Digestive System

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Digestive System

• Parts and Function

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Digestion

• All organisms are composed of four complex
  biological molecules lipids (or fats), proteins,
  carbohydrates, and nucleic acids. All are
  composed of carbon, hydrogen, oxygen and some
  have nitrogen.
• The chemical and physical breakdown of complex
  biological molecules into their component parts
  is the process of digestion.
• Lipids are broken down into fatty acids and
  glycerol, proteins to individual amino acids, and
  carbohydrates into simple sugars.

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Importance of Proteins

• Proteins are important as a structural element in
  bones, cartilage, hair,, nails, and cell
  membranes.
• They are also important as enzymes, hormones,
  antibodies, and in oxygen transport in red blood
  cells.
• Proteins are formed by the linkage of amino acids
  into polypeptides.

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Importance of Lipids

• They are fats and oils which are a fundamental
  component of cell membranes and may be used for
  energy storage or insulation
• A characteristic feature is that they do not
  dissolve in water

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Importance of Carbohydrates

• These include simple sugars such as glucose and
  sucrose and polysaccharides such as starch and
  cellulose
• They are important as structural compounds and as
  a source of energy that can be used as ATP
• Starch is a complex polysaccharide made in plants
  cells for the storage of energy
• Foods such as potatoes and pumpkins are rich in
  starch and can be good sources of energy
• Cellulose is one of the most common carbohydrates
  and can be found in the cell walls of plants
• Human digestive system is unable to break down
  cellulose and is the largest component of dietary
  fiber

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The digestive system

• In most animals the digestive system is made up
  of a tube (alimentary canal) that runs more or
  less the length of the body.
• Generally the food moves in one direction and
  different parts are responsible for doing
  different jobs in the digestive process.
• There are also accessory organs that are
  important in digestion that connect to the
  alimentary canal via ducts.

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Parts of Alimentary Canal

• Mouth- mechanical breakdown of food tasting
  secretion of salivary glands (salivary amylase)
• Esophagus- muscular tube that connects the mouth
  with the stomach
• Stomach- large muscular storage organ functions
  in storage, mixing, some secretions (hydrochloric
  acid and pepsinogen)
• Small intestine
• Duodenum (first 25-20cm), jejunum, ileum
• Receives bile and pancreatic juices
• Secretes digestive enzymes from the walls
• Absorption of nutrients

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Continuation of Alimentary Canal

• Large intestinewater reabsorption and production
  of vitamin K.
• Anusopening through which undigested material
  will be excreted.

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Accessory Organs Connected to Digestive System

• Liver- has many functions including regulation of
  amino acids in blood, production of glycogen (a
  storage molecule) and bile
• Pancreas- secretion of amylase, trypsinogen and
  bicarbonate
• Gall Bladder- storage of bile

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• Salivary Glands
• Esophagus
• Stomach
• Pancreas
• Large Intestine
• Appendix
• Small intestine
• Gall Bladder
• Liver

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Connections to the Circulatory System

• Mesenteric Veins- veins connected to capillary
  bed closely tied with the intestine
• Hepatic Portal System- major blood vessel that
  takes blood from the intestine to the capillary
  bed in the liver
• Circulatory system- major system that transports
  nutrients to the rest of the body
• Absorption of nutrients from the small intestine
  occurs with the aid of villi and microvilli in
  the small intestine which increase surface area
  for diffusion

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The Villi of the small intestine
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Digestion of Proteins

• Any enzyme that digests proteins is called a
  protease
• Chemical digestion of proteins begins in the
  stomach
• The stomach is very acidic (has a low pH, 1.5
  - 7)
• Pepsin is the primary digestive enzyme in the
  stomach
• The small intestine carries out further digestion
  with trypsin, which is secreted by the pancreas
• As proteins are digested, the polypeptide chains
  unravel and break up into small chains of amino
  acids called peptides

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Digestion of Lipids

• It begins in the small intestine by making the
  molecules more compatible with water so that the
  digestive enzymes can access them.
• This is accomplished by breaking up the lipid
  into small droplets which can be distributed in
  the water of the small intestine
• This process is referred to as emulsification
• Bile which is produced by the liver, stored in
  the gall bladder and pumped into the small
  intestine when lipids are present is responsible
  for emulsification
• Once emulsified, they may be digested into their
  subunits (glycerol and fatty acids) by digestive
  enzymes called lipases.
• They are produced in the pancreas and secreted
  into the small intestine

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Digestion of carbohydrates

• The digestion begins by converting
  polysaccharides (long chains of simple sugars)
  and disaccharides (two sugars linked together)
  into monosaccharides (simple sugar units) that
  can be absorbed by body cells
• It begins in the mouth and is completed in the
  small intestine (they are not digested in the
  stomach)
• Amylase is the enzyme responsible for digesting
  starch
• It can be found in the mouth in ones saliva as
  well as in the small intestine secreted by the
  pancreas