Most of the nutrient contained in food such as protein, lipid and carbohydrates are relatively
large molecules, which do not dissolve in water and cannot enter cell membrane. This
nutrient must be broken down to smaller unit, which can be absorbed and transported by
blood to the whole body. This is achieved through mechanical, chemical and enzymatic
How food is process to extract its nutrient is call digestive system and occur in alimentary
canal. In human, alimentary canal is divided into six region: mouth, esophagus, stomach,
small intestine, large intestine and anal region.
PROCESSES IN HOLOZOIC NUTRITION
In general, holozoic nutrition involved the following processes :
Ingestion involved intake of
complex organic food (solid
Digestion is a process of breaking large compl
molecules into simple, small and soluble
molecules. Digestion is
achieved through mechanical means (chewing)
of food into
smaller particles and hydrolysis by digestive
Soluble smaller molecules
move from digestion zone to
the absorption zone where it
crosses the epithelial lining of
small intestine. There
molecules either enter the body
tissues or transported to other
part of the body by blood.
Assimilation is a process where
the molecules absorbed is used
to generate energy or used for
growth and repairs.
Defecation is a process where
undigestible food, residue,
bacteria is expelled from the
Function of human digestive tract
Nutrient can only be absorbed into the body when complex foods are broken down by
mechanical action of teeth and digestive tract muscle followed by enzymatic digestion
(chemical digestion) to produce simple and small molecules that can be absorbed into the
blood capillaries surrounding the small intestine. Enzymetic digestion for three food classes
(polysaccharide, protein and fat) is shown in table 4.1
Table 4.1 Enzymetic digestion for three food classes (polysaccharide, protein and fat) in
human digestive tract.
Polysaccharide Amylase Disaccharide Disaccharase monosaccharide
Protein Endopeptidase Polypeptide Exopeptidase Dipeptide Dipeptidase Ami
Breaking long Aminopeptidase Breaking
chain of carboxypeptidase down
protein reacted on the dipeptide
end of carboxil into
polypeptidase amino acid
Bile salts Lipase
Fat Colloidal fat Fatty acid + glycerol
Each part of the digestive system have a specific function as shown in table 4.2
Table 4.2 Summary of function in human digestive system
Specific region Function
Mouth cavity Ingestion, mechanical digestion
Pharyngx To swallow
Esophagus Linking pharyngx to stomach
Stomach Storing food and digestion of protein
Liver (gall bladder) Fat emulsification
Pancreas (pancreatic secretion) Carbohydrate, protein and fat digestion
Illeum End of digestive process and start of
Colon Absorption of water
Rectum Formation and storage of fecal matter
Stomach surface lining is creased or crinkeled known as rugae Permukaan dalam perut
mempunyai lipatan-lipatan vang disebut rugae. Rugae membolehkan perut mengembang
untuk menampung kemasukan makanan. Fungsi perut termasuklah :
(1) Place for temporary
storage of food
(2) Secretion of gastric juice
(HCl) and enzymes
(3) Mechanical food mixing
(4) Control entry of food to
Thick mucosal layer are supplied with gastric gland and goblet cells. Gastric gland consists
of three type of cells
i) zymogen cells ii) oxintic cells iii) argentafin cells
These cells produced intrinsic factor which assist in the absorption of vitamin B12 complex
Goblet cells produced copius mucous to protect intestinal lining against corrosive nature of
HCl and digestive protein enzyme.
Collective secretion of gastric gland and goblet cells is also known as gastric secretion.
Stomach muscle will churned (mixed) foods with gastric secretion and turned it into semi-
liquid state called chyme. After about 2-3 hours the chyme is released to the duodenum.
Chyme enter small intestine in stages and mixed together with small intestine’s secretion.
There are three types of small intestine secretion:
i) intestinal secretion ii) bile secretion iii) pancreatic secretion
Most of the digestion and absorption processes taken place in small intestine. Chyme will
remain in small intestine for about 4-5 hours. When most of the nutrient are absorbed, the
remaining mixture will move to the large intestine by peristaltic action.
Table 3.2 Summary of chemical/enzymatic digestion in human.
Secretion Enzyme reaction Substrates End product
Salivary amylase 6.5-7.5 Starch (amylose) malthose
Pro(renin) Stomach 2.0 Caseinogen Casein
Pepsin (ogen) 2.0 Protein Polypeptide
2.0 Pepsinogen Pepsin
Malthase 8.5 Malthose Glucose
Small Glucose +
Lactase 8.5 Lactose
Small Glucose +
Sucrase 8.5 Sucrose
Exopeptidase Aminopeptidase 8.5 Polypeptide and Amino acid
Dipeptidase 8.5 Dipeptide Amino acid
Enterokinase 8.5 Trypsinogen Trypsin
Bile salt Small
Bile secretion 7.0 Fat Fat droplet
(Non enzymatic) intestine
Amylase 7.0 Starch (Amylose) Malthose
Small Fatty acid +
Lipase 7.0 Fat
Small Protein Amino acid
intestine Chemotrypsinogen chemotrypsin
(Endopeptidase) Chemotrypsin(ogen) 7.0 Casein Amino acid
(Exopeptidase) Carboxypeptidase 7.0 Polypeptide aminoacid
In overall digestive gland secrete about 5-6 liters of water daily on the food mixture. From
that amount only 0.2 liter that is not reabsorb by the large intestine. There are symbiotic
bacteria living in the large intestine synthesizing amino acid and vitamins especially vitamin
K from the remaining (residue). The synthesize amino acids and vitamin K is absorb into the
Reabsorption of water and the presence of bacteria turned the “residue” into solid mass
known as fecal matter. It is estimated that about 50% of the solid fecal matter consist of
bacteria. The epithelial mucosal lining of the large intestine secrete mucous to lubricate
Fecal matter can remain in large intestine for 36 hours before sending it to temporarily to
rectum. It will expelled through anus by anal sphincter muscle.
