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					Biology 2201                                     Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                      Topic: Human Digestive System
                                                                             Page 1 of 13

                        The Chemical Foundation of Digestion

All organisms, regardless of size or complexity, have some method to obtain the
essential nutrients they need for survival.
Heterotrophs:         Organisms that depend on organic molecules manufactured by
                      other living things. Humans are ingestive heterotrophs.

                     Ingestive = nutrients are taken in through the mouth.

Autotrophs :              Organisms that nourish themselves using inorganic material
                     (examples: water and carbon dioxide).

Nutrient:      Any substance used by the human body to maintain homeostasis.

Essential Nutrients
  • The basic raw materials organisms need to make their own structures, perform
     functions, and obtain energy for survival.
  • Essential = Needed and must be obtained from the organism’s diet.

Six Essential Nutrients
   • water
   • carbohydrates
   • proteins
   • fats (lipids)
   • minerals
   • vitamins

Your body functions best when these essential nutrients are present in correct
proportions. A diet that satisfies this is called a balanced diet.

1. Water
   • major role in the body is to act as a solvent.
   • Important for chemical reactions ( Hydrolysis)
   • Needed for respiration (Alveoli)
   • Needed for movement of materials (osmosis)
   • Main source: food and liquids.
Biology 2201                                       Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                        Topic: Human Digestive System
                                                                               Page 2 of 13

2. Carbohydrates
    • Consist of atoms of carbon, hydrogen, and oxygen (CHO).
    • Basic unit of Carbohydrates is called the Monosaccharide (Simple Sugar).
         o Examples of Monosaccharides: glucose, fructose, galactose
         o Disaccharide: 2 monosaccharides bonded together. Ex: maltose
         o Polysaccharide: many monosaccharides bonded together. Ex: cellulose,
            glycogen, starch
    • Function: provide a source of energy.
    • Excess carbohydrates are converted to fat and stored.
    • Food sources: glucose, rice, bread, pasta, etc.

Q.    How are monosaccharides joined together to make Di and Polysaccharides?

A.    Dehydration Synthesis Reaction

          •    This is a reaction in which two small molecules are joined together to
               make a larger molecule by removing (dehydrating) water.
          •    Dehydration reactions happen in fats and proteins as well.

      Ex: Glucose (monsaccharide) + Glucose (monosaccharide)              Maltose

                                                Water is removed

                        Water removed
Biology 2201                                       Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                        Topic: Human Digestive System
                                                                               Page 3 of 13

Q.       How do Carbohydrates provide energy?

A.       Hydrolysis Reactions

            •   This is a reaction where larger molecules are broken/split into smaller
                molecules by the addition of water.
            •   Hydrolysis reactions happen whenever large molecules such as
                polypeptides and lipids are broken down into smaller molecules.
            •   Energy is released when the bonds of the large molecule are broken in
                the process of creating the smaller molecules.

3. Proteins
    • Made up of strings of amino acids called peptides.
    • Amino Acid: The basic building block of a protein.
          o Humans have 20 amino acids that make up all the proteins in the body.
          o 8 Amino acids are essential meaning they have to come from their diet.
    • Proteins are made up of peptides created when more than one amino acid joins
          o Dipeptide = A combination of 2 amino acids.
          o Polypeptide = A combination of many (poly) amino acids.

     •   Function: include, cell growth and repair, enzymes in your body are all proteins
         (act in metabolism), and can supply energy.
     •   Food sources: meat, milk, poultry, fish, eggs, cheese, etc.
Biology 2201                                    Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                     Topic: Human Digestive System
                                                                            Page 4 of 13

4. Lipids
    • Contains Fats, Waxes and Oils.
    • Structure of a Lipid
          o Made up of a 3 Fatty Acids + Glycerol

   •   Function: Provides a source of energy, insulating the body from the cold, and
       protecting organs from injury. Also used for Cell Membranes (Cholesterol)
   •   Food sources: margarine, butter, meat, cheese, nuts, eggs, etc.

5. Minerals
   • These are inorganic compounds that your body needs in small amounts.
   • Function: carry out certain chemical reactions, and helping to maintain acid-base
      balance within the body.
   • Readily absorbed into the bloodstream.
   • Ex: Iron, calcium, potassium etc.

6. Vitamins
    • Compounds required in very small amounts in diet.
    • There are Two types of Vitamins:
          o Fat Soluble Vitamins Vitamins that dissolve into fats.
                  Vitamins A, D, E and K are Fat soluble.
          o Water Soluble Vitamins     Vitamins that dissolve into water.
                  Vitamins B and C are water soluble.
                      • These vitamins need to be replaced daily.

