Digestion

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Digestion
AP Biology Unit 6
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General Steps

• Ingestion food is taken in (eating ?)
• Digestion Food is broken down into smaller
  pieces /molecules
• Absorption Nutrient molecules are absorbed into
  body cells
• Elimination undigested material exits the body

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Types of Digestion

• There are two kinds of digestion mechanical and
  chemical
• Mechanical Food is broken down into smaller
  pieces (chemical structure not changed)
• Chemical Food is broken down into smaller
  molecules (chemical structure changed)

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Intracellular Digestion

• In the animal kingdom, only sponges (Phyla
  Porifera) do this exclusively.
• Steps
• Cells engulf food via phagocytosis or
  pinocytosis, forming food vacuole.
• Lysosomes fuse with food vacuoles hydrolytic
  enzymes break down food.

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Extracellular Digestion

• All animals (except sponges) perform this mode of
  digestion
• Digestion begins in a compartment continuous with
  the outside of the animals body (gut).
• What is the advantage of extracellular digestion?
• animal can take in a lot of food at once and
  slowly digest it

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Extracellular Digestion

• Enzymes are secreted to break food into smaller
  molecules.
• After the food has been broken down, many animals
  continue digestion intracellularly.
• Animals that perform extracellular digestion can
  have a variety of different digestive systems.

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Gastrovascular Cavity

• Gastrovascular cavity digestive sac with a
  single opening
• Animals that have a gastrovascular cavity Hydra
  
• Phyla Cnidarian (also includes jellyfish,
  anemones, corals)

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Digestion in Gastrovascular Cavities

• Tentacles sting prey and stuff it into opening
• Digestive enzymes are secreted to allow for
  extracellular digestion
• Nutritive muscular cells then engulf food
  particles and complete digestion intracellularly
• Undigestible material leaves through mouth (no
  anus)

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Complete Digestive Tracts

• Complete digestive tract digestive tube running
  throughout body (alimentary canal)
• Organisms with a complete digestive tract have
  both a mouth and an anus

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Question

• Why would having a separate entry and exit point
  be beneficial?
• digestion can be broken down into steps
• there can be specialization of digestive tissues
  for these steps

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Specialized Digestive Structures

• Certain organisms have specialized digestive
  structures
• Crop
• Function Store and moisten food
• Who has it?
• Earthworms (Phylum Annelida)
• Grasshoppers (Phylum Arthropoda)
• Birds (Phylum Chordata)

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Specialized Digestive Structures

• Gizzard
• Function Contains sand and stones As the
  muscles around it move, food is ground up
  (mechanical digestion)
• Who has it?
• Earthworms (Phylum Annelida)
• Cockroaches (Phylum Arthropoda)
• Birds (Phylum Chordata)

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Specialized Digestive Structures

• Gastric ceca
• Function Extend from midgut to help with
  absorption
• Who has it?
• Grasshoppers (Phyla Arthropoda)

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Human Digestion

• Where do the four steps in food processing occur?

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Human Digestion

• Ingestion mouth
• Digestion mouth, Stomach, Small Intestines
• Absorption Small Intestines, Large Intestines
• Elimination End of large intestines

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Oral Cavity, Pharynx, Esophagus

• Participate in ingestion and digestion
• Mechanical Digestion
• By teeth and tongue (chewing)
• Forms mass of food bolus
• Chemical Digestion
• Salivary amylase begins digestion of
  carbohydrates (starch)

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Oral Cavity, Pharynx, Esophagus

• Epiglottis moves during swallowingto cover the
  trachea, so food travels down right pipe to the
  esophagus
• Peristalsis (muscle contractions) will
  involuntarily continue movement of the bolus

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Stomach

• Important in storage digestion
• What advantage do folds in the stomach tissue
  provide?
• Allows the stomach to expand to hold more food
• Tissue is also very elastic so that it can stretch

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Stomach

• Gastric glands produce gastric juice from a
  combination of 3 cells
• Mucus Cells secrete mucus (protects stomach
  lining)
• Chief Cells secrete pepsinogen (inactive enzyme)
• Parietal Cells secrete HCl

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Stomach

• Gastric juice is churned with bolus to break down
  food and kill bacteria
• HCl converts pepsinogen into pepsin (active
  enzyme)
• Pepsin hydrolyzes (breaks down) protein.

Halfway done!
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Stomach Pepsin

• What kinds of bonds are broken by pepsin?
• Peptide bonds
• What kind of reaction is this?
• Hydrolysis

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Protection of the stomach

• Stomach protects itself from self-digestion by
• keeping pepsinogen stores away from HCl until
  pepsin is needed
• lining stomach with mucus
• Food leaves stomach as acid chyme, squirting
  slowly through the pyloric sphincter

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Small Intestine

• Participates in digestion and absorption
• Peristalsis allows for movement of chyme and
  digestive juices down the small intestine.

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Digestion in the Small Intestine

• Digestion is usually completed in the duodenum
  (first section) with the help of digestive juices

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Digestive Juices

• Digestive juices come from 4 sources, entering
  the duodenum
• Pancreas
• produces digestive enzymes
• produce basic bicarbonate solution (buffer)

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Digestive Juices

• Lining of duodenum
• produces digestive enzymes
• Liver
• Produces bile
• Gallbladder
• Stores bile

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Bile

• contains bile salts
• breaks up fat droplets into very small pieces
  called micelles
• micelles are then absorbed and taken up by the
  lymph vessel (lacteal)

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Question

• Does bile perform mechanical or chemical
  digestion?
• Mechanical breaking it up into smaller pieces

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Protection of Small Intestine

• Small intestine and adjacent organs are protected
  from digestive enzymes by producing inactive
  forms that are only activated in the duodenum.

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Absorption of Nutrients

• Occurs in the jejunum (mid-small intestine) and
  ileum (end-small intestine)
• The surface area in these regions is very large
  because of the villi and microvilli.
• Villi and microvilli are projections of the lining

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Absorption of Nutrients

• Some nutrients are absorbed into the lymph vessel
  (lacteals).

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Absorption of Nutrients

• Most nutrients are absorbed into the blood
  vessels
• capillaries ? hepatic portal vessel ? liver
• the liver converts many nutrients and regulates
  blood sugar and other nutrient levels
• Water is also absorbed here.

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Digestion Summary
Biomolecule Where Digested Key Enzymes
Carbohydrates Mouth, Small Intestine Amylase, Maltase, Lactase, Sucrase
Lipids Small Intestine Bile, Lipase
Proteins Stomach, Small Intestine Pepsin, Peptidases, Trypsin
Nucleic Acids Small Intestine Nucleases
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Large Intestine

• responsible for water recovery from digested
  material
• Feces Waste of digestive tract
• Bacteria live here (including E. coli) that live
  on feces and produce vitamins B and K (?) and
  stinky gases

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Large Intestine

• End of colon rectum
• End of rectum anus

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Ruminant Digestion

• Ruminants (ex. Cows) eat plants with
  tough-to-digest cellulose.
• In order to improve digestion efficiency,
  Ruminants have symbiotic relationships with
  microbes.

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Ruminant Digestion Steps

• Microbes in the rumen help digest the cellulose
  (products are fatty acids and cud (partially
  digested food)
• Cud is regurgitated and rechewed
• Water is removed in the omasum
• Digestive enzymes in the abomasum complete
  digestion

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Control of Digestive System

• The actions of the digestive system are regulated
  by hormones as well as it's own nervous system.

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Lower pH inhibits gastrin release
CCK slows movements of stomach down
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Diabetes

• Diabetes mellitus is a condition in which insulin
  is not produced or cells are insulin resistant

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