Lecture 3

• Digestion

  • Is the breakdown of large, complex organic materials into smaller components that are used by the body
• Four stages of Digestion:

  1. Ingestion - take food (nutrients) in 
  2. Digestion - breakdown of food into smaller molecules (physically or chemically)
  3. Absorption - the transport of the products of step 2 from the digestive system to circulatory system and then rest of the body
  4. Elimination/esgestion - removal of waste from the body 
• 2 Types of Digestion:

  Mechanical Digestion: The physical breakdown of large pieces of food into smaller pieces. 

  Chemical Breakdown: The chemical breakdown of nutrient molecules into smaller molecules by enzyme action
• • The human digestive system is made of a tube from the mouth to the anus, this is called the Gastrointestinal Tract (GI Tract)
• • Digestive systems is needed to breakdown large materials into smaller ones that can be used by the tissues of the body. 
• There are FOUR stages of Food Processing:

  • STEP 1: Ingestion
  • The act of eating or drinking (taking IN food)
  • Occurs in the mouth
• There are FOUR stages of Food Processing:

  • STEP 2: Digestion
  • The process of breaking food down into molecules that are small enough for the cells of the digestive system to absorb
  • 2 types of digestion = Mechanical (Physical) Digestion & Chemical Digestion
  • One or both types occur(s) in mouth, stomach, and small intestine
• There are FOUR stages of Food Processing:

  • STEP 3: Absorption
  • The smallest nutrient molecules from broken down (“digested”) food are absorbed into (taken into) the cells of the intestine.
  • Once absorbed, they enter into the blood (circulatory system), and are transported to body cells to be used to build bigger molecules or to provide energy.
  • If not used immediately, excess molecules are stored (often as fat).
  • Occurs MAINLY in the small intestine, as well as, some in the large intestine
• There are FOUR stages of Food Processing:

  • STEP 4: Egestion
  • The process of removing (eliminating) any undigested material from the digestive tract.
  • The undigested material travels through the large intestine and is eliminated (excreted) as feces (“solid waste”)
  • Occurs from large intestine to the anus 
• Mechanical Digestion

  • The physical breakdown of food into smaller physical pieces. Can occur through chewing in the mouth (chops & grinds food), churning in the stomach (contractions pushing food against walls) emulsifying in the small intestine (s.i)
  • Carried out by teeth, tongue, rugae (stomach wall), bile salts (s.i)
  • Importance = to INCREASE SURFACE AREA
  • The more exposed area for the food to be “attacked” by enzymes that will chemically digest it 
• Chemical Digestion

  • The chemical breakdown of nutrient molecules (ie. macromolecules) into smaller molecules, and ultimately their individual subunits.
  • Occurs by breaking the chemical bonds between the subunits (monomers) that make up the large macromolecules
  • This breaking of the chemical bonds is done through a chemical reaction called HYDROLYSIS (“hydro” = water; “lysis” = to break —--> breaking apart using water)
  • Chemical digestion via hydrolysis requires the action of ENZYMES
  • Occurs in the mouth, stomach, and small intestine
• Enzymes in Digestion

  • The digestion of each type of macromolecules is carried out by specific enzymes. The “ASE” ending is indicative of a substance being an enzyme. However, there are some exceptions that do not follow this rules (eg. pepsin)
  • Carbohydrate-digesting enzymes Carbohydrates
  • Proteins-digesting enzymes are called Proteases
  • Lipid-digesting enzymes are called Lipases 
• The Mouth

  • The tongue and teeth contribute to the physical digestion occurring in the mouth. 
• The Tongue

  • The tongue is a muscle attached to the lower jaw. It freely moves in all directions and is sensitive to touch, taste and temperature.
  • Physical digestion begins in the mouth where food is chewed and formed into bolus.
  • Bolus ~ in latin means “ball of form” and contains food and saliva. 
• The Salivary Glands

  1. These are glands which secrete saliva (which is mostly water) into the mouth.
  2. Submaxillary: base of the jaw
  3. Sublingual: under the tongue
  4. Parotid: below the ear lobe 

  * 1 and 2 helps food stick together

  * 3 helps with digestion 
• Saliva 

  • Saliva contains amylase enzymes, which breaks down starches.
  • Function:

  1. Moisten food to make swallowing easier
  2. Helps food dissolve into a solution
  3. Contains amylase which starts to break down carbohydrates
  4. Has a neutral pH (6.5 - 6.8) 
• Uvula

  • Hangs from the middle of back edge of soft palate
  • Prevents food from blocking the pharynx when swallowing
  • Initiates the gag reflex 
• Types of Teeth

  1. Incisor ~ cut
  2. Pre Molars ~ grind
  3. Canines ~ tear
  4. Molar ~ grind 
• Structures of the Tooth include

  1. Enamel ~ a hard outer coating 
  2. Dentin ~ a bone like inner layer
  3. Pulp Cavity ~ contains soft tissue with blood vessels and nerves 
• Moving Food Past The Mouth:

  • The bolus of food moves down the esophagus by rhythmic contractions known as peristalsis.

  When the food reaches the stomach, it enters through the muscular valve called the cardiac sphincter.
• • When food is swallowed the epiglottis shuts off the trachea (for breathing) letting the bolus go down the esophagus (for food). 
• The Stomach 

  • The entrance of the stomach is guarded by the cardiac sphincter which controls the flow of materials into the stomach. 
  • The stomach is a muscular bag where both chemical and physical digestion occur.
  • The exit of material from the stomach is guarded by the pyloric sphincter which controls flow into the small intestine.