Chapter 1
Osmotic Problems of Vertebrates
General Introduction
Water provides the principal physical framework in which the energy transformations characteristic of life take place; it is the solvent for life as we know it . Chemical
reactions take place in a variety of media, but the physico-chemical properties of
water make it uniquely suitable for such a role. Its mobility and plasticity provide
it with structural advantages suitable to living systems, its high heat capacity and
latent heat of evaporation buffer the organism against thermal changes in the environment, while its remarkable capaciry as a solvent, for both organic and inorganic materials, provides a vehicle for the interactions of many different substances .
Living organisms contain 40 to 99% water, the content in vertebrates ranging
from 65 to 85% of their body weight. This water is nearly all in a free uncombined
state and contains solutes that participate in transformations of energy, together
with those wh ich are structural components contributing to the stability of the system . Thus, the solubility of proteins can be facilitated by the presence of certain
concentrations of ions, while metals may act as co-factors in enzyme reactions.
Solutes may also be util ized to provide chemical gradients which act as stores of
potential energy, as in nerve and muscle cells, where electrical depolarization represents a release of stored energy due to earlier ion accumulation. In addition solutes influence the osmotic mobility of water, restricting its movements across
structural barriers within the organism or between it and the external environment.
1. Geographical and Ecological Distribution of the Vertebrates
(Osmotic Distribution)
It is uncertain whether the vertebrates originated in the sea or fresh water, but subsequently both environments, as well as the dry land, have been occupied by representatives of most of the major groups. The geographical location(s) for the
transition to a vertebrate is also unknown, but subsequent dispersal has been
widespread. The major groups of fishes, the Agnatha, Chondrichthyes and
Osteichthyes, have representatives in the sea and fresh water. The tetrapods are
primarily terrestrial, but the Amphibia, reptiles, birds and mammals all have species
that normally live in marine or freshwater environments. Osmoregulatory processes playa major role in such dispersal, and it is not surprising to observe that
the ph ysiological mechanisms by which such adaptations take place, often
differ greatly. Even among the fishes, an apparently novel occupation of a certain
medium may represent a secondary, tertiary or even further removed alteration
of the situation that was primary in the group. DARLINGTON (1957) has traced the
probable historic origins of the endemic catfishes of Australia : teleost fish originated in the sea from whence a fish related to the order Isospondyli moved into
fresh water to give rise to the order Ostariophysi. Some of this group probably reentered the sea and returned subsequently to fresh water in Australia. Among the
1
Osmotic Problems of Vertebrates
General Introduction
Water provides the principal physical framework in which the energy transformations characteristic of life take place; it is the solvent for life as we know it . Chemical
reactions take place in a variety of media, but the physico-chemical properties of
water make it uniquely suitable for such a role. Its mobility and plasticity provide
it with structural advantages suitable to living systems, its high heat capacity and
latent heat of evaporation buffer the organism against thermal changes in the environment, while its remarkable capaciry as a solvent, for both organic and inorganic materials, provides a vehicle for the interactions of many different substances .
Living organisms contain 40 to 99% water, the content in vertebrates ranging
from 65 to 85% of their body weight. This water is nearly all in a free uncombined
state and contains solutes that participate in transformations of energy, together
with those wh ich are structural components contributing to the stability of the system . Thus, the solubility of proteins can be facilitated by the presence of certain
concentrations of ions, while metals may act as co-factors in enzyme reactions.
Solutes may also be util ized to provide chemical gradients which act as stores of
potential energy, as in nerve and muscle cells, where electrical depolarization represents a release of stored energy due to earlier ion accumulation. In addition solutes influence the osmotic mobility of water, restricting its movements across
structural barriers within the organism or between it and the external environment.
1. Geographical and Ecological Distribution of the Vertebrates
(Osmotic Distribution)
It is uncertain whether the vertebrates originated in the sea or fresh water, but subsequently both environments, as well as the dry land, have been occupied by representatives of most of the major groups. The geographical location(s) for the
transition to a vertebrate is also unknown, but subsequent dispersal has been
widespread. The major groups of fishes, the Agnatha, Chondrichthyes and
Osteichthyes, have representatives in the sea and fresh water. The tetrapods are
primarily terrestrial, but the Amphibia, reptiles, birds and mammals all have species
that normally live in marine or freshwater environments. Osmoregulatory processes playa major role in such dispersal, and it is not surprising to observe that
the ph ysiological mechanisms by which such adaptations take place, often
differ greatly. Even among the fishes, an apparently novel occupation of a certain
medium may represent a secondary, tertiary or even further removed alteration
of the situation that was primary in the group. DARLINGTON (1957) has traced the
probable historic origins of the endemic catfishes of Australia : teleost fish originated in the sea from whence a fish related to the order Isospondyli moved into
fresh water to give rise to the order Ostariophysi. Some of this group probably reentered the sea and returned subsequently to fresh water in Australia. Among the
1