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6.1 Digestion
Readings 203-208
IB
Pg 209 1-8
Overview: The Need to Feed
• Heterotrophs
– dependent on a regular supply of food
• Animals fall into three categories:
– Herbivores
• eat mainly autotrophs (plants and algae)
– Carnivores
• eat other animals
– Omnivores
• regularly consume animals as well as plants or algal
matter
• An adequate diet must satisfy three needs:
– Fuel
– Materials for biosynthesis
– Essential nutrients the animal cannot make itself
Caterpillar
Feces
6.1.1
Explain why digestion of large food molecules is
essential
• Most food molecules are large polymers
and insoluble
• They must first be digested to smaller
soluble molecules before they can be
absorbed into the blood
Small
molecules
Pieces
of food
Mechanical
digestion
Chemical digestion
(enzymatic hydrolysis)
Nutrient
molecules
enter body
cells
Undigested
material
Food
INGESTION
DIGESTION
ABSORPTION
ELIMINATION
6.1.2
Explain the need for enzymes in digestion
• Enzymes
– increase the rate of reaction
– Lower activation energy
• Digestive enzymes
– are secreted into the lumen of the gut
– increase the rate of reaction of the hydrolysis
of insoluble food molecules to soluble end
products
– increase the rate of reaction at body
temperature
6.1.3
State the source, substrate, products and optimum pH
conditions for one amylase, one protease, and one
lipase
• Pancreatic amylase:
–
–
–
–
Source the Pancreas
Substrate is starch
Optimal pH 7.5-7.8
End product is the disaccharide maltose
• Pepsin is a protease:
–
–
–
–
Source is the stomach
Substrate is a polypeptide chains of amino acids
Optimal pH is 2
End product is small polypeptides
• Pancreatic lipases:
– Source is the pancreas
– The optimal pH is 7.2
– The substrate is a triglyceride
lipid
– The product is glycerol and
fatty acid chains
6.1.4
Draw & label a diagram of the digestive system
• Each organ of the mammalian digestive system
has specialized food-processing functions
• The mammalian digestive system consists of an
– alimentary canal
– accessory glands that secrete digestive juices
through ducts (a tube within a tube)
• Mammalian accessory glands?
– salivary glands, the pancreas, the liver, and the
gallbladder
• Peristalsis?
– rhythmic contractions of muscles in the wall of the
canal to move food along
Cardiac
orifice
Tongue
Oral cavity
Salivary
glands
Pharynx
Esophagus
Pyloric
sphincter
Liver
Stomach
Ascending
portion of
large intestine
Gallbladder
Pancreas
Ileum
of small
intestine
Small
intestine
Large
intestine
Rectum
Anus
Appendix
Cecum
Duodenum of
small intestine
Salivary
glands
Mouth
Esophagus
Gallbladder
Liver
Pancreas
Stomach
Small
intestines
Large
intestines
Rectum
Anus
A schematic diagram of the
human digestive system
6.1.5
Outline the function of the stomach, small
intestine, and large intestine
• The stomach stores food and secretes
gastric juice, which converts a meal to
acid chyme
• Gastric juice is made up of hydrochloric
acid and the enzyme pepsin
• Pepsin is secreted as inactive
pepsinogen; pepsin is activated when
mixed with hydrochloric acid in the
stomach
• Mucus protects the stomach lining from
gastric juice
Esophagus
Cardiac orifice
Stomach
5 µm
Pyloric sphincter
Interior surface of stomach
Small
intestine
Epithelium
Pepsinogen
Gastric gland
Pepsin
(active enzyme)
HCl
Pepsinogen and HCl
are secreted into the
lumen of the stomach.
HCl converts
pepsinogen to pepsin.
Pepsin then activates
more pepsinogen,
starting a chain
reaction. Pepsin
begins the chemical
digestion of proteins.
Mucus cells
Chief cells
Parietal cells
Chief cell
Parietal cell
Most common regulation in biological systems
The accumulation of an end product of a process
slows the process or system
Less common
End product of a reaction speeds up production
of that reaction
(a) Lumen
(b) Gastric pits (mucus)
(c) Mucus secreting cells.
(d) Parietal cells that produce
HCL which kills
microorganism that enter
the digestive system. This
also converts inactive
pepsinogen to active pepsin
(e) Chief cells: produces
pepsinogen a protease
enzyme
Gastric ulcers, lesions in the lining, are
caused mainly by the bacterium Helicobacter
pylori
Bacteria
1 µm
Mucus
layer of
stomach
The small intestine
• The first portion  duodenum,
– acid chyme from the stomach 
– digestive juices from…
– pancreas, liver, gallbladder, and the small
intestine itself
• The small intestine is the longest
section of the alimentary canal and…
– major organ of digestion and absorption
• Digestion is completed here
• The products of digestion are absorbed
into the blood stream
(a) Villus which increase the
surface area
(b) Microvilli further increases
the surface for absorption.
(c) Lacteals are connected to
the lymphatic system for the
transport of lipids.
(d) In the wall of the small
intestine are the blood
vessels to transport
absorbed products to the
general circulation.
There are also the muscle to
maintain peristalsis
WHAT TYPE OF MUSCLE?
SMOOTH MUSCLE
The LARGE intestine or
colon
• The large intestine, or colon, is connected to
the small intestine
• Its major function is to recover water that
has entered the alimentary canal
• Wastes of the digestive tract, the feces,
become more solid as they move through the
colon
• Feces pass through the rectum and exit
via the anus
• The colon houses strains of the
bacterium Escherichia coli, some of
which produce vitamins
(a) The lumen of the colon
(b) The mucus producing goblet cells
(b) Muscular walls to maintain peristalsis
6.1.6
Distinguish between ABSORTION and
ASSIMILATION
6.1.7
Explain how the structure of the villi is related to its role
in absorption and transport of the products of digestion
• The small intestine has a huge surface
area, due to villi and microvilli that are
exposed to the intestinal lumen
• The enormous microvillar surface greatly
increases the rate of nutrient absorption