The document summarizes the key processes of digestion. It describes digestion as the breakdown of large food molecules into smaller molecules that can be absorbed. The main stages are ingestion, digestion in the mouth, stomach, and small intestine, absorption in the small intestine, and egestion. It provides details on the roles of teeth, saliva, stomach acid, pancreatic juices, and intestinal enzymes in breaking down proteins, lipids, and carbohydrates into smaller molecules like amino acids and simple sugars.

1. Biology The digestive system
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• Digestion ----Digestion is the process by which large, complex, insoluble food molecules
are converted to smaller, simpler and soluble ones using enzymes.
Or, Digestion is the process by which large food molecules are broken down into small, soluble and
diffusible molecules that can be absorbed into the body cells.
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Or, digestion is the chemical and mechanical breakdown of food. It converts large insoluble
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molecules into small molecules, which can be absorbed into the blood.
• Absorption ---Absorption is the process by which food is transferred from the walls of the ileum
into the bloodstream.
• Assimilation --- assimilation is the uptake of digested food into cells from the blood and then
making use of it. E.g. amino acids are taken up to make protein in ribosome / glucose is taken up to
make energy in mitochondria.
Or, assimilation is the process whereby some of the absorbed food molecules are converted into
new protoplasm or used to provide energy.
• Ingestion--- taking the food into the mouth is called ingestion.
Or, ingestion is the act if taking food into the alimentary canal through the mouth.
 Or. Taking food into the body through the mouth is called ingestion.
• Mastication --- the act of chewing food
Or, the process of chewing food, which involves movements of the jaws and teeth. Mastication
breaks up the food into small particles, which provides a great surface area for digestion and enables
the formation of bolus, which is small enough to pass through the oesophagus.
• Defaecation --- removal of body wastes [faeces] through the anus.
Or, the expulsion of faeces from the rectum due to contractions of muscles in the rectal wall.
• Egestion --- egestion is the removal of undigested food from the alimentary canal through the
anus, i.e. food which is neither soluble of diffusible.
• Digestion involves 2 distinct processes: physical digestion and chemical digestion.
•
Physical digestion increases the surface area of the ingested food enabling digestive
enzymes to act on it more efficiently.
• Chemical digestion is the breaking down of the large molecules, such as proteins, starch
and fats, contained in food into small soluble molecules. This involves hydrolytic
reactions catalyzed by digestive enzymes.
Physical digestion --- in physical digestion, parts of the body such as the teeth, tongue, cheeks
and wall of the gut act upon food to make it easier to digest chemically. These actions include breaking
down the food into smaller pieces to increase the food surface area on which the chemicals can work,
emulsifying lipid substances to make them into small droplets on which chemicals can act, lubrication of
the food so that it moves along the gut easily.
Chemical digestion ---in chemical digestion, extracellular enzymes break down the food
molecules mainly by hydrolysis into molecules which can pass through the wall of the gut.
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2. Biology The digestive system
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Parts of the digestive system---
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Mouth [teeth, tongue and salivary gland (3 parts)]
Oesophagus [food pipe]
Stomach
Small intestine [deudenum + ileum]
Large intestine:
Appendix
Caecum
Ascending colon
Transverse colon
Descending colon
Rectum
Anus Accessory organs are --- liver, gall bladder, pancreas
Food is digested in the human alimentary canal, principally in
the mouth, stomach, and small intestine.
Food is absorbed mainly in the small intestine
Types of teeth
Type Number in each jaw Total number Function
Incisor 4 ×2 8 Chop / cuts food
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3. Biology The digestive system
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Canine 2× 2 4 Tear flesh /food
Pre molar 4 ×2 8 Crush and grind food
Molar 6 ×2 12
Functions of tongue ---
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• Have taste buds which give the taste of food
• Rolls the food into a bolus
• Mixes the food with saliva to soften it
• Helps to swallow food
• Helps in speech
Mouth has salivary glands secretes saliva
Saliva contains salivary amylase which acts on starch converts it to maltose
Q: Describe how food is digested in the mouth?
