Download 6.1 Digestion - Biology3Nash

We eat a wide variety
of foods, containing
many different
biochemicals. Most of
them are useful to
the body but cannot
be absorbed or used
in the form they are
in when they are
eaten– they have to
be digested.
Types of Nutrients
 Micronutrients- vitamins, minerals, & water
 Macronutrients- proteins, lipids, carbohydrates, etc…
Important fact on digestion.
1- Proteins, polysaccharides and lipids are
all digests.
2- Large molecules cannot be absorbed
through the wall of the gut.
3- The membranes of the cell are
impermeable to large molecules.
4- Digestion is a chemical process.
Plants, fungi, ot other animals
have produced them, to
perform functions in those
organisms. But they are unlike
to be ideally suited to perform
functions in the human body.
By breaking down proteins and
other macromolecules, the human
body can obtain all the subunits that
it needs to build op its own
macromolecules.
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There are 9 amino acids that
humans must obtain by
digesting proteins in food.
Other amino acids can be
made from these nine and all
human proteins can be
synthesized.
Essential amino acids
tryptophan
methionine
valine
histidine
theorine
phenylalanine
leucine
Isoleucine
lysine
Digestion of macromolecules
Most food molecules are large
polymers and insoluble
They must first be digested to
smaller soluble molecules
before they can be absorbed
into the blood
6.1.2 Enzymes and digestion.
Enzymes are biological catalysts
that increase the rate of
reaction.
Digestive enzymes are secreted
into the lumen of the gut.
Digestive enzyme increase the rate
of reaction of the hydrolysis of
insoluble food molecules to soluble
end products.
Digestive enzymes increase the
rate of reaction at body
temperature.
This image illustrates the
reduction in activation energy that
is achieved by the use of an
enzyme.
Notice that the normal reaction
requires a higher activation energy
which would correspond to a high
body temperature. This is usually
not possible in living organisms.
The enzyme-catalysed reaction
has a lower activation energy.
This lower activation energy
would correspond to body
temperature but is only possible
in the presence of an enzyme.
6.1.3 Types of digestive enzyme
Example 1 Pancreatic amylase:
Conditions:
Source the Pancreas
Optimal pH 7.5-7.8
Substrate is starch (amylose)
End product is the disaccharide maltose
Action: hydrolysis of 1-4 glycosidic bonds
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Example 2: Pepsin is a protease
produced in the stomach
Conditions:
Source is the stomach
Optimal pH is 2
Substrate is a polypeptide chains of amino
acids
End product is small polypeptides
Action is the hydrolysis of peptide bonds
within the polypeptide chain (endopeptidase).
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Example 3: Pancreatic lipases:
Source is the pancreas
The optimal pH is 7.2
The substrate is a triglyceride lipid
The product is glycerol and fatty acid chains
The action of pancreatic amylases also requires the
presence of bile salts that emulsify the lipid.
This emulsification has two effects:
Increases the surface area of the lipid for the digestion of
fat
Exposes the glycerol 'head' structure to the enzyme
Action: hydrolysis of ester bonds between the glycerol
molecules and the fatty acid chains.
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Enzymes and pH
Enzyme
Main secreting pH
glands
optimum
of enzyme
Region
where
enzyme
works
Amylase
1- salivary
glands
2- pancreas
Mouth
Small intestine
7
7
Protease
1- stomach wall
2- pancreas
2
7
lipase
pacreas
7
Stomach
Small intestine
Small intestine
pH of region
where enzyme
works
6.0- 7.0
7.0- 8.3
1.0- 3.5
7.0- 8.3
7.0- 8.3
The digestive System
It consists of the
digestive tract, a
tube extending
from the mouth
to the anus, and
its associate
accessory organs.
The regions of the digestive tract include
the following:
 Oral cavity or mouth
 Pharynx or throat
 Esophagus
 Stomach
 Small intestine
 Consists of the duodenum, jejunum and ileum,
with liver, gallbladder, and pancreas
 Large Intestine
 Includes cecum, colon, rectun and anal canal
 Anus
Structure of the digestive system
The part of the human body
used for digestion can be
described in simple terms as
a tube through which food
passes from mouth to the
anus.
Human digestive system
GI (gastrointestinal) tract = alimentary canal
Ingestion
 Mouth
 mechanical digestion

