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THE DIGESTIVE SYSTEM
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Humans and bacteria have similar metabolic
needs in that both require simple molecules (monosaccharides,
amino acids, fatty acids, etc.) from which to construct their own unique
complex molecules and both need energy, which may be obtained through the
degradation of molecules encountered in their environment. Thus, the ability to digest large molecules
from the environment to produce energy and to provide molecular building
blocks has been a characteristic of living things for billions of years. Bacteria and protists
can release digestive enzymes into their environment and, after chemical
digestion, absorb the simpler molecules.
Controlling the acid levels of digestion is important since many
digestive enzymes have an optimum pH at which they work best. The typical eukaryotic cell possesses intracellular
organelles called lysosomes which digest molecules
in an acidic medium. In many protists
and sponges, the intracellular digestion which follows phagocytosis
occurs first in an acidic environment, then in an alkaline environment,
interestingly similar to the sequence in the stomach and intestine of vertebrates
(Barrington, p. 172). |
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Cnidarians (such as the Hydra in the adjacentimage) do not possess
organs or systems, but they possess a number of significant characteristics
of the digestive systems of higher animals. They possess a mouth and a gastrovascular cavity. The
food which will be digested can be trapped by mucus and moved to the gastrovascular
cavity by the action of cilia. Digestive enzymes are secreted into cavity,
mostly proteinases from endodermal
gland cells. Muscle cells surround
the gastrovascular cavity and may be present
in multiple layers. Some cnidarians
have a pharynx. Some ctenophores
possess anal canals which means that indigestible material does not
need to exit the body through the mouth, which is the condition of most
cnidarians. In cnidarians, extracellular
enzymes don’t break down the food completely—small pieces undergo phagocytosis and are digested further intercellularly
(Hickman 185, Fretter). Hydra possess
2 kinds of gland cells in their digestive tracts for the production of
mucus and enzymes. The use of extracellular
digestion by primitive animals probably introduced a selective advantage
for greater differentiation of the gastrointestinal tract.
Although extracellular digestion can
aid intracellular digestion, eventually it requires a control of the pH
in the area of digestion, the ability to mix food and enzymes, and the
separation of the digestive area into separate compartments to function
well (Beklemishev). A cross
section of a hydra, with its central gastrovascular
cavity, is depicted below. |
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Flatworms of the order Acoela
lack a pharynx, an intestine, and an anus—the digestive tract has only
one opening, as in cnidarians. The
mouth is not located at the anterior end of the animal, but rather more
centrally. Although acoels are
bilateral animals with a head, the organization of their digestive system
with a single, more centrally located opening is more similar to that
of cnidarians than to other bilaterans (Hickman). |
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In acoels, food
is digested by phagocytosis by the gut lining
instead of by enzymatic breakdown. It
is unknown whether this is a reduction from the primitive state of extracellular
digestive enzymes found in cnidarians or whether Acoela
retain the most primitive type of animal digestion (Fretter, Dougherty). In more advanced flatworms (turbellarians, macrostomids), there
is a muscular pharynx which performs peristalsis (Fretter), is ciliated,
and possesses longitudinal and circular muscle layers (Dougherty, p. 197,
Beklemishev 2, p. 196; Rieger, from
Harrison, 1991). Microvilli increase the surface area of the intestine as in
higher animals (Hickman) and many unicellular glands are present (Beklemishev 2, p. 196). The
following image is of the pharynx of a planaria
which is located in the center of the body rather than in the head. |
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The
following image is a cross section of a planarian pharynx. |
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As in Acoela, the
mouth is not located in the head but is located more posteriorly
or even centrally, suggesting a link to more primitive, radially
symmetric animals. In flatworm orders
Macrostomida and Notandropora digestion
is extracellular and a stable epithelial gut lining
exists (Beklemishev 2, p. 192). Since they lack a circulatory system, flatworm
cells must be located near the gut to obtain food by diffusion, just as
in coelenterates. The gut (which
is pigmented in the following images) is highly branched as a result and
serves a distributive (vascular) function in addition to a digestive function. |
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While
some flatworms have anal canals, most have only one opening of their digestive
tracts (Beklemishev 2). Advanced flatworms possess a diverse set of
digestive glands although their products aren’t well known (Rieger,
from Harrison, 1991). Nemertine worms
show a number of advances over flatworms, to which they are thought to
be related. They possess a complete
digestive system, with a mouth on one end and an anus on the other. In a few species, the mouth is not at the anterior
end of the animal and is located more posteriorly. Food is moved primarily through ciliary action and is digested both extracellulary
and intracellulary.
