Digestion and nutrition
Campbell and Reece Chap. 41

DIGESTION (breakdown) AND NUTRITION

break down long chain proteins, polysaccharides and nucleic acids into monomers
recall hydrolysis (opposite of dehydration synthesis) (hydro-water lysis-break apart)
if not broken down, proteins which are non-self would make a big antigen invasion


Although human is covered in detail, a course called "biology" must cover comparative aspects.
In sponge, Choanocyte TRANSPARENCY Fig. 33.3
TRANSPARENCY Fig. 41.11 In Hydra and flatworm, gastrovascular - digestive and circulatory systems are combined

Tube - Alimentary canal TRANSPARENCY Fig. 41.12 (worm, grasshopper, bird)
birds have gizzard - stones to break up seeds, sort of a lapidary device (this is why you give pet bird gravel),
gizzard is near the stomach and near the bird's center of gravity -- not only do birds need to be light to fly but also balanced, and teeth, which are heavy and in the mouth, would upset the center of gravity
Crop is used for storage in birds

One emphasis will be on how the human digestive system invests many juices (hydrolases = enzymes which catalyse hydrolysis)
Some glands have ducts and these are called exocrine glands.
This is in contrast with endocrine glands (ductless, for hormones, which are also involved in digestion)

800 g food IN per day
1200 ml water
/
/ 7000 ml GLANDS
/
50 g solid OUT
100 g water

TRANSPARENCY Fig. 41.13
Mouth - teeth, lubrication, salivary amylase to disaccharide maltose - starch tastes sweet (only starch in mouth)
-ase enzymes
Pharynx swallowing
Esophagus - bolus, peristalsis
Cardiac oriface
An interesting story: rats cannot vomit. They are very good at learning in one trial to avoid tastes which make them sick and can only be poisoned by chemicals with a delayed reaction like warfarin which is an anticoagulant.

TRANSPARENCY (Fig. 41.15) Stomach - gastric mucosa - mucus protect
from parietal cell: HCl kill bacteria
stop amylase
From chief cell: pepsinogen ---(HCl, pepsin))--> pepsin (proteolytic)
(Inactive forms called zymogens)
Heartburn, antacids, ulcer (although it is now known that a specific bacterium, Helicobacter pylori, is associated with ulcer)
very little absorption in stomach - exceptions: aspirin, alcohol

TRANSPARENCY (Fig. 41.22)
Rumen - bacteria - ruminants - chew cud not acid - pensive "ruminate"
fermentation then feed to other chambers

TRANSPARENCY (Fig. 41.16)
Pyloric sphincter regulates emptying of acidic gastric juice to duodenum
there bile from liver and bicarbonate and enzymes from pancreas add to enzymes from small intestine

TRANSPARENCY (Fig. 6.16) interesting review point from last semester
optimum pH for pepsin is 2 while for trypsin, it is 8

TRANSPARENCY (Fig. 41.19)
Small intestine - intestinal enzymes:
In intestine: lactase, maltase, sucrase, others
In intestine, mitosis is frequent since cells digest themselves
food and water absorbed
Signalling by G-protein involving cAMP (covered last semester) is disrupted by cholera toxin - a life-threatening diarrhea, must replace fluids - salts and glucose (as in the electrolyte coctails athletes drink) facilitate water absorption

TRANSPARENCY (Fig. 41.18)
Pancreas
Bicarbonate (alkaline)
Protease (enteropeptidase makes trypsin which, in turn, makes chymotrypsin and carboxypeptidase
notice "pro..." and "...ogen" cut off a peptide fragment from a larger precursor to make final enzyme - there will be many examples of this sort of thing in biology
lipase
Amylase, nuclease
(most of enzymes)
The pancreas is also an endocrine gland producing Insulin (whose defect is diabetes) and Glucagon, both involved in sugar metabloism (covered later in the course)

Common bile duct
Liver gall bladder
Bile - emulsify fats,
erythrocyte iron recycling: turn feces dark
Hepatitis (disorder which spills bile into blood) - turns skin yellow (jaundice)
very few enzymes
also, via hepatic portal vein (see below absorption) microsomal (smooth endoplasmic reticulum) enzymes to detoxify
Portal blood veins (circulatory system "wired" in series is unusual, another famous example being the hypothalamus of the brain which feeds to the pituitary gland, covered later in hormones.