teeth

breaking up food
 chemical digestion

saliva




amylase
 enzyme digests starch
mucin
 slippery protein (mucus)
 protects soft lining of digestive system
 lubricates food for easier swallowing
buffers
 neutralizes acid to prevent tooth decay
anti-bacterial chemicals
 kill bacteria that enter mouth with food
mouth
break up food
digest starch
kill germs
moisten food
Mouth
 Chemical and
mechanical
digestion.
 Food is chewed
(masticated)
mechanically.
 A bolus (lump) is
formed with saliva
and the tongue.
Swallowing (& not choking)
 Epiglottis
 flap of cartilage
 closes trachea (windpipe) when swallowing
 food travels down esophagus
 Peristalsis
 involuntary muscle contractions to move food along
Which type of digestion is the
following?
1.
Chewing a saltine? -
2. Saliva breaking the saltine down into molecules of
glucose? 3. Your tongue breaking pieces of a hamburger apart?
4. Pepsin (an enzyme) in your stomach breaking the
hamburger into amino acids?
Pharynx
 The back of the
throat.
 Larynx- passage
for air, closes
when we
swallow.
 Is approximately
15cm long.
Esophagus
The part of the digestive
tract that extends between
the pharynx and the
stomach. It is about 25 cm
long and lies in the
mediastrinum, anterior to
the vertebrae and posterior
to the trachea. The
esophagus transports food
from the pharynx to the
stomach.
The wall of the tube is made of living
tissues, which include:
Muscle fibers, to push the food
along
Secretory cells, which produce
mucus for lubrication and enzymes for
digestion.
A single layer of epithelium cells,
which form the inner lining and absorb
digested food.
Peristalsis
Series of
involuntary
wave-like muscle
contractions
which move food
along the
digestive tract
Peristalsis
propels
material
through most
of the
digestive tract.
Stomach
 Food is temporarily
stored here.
 Gastric juices are
secreted.
 Has layers of muscle
that line the inside.
 Mechanically and
chemically breaks
down food.
Peristaltic waves
Muscular contractions consisting of a wave of
relaxation of the circular muscles in front of
the bolus, followed by a wave of strong
contraction of the circular muscles behind the
bolus, which force the bolus along the
digestive tube.
An enlarged segment
of the digestive tract
that primarily function
as a storage and
mixing chamber. It is
located in the left
superior part of the
abdomen.
Function of the stomach
1- The wall of the stomach is glandular,
secreting mucus, hydrochloric acid
and protease.
2- Hydrochloric acid creates very acid
conditions that kill bacteria in food
that has been swallowed. Without
this, we would often catch food
poisoining or other diseases from the
food that we eat.
Function of the stomach
3- Acid conditions denature
proteins and suit protein digestion
by hydrolysis. Protease secrete in
the stomach, called pepsin, begins
the process by breaking up
polypeptides into shorter chains of
amino acids.
Function of the stomach
4- The wall of the stomach is elastic,
so large meals can be swallowed
quickly, stored and then gradually
released into the small intestine.
5- The wall of the stomach is muscular
and contracts rhythmically to squeeze
the food, mixing it and breaking it
into smaller lumps.
Stomach
 Functions
 food storage