•Incisors cut / chop the food
•Canine tears food
•Premolars and molars crushes and grinds the food
•The tongue rolls the food in to a bolus
•Salivary glands secrete saliva which contains salivary amylase
•Salivary amylase acts on starch and converts it to maltose
Oesophagus --- tube through which food passes from the mouth to the stomach
The gut wall from the oesophagus to the rectum composed of 2 main layers:
A mucous coat or membrane lined with epithelium, continuously moistened with mucus
and with numerous folds.
A muscle coat of inner circular and outer longitudinal involuntary smooth muscle
Q: What are the functions of the stomach?
• Churns the bolus into a chyme
• Secretes concentrated Hydrochloric acid
• Which –
• Kills germs in the food
• Lowers the pH
• Activates the inactive pepsinogen to pepsin
• Gastric glands secrete gastric juice which contains pepsin [pepsin requires an acidic
pH]
• Pepsin acts on protein and converts it to peptides
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4. Biology The digestive system
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Figure: first part of the small intestine
Deudenum has no glands of its own
Functions of pancreatic juice -----
• Has NaHCo3 [sodium hydrogen carbonate] which makes the food alkaline
• Has enzymes :
Trypsin acts on remaining protein and converts it to peptide
Amylase acts on remaining starch and converts it to maltose
Lipase acts on lipid and converts it to fatty acids and glycerol
Enzymes according to pH sensitivity are classified into 3 groups:
Acidic [pepsin], neutral [salivary amylase] and alkaline [Trypsin, amylase and lipase]
Most enzymes require a neutral pH
What are the functions of small intestine [ileum]?
• Ileum has ileal glands
• Secretes ileal juice [succus entericus]
• Which contains:
acts on and breaks them down to
Lipase Remaining Fatty acids and glycerol
lipids
Act on and forms Amino acids
Peptides
Peptidase
Sucrase Acts on Sucrose and forms Glucose + fructose
Acts on Maltose and breaks it to 2 molecules of
Maltase glucose
Acts on Lactose and breaks it to Glucose + galactose
Lactase
Trace the digestion of lipid
• Digestion of lipids begin in the deudenum
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5. Biology The digestive system
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• In the deudenum, the enzyme lipase acts on lipids and converts it to fatty acids and
glycerol
• Final digestion takes place in the ileum where the enzyme lipase present in the ileal juice
converts the remaining lipids to fatty acids and glycerol.
Trace the digestion of protein
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• Digestion of protein begins in the stomach
• Stomach has gastric glands which secretes gastric juice
• Gastric juice contains an enzyme called pepsin
• Pepsin requires an acidic pH
• So the walls secretes concentrated HCl
• Pepsin acts on protein and converts it to peptides
• In the deudenum, the enzyme Trypsin acts on remaining protein and converts them to
peptide
• The final digestion of protein takes place in the ileum, where the enzyme peptidase present
in the ileal juice converts the peptides to amino acids
Trace the digestion of starch
• Digestion of starch begins in the mouth
• In the mouth, the enzyme salivary amylase present in the saliva converts the starch into
maltose
• Since the food does not remain in the mouth for long, so a little starch is converted by
salivary amylase to maltose
• No digestion of starch takes place in the stomach
• In the deudenum, the enzyme amylase acts on remaining starch and converts it to
maltose
• Final digestion takes place in the ileum, where the enzyme maltose present in the ileal
juice converts the maltose to glucose
Ileum --- the region of the alimentary canal between the deudenum and colon,
where digestion is completed and absorption takes place.
ADAPTATIONS OF ILEUM TO ITS FUNFTIONS---
• It is fairly long and presents a large absorbing surface to the digested food.
• Its internal surface is greatly increased by circular folds bearing thousands of tiny
projections called villi. These villi are about 0.5 mm long and may be finger like or
flattened in shape.
• The lining of the epithelium is very thin and the fluids can pass rapidly through it. The
outer membrane of each epithelial cell has microvilli which increase by 20 times of the
exposed surface of the cell.
• There is a dense network of blood capillaries [tiny blood vessels] in each villus.
Swallowing ---
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6. Biology The digestive system
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• The tongue presses upwards and back against the roof of the mouth, forcing a pellet of
food ,called a bolus, to the back of the mouth.
• The soft palate closes the nasal cavity at the back.
• The larynx cartilage round the top of the windpipe is pulled upwards so that the opening
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of the windpipe [the glottis] lies under the back of the tongue.
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• The glottis is also partly closed by the contraction of a ring of muscle.