In some, the larval forms lack an anus, as is the condition in
flatworms (Hickman, p. 227). Closely related invertebrates often differ
in their source of nutrition. For
example, although most ascidian urochordates
are filter feeders, some species in the family Octacnemidae
are carnivores which feed on small invertebrates (Burighel,
from Harrison, 1997, p. 221). THE GASTROINTESTINAL TRACT In chordates,
such as in Amphioxus pictured
below, the gastrointestinal tract is ventral to the nervous system and
notochord. |
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The gastrovascular
cavity of cnidarians is lined by epithelial cells which secrete enzymes
and absorb nutrients. In hemichordates
and tunicates, as in higher chordates, microvilli
in the GI tract form brush border extending columnar epithelial cells to
increase surface area for secretion and absorption (Benito, form Harrison
1997, p. 88). In vertebrates, the
absorptive cells may be located on villi which
further increase the surface area of the gastrointestinal tract. |
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lamprey |
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frog intestine |
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While
the intestinal wall of tunicates is typically smooth, a few species possess
grooves (Burighel, from Harrison, 1997, p. 256). Folds can also be seen in the lining of the
GI tract in the annelid worm pictured below.
Also note the presence of cilia on the epithelial cells to propel
the ingested material. The mucus that worms developed to help them
burrow could also be used to trap and move particles of food (or vice versa).
The cilia which primitive worms used for movement could help move
this mucus and food. Among primitive deuterostomes,
the use of cilia to propel food is commonly used. Primitive echinoderms and pterobranchs
possess ciliated tentacles while enteropneusts
possess a ciliated proboscis. Primitive
chordates utilize cilia and mucus for food capture in their pharynx (in
Amphioxus, the endostyle
is a ciliated groove which makes mucus to trap food) ( |
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In
the primitive condition (as in the adjacent image of the worm), ciliary
movement propelled food which had been filtered from sea water and had been
lodged in mucus. In tunicates, there is very little muscle along the GI
tract and most material is moved through ciliary
action (Burighel, from Harrison, 1997, p. 255) Muscle
along the GI tract was needed when larger, more solid food items were ingested.
The muscularis layer of the GI tract of a frog
is pictured below. |
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Many
invertebrates possess circular and longitudinal layers of muscle along
the GI tract, although in many (such as squid, sea cucumbers, and some
insects), the circular layer is outside the longitudinal layer unlike
the organization in vertebrates (Hoar, 1983, p. 425). Peristaltic waves of muscular contraction occur
in worms and echinoderms (Hoar, 1983, p. 425). Jawless fish only have thin muscle layers around
the GI tract and rely on cilia to move food. In gnathostomes, there
are inner circular and outer longitudinal muscle layers. Cilia exist in
the gastrointestinal tracts of some fish, amphibians, and reptiles (Stevens,
p. 20). A few fishes such as the
tench (Tinca) possess striated muscle along their GI tract. (Hoar, 1983, p. 425). The layers of the human GI tract are essentially
the same as those of the frog depicted in the following images. The lumen of the GI tract is lined by a mucosa
layer which contains the epithelia which secretes digestive enzymes and
absorbs food. The human duodenum
possesses simple columnar epithelia located on villi,
just as in the frog duodenum below. Beneath
the mucosa is the submucosa, the bands of longitudinal
and circular muscle of the muscularis layer,
and the serosa lining of the GI tract (Stevens,p. 273). |
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mammal |
HUMAN MODEL |
