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Digestive Systems
• Functions of digestive system:
– Break down food into organic molecules
• Physically (teeth, gizzard)
• Chemically (by hydrolysis in stomach, intestines)
– polysaccharides
H2O
– fats
– proteins
HO
HO
H
H
HO
– Absorb subunits of organic molecules
• Monosaccharides, amino acids, fatty acids
H
Digestive Systems
• Types of digestive systems:
– None (intracellular digestion)
• Phylum Porifera (sponges)
Digestive Systems
• Types of digestive systems:
– Open ‘sac’ (gastrovascular cavity)
• Phylum Cnidaria (jellyfish, sea anemone, Hydra)
– Enzymes
secreted from
cells lining
cavity
– Two-way system
(food and waste
enter and leave
mouth)
– No specialization
Digestive Systems
• Types of digestive systems:
– One-way digestive systems
• Phylum Annelida (earthworm)
• Phylum Arthropoda (grasshopper, crayfish)
• Phylum Chordata (frog)
– one-way system
– Tube-within(food enters mouth,
tube body plan
leaves anus)
– Specialization of
digestive tasks
Digestive Systems
• Specialization in vertebrate digestive systems:
– Gastrointestinal tract
• Mouth and structures to grind
or break apart food
– Mammals: teeth
» Sharp, pointed canines to
tear flesh
» Large, flat teeth to grind
plants
– Birds: gizzard (swallow grit)
• Saliva released in mouth -- moisten food and releases
salivary amylase
• Food swallowed and passes through pharynx to
esophagus
Digestive Systems
• Specialization in vertebrate digestive systems:
– Gastrointestinal tract
• Esophagus
– Muscular tube that connects
pharynx to stomach
– Successive waves of
contractions (peristalsis)
Digestive Systems
• Specialization in vertebrate digestive systems:
– Gastrointestinal tract
• Stomach
– Saclike portion of digestive tract
– Inner surface convoluted (able
to expand > 50x empty size)
– Gastric glands release HCl
(stomach pH=2)
» Helps break down proteins
» Kills most bacteria
– Gastric glands also release pepsinogen (activated into
protein-digesting enzyme called pepsin)
– Partially digested food and gastric ‘juice’ passes into small
intestine
Digestive Systems
• Specialization in vertebrate digestive systems:
– Gastrointestinal tract
• Small Intestine
– Tube for chemical
digestion and absorption of
organic molecules
– Small intestine receives:
» Chyme from stomach
» Digestive enzymes and
bicarbonate from pancreas
» Bile from liver
– Inner surface of small intestine lined with fingerlike villi
(and microvilli on cells)
» Increases surface area for digestion (by enzymes in
microvilli) and absorption (by epithelial cells)
Digestive Systems
• Specialization in vertebrate digestive systems:
– Gastrointestinal tract
• Large Intestine (colon)
– Large diameter but short
length
– Some absorption of ions and
vitamins, but main function is
to concentrate digestive
wastes (absorb water)
» Many bacteria live within
large intestine -- bacteria
fermentation produces
‘gas’
– Feces produced in large intestine passed to rectum and exit body
through anus
Digestive Systems
• Specialization in vertebrate digestive systems:
– Accessory organs
• Pancreas
• Liver
• Gall bladder