   •   Function: act as coenzymes, chemicals needed to make enzymes function.
Biology 2201                                   Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                    Topic: Human Digestive System
                                                                           Page 5 of 13

                      The Human Digestive System
Function of Digestive System
  • Break down food into small useful substances that can be absorbed into the
      circulatory system where nutrients are transported to individual cells.

   • The process of breaking necessary substances into smaller molecules so that
      they can be absorbed in the body.

Digestion occurs in the alimentary canal, a continuous tube beginning at the mouth and
ending at the anus.

Structure of the Human Digestive System
Biology 2201                                     Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                      Topic: Human Digestive System
                                                                             Page 6 of 13

Two basic types of digestion

A.       Mechanical Digestion
     •   Initial stage of physically breaking down food into smaller pieces.
     •   Occurs mainly in the mouth where teeth chew food and tongue manipulates it.
     •   Some mechanical digestion occurs in the stomach (rugae).

B.       Chemical Digestion
     •   Separation of food into molecular components by chemical means.
     •   Process begins in the mouth with the secretion of saliva which contains the
         digestive enzyme amylase.
     •   Chemically breaking down food continues through the stomach and is completed
         in the small intestine.

Mechanical and chemical digestion act together to speed up the digestion process.
Mechanical digestion aids chemical digestion by exposing more food particles to
digestive enzymes.

           Major organs and glands involved in digestive process

1. Mouth
   • Digestion begins here.
   • Contains teeth used for tearing food apart (mechanical digestion).
   • Contains tongue that moves food into position for digestion.
   • Contains Salivary glands that secrete the enzyme Saliva.
         o Saliva: Liquid in the mouth that lubricates and moistens food. Contains
            the enzyme Salivary Amylase that converts Starch into Maltose.
                                  Salivary Amylase
                      Starch ------------------------------------>      Maltose
     Digestion in the Mouth

     •Food enters the mouth where teeth begin breaking it down. The salivary glands
      secrete mucin which makes the food slippery and helps prepare it for further
      digestion. Salivary Amylase breaks down starches into maltose.
    • At the same time the tongue shapes the food into a round ball called a bolus.
      Swallowing (an involuntary response) is initiated when food touches the back of
      the tongue.
    • Food is passed into the esophagus.
2. Esophagus
Biology 2201                                    Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                     Topic: Human Digestive System
                                                                            Page 7 of 13

   •   A hollow tube leading from the mouth to the stomach.
   •   No digestion occurs here.
   •   Lined with circular and longitudinal muscles that undergo a series of muscle
       contractions called Peristalsis.
          o Peristalsis: Series of muscle contractions that forces food through the
              alimentary canal.
   •   Contains the Esophageal sphincter muscle at the lower end of the esophagus
          o Function of Esophageal Sphincter: Controls amount of food entering

3. Stomach
   • A muscular, j-shaped, sac-like organ.
   • Protein digestion occurs here.
   • Has two distinct regions:
        o Cardiac Region: Region of stomach closest to heart.
        o Pyloric Region: Lower portion of the stomach. (Closest to the feet)
   • Contains folds of smooth muscle called Rugae.
        o Rugae: Folds of muscle in the stomach that increases the surface area
            for digestion and also contract to break up food into smaller pieces in a
            process called churning.
                   Churning: Contractions of muscles in the stomach in order to
                   break up food. (Mechanical digestion)
   • The stomach secretes Gastric Juice that digests proteins.
        o Gastric Juice: Fluid in the stomach composed of HCl (hydrochloric acid),
            water and Pepsinogen.
                   Pepsinogen: Inactive enzyme in the stomach that changes into
                   pepsin in the presence of HCl.
                   Pepsin: Enzyme that breaks proteins into long polypeptides.

               Proteins      --------------------------------------> long polypeptides

   •   Contains the pyloric sphincter at the bottom of the stomach at the entrance of
       the small intestine.
          o Pyloric Sphincter: Ring of muscle that controls the amount of food
             entering the small intestine.

Digestion in the stomach
Biology 2201                                      Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                       Topic: Human Digestive System
                                                                              Page 8 of 13

   •   A bolus enters the stomach via the cardiac sphincter.
   •   Gastric glands secrete gastric juice that aid in the digestion of proteins.
   •   Pepsin changes proteins into long polypeptides.
   •   When digestion in the stomach is finished, an acidic soupy liquid remains called
   •   Chyme enters the small intestine by passing through the pyloric sphincter.

