IB Assessment Statement
• H.2.1 State that digestive juices are secreted into
the alimentary canal by glands, including salivary
glands, gastric glands in the stomach wall, the
pancreas and the wall of the small intestine.
The Major glands secreting digestive enzymes in
the alimentary canal are:
• Salivary Glands in the Mouth
–
Enzyme: Salivary amylase (digests carbohydrates)
• Gastric Glands in the stomach
–
Enzyme: Pepsin (digests proteins)
• Exocrine Glands of the pancreas
•
Enzyme: Pancreatic lipase (digests lipids) and pancreatic amylase
(digests carbohyrdates)
• The cells of the walls of the small intestines:
Enzyme: Trysinogen/ trypsion (digests proteins)
Secretion of Digestive Juices
• Digestive Juices and
enzymes are secreted by
secretory glands of the
exocrine system.
IB Learning Objective: Identify and describe the
parts of the digestive system that secretes
digestive enzymes.
Gastric secretion Animations
• http://highered.mheducation.com/sites/007243731
6/student_view0/chapter43/animations.html#
Summary of Saliva & Salivary Gland
• Secrete in mouth
• pH 6.5-7.5
• Contents
– Amylase – function to digest starch
– Mucus –lubricate food.
Summary of Gastric Juice & Stomach gastric
glands
• Secrete in stomach
• pH 2
• Contents
– Pepsin – function to digest protein
– Hydrochloric Acid – creates acidic
environments to kill bacteria
Summary of Pancreatic Juice & Pancreas
• Secrete into small intestine
• pH 7
• Contents
– amylase – function to digest carbohydrates
– Protease (trypsin) – function to digest proteins
– Lipase – functions to digests lipids
– Nucleases – functions to digest nucleic acid
IB Assessment Statement
• H.2.2 Explain the structural features of exocrine
gland cells.
– Include the secretory cells grouped into acini
and ducts, and the ultrastructure of secretory
cells as seen in the electron micrographs
IB Learning Objectives
Explain the structural features of exocrine gland
• Secretory cells of glands are one cell thick
• Large Surface areas due to invagination.
• Digestive juice is released via exocytosis into
ducts
• Juices travel through the duct
• At the end of the ducts of the pancreas is the
acinus, which secretes pancreatic Juices.
Secretion of Digestive Juices
• An example of a secretory cell is a groups of
cells clustered around the end of a duct in the
pancreas called the acinus.
Secretion of Digestive Juices
• Digestive Juices and
enzymes are secreted by
secretory glands of the
exocrine system.
• Secretory glands are made
up of secretory cells that
line the pancreas and
stomach.
• These cells are one layer
thick.
Secretion of Digestive Juices
• The digestive juice is released from cells via
exocytosis.
• Then they are discharge into the ducts of the
excretory gland,
– A duct is a channel that transports
substances from gland to organ.
Secretion of Digestive Juices
• The surface area of the secretory cells is large
due to the invagination and branching of the
pancreas and stomach lining
Exocrine Gland Cells
• Features of Exocrine gland cells
– One or two prominent nucleus for the
production of ribosomes.
Exocrine Gland
• Features of Exocrine gland cells
– Many Golgi aparatus for processing proteins
(example: enzymes)
Golgi
apparatus
Exocrine Gland
• Features of Exocrine gland cells
– Many large vesicles, for storage of the
substance being secreted and transport.
Large
vesicles
Exocrine Gland
• Features of Exocrine gland cells
– Many mitochondria to provide ATP for Protein
Synthesis.
mitochondria
IB LEARNING OBJECTIVE
• H.2.3 Compare the composition of saliva, gastric juice
and pancreatic juice.
IB LEARNING OBJECTTIVE
• H.2.4 Outline the control of digestive juice
secretion by nerves and hormones, using the
example of secretion of gastric juice.
The control of Secretion of Digestions Juices
• The secretion of digestive juices is regulated by
nervous or/ and hormonal mechanisms.
The control of Secretion of Digestions Juices
Nervous Response
• The sight or smell of food stimulates a nervous
impulse from the brain to the exocrine glands of
the walls of the stomach to start gastric secretions.
• More gastric secretions are released when food
enters the stomach
– Chemoreceptors and touch receptors in the
lining of the stomach
• These receptors send impulse to brain, which
sends impulses to the exocrine gland cells.
