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The Digestive
System and
Process
BIOLOGY 11
The Components of the Digestive System
Figure 24.1
Functions of the digestive system
 Ingestion
 Mechanical
 Digestion
 Secretion
 Absorption
 Excretion
processing
Explain why digestion of large food molecules is essential

Most food is solid and in the form of large complex molecules
which are insoluble and chemically inert (not readily usable)
 As
food was synthesized by other organisms, it
contains materials not suitable for human tissue these need to be separated and removed
 Large
molecules need to be broken down into
smaller molecules that can be readily absorbed
across membranes and into cells
 Small
molecules can be reassembled into new
products (e.g. amino acids can be reassembled to
make new proteins)
Digestive enzymes

Enzymes are globular proteins that control
biological reactions.

Digestive enzymes speed up the breakdown
(hydrolysis) of food molecules into their ‘building
block’ components.

These reactions occur outside of the cells lining
the gut.
Explain the need for enzymes in
digestion

Enzymes are biological catalysts which speed up the rate of a
chemical reaction (e.g. digestion) by lowering the activation
energy

Enzymes allow digestive processes to occur at body temperature
and at sufficient speed to meet the organism's survival requirements

Enzymes are specific for a given substrate and so can allow
digestion of certain molecules to occur independently of others
Naming and classification of
enzymes

There are 2 systems used for naming enzymes:

The suffix ‘-ase’ is used with the root name of the substance being acted upon, for
example, when sucrose (sugar) is digested, it is acted upon by an enzyme called
sucrase.

The type of chemical reaction involved as the enzyme functions, for example, when
sucrase acts on sucrose, it breaks it into a molecule of glucose and a molecule of
fructose. This reaction involves adding a water molecule to break a chemical bond
and so the enzyme is a hydrolase. All digestive enzymes belong to this hydrolase class.

Enzymes are classified according to the type of chemical reaction catalysed.

All digestive enzymes are hydrolases, whereas most of the enzymes involved in
energy release for muscular contraction are oxidation-reduction enzymes such
as oxidases, hydrogenases and dehydrogenases.
Digestive Juices and
Enzymes
Saliva
Gastric juice
Pancreatic juice
Intestinal enzymes
Bile from the liver
Substance Digested
Product Formed
The Structure of the Digestive Tract
Figure 24.3
Movement of digestive materials
 Visceral
smooth muscle shows rhythmic cycles
of activity
 Pacemaker
cells
 Peristalsis
 Waves
that move a bolus
 Segmentation
 Churn
and fragment a bolus
Peristalsis
Figure 24.4
Control of the digestive system
 Movement
of materials along the digestive
tract is controlled by:
 Neural
mechanisms
 Parasympathetic
 Hormonal
 Enhance
 Local
and local reflexes
mechanisms
or inhibit smooth muscle contraction
mechanisms
 Coordinate
stimuli
response to changes in pH or chemical
The Regulation of Digestive Activities
Figure 24.5
The mouth opens into the oral or
buccal cavity
 Its
functions include:
 Analysis
of material before swallowing
 Mechanical
processing by the teeth, tongue, and
palatal surfaces
 Lubrication
 Limited
digestion
The tongue
 primary
functions include:
 Mechanical
 Assistance
 Sensory
processing
in chewing and swallowing
analysis by touch, temperature, and
taste receptors
The pharynx
 Common
and air
 Lined
passageway for food, liquids,
with stratified squamous epithelium
 Pharyngeal
muscles assist in swallowing
 Pharyngeal
 Palatal
constrictor muscles
muscles
Histology of the esophagus
 Distinctive
include
features of the esophageal wall
 Non-keratinized,
 Folded
stratified squamous epithelium
mucosa and submucosa
 Mucous
secretions by esophageal glands
A
muscularis with both smooth and skeletal muscle
portions
 Lacks
serosa
 Anchored
by an adventitia
The Esophagus
Figure 24.10a-c
The Swallowing
Process
Figure 24.11ah
Functions of the stomach
 Bulk
storage of undigested food
 Mechanical
breakdown of food
 Disruption
of chemical bonds via acids
and enzymes
 Production
of intrinsic factor
Digestion and absorption in the
stomach

Preliminary digestion of proteins
 Pepsin

Permits digestion of carbohydrates

Very little absorption of nutrients
 Some
drugs, however, are absorbed
 Mucous

secretion containing several hormones
Entero-endocrine cells
G
cells secrete gastrin
D
cells secrete somatostatin
The Stomach
Figure 24.12b
The Stomach Lining
Figure 24.13a,
b
The Stomach Lining
Figure 24.13c, d
Histology of the stomach
 Gastric
glands
 Parietal
cells
 Intrinsic
 Chief
factor, and HCl
cells
 Pepsinogen
 Pyloric
glands
The Secretions of Hydrochloric Acid
Figure 24.14
The Phases of Gastric Secretion
Figure 24.15a
The Phases of Gastric Secretion
Figure 24.15b
The Phases of Gastric Secretion
Figure 24.15c
Small intestine