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The nervous plexuses of the submucosa and muscularis layers
of the gastrointestinal tract (referred to as the enteric nervous system)
consist of a diffuse nerve net which is largely independent of the central
nervous system. This nerve net
is similar to the nerve net which surrounds the gastrovascular
cavity of cnidarians and evolved prior to the brain and nerve cords of
bilateran animals. The
connections between the enteric nervous system and the central nervous
system evolved gradually in vertebrates. In fish, there is no vagal
stimulation of the GI tract beyond the stomach. (Stevens,p. 273). In
amphibians, there is some sacral innervation
of this system. In amniotes the
only cholinergic excitation of the gut comes from the parasympathetic
division of the ANS and there is sacral parasympathettic
stimulation of the hindgut (Stevens, p. 274) ENZYMES Digestion relies on the function of enzymes
which are able to break down specific molecules in the diet. The enzymes which humans use to break down their
food are not unique to humans and many belong to gene families which originated
before the evolution of animals. Serine
proteases are a large family of enzymes in the human genome which function
in diverse physiological processes ranging from digestion to coagulation
(OMIM; Yosef, 2003). This is an ancient gene family which includes
eubacterial digestive enzymes and the vertebrate digestive
enzymes trypsin and chymotrypsin.
Cnidarian digestive enzymes include a trypsin-like digestive enzyme that functions in an alkaline
environment (Hyman, 393). Trypsin is produced
in the pancreas of all gnathostomes and in the
intestinal mucosa of hagfish (Stevens).
Vertebrates
use both endopeptidases (which cut proteins
in the middle of the chain but only at specific amino acids) and exopeptidases
(which remove amino acids from the ends of chains, regardless of the amino
acid). No pepsin is known in jawless
fish but this enzyme is known in gnathostomes. Chymotrypsin is known
in bony fish and higher vertebrates; elastase
is known from gnathostomes. Aminopeptidase
and carboxypeptidase are known from jawless
fish (Stevens). Cnidarians can digest most types of biomolecules but most cannot digest starches (Hickman, 137;
Hyman). Amylase is secreted from
pancreas in all vertebrates and from the salivary glands in many mammals,
including the echidna (Stevens, p. 160).
Maltase, isomaltase, and trehalase
are known in all tetrapods. Amphibians lack sucrase
but it is present in some fish (Stevens). The echidna and some marsupials
also lack sucrase (Stevens).
Low levels of lactase are present in birds which
is interesting since mammals use lactase to digest the milk sugar
lactose. Chitinase can break
down the exoskeleton of arthropods and is present in jawless fish, cartilaginous
fish, bony fish, and all groups of tetrapods
including some mammals (but not humans) (Stevens). In vertebrates, pancreatic lipase is the
most important enzyme in the digestion of lipids. (Stevens,
p. 168). In amniotes, the primary bile salt is cholesterol (as
opposed to sulfated alcohols in fish and amphibians) (Stevens). The
myelin P2 superfamily of proteins includes proteins
known as fatty acid-binding proteins (FABPs)
known from nematodes through mammals.
Vertebrate proteins seem to belong to distinct groups functional
in the heart, liver, and intestine. The
FABPs expressed in the livers of lampreys and sharks belong
to the heart subgroup, suggesting that a change in the FABPs
expressed in livers occurred early in the vertebrate line after the shark
lineage diverged from that leading to bony fish (Baba, 1999). Goblet
cells exist in acorn worms and secrete some enzymes. (Benito,
form Harrison 1997, p. 26). In tunicates, there are 6 types of
cells lining the stomach region. Some possess cilia, others are absorptive,
others seem to be similar to acinar cells of
the vertebrate pancreas, and others are endocrine cells, whose peptides
include secretin, gastrin, and somatostatin (Burighel, from Harrison,
1997, p. 255). |
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