The control of Secretion of Digestions Juices
Hormonal Response – ENDOCRINE SYSTEM
RESPONSE
– When food enters the stomach nervous
impulse are sent to endocrine glands in
stomach lining
– Endocrine glands release gastrin hormone
– Gastrin stimulates exocrine gland cells to secret
hydrochloric acid (HCl)
• This lowers the pH in stomach to 3
IB Learning Objective:
• H.2.5 Outline the role of membrane bound enzymes on
the surface of epithelial cells in the small intestine in
digestion
Membrane-Bound Digestive Enzymes
• Some enzymes secreted by the exocrine glands
become mixed with the food in the alimentary canal.
• Other enzymes are produced in the wall of the small
intestines, but are not secreted
– These enzymes are embedded in the plasma
membrane of the villi in the small intestine.
– These enzymes have their active sites are exposed
to the food inside the small intestines.
Membrane-Bound Digestive Enzymes
– These embedded enzymes can digest their
substrate and the cells of the villi can then
immediately absorb the product.
Membrane-Bound Digestive Enzymes
– These villi cells with enzymes embedded in their
membrane sometimes get rubbed off via abrasion.
– Once rubbed off, the enzymes continue to work and
digest food molecule, as they become mixed with food
in the small intestine
– The combined affect of all enzymes complete the
process of digestions.
IB Assessment Statement
• H.2.6 Outline the reasons for cellulose not being
digested in the alimentary canal.
IB Assessment Statement
• H.2.9 Explain the problem of lipid digestion in a
hydrophilic medium and the role of bile in
overcoming this.
LIVER
• Assisting the pancreas is the liver, which
produces bile.
• Bile dissolves and disperses droplets of fat in
fatty foods. This process is called emulsification
– Emulsification increases surface area of lipid
droplets
– This enables enzymes (lipase) to break down
smaller fat molecules.
Liver – Bile -- Digestion of Lipids
• Bile is stored in the gallbladder.
– and released into the small intestines.
– Lipid molecules tend to bond together and
are only accessible to lipase (enzyme) at the
lipid–water interface/ boundary.
Liver – Bile -- Digestion of Lipids
– Bile molecules have a hydrophilic end and a
hydrophobic end, and thus prevent lipid droplets
bonding together.
– The maximum surface area of lipid is exposed to
lipases (Enzyme). Lipase is water-soluble, but has
an active site to which a hydrophobic substrate
binds
Digestion of Lipids Animation
• http://www.wiley.com/legacy/college/boyer/047000
3790/animations/fatty_acid_metabolism/fatty_acid
_metabolism.htm
IB Learning Objective
H.2.7 Explain why pepsin and trypsin are initially
synthesized as inactive precursors and how they
are subsequently activated
Pepsin vs. Pepsinogen
• Since pepsin digests proteins, it could be harmful
to the cell of the glands that secrete them.
• They are therefore secreted as an inactive precursors called Pepsinogen.
• Pepsinogen is an inactive form of pepsin.
• Pepsinogen becomes pepsin when it is released
in the hydrochloric acid (HCl) of the stomach
H.2.7: Why pepsin and trypsin are initially
synthesized as inactive precursors?
• Pepsin and trypsin are initially synthesized as
inactive precursors known as zymogens.
• They are activated once secreted into the
alimentary canal.
• Pepsin and trypsin are activated under different
conditions in different parts of the alimentary canal
explain on next slide.
H.2.7: Why pepsin and trypsin are initially
synthesized as inactive precursors?
IB Learning Objective
H.2.8 Discuss the roles of gastric acid and
Helicobacter pylori in the development of stomach
ulcers and stomach cancers
Stomach Ulcers
• Stomach Ulcers are damaged
part of the stomach lining.
• In the past, Doctors use thought
ulcers were caused by an
excessive production of
stomach acid (HCl).
– Doctors use to use antacid
treatments to neutralize
stomach acid.
• Now doctors know that many
stomach ulcers are caused by an
acid tolerant bacteria called,
Helicobacter pylori (H. pylori).
H.2.8: The development of stomach ulcers and
stomach cancer
• This is an example of a
paradigm shift, where existing
ideas of scientists and doctors
about the tolerance of bacteria
to stomach acid were incorrect
in spite of the evidence.
• Australians Robin Warren and
Barry Marshall made the
discovery of the role of
Helicobacter pylori and stomach
acid in the formation of ulcers
but struggled to convince the
scientific and medical
community.
The affects of Helicobacter pylori
• Helicobacter pylori is strongly
associated with stomach ulcers.
• Helicobacter pylori releases
enzymes and other toxins which
damage the stomach lining.
• Many people with Helicobacter
pylori have developed stomach
cancer.
•
Helicobacter pylori if diagnosed
can be permanently treated with
antimicrobial treatments that
eliminate this bacterial
infection.