive to gravity; and, under weightlessness, the separation of
different density components might be irregular, leading to aberrant
development. When certain aquatic insect eggs are inverted, subsequent
development results in shortened abnormal larvae.
The directional growth of plant shoots and plant roots is probably due
to this sedimentation phenomenon, particularly the effect on movement of
auxins ([ref.65]).
Free convection flow is a major transport process, and under its
influence the mixing of substances is much more effective than when
diffusion operates alone. Free convection flow is a macroscopic
phenomenon which increases not only with g, but varies also
approximately with the five-fourths power of the bulk concentration
involved. Whether or not convection is important at the microscopic
level remains an experimentally unsolved question. The Grashoff number
limits free convection to the macroscopic domain. It would appear in
weightlessness that the contribution of free convective flow would be
small and that only diffusion should occur. This phenomenon would cause
equilibration to occur much more slowly than that occurring with free
convection and diffusion. The absence of convective transfer raises a
problem as to how nutrients may be obtained and waste products removed
in living cells during weightlessness. In a liquid substrate, nutrients
and oxygen would be depleted, and waste products would accumulate around
the cell.
Absence of gravity may have far-reaching consequences in the homeostatic
aspects of cell physiology. The outstanding characteristics of living
cells which are most likely to be influenced by the absence of gravity
are the ability of the cell to maintain its cytoplasmic membrane in a
functional state, the capacity of the cell to perform its normal
functions during the mitotic cycle, and the capacity of the cytoplasm to
maintain the constant reversibility of its sol-gel system ([ref.66]).
Two-phase systems, e.g., air-in-water and air-in-oil, possess entirely
different characteristics at zero g than at 1 g. These physical
differences in phase interaction could well be suspected of interfering
with the orientation and flow pattern of cell constituents, thus
hindering the cellular processes involved in the movement, metabolism,
and storage of nutrients and waste.
On the basis of theoretical calculations, weightlessness can be expected
to have some effect even on one individual cell if its size exceeds 10
microns in diameter ([ref.64]). Cell colonies might be affected. In
larger cells there may be a redistribution of enzyme-forming systems
which give rise to polarization. The low surface tension of the cell
membrane lends itself to hydrostatic stress distortion, implying an
alteration in permeability and thus an almost certain alteration of cell
properties under low gravity conditions.
Another aspect of gravity that affects the growth and development of
living organisms is the directionality of the gravitational field. In
fact, some plants are so sensitive that they are able to direct their
growth with as little stimulus as a 1×10⁻⁶ gravitational field.
Investigations of plant growth in altered gravitational fields are
underway at Argonne National Laboratory and Dartmouth College.
The Argonne Laboratory has designed and developed a 4-pi, or
omnidirectional, clinostat. By rotating a plant so that the force of
gravity is distributed evenly over all possible directions, the
directional effects of gravity are eliminated, simulating some aspects
of the zero-g state. It was shown that certain plants grew more slowly
and had fewer and smaller leaves, while others had about 25 percent
greater replication of fronds and had greater elongation of certain
plant parts. It will be extremely interesting to compare these effects
under zero-g conditions in orbiting spacecraft.
The effect of gravity in transporting growth hormones in plants has been
demonstrated at Dartmouth College using radiocarbon-labeled growth
hormones. Plant geotropisms and growth movements have been studied and
biosatellite experiments developed.
Anatomy is considered a derivative adaptation to gravity ([ref.67]). A
large background of plant research exists on the effect of orientation
on plant responses. Information from clinostat experiments is considered
susceptible of extrapolation to low gravity conditions because the
threshold period for gravitational triggering is relatively long.
Once over critical minimum dimensions, the major effects of low gravity
would be assumed to occur in those heterocellular organisms that develop
in more or less fixed orientation with respect to terrestrial gravity
and which respond to changes in orientation with relatively long
induction periods; these are the higher plant orders. On the other
extreme are the complex primates which respond rapidly, but whose
multiplicity of organs and correlative mechanisms are susceptible to
malfunction and disorganization. It may be suggested that the
heterocellular lower plants and invertebrates will be less affected.
Perturbations of the environment to which the experimental organism is
exposed must be limited or controlled to reduce uncertainties in
interpretation of the results. At the same time, the introduction of
known perturbations may assist in isolating the effects due solely to
gravity. Study of _de novo_ differentiation and other phenomena
immediately after syngamy may be of particular importance. Study of
anatomical changes after exposure of the organism to low gravity is
important.