1. Option D: Human Physiology
D.2 Digestion

2. D.2
Essential idea: Digestion is controlled by nervous and hormonal mechanisms.

3. This helps to conserve energy!
D.2.U1 Nervous and hormonal mechanisms control the secretion of digestive juices.
Typically there are gaps in between meals which means that digestive juices only need to be secreted at certain times.
The nervous system and hormones work together to control the secretion of digestive juices.
This helps to conserve energy!

4. There are two types of glands: Exocrine glands & Endocrine glands
D.2.U2 Exocrine glands secrete to the surface of the body or the lumen of the gut.
There are two types of glands: Exocrine glands & Endocrine glands
Examples of digestive exocrine glands:
Salivary glands, pyloric glands (stomach), pancreas, goblet cells (small intestine)
Some other examples:
Sweat glands, Moll’s glands (eyes), mammary glands, sebaceous glands (skin)
Acinus = rounded secretory unit & secrete either enzymes or mucin/lubricant (
Basement membrane separates the acinus from everything else (connective tissue, blood vessels, nerves, etc.)

5. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.

7. 2) Medulla sends message to stomach via vagus nerve
D.2.U3 The volume and content of gastric secretions are controlled by nervous and hormonal mechanisms.
CEPHALIC PHASE
1) Thought, smell, sight or taste of food causes impulse to be sent to medulla
2) Medulla sends message to stomach via vagus nerve
3) Stomach begins producing HCl & pepsinogen.
Also stimulates production of Gastrin (hormone) which is made in lower stomach and increases production of HCl & pepsinogen in other stomach cells
INTESTINAL PHASE
6) Sphincter opens to duodenum and a signal is sent to terminate production of HCl & pepsinogen.
Also stimulates production of secretin (hormone) that lowers gastric pit activity.
GASTRIC PHASE
5) Medulla sends message back to continue HCl & pepsinogen production
4) After eating, stomach becomes distended & sends message to medulla

9. Secretion of the gastric pits include: mucus, pepsinogen and HCl
D.2.U4 Acid conditions in the stomach favour some hydrolysis reactions and help to control pathogens in ingested food.
Secretion of the gastric pits include: mucus, pepsinogen and HCl
Mucus protects the stomach cells from HCl

10. Stomach acid has several purposes: HCl + Protein = Denatured protein
D.2.U4 Acid conditions in the stomach favour some hydrolysis reactions and help to control pathogens in ingested food.
Stomach acid has several purposes:
HCl + Protein = Denatured protein
Easier for enzymes to act on
HCl + Pepsinogen = Pepsin
Pepsin + Large polypeptide chains = Smaller peptides
Smaller peptides get broken down further by other enzymes later
HCl kills pathogens that are consumed (stomach acid can have a pH as low as 2)

11. Stomach ulcers: a paradigm shift in medicine
D.2.A3 Helicobacter pylori infection as a cause of stomach ulcers.
Stomach ulcers: a paradigm shift in medicine
A paradigm shift can be described as a huge change in scientific thought or consensus.
The case of stomach ulcers is a rare example of how one discovery caused many experts to change their minds.
It also led to the Nobel Prize for Medicine in 2005 for its discoverers.

12. D.2.A3 Helicobacter pylori infection as a cause of stomach ulcers.

13. D.2.A3 Helicobacter pylori infection as a cause of stomach ulcers.

14. Some people suffer from a condition called acid reflux
D.2.A1 The reduction of stomach acid secretion by proton pump inhibitor drugs.
Some people suffer from a condition called acid reflux
Esophagus becomes irritated by HCl when the muscle at the top of the stomach does not function properly
The acidic environment in the stomach is achieved by a proton pump called the H+, K+-ATPase.
It uses one ATP to exchange two protons from the cytoplasm for two potassium ions in the lumen surrounding the parietal cell (cell that secretes HCl)

15. These drugs are known as proton pump inhibitors (PPIs).
D.2.A1 The reduction of stomach acid secretion by proton pump inhibitor drugs.
Late in the 20th century, researchers discovered a class of drugs that inhibit the production of acid by cells in the gastric pits.
These drugs are known as proton pump inhibitors (PPIs).
PPIs bind irreversibly to proton pump blocking the secretion of H+
The effect is not permanent however, as the pumps are normally recycled and replaced with new pumps.

16. D.2.S1 Identification of exocrine gland cells that secrete digestive juices and villus epithelium cells that absorb digested foods from electron micrographs.
Chyme from the stomach enters the small intestine for further digestion and absorption.
The ileum is the section of the small intestine where most absorption occurs.
The inner surface of the ileum has many folds, each covered in projections called villi.
An electron micrograph at very high magnification: these are epithelial cells covering the villus; the microvilli on the surface of a single villus can be seen

19. D.2.U5 The structure of cells of the epithelium of the villi is adapted to the absorption of food.1 2 3 4
Infoldings (invagination) on side of epithelial cell closest to capillary bed increases surface area which increases absorption into capillary or lacteal

21. Fiber helps by providing bulk so peristalsis can function optimally
D.2.U6 The rate of transit of materials through the large intestine is positively correlated with their fiber content.
Fiber is composed mostly of cellulose and lignin in plant material. Both are components of the cell wall.
There is a positive correlation between the amount of fiber in one’s diet and the rate of movement of material through their digestive tract.
Fiber helps by providing bulk so peristalsis can function optimally
Muscles that push food along need to have a sufficient volume of material to apply pressure to
High fiber diets can also help you manage your weight in part because it fills you up, giving a feeling of satiety without introducing excess energy.

22. D.2.U7 Materials not absorbed are egested.
Some substances you consume do not get digested and therefore are not absorbed by the intestines. Instead they pass into the large intestine and become part of solid waste (feces).

23. Cholera is a disease caused by the bacterium Vibrio cholera
D.2.A2 Dehydration due to cholera toxin.
Cholera is a disease caused by the bacterium Vibrio cholera
Bacteria releases toxin
Binds to intestinal cell receptors and is taken into the cell by endocytosis
Chain of reactions occurs that ultimately causes the efflux of Cl- and HCO3- ions from the cell into the intestine
Water follows by osmosis leading to watery diarrhea
Water moves from blood to intestinal cells to replace water lost
Severe dehydration that can lead to death
Cholera is spread by drinking water or eating food that is contaminated with the bacteria (often contaminated with feces).