The Digestive
System
Chapter 45
Animals are heterotrophs
• Require fuel
– Chemical energy is obtained from the oxidation of
complex organic molecules
• Require essential nutrients
– Chemicals an animal requires but cannot synthesize
– varies from species to species
– Essential amino acids – human adults can produce
12 of the 20, making 8 essential
– Essential fatty acids – deficiencies are very rare
– Vitamins and minerals (required in small amounts)
Food Processing
• Ingestion
– The act of eating
• Digestion
– Breaking down food into smaller molecules
– Polymers  monomers
– Mechanical digestion: physically breaks food up and increases
its surface area
– Chemical digestion: enzymatic hydrolysis uses water to break
bonds in polymers
• Absorption
– The uptake of small molecules into the body
• Elimination
– Undigested material passes out of the body
The Mammalian Digestive
System
• The oral cavity
– Begins both physical and chemical digestion
• Teeth + saliva produce bolus
• The pharynx
– Epiglottis helps prevent food in trachea
• The esophagus
– Peristalsis – rhythmic contractions that move
food; starts as voluntary and changes to
involuntary
The Mammalian Digestive
System…
• The stomach
– Stores food and performs preliminary digestion
• Churning to mix food into chyme
• Gastric juice works mainly on proteins
• The small intestine
–
–
–
–
Most enzymatic hydrolysis of food and absorption of nutrients
Also involved: pancreas, liver, gall bladder
Huge surface area provided by villi and microvilli
Nutrients cross into bloodstream by diffusion and active
transport
• The large intestine
– Reclaiming water from feces is the major job
– Intestinal bacteria live on organic material – produce vitamin K