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Chapter 21 Nutrition and Digestion
1) Animals ingest there food in a variety of ways
a) Suspension feeders
i)
extract food particles suspended in surrounding
water
ii) Examples
(1) baleen whale – krill (Intro)
(2) Clams, oysters, scallops
b) Substrate feeders
i)
ii)
live in or on food and eat their way through it
Examples
(1) Earthworms through soil
(2) Caterpillars through leaves
c) Fluid Feeders
i)
suck nutrients from a living host
ii) Example
(1) aphids, mosquitoes, ticks
d) Bulk Feeders
i)
ingest relatively large pieces of food
ii) tentacles, pincers, jaws, teeth, etc… to bite into prey
2) Food processing occurs in four stages
a) ingestion
b) digestion
i)
mostly ingest macromolecules (proteins, fats,
carbohydrates, nucleic acids)
ii) need to break macromolecules down to amino acids,
glycerol and fatty acids, simple sugars, nucleotides)
iii) Two reasons:
(1) macromolecules too large to cross plasma membrane
(2) animals need monomers to make their polymers
iv)
Two phases in digestion
(1) mechanical breakdown (increase surface area)
(2) chemical breakdown (enzymes - hydrolysis)
c) absorption
i)
cells lining digestive tract absorb building blocks
ii) building blocks circulate in blood and are
(1) used by other cells for building
(2) burned for energy
(3) stored as glycogen or fat
d) elimination (EGESTION)
i)
passing of undigested material
3) Digestion occurs in specialized compartments
a) needs to be contained – don’t want to digest yourself
b) Even single-celled organisms must do this
i)
food vacuoles and lysosomes
c) sponges – like single-cells – digests in individual cells
d) Gastrovascular cavity
i)
Hydra
(1) gastrovascular cavity with a single mouth opening
(a)digestion and distribution of nutrients to body
(b) Why do this? Why not just do what a sponge does?
(c)allows for ingestion of prey much larger than a single
cell could handle
(2) Main Food-Processing events
(a)stings prey and stuffs in mouth
(b) cells secrete digestive enzymes into GV cavity
(c)flagella keep food mixed with enzymes
(d) cells engulf now tiny particles (endocytosis) into
food vacuoles, which have additional enzymes for
further breakdown into monomers
(e)undigested material leaves through mouth
e) Alimentary canal
i)
tube between two openings – mouth and anus
ii) adapted along the way for specialized functions since
it is one-way
(1) mouth
(2) pharynx (throat)
(3) esophagus
(4) depending on species, food may enter a crop, gizzard or
stomach
(a)crop
(i) organ where food is softened and temporarily
stored
(b) stomachs and gizzards
(i) may also store food temporarily, but are more
muscular than crops
(ii) use muscles to grind and churn food
(iii)gizzards have teeth or grit to assist grinding
(c)intestine
(i) site of nutrient absorption
(d) anus site of egestion
iii) regions vary with diet
(1) Earthworm
(a)omnivorous substrate feeder
(i) muscular pharynx sucks soil in mouth
(ii) stored and moistened in crop
(iii)gizzard retains small bits of sand to pulverize food
(iv) nutrients digested and organic matter absorbed in
intestines
1. shape increases surface area
2. greater number of cells exposed to food
(v) indigestible material (sand, gravel, inorganic
substances) exits anus
(2) Grasshopper
(a)herbivorous
(i) jawlike mouth to cut and chew plant leaves
(ii) crop similar to earthworm
(iii)gizzard with hard teeth
– where most chemical digestion occurs
(v) nutrients absorbed in stomach and gastric pouches
1. gastric pouches are extensions of stomach
(vi) intestines – short – absorb water and compact
undigested solids
(vii)
anus – same old story
(3) bird
(a)same major organs as earthworm and grasshopper
(b) crop – enable them to eat a large amount quickly
(c)no teeth
(i) food swallowed whole into crop
(d) crop stores and softens and passes to stomach
(e) stomach – beginning of mechanical and chemical
digestion and continues in gizzard
(i) many birds eat gravel – collects in gizzard and
helps pulverize tough plant parts and hard parts of
insects
(f) intestines
(i) chemical digestion is completed here
(ii) nutrients and water absorbed
(g) anus
(i) same old with some gravel
(iv) stomach
4) Humans digestive system – alimentary canal and some
specialized glands
a) Main parts of canal
i)
mouth
ii) oral cavity
iii) tongue
iv) pharynx
v)
esophagus
vi) stomach
vii) small intestine
viii) large intestine
ix) rectum
x)
anus
b) digestive glands
i)
salivary glands
ii) pancreas
iii) liver (secretions stored in gall bladder)
5) Main food processing events
i)
Digestion starts in the mouth
(1) salivary glands
(a)can secrete over a liter of saliva a day
(b) activate in response to sight or smell of food
(i) tingling in jaw
(c)contents of saliva
(i) slippery glycoprotein
1. protects lining of mouth, lubricates solid food
for easier swallowing
2. Buffers
a. neutralize food acids – help prevent tooth
decay
3. antibacterial agents (antibodies)
4. salivary amylase (chemical digestion)
a. digestive enzyme that starts to hydrolyze
starch (start – starch)
(2) chewing (mechanical digestion)
(a)makes food easier to swallow and increases surface
area for enzyme digestion
(b) four kinds of teeth
(i) two bladelike incisors - biting
(ii) canine tooth – tearing
(iii)premolar and molars
1. grind and crush
(3) tongue
(a)muscular organ covered with taste buds
(b) manipulates food
(c)shapes
into a ball (bolus)
(d) pushes bolus into back of oral cavity and into
pharynx during swallowing
6) Food and breathing passages both open into the pharynx
a) esophagus and trachea opening are both in pharynx
b) esophageal sphincter
i)
closes off esophagus most of the time
ii) trachea open for breathing
c) swallowing reflex
i)
triggered by bolus entering pharynx
(1) esophageal sphincter relaxes
(2) larynx (voice box) moves upward and tips epiglottis
(flap of cartilage and fibrous connective tissue) over
trachea
(3) feel your larynx (adam’s apple) when you swallow
(4) after swallowing everything goes back to original
positions
7) esophagus squeezes food along the stomach
a) muscular tube
i)
muscles run in two layers (smooth muscle)
(1) circular muscle layer
(a)constriction constricts the esophagus
(2) longitudinal muscle layer
(a)constriction shortens the esophagus
(3) whenever one contracts, the other relaxes
(4) RESULT: peristalsis
(a)circular muscles contract above bolus
(b) longitudinal muscles contract below to shorten
distance to stomach
(c)Time from pharynx to stomach – 5 to 6 seconds
8) Stomach stores food and breaks it down with acid and enzymes
i)
ii)
stomach is main reason we dont need to constantly eat
can hold two liters of food and drink
(1) enough for several hours
iii) chemical digestion
(1) secretes gastric juice
(a)mucus, enzymes, and strong acid
(2) surface is highly folded
(3) many pits leading to tubular gastric glands
(4) gastric glands
(a)three types of cells
(i) mucous cells
1. secrete mucous – lubricates and protects cells
lining stomach
2. parietal cells – secrete hydrochloric acid (HCl)
a. pH of stomach b/w 1 and 2
3. chief cells – secrete pepsinogen (inactive form
of digestive enzyme pepsin)
(5) Activation of pepsinogen
(a)HCl and pepsinogen secreted into gastric gland
(b) HCl converts pepsinogen to pepsin
(c)Pepsin activates more pepsinogen (positive feedback)
(6) Pepsin begins chemical digestion of proteins
(a)(pepsin – proteins)
(7) Mucous protects cells from pepsin
(8) stomach lining needs to be replaced every three days
(mitosis) – it is continuous
(9) gastric glands are not always on
(a)regulated by nerve signals and hormones
(b) initial signal
(i) see, smell, taste food sends nerve signal to glands
(c) continuous signal
(i) food in stomach stimulates cells to release
hormone gastrin into blood of circulatory system
(ii) gastrin
circulates and upon return causes further
release of gastric juice
(iii)keeps gastric juice going while food is in stomach
(iv) negative feedback – if contents become too acidic
(too much gastric juice), gastrin release is
inhibited
iv) mechanical digestion
(1) contraction of muscle in stomach wall
(2) acid chyme formed – churned food with gastric juice
v)
stomach usually closed at both ends
(1) keeps acid chyme from going up esophagus
(a)heartburn (acid reflux)
(2) pyloric sphincter
(a)regulates passage of acid chyme to small intestines
(b) leaves one squirt at a time
(c)takes 2 to 6 hours to empty stomach
vi) Vomiting
(1) reverse peristalsis
9) Small intestines – the major organ of chemical digestion and
nutrient absorption
a) Recap – starch and protein digestion began, but nowhere near
complete, acid chyme is a thick soup
b) virtually all chemical digestion occurs here
c) nutrients are absorbed here
d) over 6 meters long
i)
longest organ of alimentary canal
ii) small refers to diameter (2.5cm)
e) pancreas and liver help with digestion here
i)
pancreas
(a)produces
(i) digestive enzymes (trypsin, chymotrypsin,
Pancreatic Lipase, Pancreatic amylase,
Ribonuclease, deoxyribonuclease,
carboxypeptidase)
(ii) alkaline solution rich in bicarbonate – neutralizes
acid chyme
ii) liver
(1) produces bile
(a)NO ENZYMES
(b) contains bile salts that help weaken fats to enzyme
attack (break globules to tiny droplets)
(2) gallbladder stores bile until needed
f) duodenum
i)
first 25 cm of small intestines
ii) site where acid chyme meets bile salts, pancreatic
enzymes, and enzymes from wall of intestines itself
iii) all for types of macromolecules digested
iv) Carbohydrates:
(1) pancreatic amylase
(a)hydrolyzes starch into maltose
(2) maltase (from intestine wall)
(a)hydrolyzes maltose into two glucose monomers
(3) sucrase (intestine wall)
(a)hydrolyzes sucrose
(4) lactase (intestine wall)
(a)hydrolyzes lactose
v)
proteins
(1) trypsin and chymotrypsin
(a)break polypeptide products of pepsin into even
shorter chains
(2) aminopeptidase (intestine wall) and carboxypeptidase
(a)hydrolyze one amino acid at a time from the ends
(3) dipeptidase (intestine wall)
(a)hydrolyzes fragments only 2 or 3 AA long
(4) work as a team
vi) Nucleic acids
(1) nucleases
(a)split
DNA and RNA into nucleotides
(2) nucleotides then broken down to nitrogenous base,
sugar, phosphate by other enzymes
vii) Fat
(1) special problem due to insolubility
(2) emulsification
(a)bile salts coat tiny droplets and keep them separate
from each other so they don’t form large ones
(b) increases surface area exposed to lipase
(3) lipase
(a)breaks fat to glycerol and fatty acids
g) beyond the duodenum, the small intestine is all about
absorption
i)
surface area about 300 m2 – size of a tennis court
(1) due to fold and projections
(2) villi – fingerlike projections
(3) each epithelial cell of the villi has tiny surface
projections called microvilli
h) lymph vessels and capillaries penetrate villi
i)
nutrients pass into capillaries or lymph vessels
(1) some by diffusion
(2) some by active transport
ii) nutrients head first to liver (hepatic portal system)
(1) processes nutrients to new substances that body needs
(2) removes excess glucose and stores as glycogen
iii) next they go to heart and then to rest of your cells
10)
Large Intestines Reclaim water
a) large intestines = colon
b) 1.5m long and 5cm diameter
c) another sphincter controls entrance to colon
d) cecum
e) appendix – fingerlike extension of cecum
i)
contain a mass of WBCs that make a minor
contribution to immunity
ii) prone to infection
f) main function
i)
absorb water from alimentary canal
g) 7 liters enter canal each day in digestive juices
i)
90% absorbed back into blood by small intestines
ii) large intestines finished job
h) as water is removed, indigestable material becomes more
solid
i) peristalsis moves it along
j) feces
i)
mostly indigestible plant fibers and prokaryotes living
in your colon
k) E. coli and other colonic flora
i)
produce important vitamins
(1) biotin
(2) folic acid
(3) several B vitamins
(4) vitamin K
(a)absorbed into blood through colon
l) rectum
i)
terminal portion of colon
ii) storage site for feces
iii) strong colon contractions create urge to defecate
m)
anus
i)
two rectal sphinctors regulate opening
(1) one voluntary and one involuntary
n) If lining is irritated by bacteria or virus less water absorption
occurs = diarrhea
o) peristalsis is too slow
i)
too much water absorbed = constipation
(1) lack of exercise, too little plant fiber