1.0 this is because enzymes only react with food

1.0  Introduction

This assignment will discuss in great
detail about the three macronutrients, the digestive system which includes the
digestion tract and its organs, which process food into molecules that can be
absorbed and used by the cells of the body (Tortora and Derrickson, 2011). The
assignment will include information about the passage of inert ingesta (food
components that cannot be digested such as NSP) starting from absorption to the

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The Digestive system involves the gastrointestinal
(GI) tract, which starts in the mouth and ends at the anus. It includes the
stomach, small intestines, large intestines and rectum.  Organs of the digestive system include,
salivary glands, pancreas and liver. All of these produce secretions, which
enter the GI tract, and each plays a role in the digestive process (Colbert et
al., 2012).


2.0 Carbohydrates

2.1 Digestion of Carbohydrates In
the mouth

Mechanical digestion begins in the mouth,
where the tongue, grinded by the teeth and mixed in with saliva, controls food.
The food is made into smaller pieces, which can be swallowed easily, mass
called the bolus.

Molecules of food dissolves in water
saliva, this is because enzymes only react with food molecules in a liquid only
Salivary amylase, an enzyme in saliva which breaks the chains of starch called
polysaccharides into disaccharide molecules called maltose. They are made up of
two units of glucose; maltose needs to be broken into single molecules of
glucose in order to provide energy.

The salivary amylase cannot finish this so
the disaccharides move down into the stomach (Tortora and Derrickson, 2011).


2.2 Digestion of Carbohydrates In the Oesophagus

Once the food from the mouth has been chewed
and mixed in with the saliva, it is then passed through to the pharynx into the
oesophagus. The opening of the food from the oral cavity into the oesophagus
starts swallowing, swallowing can be spilt into different stages as in
voluntary, pharyngeal and esophageal, this is a reaction response that has been
started by the voluntary action and it is controlled by the swallowing center
in the medulla of the brain. In order to allow the food to be swallowed, the
esophageal sphincter relaxes and then allows the oesophagus to open (Gropper
and Smith, 2013).


2.3 Digestion of Carbohydrates In
the stomach

Enzyme amylase presented in the masticated
food from the mouth continues to break down the carbohydrates in the stomach.
However, the food and salivary enzymes continue the digestion until the
secretion of stomach acid reaches the pH to 3.0. Normally when the food arrives
into the stomach the pH is between 5.0 and 6.0.

Studies have shown that young and health
adults it takes about 45 minutes before enough acid generated to drop the pH to
3.0. This is due to the stomach acid that is secreted into the stomach in
response to the expansion the stomach wall (Loomis and Mann, n.d.).

2.4 Digestion of Carbohydrates In
the small Intestines.

2.4.1 Digestion

Majority of carbohydrate digestion takes
place in the small intestines the pancreas secretes pancreatic amylase along
the pancreatic duct into the small intestine. Pancreatic amylase maintains the
digestion of starch, breaking it down into the disaccharide maltose.

The three brush borders digest
disaccharides into monosaccharide. Sucrase breaks sucrose into molecules of
fructose. Lactase digests lactose into molecules of glucose and molecules of
galactose. Maltase splits maltose and maltotriose into three molecules of

Digestion of carbohydrates ends in the
production of monosaccharide, where the digestive system is able to absorb
(Tortora and Derrickson, 2011).

2.4.2 Absorption

The digestion phase of chemical and
mechanical from the mouth to the small intestine is directed towards changing
the food into forms that can be passed through the absorptive epithelial cells
lining the mucosa and into the underlying blood and lymphatic vessels.

When carbohydrates are condensed and made
into smaller pieces, it can then be easily absorbed into both upper and lower
parts of the small intestine (Lunn and Buttriss, 2007).

About 90% of all the absorptions of
nutrient take place in the small intestine, and 10% of the nutrients take place
in the stomach and the large intestine. 
Any of the materials that were left behind in the small intestine in
terms of digestion and absorption will pass on to the large intestine (Tortora and
Derrickson, 2011).

3.0 Protein

3.1 Digestion of protein In the

digestion takes place when food is mixed in with saliva to form a bolus (see section

3.2 Digestion of
Protein In the Oesophagus

Bolus is moved down the esophagus by
recurring muscle contracts called a peristalsis. At the end of the esophagus is
a thick ring of a muscle called a sphincter that allows food to enter the

3.3 Digestion of
Protein In the stomach

3.3.1 Digestion

Protein digestion starts in the stomach,
where gastric cells release the hormone gastrin, which then enters the blood,
causing the release of gastric juices.

Hydrochloric acid in gastric juice disrupts
protein and then converts pepsinogen to pepsin; this then starts to digest
protein by hydrolyzing peptide bonds. Some of the digested proteins enter the
small intestine, which then causes the release of the hormone secretion and
cholecystokinin (Tortora and Derrickson, 2011).


The hormone stimulates the pancreases so
they can release the pro-enzymes and bicarbonate into the intestines. This is
because these enzymes will digest the polypeptides into tripeptides, dipeptides
and free amino acids.

