D.2: Digestion
Review
Draw and annotate the
digestive system
Nervous & hormonal control
The volume and content of gastric secretions are controlled by nerves and hormones.
The sight/smell of food causes the brain to stimulate a response via the vagus nerve from the
medulla. Gland cells in the stomach are stimulated to secrete components of gastric juice.
1.Chemoreceptors in the stomach detect peptides or ‘stretch’ receptors detect distension of
the stomach.
2.Impulses sent to the brain
3.Brain sends signal via the vagus nerve to endocrine cells in the duodenum wall & cells in
the stomach (close to the duodenum) cells secrete gastrin.
4.The hormones gastrin stimulates the secretion of acid and pepsinogen.
5.The hormones secretin & somatostatin inhibit gastrin secretion if the pH of the stomach
becomes to low.
Exocrine secretions
Salivary glands
Gastric glands
Pancreas
Liver
Intestinal glandular
cells
Water, electrolytes, salivary amylase, mucus, lysosome
Water, mucus, enzymes inc. pepsin, rennin and hydrochloric
acid
Water, bicarbonate, enzymes inc. amylase, lipase,
carboxypepidase, trypsinogen
Bile, can be stored in the gall bladder. Emulsifies lipids
(increases SA for lipase action)
Variety of digestive enzymes
Example of an exocrine gland
Defined as ‘a collection of cells that produce and
secrete a product which is then carried to a specific
region of the body by a duct e.g. a digestive enzymes’
Exocrine glands often contain a lot of endoplasmic
reticulum, ribosomes, golgi bodies, vesicles/granules
and mitochondria. Why?
If the exocrine gland is producing enzymes, all of
these things are required for protein synthesis,
packaging and transportation
Adaptations of the villus
• Microvilli greatly increase surface area for absorption, face the lumen
(interior of the SI).
• Mitochondria provide the energy for active transport of molecules
through the plasma membrane.
• Pinocytotic vesicles are visible near the plasma membrane, pinocytosis is
a form of active transport.
• Tight junctions are membrane to membrane ‘seals’ between epithelial
cells, this eliminates the possibility of molecules passing between the
cells.
Micrograph: villus epithelium
1. Google image search ‘ micrograph of villus epithelium’
2. Screen shot the image into your notes
3. Identify and label: villus, mitochondrion, vesicles (hopefully you can
see some), tight junction
4. What are the function of these organelles/structural components?
Role of acid
Acidic conditions of the stomach favor some hydrolysis reactions and help to
control any pathogens present in ingested food.
Acid is secreted by the parietal cells of the stomach. Acid denatures proteins,
exposing polypeptides chains to the enzyme pepsin.
Pepsin is released by chief cells as inactive pepsinogen. The acidic conditions
converts pepsinogen  pepsin.
Defined as a defect in the lining of the stomach
Research into stomach ulcers 1982 -1983, Dr Barry J. Marshall & Dr J. Robin Warren,
proved the existence of Helicobacter pylori in the inner lining of the stomach. These
bacteria secrete urease (creating ammonia) neutralising stomach acid, resulting in a
habitable environment.
More information page 675
Egestion
Excretion of undigested materials.
Fibre is not digested by the body.
Rate materials move through the intestines is positively correlated
with the amount of fibre present.
DBQ page 677.