HOLOZOIC NUTRITION 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 reaction.

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.

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PROCESSES IN HOLOZOIC NUTRITION

In general, holozoic nutrition involved the following processes : a) Ingestion

Ingestion involved intake of complex organic food (solid and liquid)

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b) DigestionDigestion is a process of breaking large complex and insoluble molecules into simple, small and soluble molecules. Digestion is achieved through mechanical means (chewing) of food into smaller particles and hydrolysis by digestive enzymes.

c) AbsorptionSoluble 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.

d) AssimilationAssimilation is a process where the molecules absorbed is used to generate energy or used for growth and repairs.

e) DefecationDefecation is a process where undigestible food, residue, bacteria is expelled from the body.

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

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Protein Endopeptidase

Polypeptide

Exopeptidase Dipeptide Dipeptidase

Amino

acidBreaking

longchain of protein

Aminopeptidase

carboxypeptidase

reacted on the end of carboxil

polypeptidase

Breaking down

dipeptide into

amino acid

Fat

Bile saltsColloidal

fat

Lipase

Fatty acid + glycerolFor emulsificatio

n

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 FunctionMouth cavity Ingestion, mechanical digestionPharyngx To swallowEsophagus Linking pharyngx to stomachStomach Storing food and digestion of proteinDuodenum:Liver (gall bladder) Fat emulsificationPancreas (pancreatic secretion) Carbohydrate, protein and fat digestionIlleum End of digestive process and start of

absorption processColon Absorption of waterRectum Formation and storage of fecal matterAnus Defecation

Stomach

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

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small intestine.

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 molecules.

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.

Small Intestine

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 EnzymeSite where reaction

takes place

Optimum pH

SubstratesEnd

product

Salivery secretion

Salivary amylaseMouth cavity

6.5-7.5 Starch (amylose) malthose

Gastric secretion

Pro(renin) Stomach 2.0 Caseinogen Casein

Pepsin (ogen) 2.0 Protein PolypeptideHCl (non

enzymatic)2.0 Pepsinogen Pepsin

Intestinal secretion

MalthaseSmall

intestine8.5 Malthose Glucose

LactaseSmall

intestine8.5 Lactose

Glucose + galactose

SucraseSmall

intestine8.5 Sucrose

Glucose + fructose

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Exopeptidase AminopeptidaseSmall

intestine8.5 Polypeptide and Amino acid

DipeptidaseSmall

intestine8.5 Dipeptide Amino acid

EnterokinaseSmall

intestine8.5 Trypsinogen Trypsin

Bile secretionBile salt

(Non enzymatic)Small

intestine7.0 Fat Fat droplet

Pancreatic secretion

AmylaseSmall

intestine7.0 Starch (Amylose) Malthose

LipaseSmall

intestine7.0 Fat

Fatty acid + glycerol

Trypsin(ogen)Small

intestine7.0

ProteinChemotrypsinoge

n

Amino acidchemotrypsi

n

(Endopeptidase)

Chemotrypsin(ogen)

Small intestine

7.0 Casein Amino acid

(Exopeptidase) CarboxypeptidaseSmall

intestine7.0 Polypeptide aminoacid

Large intestine

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 blood stream.

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.

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.

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Absoption and transport of nutrient

The structure of small intestine is optimized for nutrient absorption.

Internal small intestine can be increased by the following methods:

(a) Small intestine is a long slender tube.(b) The internal surface is folded to increase surface area.(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 nutrient. 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.

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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 materialFat 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 tractDigestive 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 digestion.

(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 :

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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

Shark :

Digestive system of shark consist of esophagus, stomach and duodenum. Duodenum has spiral valve to slows down food movement and increase absorption area.

Bird :

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.

Cow. :

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 "four stomachs" 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 "honeycomb tripe" 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 "indigestible" 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.

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