can stretch to fit ~2L food
 disinfect food
 HCl = pH 2

kills bacteria
 chemical digestion
 pepsin

enzyme breaks down proteins
But the stomach is made out of protein!
What stops the stomach from digesting itself?
mucus secreted by stomach cells protects
stomach lining
mouth
break up food
digest starch
kill germs
moisten food
stomach
kills germs
break up food
digest proteins
store food
sphincter
sphincter
Gastric Juices
 Secreted by the stomach.
 Acidic (pH 1.5-2.5) (HCl).
 Pepsin- an enzyme that
breaks down large
proteins into amino
acids.
 Food is further broken
down into a thin liquid
called chyme.
Accessory Organs
Pancreas
Gall Bladder
Spleen
The gallbladder is a
saclike structure on
the inferior surface of
the liver.
The liver continually
secretes bile, which
flows to the
gallblader where it
stores.
Gall bladder
 Pouch structure located near the liver
which concentrates and stores bile
 Bile duct – a long tube that carries
BILE. The top half of the common bile
duct is associated with the liver, while
the bottom half of the common bile
duct is associated with the pancreas,
through which it passes on its way to
the intestine.
BILE
 Bile emulsifies lipids (physically breaks apart FATS)
 Bile is a bitter, greenish-yellow alkaline fluid, stored in
the gallbladder between meals and upon eating is
discharged into the duodenum where it aids the process
of digestion.
Pancreas
A complex organ composed of both
endocrine and exocrine tissue that perform
several functions.
The exocrine secretions of the pancreas,
called pancreatic juice, have an aqueous
component and an enzymatic component.
Pancreatic juice is delivered to the small
intestine through the pancreatic ducts,
where it functions in digestion.
Pancreas
Pancreas
 An organ which secretes both digestive enzymes (exocrine)
and hormones (endocrine)
 ** Pancreatic juice digests all major nutrient types.
 Nearly all digestion occurs in the small intestine & all
digestion is completed in the SI.
Pancreas
 Digestive enzymes
 digest proteins

trypsin, chymotrypsin
 digest starch

amylase
 Buffers
 neutralizes
acid from
stomach
Liver
It performs
important digestive
and excretory
functions, stores
and processes
nutrients, detoxifies
harmful chemicals,
and synthesizes
new molecules.
Liver
 Function
 produces bile


