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Human Digestive System
DIGESTION
Topic:6.1
Explain why digestion of large food molecules is essential.
Explain the need for enzymes in digestion.
State the source, substrate, products and optimum pH conditions for one
amylase, one protease and one lipase
Draw and label a diagram of the digestive system.
Outline the function of the stomach, small intestine and large intestine.
Distinguish between absorption and assimilation.
Explain how the structure of the villus is related to its role in absorption
and transport of the products of digestion.
NUTRITION
Nutrition: The process by which an organism takes the matter and
energy from its environment.
All organisms need nutrient to survive. There are two main types of
nutrition.
1- Autotrophic nutrition
2- Heterotrophic Nutrition
Classification of Nutrition
Heterotrophic Nutrition
Autotrophic Nutrition
photosynthesis
Holozoic
nutrition
Parasitic nutrition
Saprotrophic Nutrition
chemosynthesis
AUTOTROPHIC NUTRITION
Autotrophic organisms are self feeding organisms.
They can produce their own food by using inorganic
substances.
1- Photosynthetic nutrition: Organisms produce
their own food by using light energy and inorganic
substances. Plants, Protista, Bacteria
2- Chemosynthetic nutrition: Organisms produce
their own food by using energy from oxidation of
inorganic substances and inorganic substances.
Chemosynthetic Bacteria
HETEROTROPHIC NUTRITION
Heterotrophic nutrition: feeding on complex ready-made
food. Feeding on other organisms.
1-Holozoic nutrition: Animals take large food pieces into the
digestive system and digest them in the gut.
2-Parasitic nutrition: Organisms feed on/in a host
organisms. They take digested nutrients from host
organisms. These organisms do not have digestive
enzymes. Example: tape worm, thick, lice ……..
3-Saprotrophic Nutrition: Organisms feed on dead bodies.
They convert them into inorganic molecules. Bacteria and
fungi
WHAT IS FOOD?
Foods can be divided into six groups
according to its composition
A- carbohydrates.
B- lipids
C- proteins
D- vitamins
E- minerals
F- water
All of the molecules above are essential for a
balanced diet.
Which group of foods are macromolecules?
Foods are
a. ingested
b. digested
c. absorbed
d. assimilated
e. egested
Explain why digestion of large food molecules
is essential.
a. Ingestion: food taken into mouth for
processing in the gut.
b. Digestion: Process in which large molecules of
foods are broken down into smaller pieces to cross
plasma membrane easily
c. Absorbtion: Soluble products of digestion are
taken into the blood/lymph circulation.
d. Assimilation: Products of digestion are
absorbed from blood into body cells and used
or stored.
e. Egestion: Undigested food and dead cells of
the gut, together with bacteria from the gut
flora, expelled from the body as faeces.
What do we need for digestion of
food?
• Digestion is a hydrolysis reaction.
• Enzymes are required.
• Water is required for hydrolysis
Do we use ATP for digestion?
Does digestion take place outside of the body?
Where does digestion take place in the body?
In the cell or outside of the cell?
DIGESTION
Mechanical digestion: Physical breakdown of the
food (cheewing by teeth, muscular
contraction of stomach…)
Chemical digestion: chemical breakdown of the
food into smaller molecules (sub units) usually
by the help of the enzymes
Where does digestion take place?
1- Intracellular digestion:
Large molecules are digested in the
food vacuole by lysosome
enzymes. Some unicellular
organisms digest food by
intracellular digestion. Foods are
taken into the cell by phagocytosis
and digested by lysosme.
Disadvantage:
They cannot benefit from the bigger molecules than
they cannot take into cell.
2- Extracellular Digestion:
Animals take the food into a body cavity that is
continuous with the outside environment, into which
they secrete digestive enzymes. Enzymes act on
food, reducing it to nutrient molecules that can be
absorbed by the cells lining the cavity. e.g: all
vertebrates, some protists and some invertebrates
Advantage:
They can benefit from the complex molecules that are present in the
environment.
What are the organs of human
digestive system
In the digestive system there are mouth,
pharynx, esophagus, stomach, small and
large intestines.
There are glands that secrete enzymes to
assist digestion such as salivary glands,
gastric glands (stomach), intestinal glands,
liver and pancreas.
Figure 21.UN02
a.
g.
b.
h.
c.
d.
e.
i.
j.
f.
k.
l.
Figure 21.4_2
Nasal cavity
Oral cavity
(mouth)
Tongue
Salivary
glands
Pharynx
Esophagus
Liver
Esophagus
Sphincters
Gallbladder
Stomach
Pancreas
Small
intestine
Large
intestine
Rectum
Anus
Small intestine
DIGESTIVE ORGANS
Mouth: mechanical digestion by chewing food
and chemical digestion of the starch into
maltose.
Oesophagus: connects mouth
to stomach. Food is transported
by peristalsis ( wave like
Movement to push food to
Stomach).
Stomach:
It is the largest portion of the digestive
system. It is saclike structure between
esophagus and intestine.
.
Functions of stomach;
• To store ingested food
• To produce gastric juice and continue the
digestion of food
• To split nucleoproteins to nucleic acid and
protein
• To activate pepsin and rennin
• To kill bacteria in the ingested food
• To denaturate proteins
Small Intestine:
The human small intestine is 5-6 meters. It
has 3 parts.
• duodenum
• jejunum
• ileum
Duodenum: 26cm in length, 3-4 cm in
width. Wirsung channel (from pancreas)
attached to the duodenum. It has
important function in digestion, chemical
digestion takes place in duedonum.
Jejunum: Longest part. It carries out
digestion and absorption in this part.
Ileum: It is the final portion that connects
small intestine to large intestine and
absorption of nutrients takes place here..