Liver not just exocrine gland, but recycling and detoxifying center for circulation: e.g. hemoglobin
Portal blood flow picks up from small intestine to liver
detoxify alcohol (metab, fat, scar, cirrhosis), drugs,
dump wastes (into feces)

TRANSPARENCY Fig. 41.17 to summarize digestive enzymes, but use caution since this table does not show where secretions come from

local hormones control digestion - Many found later in other places
from stomach:
food stimulates gastrin which, in turn, stimulates gastric juice until there is a low (acidic) pH
from duodenum:
Cholecystokinin (CCK) - liver and pancreas
Secretin for bicarbonate release
Enterogastrones to slow gastric emptying

It is worth mentioning that hunger and satiety are complex
In old work on brain lesions, LH (lateral hypothalamus) was called the hunger center, while the VMH (ventromedial hypothalamus) was considered to be the satiety center, but it never turned out to be so simple.
Hypothalamus is important in many motivated behaviors including thirst and sex drive
Affect (the aspect of perception of goodness or badness of a stimulus) is linked through the nigrostriatal tract (bundle of nerve axons) which uses the neurotransmitter dopamine and which is deficient in patients who have Parkinson's disease. Now it appears that there is a hormone which is called leptin which is released by (well-fed) fat cells which causes the brain to decrease apetite.
Specific appetite for salt after adrenalectomy eliminates aldosterone

so far DIGESTION. also ABSORPTION
TRANSPARENCY (Fig. 41.19)
here is a micrograph from our histology course dramatizing the tremendous increase in apical surface area of intestinal cells caused by the microvillar brush border
blood vessels and lacteals
fats - globules called chylomicrons or lipoproteins
fats with carrier proteins important

Large intestine
bacteria make vitamin K and some B vitamins
hydrogen, methane (gas "flaming")
feces
E. coli in gut, molec. biol. NIH regulations.
Superinfection if broad spectrum antibiotic

NUTRITION

Energy (calories - kilocalories)
2000-3000/day 250 extra/day add 25 lb/yr regulation
do not lose calories in feces, urine (except diabetes)
sweat (all systems efficient)

"Precursors" building blocks 8 essential amino acids
TRANSPARENCY (Fig. 41.4)
corn very low in lysine and isoleucine,
beans low in tryptophan and methionine
Kwashiorkor - Africa - disease of child new baby born
some essential fatty acids
even cholesterol essential to life, essential in insect diet

Vitamins not synthesized (in most cases), and are not metabolized (in the sense of sugars being catabolized into carbon dioxide + water + energy
TRANSPARENCY Big table 41.1

A is usuaal considered to be the "chromophore" (pigmented portion) of the visual pigment (rhodopsin)
retinoic acid and steroid hormone receptors act by regulating transcription (into mRNA) of specific genes
zinc fingers are used by the retinoic acid binding protein to grab onto DNA, so Zn is an important mineral (but otherwise some heavy metals like lead and mercury are very toxic)
E (antioxidant) Oxygen is a necessary evil. fat soluable
K (coagulation), normally made by gut bacteria, can have hypervitaminosis which would cause thromboses
D - also like a hormone, affects bone, D deficiency leads to a disorder in young people known as rickets with skeletal abnormalities witnessed as bowleggedness, sunshine creates vitamin D in the skin, and hence the expression "sunshine vitamin D." Rickets was in cool climate areas where people are dressed and/or indoors. Now the problem is solved by adding vitamin D to milk, but in the old days thay gave cod liver oil, and fish (especially livers) are high in the fat soluable vitamins most notably A and D
C colds? The Nobel Prize winning chemist Linus Pauling (later in his career) became a strong advocate of the wonders of vitamin C. Does it help for cancer? Scurvey is the name of the vitamin C deficiency, and all sorts of things go wrong. On voyages, sailers were deprived of fresh fruits and came down with scurvey. In the1700's the British figured this out and stocked the ships with citrus fruits and hence British sailers were called "Limeys"
B complex there are 12 B vitamins and some are Kreb's cycle coenzymes, hence these vitamins are important in metabolism
Beriberi - B1 thiamin

Some of my main research interests concern vitamin A: see site 1, site 2, site 3, and site 4

TRANSPARENCY Minerals Table 41.2
Ca - used in bone, but really importantly in muscle and nerve
P - phosphate - used in ATP etc.
Na, Cl, K - electrolytes which regulate cell electrophysiology
Iodine - used in the hormone thyroxine - deficiency causes goiter (thyroid gland in neck gets very big), iodine is in seafood so deficiencies used to be found mainly inland, now iodine is added to iodized salt
Iron, cobalt, copper - hemoglobin - anemia

return to Stark home page

return to syllabus

this page was last updated 1/20/03