bile stored in gallbladder until needed
breaks up fats
 act like detergents to breakup fats
bile contains
colors from old
red blood cells
collected in liver =
iron in RBC rusts &
makes feces brown
mouth
break up food
digest starch
kill germs
moisten food
liver
produces bile
- stored in gall bladder
break up fats
pancreas
produces enzymes to
digest proteins & starch
stomach
kills germs
break up food
digest proteins
store food
Small Intestine
It consists of the three parts: the
duodenum, the jejunum and the ileum.
The entire small intestine is about 6 m
long. Two major accessory glands, the liver
and the pancreas, are associated with the
duodenum.
The small intestine is where the greatest
amount of digestion and absorption
occurs.
Small Intestine
a) Villus which increase the
surface area for absorption of the
products of digestion
(b) Microvilli border of the
epithelial cell increases the
surface are for absorption.
(c) Lacteals are connect to the
lymphatic system for the
transport of lipids.
(d) In the wall of the small
intestine are the blood vessels to
transport absorbed products to
the general circulation, There
are also the muscle to maintain
peristalsis
Function of the small intestine
1- Pancreatic juice is secreted
into the small intestine. It
contains protease, amylase and
lipase, so the three main groups
of the macromolecules are all
digested here.
Function of the small intestine
2- The wall of the small intestine
secretes a variety of other enzymes,
which digest other substances.
Nucleases, for example, are secred to
digest DNA and RNA in food eaten.
Because of the great length of the
small intestine, food spends hours
passing thorugh, giving time for
digestion to be completed.
Function of the small intestine
3- Digested foods are absorbed by the small intestine.
4- Areas in the wall of the small intestine called
Peyer´s patches contain lymphocytes that detect
pathogenic organisms in the food in the small
intestine and help in the production of antibodies
against them.
Small Intestine
 Most chemical digestion
takes place here.
 Simple sugars and
proteins are absorbed
into the inner lining.
 Fatty acids and glycerol
go to lymphatic system.
 Lined with villi, which
increase surface area for
absorption, one cell thick.
Small intestine
 Function
 chemical digestion
 major organ of digestion & absorption
 absorption through lining
 over 6 meters!
 small intestine has huge surface area = 300m2 (~size of
tennis court)
 Structure
 3 sections
 duodenum = most digestion
 jejunum = absorption of nutrients & water
 ileum = absorption of nutrients & water
Duodenum
 1st section of small intestines
 acid food from stomach
 mixes with digestive juices from:
 pancreas
 liver
 gall
bladder
mouth
break up food
digest starch
kill germs
moisten food
pancreas
produces enzymes to
digest proteins & starch
stomach
kills germs
break up food
digest proteins
store food
Absorption in the SI
 Much absorption is thought to occur directly
through the wall without the need for special
adaptations
 Almost 90% of our daily fluid intake is
absorbed in the small intestine.
 Villi - increase the surface area of the small
intestines, thus providing better absorption of
materials
Absorption by Small Intestines
 Absorption through villi & microvilli
 finger-like projections
 increase surface area for absorption
Structure and function of the villus
The structure of the villus increases the surface
are for the absorption of digested food
molecules.
(a) folds increase SA:VOL ration by X 3
(b) Villi project into the lumen of the gut increasing the surface
area by X 10
(c) Microvilli are outward folds of the plasma membrane
increasing the surface area another X10
This sequence of light microscope and electron
micrograph images show the same sequence as the
diagram above.
This sequence of light microscope and electron micrograph images show the same sequence as the diagram above.
Histological adaptations within the
villus.
•Blood supply in the villus which absorb the end
products of digestion from the epithelial cells
•The lacteals (green) that receive the lipoproteins
before transporting them to the circulatory
system.
•Muscular walls that maintain the movement of
chyme by peristalsis.
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Inside the small intestine
VILLI
Large Intestine
The movement of the large intestine are
more sluggish than those of the small
intestine. While in the colon, chyme is
converted to feces. The formation of
feces inclives the absorption of water and
salts, secretion of mucus and extensive
action of microorganisms. The colon
stores the feces until they are eliminated
by defecation.
Large intestines (colon)
 Function
 re-absorb water
 use ~9 liters of water every
day in digestive juices
 > 90% of water reabsorbed
 not enough water
absorbed
 diarrhea
 too much water absorbed
 constipation
Large Intestine
 Solid materials pass
through the large intestine.
 These are undigestible
solids (fibers).
 Water is absorbed.
 Vitamins K and B are
reabsorbed with the water.
 Rectum- solid wastes exit
the body.
You’ve got company!
 Living in the large intestine is a
community of helpful bacteria
 Escherichia coli (E. coli)
produce vitamins
 vitamin K; B vitamins
 generate gases
 by-product of bacterial metabolism
 methane, hydrogen sulfide

Appendix
Vestigial organ
Rectum
 Last section of colon
(large intestines)
 eliminate feces
 undigested materials
 extracellular waste
 mainly cellulose
from plants
 roughage or fiber
 masses of bacteria
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Defecation
 Stretch receptors in rectal wall initiate
spinal reflex
 Motor through parasympathetic fibers
 Longitudinal muscles contract
 Aided by voluntary contraction of
diaphragm and abdominal muscles and
relaxation of external anal sphincter
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Insoluble food molecules
are digested to soluble
products in the lumen of
the gut.
Absorption:
The soluble products are
first taken up by various
mechanisms into the
epithelial cells that line the
gut.
These epithelial cells then
load the various absorbed
molecules into the blood
stream.
Assimilation:
The soluble products of
digestion are then
transported to the various
tissues by the circulatory
system.
The cells of the tissues then
absorb the molecules for use
within this tissues