Structure of small intestine;
• Mucosa layer has fingerlike projections
called villi. Mucosa cells secrete mucus
and enzymes and absorbs the nutrients.
• What is the difference between absorption
and assimilation?
Large
Intestine:
It is almost 1,5 – 2 meters in length and 6-8 cm
width. It is seperated from the small intestine by a
valve.
Cecum: small portion
water absorption takes place
Colon: the mid part
& Vit.K is produced by useful
Rectum: the last part
bacteria
There is no villi in the large intestine. Mucosa
contains lymphoid structures and glands.
DIGESTIVE SECRETIONS
Digestive secretions originate from the
salivary glands, gastric glands,
pancreas, gall bladder and small
intestine.
a) Salivary Glands and Their Secretions
• They produce saliva and secrete into a special duct
that open into mouth.
Saliva contains amylase (ptyalin), P, Ca.
FUNCTION:
• The initiation of carbohydrate digestion
• As a solvent for detection of taste
• The formation of bolus by mixing food
• Lubrication of the pharynx to swallow the food easily.
b) Gastric Glands
These glands secrete gastric juice. It is an acidic fluid,
that has pH almost 1,5. Because it consist of HCl
Function of HCl:
• It converts pepsinogen  pepsin
• It stimulates secretion of pancreas and intestinal
glands
• It destroys bacteria that are ingested with food
• It facilitates protein digestion by denaturation
c) The LIVER and bile secretion
[Click on the picture for liver animation]
Liver has many lobules …Liver is the largest organ of the body (1,52 kg). It las four main lobes(each lobe has lobules)
and gall bladder.
The liver cells in the lobules receive double blood
supply;
Oxygenated blood from hepatic artery and blood
containing newly absorbed nutrients from the gut
(via hepatic portal vein).
Also there are bile canals in the lobules and bile is
sent to the bile duct via these canals.
Bile and blood in the veins that goes to the central
hepatic vein move in opposite directions.
Liver is a multifunctional organ and
has more than 100 functions
Here are some major functions;
• Formation and secretion of bile
• Regulation of blood sugar level
• Formation of fibrinogen and thrombinogen for
blood clotting
• Conversion of pro-vit. A into vit.A by the
enzyme carotinase
• Deposition of Fe, Cu, proteins and vit. A, D and
K
• Degradation of old RBC by Kuppfer cells
• Elimination of foreign substances which enter the
liver by the blood (alcohol, medicine…)
• Detoxification of substances
(H2O2  H2O + O2 )
• Synthesis of urea
• Heparin secretion
• Synthesis of RBC by reticulo endothelial cells.
Functions of bile …
• Bile has bile salts, water,
pigments and cholesterol in
it.
• It neutralizes the acidity of
chyme entering the
duedonum from the
stomach
• It emulsifies lipids and
facilitates the activity of
enzyme lipase
• It helps absorption of vit. A,
D and K.
• Excessive cholesterolin
the bile may precipitate
out of solution with
calcium and bile salts,
forming hard crystals
called gallstones.
Pancreatic secretions;
• Digestive enzymes
• HCO3- for
neutralization of acidic
food.
Intestinal glands;
• Mucus for protection
• Digestive enzymes
Pancreatic secretions;
• Digestive enzymes
• HCO3- for
neutralization of
acidic food.
Intestinal glands;
• Mucus for protection
• Digestive enzymes
DIGESTION OF FOODS
CARBOHYDRATES:
Mouth:
Starch
amylase
maltose + dextrin
Stomach: No chemical digestion of Carbs.
Small Intestine: Digestion of Carbs. is
completed in small intestine
Dextrin + H2O amylase
maltose
Maltose+ H2O
maltase
glucose
lactase
Lactose+H2O
Sucrose+H2O
glucose+galactose
sucrase
glucose+ fructose
PROTEINS
They are digested in the stomach and
small intestine.
Stomach:
Proteins+ H2O
pepsin
polypeptide
Gastrin (hormone that secrete from stomach glands)
Gastric gland
HCL
Pepsinogen
pepsin
Protein + H2O
polypeptides
Small intestine:
Chyme
duodenum cells
Pancreas secretes;
a) Tyripsinogen Enterokinase
a) Chymotyripsinogen
Secretin(hormone)
İntestinal gland
Enterokinase
tyripsin
chymotyripsin
Tyripsin and chymotyripsin
Proteins
Polypeptides
polypeptides
aminopeptidase
carboxypeptidase
dipeptidase
tripeptidase
a.a’s
Lipids:
Lipids are digested only in small intestine
Lipids+Lipase+Bile+water
Glycerol
F. acids
Absorption of Nutrients
a) Mouth:
Some medicine, ions, drugs (cocaine)
and some poisons are absorbed in the
mouth
b) Stomach:
Alcohol, some medicine, ions, some
poisons are absorbed
c) Small intestine:
Most of the nutrients are
absorbed in the small
intestine.
Amino acids, glucose,
minerals, water soluble
vitamins are absorbed by
epithelial cells of mucosa
layer and are given to the
capillary blood vessels.
Before they enter the
general circulation, they
are transported into liver
by portal vein.
Glycerol, fatty acids and lipid soluble
vitamins are absorbed by epithelial cells
and they are combined with each other in
the mucosa cells and they are called
chylomicrons.
Then, these chylomicrons are given to the
lymph vessels (lacteals). That means they
are transported by lymph vessel, not blood
vessels.
d) Large intestine:
Water, electrolytes (Na, Cl, K) are
absorbed. Bacteria produce Vit. B & K
Figure 21.11
Heart
Liver
Hepatic
portal
vein
Intestines