Intestinal enzymes in the lumen of the
small intestine and within the mucosal cell complete the protein digestion
(Gropper and Smith, 2013).


3.4 Digestion of Protein In the small

Pancreatic juice contains inactive enzymes
that digest proteins (trypsin, chymotrypsin and carboxypeptidase) (Tortora
& Derrickson, 2012). 

When pancreatic juice enters the small
intestine, enterokinase activates trypsin, which then activates all of the
other proteins digesting enzymes (Kohlmeier, 2015). Trypsin, chymotrypsin and
carboxypeptidase break the peptide bonds in between amino acids. The final
digestion of the protein happens when peptidase released from the absorptive
cells in the villi break the proteins into amino acids, dipeptides and

3.4.1 Absorption

Proteins are unable to be digested by
adults, therefore they are metabolised to small peptides and dipeptides in the

Enzymes that include trypsin and
chymotrypsin, peptides: the pancreases and amino peptidase secrete
carbozypeptidase, which are located in the brush boarder of the epithelial
cells. They both release amino acids from different sides of the peptide
chains, increasing the hydrolysis reaction. Transport proteins that are found
in the brush boarder act on the small peptides are also on the amino acids to
move the molecules across the plasma membrane where they are released into the
blood stream (Bradley and Calvert, 2013).

4.0 Lipids

4.1 Digestion of Lipids In the

Mouth is the first stage of the digestion
when lipids are broken down into tiny droplets, in order to make the digestion
process easier. Lipids are mainly not water-soluble even though that the digestive
secretions are all water base with the helps of chewing, peristalsis and
segmentations along with combined various emulsifiers in the mouth (Marieb,


According to (Zafra, Molina and Puerto, 2006)
saliva is released before food is present, it contains an enzyme called lingual
lipase which is important for fats to breakdown from triglycerides to fatty
acids and glycerol.


4.2 Digestion of lipids In the Oesophagus

Oesophagus receives food from the mouth when it is swallowed. Peristalsis
takes place, Lingual lipase carry on breaking down the fat until it is able to
be absorbed into fatty acids and glycerol.


4.3 Digestion of Lipids In the

Lipid digestion starts with the secretion
of gastric lipase in the stomach. Lingual lipase continues to digest
triglycerides and diglycerides into fatty acid, when it is exposed to
hydrochloric acid in the stomach, enzymes stop functioning properly as lingual
lipase is active when pH is either acidic or alkaline. Chief cells secrete
gastric lipase in gastric juice, as they can tolerate the acidic pH of the
stomach, which then breaks triglycerides into fatty acids and mono-diglycerides
(Kohlmeier, 2015).

4.4 Digestion of Lipids In the
small Intestine


4.4.1 Digestion

When lipids are presented in the duodenum
it stimulates the release of store bile from the gallbladder. This is because
fats are not water-soluble and they do not mix with water so at that point bile
starts to react as a emulsifier to keep the lipid molecule combined with water
and digestive secretion (Rolls & Bell, 1999).


However, (David A. Bender, 2014) stated
that lipase would require a pancreatic protein as in a colipase for activity.
Pancreatic lipase removes two fatty acids from triglycerides, allowing the
fatty acids to be absorbed through the brush boarder of the intestines. Monoacylglycerols
are digested into the glycerol to free the fatty acids by the pancreatic
esterase in the intestinal lumen and intracellular lipase within the intestinal
mucosal cells.


4.4.2 Absorption

Fatty acids and monoglycerols are the two
digestive products of lipids; they are produced when lipids are broken down by
the pancreatic lipase. Fatty acids and monoglycerols are easier for the small
intestine to handle as well as they are easily absorbed from the digestive
tract when passing through the mucosal lining of the small intestine and into
the epithelial cells (Rhees et al, 2009).


Once inside, fatty acids and monoglycerols
form a new triacylglycerol molecule, combining it with proteins and
phospholipids to make chylomicron (Kohlmeier, 2015).  They are then released from the lacteal to
the lymphatic system before it gets passed through the thoracic duct and then
into the bloodstream (David A. Bender, 2014).

5.0 Inert Ingesta

Dietary fibres can’t be broken down by the
digestive system, so therefore it gets passed through the intestine, which
absorbs water and increases in the bulk.

There are two types of dietary fibres,
insoluble and soluble and they both function differently.


Insoluble fibres- they absorb water, and
increases in bulk, it also helps the stool to become soft which helps the gut
to be kept in good shape. Foods that’s are rich in insoluble fibres are whole
meal bread, flour, pasta and some fruits and vegetables.


Soluble fibres- they are known to slow down
digestion and absorption of carbohydrates, which then helps to control the
sugar levels, this helps from feeling hungry. They can also reduce blood
cholesterol levels that can reduce the risk of coronary heart disease.  Sources of soluble fibres include oats, peas
and beans (Ridgwell, 1996). 


Benefits of high fibre diets: there is less
chance of getting hemorrhoids and small pouches in the colon. Liver converts
cholesterol in bile salts, this releases in the small intestine, this helps the
fat digestion (Insel, Turner & Ross, 2005).