Terrestrial animals have their problems too. We have already seen
that one of the conditions of animal life in freshwater is a relatively
impermeable covering for all but certain small portions of the body surface,
as an aid in preventing excessive absorption of water. For this reason
freshwater animals had an important preadaptive advantage over primitively
marine animals in colonizing the terrestrial environment. The evidence
strongly supports the view that the movement to land was by way of freshwater,
not directly from the sea.
On land the greatest threat to life is desiccation. Water is
lost by evaporation from the respiratory surfaces (lungs, tracheae, etc.),
by evaporation from the general body surface, by elimination in the feces,
and by excretion in the urine. The lost water must obviously be replaced
if life is to continue. It is replaced by drinking, by eating foods containing
water, and by the oxidation of nutrients, (remember that water is one of
the products of cellular respiration). Taking in of O2 and releasing CO2
across a gas membrane is breathing, not respiration. Respiration
is the release of energy by oxidation of fuel molecules, or, the various
energy yielding oxidative reactions in living matter that typically involve
transfer of oxygen and production of carbon dioxide and water as end products.
A fish breathes by passing water over the gill membranes, a man breathes
by passing air over the lung membranes. Some freshwater fish have gills
and lungs and can breath in water and in air, hence the apparent ability
to move on to the land, of some early freshwater animals.
We saw that ammonia is a satisfactory nitrogenous excretionary
product for aquatic animals. It is far from satisfactory for terrestrial
animals because of the difficulty of getting rid of this highly toxic substance
on land, where an unlimited supply of water is not available. As a child
I remember the ammonia operated refrigerators, and the panic caused by
an ammonia leak and the evacuation of the home. All the plants in the home
died. We were not allowed in the home until the gas had been cleared after
the leak was repaired. Amphibians and mammals, therefore, rapidly convert
ammonia into urea, a compound that, though very soluble, is relatively
nontoxic. Urea can remain in the body for some time before being excreted,
and we can regard its production as an adaptation to the conditions of
water shortage characteristic of terrestrial existence.
Although urea is a far more satisfactory excretionary product
than ammonia for land animals, it has the disadvantage of draining away
some of the critically needed water, for, being highly soluble, it must
be released in an aqueous solution (urine). If, however, uric acid, a very
insoluble compound, is excreted instead of urea, almost no water need be
lost. It is not surprising, therefore, that many terrestrial animals--most
reptiles, birds, insects and land snails--excrete uric acid or its salts.
The excretion of this substance not only allows them to conserve water,
but has another advantage, which may have been even more important in the
evolution of uric acid metabolism. All these animals lay eggs enclosed
within a relatively impermeable shell or membrane. If the embryos excreted
ammonia, they would rapidly be poisoned, and if they produced urea, the
concentration in the egg by the latter part of development would become
decidedly harmful. Uric acid, on the other hand, is so insoluble that it
can be precipitated in almost solid form and stored in the egg without
exerting harmful toxic or osmotic effects. In the nitrogen metabolism of
fully terrestrial animals, uric acid excretion is correlated with egg laying,
while urea excretion is correlated with viviparity (giving live birth to
the young).
Lets take a closer view of the cellular environment. All
living cells are bathed by liquid. This statement may seen perfectly obvious
when one considers the protists, plants, and animals that live in fresh
or salt water. Also many organisms live in a terrestrial environment---in
the earth, on the surface of the earth, or in the clouds ( many organisms
live in the atmosphere and clouds). A close examination reveals that in
all these cases, each living cell is continually bathed in water. The skin
which man and his fellow vertebrates expose to the air consists of layers
of dead cells which protect the living cells beneath from, among other
things, the drying effects of the air. Where living cells are exposed to
the environment, as in the epithelium that lines our air passages and the
transparent cornea at the front of the eye, secretory cells bath the exposed
surfaces in a continuos supply of moisture (water). In a similar way, the
exoskeleton of insects, the bark of trees, and the waxy cuticle of leaves
all consist of dead cells or waterproof secretions of cells which permit
the underlying living cells to remain protected by at least a film of moisture.
This is generally referred to as the impermeable membrane. Within the soil
itself, living cells may be directly exposed to the environment, but here
again the environment is liquid. Each soil particle is surrounded by a
film of water, the delicate root hairs of vascular plants, and myriads
of protists and tiny invertebrates that live in the soil are in contact
with this moisture (water). If the moisture is removed the cells and animals
of the soil die, as in drought.
In considering the cellular environment of more complex plants
and animals, one must consider also the environment of the cells that are
not close to the exterior of the organism. These deep-lying inner cells
are also in contact with liquid. Sap in plants, the blood of insects, and
lymph in man are examples of fluids that bathe the inner cells of higher
organisms. The blood in man never contacts the cells directly, only the
lymph contacts the cells. Refer to the 24 part series called "intestine"
in the archives. Lymph and the blood plasma from which it is derived make
up about twenty percent of the body weight of man. Because these
fluids are outside of the cells, we refer to them as the "extra cellular
fluid" or ECF. To distinguish between the external environment of our bodies
(air and water), and the actual environment of our cells ( the lymph),
the French physiologist Claude Bernard referred to the latter as the internal
environment. We now refer to it as the intracellular space which
is filled with lymph, as distinguished from the fluid inside of the cells
proper.
---to be continued---
Bless you, Bob Lee
--
oozing on the muggy shore of the gulf coast
l...@fbtc.net
--
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