CSEC Biology Notes
Ms. K. Armstrong
The Digestive System
CSEC Biology Notes
Ms. K. Armstrong
The stages through which food passes as it travels the alimentary canal are:
1. Ingestion: The introduction of food into the alimentary canal, through the mouth.
2. Digestion: The breaking down of complex insoluble food into simple soluble substances. There are two
types of digestion
1. Mechanical Digestion: This changes the physical form of the food. Mechanical digestion
involves chewing, tearing, grinding, mashing and mixing.
2. Chemical Digestion: This type of digestion changes the chemical composition of food. This is
done with the aid of digestive enzymes. Digestive enzymes are special proteins that help break
up large molecules of food into very tiny molecules that can be absorbed and used by the cells in
the form of nutrition.
3. Absorption: The passage of products of digestion across the lining of the small intestine into the blood
stream.
4. Assimilation: The building of material from the products of digestion by the body cells.
5. Egestion: The removal or elimination of indigestible food from the body through the anus.
The Importance of Digestion
Digestion is important because it changes solid, insoluble food substances into soluble ones which can be
absorbed and transported around the body where they can be used to build, repair or provide energy for the
body.
Structures of the Alimentary Canal Involved in Ingestion and Digestion
Mouth: The mouth (also known as the buccal cavity) is the organ where ingestion occurs. It is also the region
where digestion begins. The teeth being the digestion process by grinding food into smaller pieces during
chewing or mastication by the teeth. This is form of mechanical digestion and it functions to break the large
food molecules into smaller ones which increases the surface area of the food particles.
Chemical digestion also occurs in the mouth. Saliva is secreted into the mouth from the salivary glands and this
contains salivary amylase which begins chemical digestion by breaking starch into maltose. Saliva also
functions to provide the alkaline environment for this action while also lubricating and moistening food. After
the food is thoroughly chewed, the tongue rolls it into a ball, called a bolus, and sends it to the back of the throat
where it is swallowed and enters the oesophagus. The bolus is prevented from entering the lungs by a flap of
skin called the epiglottis which covers the entrance to the trachea when you swallow.
Oesophagus: The oesophagus is a long mucus-membrane lined tube that runs from the mouth to the stomach. It
uses rhythmic, wave-like muscle movements called peristalsis to force food from the throat into the stomach.
Peristalsis is an involuntary process of muscular contraction which forces the bolus (food) down to the stomach.
This muscle movement gives us the ability to eat or drink even when we are upside-down.
CSEC Biology Notes
Ms. K. Armstrong
Stomach: This is a large, hollow, muscular J-shaped organ. The stomach is lined with a layer of mucus which
protects it from being digested. Hydrochloric acid is also present in this organ and it helps break down food and
kill any bacteria that may have come along with the food. The stomach has inner folds called rugae that
increases the surface area. Food in the stomach that is partly digested and mixed with stomach acids and
enzymes is called chyme. In the stomach both mechanical and chemical digestion occur:
✓ Mechanical Digestion – The physical movement (churning) of the food in the stomach cavity breaks
large pieces of food into smaller pieces.
✓ Chemical Digestion – Gastric juice is produced by special cells which line the stomach. Gastric juice
contains a variety of enzymes which work on specific substrates:
o Pepsin: This breaks proteins into polypeptides
o Rennin: This clots soluble milk proteins
Gastric juice also contains hydrochloric acid which creates an optimum pH of 2 for these enzymes to
function in.
Small Intestine: This is a tube which is about 20 feet long. It is so named because its diameter is much smaller
than that of the large intestine. The small intestine finishes the process of digestion, absorbs the nutrients, and
passes the residue on to the large intestine. It consists of three regions:
1. Duodenum
2. Jejunum
3. Ileum
Duodenum and Jejunum: These are the 1st and 2nd regions of the small intestine. They do not produce any
digestive enzymes. Enzymes are secreted into this area from the pancreas and include:
✓ Carbohydrases: pancreatic amylase, maltase, sucrase, lactase
✓ Proteases: trypsin
✓ Lipase: pancreatic lipase
Bile (an alkaline green liquid) is also secreted from the gall bladder where it is stored (it is made in the liver)
into this area for the process of emulsification. Emulsification is the breaking down of large fat droplets into
smaller ones. This increases the surface area for the enzyme lipase to act on ensuring digestion occurs at a
faster rate. Another substance secreted into the duodenum is sodium hydrogen carbonate which is used to
maintain the optimum pH (8) for the enzymes in this area to function in.