• The epiglottis, a flap of cartilage [gristle] helps to prevent the food from going down the
windpipe instead of gullet
A summary of digestion and absorption ---
Tongue --- mixes the food with saliva and takes part in swallowing.
Teeth --- masticate [break food into small fragments] and mix food with saliva.
Salivary glands --- produce saliva, which moistens food and contains an enzyme called salivary
amylase which digests starch.
Oesophagus --- tube through food passes from the mouth to the stomach.
Liver ---produces bile, stores vitamins and minerals, regulates blood sugar and has many other
functions.
Gall bladder--- stores bile. Bile emulsifies fats and oils and neutralizes stomach acid.
Bile duct --- carries bile to the deudenum.
Deudenum --- food is mixed with bile which emulsifies fats and oils.
Ileum --- glands in the ileum wall produce enzymes which continue the digestion of starch, proteins
sugars, fats and oils. Completely digested food is absorbed into the blood-stream through villi which line
the ileum.
Colon ---absorbs water and salt from faeces.
Rectum --- holds indigestible [faecal] matter prior to defecation.
Anus --- indigestible matter [faeces] passed out of the body [defecation]
Q: How the structure of ileum adapted to its function?
• The walls of the ileum has infolding to produce long finger like projections called the villi
and bearing microvilli [hair like projections] which 2 together provides a large surface
area for easy absorption of digested food.
• A network of capillary ensures easy absorption of glucose & amino acids.
• A centrally placed lacteal ensures easy absorption of fatty acid & glycerol.
• A thin epithelium [one cell thick] offers a short distance for easier absorption.
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7. Biology The digestive system
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Structure of a villus
Bile --- it is a greenish yellow fluid
Composition --- bile is composed of:
• Bile pigments like bilirubin
• Bile salts like sodium hydrogen carbonate
• Worn out RBC
• cholesterol
• Formation – liver
• Stored – gall bladder
Functions ---
Contains sodium hydrogen carbonate used to neutralize the acidity.
Emulsify fat i.e. large droplets of fats are broken down to smaller droplets of fats, So that the enzyme
lipase can work.
Activate the enzyme lipase
• Functions of large intestine ---
Absorbs water and salt and make faeces
Stores faeces temporarily [rectum]
Expels faeces [anus]
• Peristalsis ---
The walls of the oesophagus [gullet]
The walls of the walls of the oesophagus/ gut has a pair of smooth muscle –
• Longitudinal muscle
• Circular muscle
• The alternative contraction of these 2 muscles creates a wave emotion which helps to
squeeze through the gut / oesophagus. It is the wave emotion created by the alternative
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8. Biology The digestive system
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contraction of smooth muscle to move food through the gut
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• The muscular contractions which move food down the gullet and along the alimentary canal
are called peristalsis.
• the rhythmic wave like contractions of the walls of the gut is called peristalsis
Digestive juice Source Contents
Saliva Salivary glands Salivary amylase and mucin
Gastric juice Gastric glands in stomach Rennin, pepsin and hydrochloric acid
Pancreatic Intestinal glands in small Pancreatic amylase, Trypsin and lipase
juice intestine
Intestinal Intestinal glands in small Enterokinase, maltase, lactase,
juice intestine Sucrase, erepein/ peptidase and lipase
• Utilization of digested food---
The products of digestion are carried round the body in the blood plasma. From the blood most living
cells are able to absorb and metabolize glucose, fats and amino acids.
Glucose : during respiration, in the protoplasm [mitochondria], glucose is oxidised to carbon
dioxide to carbon dioxide and water. This reaction releases energy to driver many chemical processes in
the cell, and in electrical changes [nerve cells]
Fats : fats are incorporated into cell membranes and other structures. The fats not used for
growth and maintenance in this way are oxidised to carbon dioxide and water releasing energy for the3
vital processes of the cells. Twice as much energy can be obtained from fats as from glucose.
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9. Biology The digestive system
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Amino acids : amino acids are absorbed by cells and reassembled to make proteins. These
proteins may form visible structures such as cell membrane and other components of the protoplasm of
the protoplasm or the proteins may be enzymes which control and co- ordinate the chemical activity
within the cell.
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Amino acids which are not required for building proteins are deaminated in the liver.
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