Absoption and transport of nutrient
The structure of small intestine is optimized
for nutrient absorption.
Internal small intestine can be increased by the
(a) Small intestine is a long slender tube.
(b) The internal surface is folded to increase
(c) The folded internal surface has papilae
outgrowth called villus.
(d) Numerous hair like projection on each
villus called microvillus.
Figure 4.4 How the internal surface area of intestine is increased
In each villus is supplied with blood capillaries network and lacteal to absorb and transport
Monosaccharide and amino acid is absorbed through simple diffusion or active to the blood
capillaries. These nutrient is sent to the liver. Most of the monosaccharides (glucose) is
stored in liver or muscle as glycogen and fats. Part of the absorbed glucose is distributed to
the rest of the body and oxidised in cellular respiration for energy generation.
Amino acid is used to synthesis new protoplasm, replacement of dead body tissues,
manufacturing enzymes and hormones. Excess amino acid cannot be stored in liver but
deaminated that is the amino group is removed and turned into urea, uric acid and other
nitrogenous excretory product. These materials is removed through urine and remaining
amino acid is converted into stored glycogen.
Fatty acid and glycerol enter epithelial cells where it is turned into fat. The fat will enter
lacteal where it is surrounded by protein layer to form lipoprotein and known as
cholymicron. Lipoprotein than enter the blood stream. In the blood, lipoprotein is
hydrolyzes by enzymes to form fatty acids and glycerol. These two lipid componen (fatty
acids and glycerol) will enter cells and used in cellular respiration or stored as fat in liver,
muscles, adipose tissues underneath skin, around heart and kidney and mesentery. The
mesentery is a structure that envelops the internal organs, such as the small intestine,
lending support, blood supply, and lymph vessel drainage. Some of the lipoprotein is used as
cell’s membrane and muclear membrane.
Advantageous of fat as storage material.
Fat is important as reserve energy. It contain significant amount of carbon-hydrogen bond
which is rich in energy compared to carbohydrate. Fat produce 9.3 kcal energy per gram
compared to 3.79 kcal for carbohydrates and 3.12 kcal for protein. Therefore more energy
can be stored in compact package.
The advantageous of fat as storage material
Fat is an important source of reserve energy. Fat contain significant amount of high energy
carbon-hydrogen bond compared to carbohydrates. Fats produce 9.3 kcal of energy /g
compared to 3.79 kcal for carbohydrate and 3.12 kcal for protein. Therefore fat is a compact
high-energy package, which is more suited for animal, which require mobility. Fat also
produced 2 times more water during cellular respiration compared to carbohydrates and
proteins. That is why it is an important for desert.
Variation of digestive tract
Digestive tract of higher organism consist of long elongated tube with one opening at one
end (mouth) for entery of food and another opening at the end (anus) for excretion of
undigested food. This tube is divided into specialized region with specific function as shown
in table 3.2
Variation of the digestive tract in vertebrates is closely associated with the following:
(1) Types of food :
Whether the food is easily absorbed or requires extensive mechanical and enzymatic
(2) Quantity of food :
Its depends whether food supply or food source is easily available or otherwise. If food is
difficult to obtain the animal must consume large quantity and digestive tract must be able to
accommodate it until it can be digested later.
3) Body shape :
If the body is long , it is most likely that the digestive tract is long. If the body is compact,
the digestive can be short of in coiled form to provide increase absorption area.
Cyclostoma is a primitive jawless fish, which has the simplest digestive tract. It is an
elongated tube with esophagus and intestine. The intestine is equipped with spiral valve
known as tiflosol to slows down food movement and increase absorption area. Ikan
Digestive system of shark consist of esophagus, stomach and duodenum. Duodenum has
spiral valve to slows down food movement and increase absorption area.
The lower end of esophagus enlarge to from crop where food is stored and moistened. The
stomact consist of two parts that is proventriculus and gizzard. Proventriculus secretes
gastric secretion whereas gizzard is a thick wall and muscular which provide mechanical
digestion. For grain eating bird, mechanical digestion is aided with small stone and sand.
A system that could process cellulose before food travelled through the main digestive
organs would be more efficient. The ungulates - cows, deer sheep and their relatives - have
found a way. The lower part of the oesophagus and the stomach of a cow are highly
modified (the quot;four stomachsquot; of a cow). A large chamber, the rumen, is dedicated to
storing grass and mixing it with a variety of symbiotic bacteria. This opens into a second
chamber, the reticulum, with highly folded inner walls that provide a massive area for
bacterial growth. The walls of this chamber are sold in grocery stores as quot;honeycomb tripequot;
The bacteria feed on the cellulose and grow to massive populations. Hours later, the cow
will regurgitate a combination of caked bacteria and undigested food called cud. It will chew
the cud and swallow it, this time diverting it to the third chamber, the omasum. Here water
is withdrawn for recycling. From the omasum the cud enters the true stomach, or
abomasum, where chemical digestion begins.
Because this all occurs before the food enters the stomach, digestive enzymes can now break
down the various proteins, carbohydrates and lipids found in the bacteria. This extremely
rich mixture of nutrients can sustain quite large animals on a diet mainly consisting of
quot;indigestiblequot; grass. In fact, a cow is not living directly on grass, but an a diet of bacteria. It
is little wonder, with this special design for getting the best out of an abundant resource, that
the wildebeest, bison, antelopes and caribou have become the dominant large mammals of
their respective environments.