Download File

Option D Human Physiology
D. 2 Digestion
D.2
Essential idea: Digestion is controlled by nervous and hormonal mechanisms
Nature of science: Serendipity and scientific discoveries- the role of gastric acid in digestion was
established by William Beaumont while observing the process of digestion in an open wound caused
by gunshot.
D.1 U1 Nervous and hormonal mechanisms control the secretion of digestive juices.
o
o
o
Naturally there are gaps between meals, some animals go long periods of time. To conserve
energy, animals do not have digestive systems active constantly.
Need to conserve energy for muscle use in case of fight or flight response.
Nerves and hormones ensure resources are devoted to digestion only when needed
D.1 U3 The volume and content of gastric secretions are controlled by nervous and hormonal
mechanisms.
o
o
o
o
Both nerves and hormones control the secretion of digestive juices.
Ex. Gastric juices- sight or smell of food causes brain to send nerve impulses via the vagus
nerve from the medulla. Gland cells in stomach wall are then stimulated to secrete gastric
juice.
If chemoreceptors in stomach detect distension of stomach, impulses sent to brain and then
brain sends impulses via vagus nerve to endocrine cells in wall of duodenum and stomach to
secrete gastrin (a hormone that stimulates secretion of acid and pepsinogen)
Other hormones (secretin and somatostatin) inhibit gastin secretion if the pH falls to low.
D.1 U2 Exocrine glands secrete to the surface of the body or the lumen of the gut.
o
Digestive juices are added to alimentary canal at several points.
o Exocrine glands secrete juices. Ex. Salivary glands, pancreas, gland cells in stomach
wall and in small intestine.
Digestive fluid
Saliva
Source
Salivary glands
Gastric juice
Stomach
Pancreatic juice
Pancreas
Composition
Water, electrolytes, salivary
amylase, mucus, lysozyme
Water, mucus, enzymes
including pepsin, rennin and
hydrochloric acid
Water, bicarbonate, enzymes
including: amylase, lipase,
carboxypeptidase, trypsinogen
D.1 U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food.
o
The inner surface of ileum has numerus folds. Each is covered in villi. Absorption takes place
through the epithelia cells covering the villus.
o Tight junctions between epithelial cells holds them together tightly. This makes materials
pass through cells and then into blood stream.
o Surface membrane of cells lining lumen side have extensions called microvilli. This side is
called brush border and it increases surface area for absorption.
o High amounts of ATP are required for active transport to move substances into cell so
these cells have lots of mitochondria
o Pinocytic vesicles present in large numbers due to absorption of some food by
endocytosis.
o Surface of lumen called apical surface and side facing blood vessels is basal surface.
The each have different types of proteins involved in material transport.
Notice:
o
Tight junctions between cells
o
Apical surface in relationship
to lumen
o
Basal surface in relation to
lumen
Notice:
o
What type of transport is the
rectangle protein doing?
Notice:
o
Glucose concentration on
right
Skill: Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that
absorb digested foods from electron micrographs. Page 674 in book.
o
o
Exocrine gland cells: often pyramidal-shaped cells. Vesicles and granules will often be found
at the surface next to the duct.
Villus epithelium cells: contain villi on one surface of the cell.
Identify the following pictures as an exocrine gland cell or vilius epithelium cell. If it is an exocrine
gland cell put a star on the side of the picture that is close to the duct.
D.1 U4 Acid conditions in the stomach favor some hydrolysis
reactions and help to control pathogens in ingest food.
o
o
o
o
o
Acid secreted by parietal cells in stomach. This acid
disrupts extracellular matrix that holds cells together in
tissues and denatures proteins so that enzyme pepsin
can hydrolyze the bonds within polypeptides (break
them into smaller pieces)
Pepsin is released by chief cells as the inactive
pepsinogen. Acid conditions in stomach convert it into
active pepsin. This ensures that the cells that produce it
don’t get digested at the same time as the protein.
Application: Helicobacter phylori infection as a cause of stomach ulcer
Ulcers are open sores
caused by partial
digestion of stomach
lining by the enzyme
pepsin and hydrochloric
acid in gastric juice.
Stomach cancer is the
growth of tumors in wall
of stomach.
Stomach ulcer where
one believed to be
caused by stress and
excessive gastric juices
but a bacterium,
Helicobacter phylori, as
been shown to be a
more significant cause
and it is associated with
stomach cancer.
Application: The reduction of stomach acid secretion by proton pump inhibitor drugs.
o Several disease involving the stomach are made worse by the release of stomach acid. The
acid is corrosive so the body make a mucus barrier that protects the lining of the stomach.
o In some the barrier may bread down allowing damage to the stomach, causing bleeding.
This is an ulcer.
o In some their may be a problem with circular muscles at top of stomach that prevents fluid
from escaping and it may get into esophagus. This is called “acid reflux” and can cause
heartburn.
o The acidic environment is achieved by proton pump called the H+, K+-ATPase. This pump uses
one ATP to exchange two protons from cytoplasm for two potassium ions in lumen. Some
medicine to treat gastric diseases are proton pump inhibitors (PPIs)
o PPIs bind irreversibly to single pump. The effect on overall acid production is not permanent as
pumps normally recycled and replaced.
o The PPIs are consumed in inactive form. The acid condition converts them to active form
close to their target.
D.1 U7 Materials not absorbed are egested.
o
o
o
o
Dietary fiber is the edible parts of plants that are resistant to being digested and not absorbed.
So some food never leaves lumen. Ex. Cellulose and lignin.
Secretion into digestive tube also becomes excretory products such as bilirubin from
breakdown of red blood cells.
Water is added to tube by secretions in mouth, stomach and small intestines and must be
reclaimed in large intestines.
The excretory products and unabsorbed water, and undigested fiber are egested as feces.
D.1 U6 The rate of transit of materials through the large intestine is positively correlated with the fiber
content.
o
o
o
o
o
Fiber includes cellulose, lignin and pectin that cannot be digested.
Fiber can be classified as soluble and insoluble:
Balanced diet contains fibers to increase bulk passing through intestines and helps prevent
constipation as it draws water into intestine. The higher water content of intestine the faster
the fecal material moved.
Other benefits of fiber: risk of intestinal disease reduced (bowel cancer, hemorrhoids,
appendicitis); decreases desire to eat because you fill full so reduces risk of diseases; slows
down absorption of sugars (reduced diabetes)
Plant foods contain fiber, especially whole-grain bread and cereals, veggies like celery. Also
foods from cultured fungi also contain fiber.
Application: Dehydration due to cholera toxin
o Cholera is a diseased caused by infection of bacterium Vibrio cholera.
o Bacterium releases toxin that binds to receptor on intestinal cells. Toxin is brought into cells by
endocytosis and then triggers a cascade response that leads to an efflux of Cl- and HCO3ions from cell into intestine. Water follows by osmosis and leads to watery diarrhea. Water is
drawn from blood into cells to replace loss fluid from there and quick, severe dehydration can
result in death in patients that do not receive rehydration.
Study diagram: Add to the diagram names of enzymes produced at the different locations. Also
add the pH changes into the different locations.