Download The living world is an amazing and complex world. It is a vast

The living world is an amazing and complex world. It is a vast
assortment of different organisms interconnected in a network of life,
which has evolved over a long time. To function, every life needs
energy to maintain itself, to grow, and to reproduce. For animals this
harnessed energy is acquired in the food they eat, which also
supplies the building blocks that make up the mass of their bodies.
Life depends on the balance between building organic material from
carbon dioxide, water and minerals, and its ultimate breakdown into
the initial parts. Green plants are able to synthesize organic
compounds from inorganic starting materials using sunlight, as a
source of energy. They are knows as autotrophic organisms.
Animals and most bacteria are heterotrophic organisms. This means
they break down organic materials to provide carbon and most of
their own organic parts. Some bacteria may also act as synthesizers,
using energy from chemical processes.
Plants, animals, and bacteria interact mutually, as well as with their
non-living environment, to form communities of living organisms.
These ecosystems are organized in food chains and food webs. They
are built up by plants, the plants eaters or herbivores and the
carnivores that feed on the fleshy tissue of other animals. The
community patterns are determined by who eats whom.
All members of the deer family are herbivores. Some deer prefer to
feed on the leaves, shoots, twigs, flowers, and fruit of tree, herbs, and
shrubs. This feeding is called browsing and deer that eat like this are
called browsers. Other deer prefer to graze on grasses. These deer
are referred to as grazers. Plants on land grow to large size, and
animals that feed on grasses, herbs, and leaves have no difficulties in
spotting the plants on which they graze or browse. The food is
sometimes hard to digest, even eat. The living cells in the higher
plants are encased in thick cell walls built of cellulose and other
polysaccharides to give them rigidity. Most animals cannot digest
these materials because they lack the necessary enzymes. Higher
plants have often developed protective resources against consumers,
varying from toxins to thorns. In fact, only a small part of the primary
production on land is eaten, and most vegetation dies and begins to
decompose under the action of variety of enzymes from the bacteria
and other soil microorganisms. During this process the plant
material, including cellulose and other indigestible substances, is
converted into microbial mass, bacteria, protozoan and fungi, which
can be eaten and digested by animals. The microbial activity converts
those parts of the primary production that is not accessible to most
animals into matter of high nutritional value for the rich fauna that
feeds in the soils of fields and forests or on the sediments of coastal
waters and lakes.
In the presence of oxygen the microbial decomposition proceeds to
the complete oxidation of the materials on which the microorganisms
live. Under anaerobic conditions the microorganisms will ferment
their substrate and convert most of the structural polysaccharides into
small organic molecules. Such anaerobic conditions prevail in the
digestive trait of larger animals, where the anaerobic microbial
breakdown has resulted repeatedly in the establishment of symbiotic
associations between herbivores and decomposing microorganisms.
Digestion by means of symbiots may be the major adaptation in the
utilization of plant food among terrestrial animals.
Symbiont aided digestion has reached its highest development in the
ruminants. Animals that ruminate include cattle, sheep, goats,
antelopes, giraffes, deer, camels, llamas, hippopotamus, sloths, and
kangaroos. Large-scale symbiont digestion of cellulose and other
insoluble and resistant polysaccharides needs space and is time
consuming. The digestive tracts of animals that have adopted this
feeding habit are equipped with greatly expanded sections in which
voluminous mixture of microorganisms and food is stored for the long
period needed to break down the plant material (often several days).
In ruminants, the rumen serves as the fermenting chamber.
The ruminants owe their name to their habit of returning some of the
content of the rumen to the mouth for further chewing. This chewing
increases the surface area of the food exposed to the action of the
digestive enzymes in the rumen. In the mouth the repeated returns of
the rumen contents further mix the cud with large amounts of saliva.
The saliva is rich in bicarbonate, which serves to neutralize the fatty
acids produced by the fermentation.
Digestive enzymes secreted by the microorganisms carry out the
initial stages in the breakdown of the food in the rumen. The
enzymes split the variety of polysaccharides, including cellulose, into
simple sugars. These sugars are largely absorbed by the
microorganisms and fermented into fatty acids, which constitute the
host’s share of the carbohydrates in the food.
The proteins in the food are also split by digestive enzymes secreted
by the microorganisms, which also absorb most of the liberated
amino acids to use for their growth and multiplication. The protein is
not lost to the ruminant. When the microorganisms themselves pass
further down the digestive tract and reach the stomach, they are
attacked by the host’s own enzymes and digested. The
microorganisms represent a most important source of protein to the
host.
Digestion by means of symbionts endows the host with other
advantages. The rumen microorganisms are able to reuse the
products of excretion, ammonia and urea, for the synthesis of
proteins. Moreover, they synthesize many vitamins; especially those
of the B group, reducing the dietary vitamin requirements. The price
of digesting food by means of symbionts depends both upon the
energetic cost of maintaining the microbial population and the
efficiency with which this population utilizes the food.
The stomach of ruminants has four compartments: the rumen,
reticulum, omasum and abomasum, as shown in the following
diagram: