Download teacher

1
AP Biology
March 2008
Digestion
Chapter 41
Homeostatic mechanisms manage an animal’s energy budget.
1) Four main feeding mechanisms of animals
a) Suspension feeders
i) animals that sift small food particles from water.
(1) Humpback whales
b) Substrate feeders
i) live in or on their food source, eating their way
through the food.
(1) Moth larvae
c) Fluid feeders
i) pull nutrient-rich fluid from a living host.
(1) Mosquitoes
d) Bulk feeders
i) eat relatively large pieces of food.
(1) Humans
ii) Three types of bulk feeders:
(1) Herbivores eat mainly autotrophs.
(2) Carnivores eat other animals.
(3) Omnivores eat both plants and animals.
2) Glucose regulation is an example of homeostasis.
a) Animals store excess calories as glycogen in the liver
and muscles and as fat in fat tissues.
b) These energy stores can be tapped when an animal is
in need of ATP.
c) Blood glucose level is maintained within a relatively
narrow range by a negative feedback mechanism.
d) Diabetes mellitus
3) Calorie imbalance
a) result in undernourished animals that have diets
deficient in calories, or overnourished animals that
consume more calories than they need.
Diet must supply carbon skeletons and essential nutrients.
2
1) Essential nutrients
a) must be obtained in preassembled form because the
animal cannot produce them.
2) Essential amino acids
a) About half of the 20 amino acids are required by
animals
b) Must be obtained from food.
c) There are also essential fatty acids that animals cannot
make and must ingest.
3) Vitamins
a) Organic molecules required in small amounts.
4) Minerals
a) Simple inorganic nutrients required in small amounts.
Four main stages of food processing
1) Ingestion
a) Act of taking down food.
2) Digestion
a) Breakdown of food into small molecules capable of
being absorbed by the cells of the body.
b) Enzymatic hydrolysis is the reaction by which
macromolecules are broken up. It involves the
addition of water.
3) Absorption
a) body’s cells take up small molecules from the digestive
track.
4) Elimination
a) passing of undigested material from the digestive track.
Types of digestion
1) Intracellular digestion
a) Occurs within a cell enclosed by a protective
membrane.
b) Sponges digest their food this way.
2) Extracellular digestion
a) Carried out by most animas
b) Food is broken down outside of cells.
3) Gastrovascular cavity
a) Simple animals have a single opening through which
food enters and waste is eliminated.
4) Complete digestive tracks (alimentary canals)
3
a) One-way digestive tubes that begin at the mouth and
terminate at the anus.
The movement of food through the digestive system.
1) Food in the mouth
a) Nervous reflex occurs that causes saliva to be secreted
into the mouth.
b) Saliva lubricates the food to facilitate swallowing.
i) Contains the enzyme salivary amylase, which
hydrolyzes starch and glycogen into smaller
polysaccharides and the disaccharide maltose.
2) Chewed food is shaped into a ball – bolus.
a) After swallowing, the bolus enters the pharynx – a
junction that opens to the esophagus and the trachea.
b) During swallowing, the epiglottis (the flap made of
cartilage) covers the trachea. This diverts the food
down the esophagus.
3) The esophagus moves food from the pharynx down to the
stomach through peristalsis.
a) Peristalsis is rhythmic waves of contraction by smooth
muscle in the walls of the esophagus.
4) The stomach’s functions include storing food and secreting
gastric juice.
a) Gastric juice contains hydrochloric acid, which is very
acidic. Breaks down the extracellular matrix of meat
and plant materials. Kills most of the bacteria ingested
with food.
5) Pepsin is an enzyme in gastric juice that begins to
hydrolyze proteins into smaller polypeptides.
a) Secreted in an inactive form called pepsinogen,
activated by hydrochloric acid in the stomach.
6) The result of stomach digestion is acid chime.
7) Acid chime is shunted from the stomach into the small
intestine via the pyloric sphincter.
8) The first section of the small intestine is the duodenum.
a) In the duodenum, the acid chime mixes with secretions
from the pancreas, the gallbladder, and the intestinal
wall itself.
b) The rest of the small intestine is responsible for the
absorption of nutrients.
9) Particular macromolecules are broken down in the small
intestine by the following processes:
a) Carbohydrates
4
i) The breakdown of starch and glycogen begins with
salivary amylase in the mouth. In the small intestine,
pancreatic amylases break starch, glycogen, and
small polysaccharides into disaccharides. The
breakdown of these disaccharides occurs at the
wall of the intestinal epithelium, and the
monosaccharides are quickly absorbed.
b) Proteins
i) Pepsin begins the breakdown of proteins in the
stomach, and in the small intestine; trypsin and
chymotrypsin break polypeptides into smaller
chains. Dipeptidases, carboxypeptdase, and
aminopeptidase break apart proteins into amino
acids.
c) Nucleic acids
i) The breakdown of nucleic acids is similar to that of
proteins. In the small intestine, nucleases break
them down into nucleosides, nitrogenous bases,
sugars, and phosphate groups.
d) Fats
i) Digestion of fats starts in the small intestine. Bile salts
coat the fat droplets and keep them from clumping
and lipase hydrolyzes them.
10)The epithelial lining of the small intestine has folds called
villi, which in turn bear projections called microvilli
a) Both of which radically increase the surface area
available for absorption.
11)In each villus is a set of tiny blood vessels called capillaries
and a lymph vessel called a lacteal, which absorbs small
fatty acids.
12)Monosaccharides, such s glucose, cross the lining via
passive diffusion, whereas amino acids and dipeptides are
pumped across in active transport.
13)The capillaries and veins that drain the nutrients away from
the villi all join the hepatic portal vessel, which brings them
to the liver.
a) The liver metabolizes the organize molecules in various
ways.
14)Some other hormones involved in digestion are:
a) Gastrin
i) Stimulates the secretion of gastric juice
b) Enterogastrone
i) Slows digestion
c) Secretin and cholecystokinin (CCK)
5
i) Secreted by the walls of the duodenum and that
prompt the digestion of various macromolecules.
15)The large intestine, also called the colon, is connected to
the small intestine by a sphincter.
a) The point of the connection is the site of the cecum, a
small pouch with an extension called the appendix.
16)The main function of the large intestine is to compact
waste and recover water.
a) The wastes become more solid as they travel along
and form feces.
17)At the end of the colon is the rectum, where feces are
stored until they are eliminated.
Adaptations of digestive systems are associated with diet.
1) A mammal’s dentition is greatly correlated with its diet.
a) Mammals have specialized dentition that best enables
them to ingest their usual diet.
2) Herbivores generally have longer alimentary canals than
carnivores. Why?
a) It reflects the longer time needed to digest vegetation.