CSEC Biology Notes
Ms. K. Armstrong
rd
Ileum: This is the 3 and final region of the small intestine. It is responsible for the absorption of the
digested food that exits the previous regions of the small intestine. The table below shows the features of the
ileum that make it suitable for its function in the digestive process.
Table Showing the Adaptations of the Ileum for Absorption
Feature
Function
It is very long.
This gives plenty of time for digestion to be
completed, and for digested food to be absorbed as it
passes through.
It contains villi. Each villus is covered with cells This gives the inner surface of the small intestine a
which have even smaller projections on them called very large surface area. The larger the surface area,
microvilli.
the faster food can be absorbed.
Villi contain blood capillaries.
Digested food passes into the blood, to be taken to
the liver and then around the body.
Villi contain lacteals, which are a part of the Fats and fat soluble substances (e.g. vitamins A, D,
lymphatic system.
E and K) are absorbed into the lacteals.
Villi are only one cell thick.
The digested food can easily cross the wall to reach
the blood capillaries and lacteals.
N.B: All absorbed food goes straight to the liver via the hepatic portal vein.
CSEC Biology Notes
Ms. K. Armstrong
Summary Table Showing the Digestive Process
Organ
pH
Mouth
7-8
Stomach
1-2
Digestive
Juice
Saliva
Gastric
Juice
Source
Contents
Functions
Salivary glands
Water and
mucus
NaCl and
NaHCO3
Salivary
amylase
Moistens and lubricates the food
allowing for tasting and swallowing
Maintains the pH as this is the optimum
pH for salivary amylase
Begins the digestion of starch to maltose
Hydrochloric
acid
Maintains pH as this is the optimum for
the action of pepsin and rennin
Stops the action of salivary amylase
Lubricates food
Protects the stomach wall from acid
Clots soluble milk proteins to be retained
in the stomach
Begins the digestion of proteins into
polypeptides
Gastric glands in the
stomach wall
Mucus
Rennin
Pepsin
salivary amylase
Starch
→
Maltose
pepsin
→ Polypeptides
Proteins
Small
Intestine
7-8
Bile
Gall bladder
Manufactured in the liver
Enters the duodenum via
the bile duct
Bile pigments
NaHCO3
Organic bile
salts
Pancreatic
Juice
Cells of the pancreas
Enters the duodenum via
the pancreatic duct
Pancreatic
amylase
Excretory products from the breakdown
of haemoglobin (no use in digestion)
Neutralises the stomach acid
Maintains the pH as this is the optimum
for the pancreatic enzymes
Emulsifies fats by lowering the surface
tension which increase their surface area
for digestion
Continues the digestion of starch to
maltose
pancreatic amylase
→
Starch
Trypsin
Maltose
Continue the digestion of proteins to
polypeptides
trypsin
Proteins
Pancreatic
lipase
→
Polypeptides
Begins the digestion of fats to fatty acids
and glycerol
pancreatic lipase
Fats
Intestinal
Juice
Intestinal glands in the
wall of the small intestine
Maltase
→
Fatty acids and glycerol
Digests maltose into glucose
maltose
Maltose
Sucrase
→
Glucose
Digests sucrose into glucose and fructose
sucrase
Sucrose
Lactase
→
Glucose + Fructose
Digest lactose into glucose and galactose
lactase
Lactose
Peptidase
→
Glucose + Galactose
Digests polypeptides into amino acids
peptidase
Polypeptides
N.B: The words in bold are the enzymes.