Important digestive and absorptive functions
 Secretions
bladder

and buffers provided by pancreas, liver, gall
Three subdivisions:
 Duodenum
 Jejunum
 Ileum

Ileocecal sphincter
 Transition
between small and large intestine
Regions of the Small Intestine
Figure 24.16a
Histology of the small intestine

Plicae
 Transverse

Villi
 Fingerlike

projections of the mucosa
Lacteals
 Terminal

folds of the intestinal lining
lymphatic in villus
Intestinal glands

Lined by entero-endocrine, goblet and stem cells
The Intestinal Wall
Figure 24.17a
The Intestinal Wall
Figure 24.17b,
c
The Intestinal Wall
Figure 24.17d,
e
Intestinal juices

Moisten chyme

Help buffer acids

Maintain digestive material in solution
Small Intestine
 Duodenal
 produce
glands (Brunner’s glands)
mucus, buffers, urogastrone
 Ileum
 aggregated
lymphoid nodules (Peyer’s patches)
Intestinal movements

Peristalsis

Segmentation

Gastroenteric reflexes
 Initiated

by stretch receptors in stomach
Gastroileal reflex
 Triggers
relaxation of ileocecal valve
The pancreas

Pancreatic duct penetrates duodenal wall

Endocrine functions
 Insulin

and glucagons
Exocrine functions
 Majority
of pancreatic secretions
 Pancreatic
juice secreted into small intestine
 Carbohydrases
 Lipases
 Nucleases
 Proteolytic
enzymes
The Pancreas
Figure 24.18ac
The liver

Performs metabolic and hematological regulation
and produces bile

Histological organization
 Lobules
containing single-cell thick plates of hepatocytes
 Lobules
unite to form common hepatic duct
 Duct
meets cystic duct to form common bile duct
The Anatomy of the Liver
Figure 24.19a
The Anatomy of the Liver
Figure 24.19b,
c
Liver Histology
Figure 24.20a,
b
The gallbladder
 Hollow,
 Stores,
PLAY
pear-shaped organ
modifies and concentrates bile
Animation: Accessory Organ
The Gallbladder
Figure 24.21a,
b
Coordination secretion and absorption

Neural and hormonal mechanisms coordinate glands

GI activity stimulated by parasympathetic innervation


Inhibited by sympathetic innervation
Enterogastric, gastroenteric and gastroileal reflexes
coordinate stomach and intestines
The Activities of Major Digestive Tract
Hormones
Figure 24.22
Functions of the large intestine
 Reabsorb
feces
 Absorb
 Store
water and compact material into
vitamins produced by bacteria
fecal matter prior to defecation
The Large Intestine
Figure 24.23a
The Large Intestine
Figure 24.23b, c
The rectum
 Last
portion of the digestive tract
 Terminates
 Internal
at the anal canal
and external anal sphincters
Histology of the large intestine
 Absence
of villi
 Presence
of goblet cells
 Deep
intestinal glands
Physiology of the large intestine

Reabsorption in the large intestine includes:
 Water
 Vitamins
– K, biotin, and B5
 Organic
wastes – urobilinogens and sterobilinogens
 Bile
salts
 Toxins

Mass movements of material through colon and
rectum
 Defecation
reflex triggered by distention of rectal walls
The Defecation
Reflex
Figure 24.25
Processing and absorption of
nutrients

Disassembles organic food into smaller
fragments

Hydrolyzes carbohydrates, proteins, lipids and
nucleic acids for absorption
Carbohydrate digestion and
absorption

Begins in the mouth
 Salivary
and pancreatic enzymes
 Disaccharides
 Brush
and trisaccharides
border enzymes
 Monosaccharides

Absorption of monosaccharides occurs across
the intestinal epithelia
Lipid digestion and absorption

Lipid digestion utilizes lingual and pancreatic
lipases
 Bile
salts improve chemical digestion by emulsifying lipid
drops
 Lipid-bile
 Micelles
salt complexes called micelles are formed
diffuse into intestinal epithelia which release
lipids into the blood as chylomicrons
Protein digestion and absorption

Low pH destroys tertiary and quaternary
structure

Enzymes used include pepsin, trypsin,
chymotrypsin, and elastase
 Liberated
amino acids are absorbed
Absorption

Water
 Nearly

all that is ingested is reabsorbed via osmosis
Ions
 Absorbed
transport

via diffusion, cotransport, and active
Vitamins
 Water
soluble vitamins are absorbed by diffusion
 Fat soluble vitamins are absorbed as part of micelles
 Vitamin
B12 requires intrinsic factor
Digestive Secretion and
Absorption of Water
Figure 24.27