→
Amino acids
CSEC Biology Notes
Ms. K. Armstrong
Assimilation of Absorbed Food
Monosaccharides are taken by the blood to the liver and then into general circulation where they are used in
respiration by body cells to provide energy. Excess monosaccharides can either be:
✓ Stored in liver or muscle cells in the form of glycogen
✓ Converted to fat by the liver and stored under the skin.
Amino acids are taken by the blood to the liver and then into general circulation and:
✓ Used by cells for growth and repair
✓ Used to make enzymes, hormones, antibodies and plasma proteins
Excess is deaminated in the liver. During deamination, the amino groups is converted to urea and excreted in
the kidneys and the remainder is stored as glycogen or fat.
Fatty substances are carried in the lymph into general circulation and:
✓ Used to form new cell membranes.
✓ Used in respiration when monosaccharides are in short supply.
Excess is stored under the skin (insulation) and around organs (protection).
Egestion
Any food which cannot be digested continues along the small intestine to the large intestine. The large intestine
is also known as the colon. It is larger in diameter than the small intestine but shorter in length (5ft). The colon
is the region where food that has not been digested is transported. The colon is responsible for absorbing most
of the water from the undigested food for reabsorption back into the bloodstream. Water along with mineral
salts are absorbed from the indigested waste and it becomes progressively more solid as it moves to the rectum
for egestion from the body. Waste stays in the large intestine for 10 – 12 hours. The indigestible remains then
form a semi-solid mass which is known as faeces. Faeces consists of water, inorganic salts, epithelial cells,
bacteria and the by-products of digestion. Undigested food in the form of faeces is stored in the rectum until it is
time for it to exit the body. The anus is the region of the digestive system where the faeces is removed from the
body.
CSEC Biology Notes
Ms. K. Armstrong
Summary Table Showing Mechanical and Chemical Digestion in the Regions of the Alimentary Canal
Region
Mouth
Mechanical Process
Mastication
Oesophagus
Peristalsis
Stomach
Churning
Peristalsis
Small Intestine
Segmentation
(mixing contraction)
Mechanical Digestion
Chewing movements reduce
the size of the food particles
and mixes them with saliva.
Wormlike
movements
squeeze the food downward
in the tract. Constricted rings
form first in one section, then
the next, etc., causing waves
of contraction to spread along
the entire canal.
Forward
and
backward
movement of the stomach
contents mixes food with
gastric juices to form chime.
Waves start in the body of the
stomach and sweep toward
the closed pyloric sphincter.
At intervals, strong peristaltic
waves press chyme past the
pyloric sphincter into the
duodenum.
Forward
and
backward
movement
within
the
segments of the small
intestine mix food and
digestive juices thoroughly.
Food comes into contact with
the intestinal mucosa and this
facilitates transport across the
intestinal wall (absorption).
Emulsification
Peristalsis
Large Intestine
Segmentation
Peristalsis
Mass peristalsis
Defecation
Bile breaks large fat droplets
into
smaller
ones
(emulsification).
Intestinal
contents
are
propelled along the digestive
tract.
Churning movements occur
within the sacs.
The contents are propelled
along the digestive tract as
water is absorbed.
The remaining undigested
material is moved into the
rectum. This occurs three or
four times a day, usually after
a meal.
The contents of the rectum
are emptied and removed
from the body via the anus
(bowel movement).
Chemical Digestion
Salivary amylase changes
starch to maltose.
Salivary amylase continues to
change starch to maltose.
Gastric juices partially digest
proteins changing them into
polypeptides.
The action of salivary
amylase is stopped because of
the acidic nature of the
stomach.
Pancreatic juices digest:
✓ Polypeptides
into
peptides
✓ Fats into fatty acids and
glycerol
✓ Starch into maltose
Intestinal juices change:
✓ Peptides into amino acids
Fats into fatty acids and
glycerol
✓ Sucrose into glucose and
fructose
✓ Lactose into glucose and
galactose
✓ Maltose to glucose
Absorption of water