4. Small Intestine
    • Most important organ of the digestive tract.
    • Approximately 6 metres long.
    • Site of most chemical digestion and absorption of food molecules into the blood.
    • Subdivided into three parts:

   A. Duodenum
      • First region of the small intestine.
      • Pancreatic and bile ducts enter here making it a site where chemical digestion
      • Site of final Carbohydrate, Protein and Lipid digestion.
      • Contains folds with finger-like projections called villi (microvilli) that serve to
         increase the surface area for digestion.

                            Structure and Function of a Villus

       •   Finger like structure that serves to absorb nutrients.
       •   Contains a central structure called a lacteal that absorbs fats and send them
           to the lymphatic system.
       •   Lacteal is surrounded by blood vessels that absorb Amino acids and
           monosaccharides into the blood.

   B. Jejunum
      • Contains many folds and intestinal glands in order to breakdown any
          remaining protein or carbohydrates.

   C. Ileum
       • Contains few and very small villi.
       • main function is to absorb nutrients
       • Pushes remaining undigested material through to the large intestine.
Biology 2201                                    Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                     Topic: Human Digestive System
                                                                            Page 9 of 13

5. Large Intestine
    • Divided into different structures: caecum, colon, rectum, and anus.
    • Much shorter than the small intestine although the diameter is much larger (basis
       for name).
    • Separated from small intestine by a valve.
    • Caecum is a sac-like structure at one end of the small intestine where the
       appendix is found.
    • Colon is the main portion of the large intestine where water and dissolved
       minerals are absorbed from undigested food. The colon has three parts.
                      Ascending Colon – Portion of colon on the right side of the body.
                      Transverse Colon – Portion of colon going across the abdomen.
                      Descending Colon – Portion of the colon on the left side of the
    • Colon also contains intestinal bacteria that help breakdown undigested material
       to provide more nutrients and can produce vitamins B-12 and K.
    • The mass of indigestible material in the large intestine is known as feces.
    • Feces passes through the rectum and out the body through the anus.
    • The anus contains rings of muscle called the anal sphincter that allow the body
       to control timing of elimination to some extent.

6. Pancreas
    • Organ located under the stomach.
    • Secretes several enzymes into the duodenum that aid in digestion of lipids,
      proteins and carbohydrates.

               Enzymes and Chemical Secretions of the Pancreas
       Name of Enzyme/Secretion                               Function
Bicarbonate ions                                •   Neutralize acidity of chyme
                                                •   Provide a basic pH so enzymes can

Pancreatic Amylase                              •   Break Polysaccharides into
Pancreatic Lipase                               •   Break lipids into fatty acids and
Trypsin/Chymotrypsin                            •   Break polypeptides into shorter
Biology 2201                                      Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                       Topic: Human Digestive System
                                                                             Page 10 of 13

7. Liver

   •   Produces Bile that aids with digestion of fats.
   •   Converts unused glucose into glycogen that is stored in the liver.

       Green liquid substance made from cholesterol.

       Function:    Emulsifies (breaks up) fats into small droplets called micelles to
                    increase surface area for digestion.

8. Gall Bladder
   • Structure located under the liver.
   • Stores bile.

Pathway of food through the Human Digestive System

1. Mouth
2. Esophagus
3. Esophageal Sphincter
4. Stomach
5. Pyloric Sphincter
6. Small Intestine
7. Large Intestine
8. Anus
Biology 2201                                          Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                           Topic: Human Digestive System
                                                                                 Page 11 of 13

                        Chemical Digestion and Enzymes

Organ          Associated          Chemical Digestion           Mechanical           Other
               Glands              (Enzyme Action)              Digestion            Secretions
                                   Salivary amylase breaks
   Mouth       Salivary glands
                                     starch into maltose
                                                                Teeth and tongue     Mucin and water
                                                                                     HCl kills bacteria,
                                                                                       breaks down
                                                                                     cellulose, lowers
                                                                                      pH for pepsin
                                    Pepsin breaks proteins         Peristalsis 3
  Stomach       Gastric glands
                                   into shorter polypeptides     times a minute

                                                                                      Mucus protects
                Liver and Gall
                                      further break down
                                   polypeptides into shorter

                                    Erepsins break simple
                                   polypeptides into amino

                                   Pancreatic lipase breaks
                                    fats into fatty acids and
                                                                                      Bile emulsifies
                  Pancreas                   glycerol
    Small                            Pancreatic amylase
  Intestine                        breaks down starch into
                                   Peptidases break simple
                                                                Peristalsis occurs         chyme
                                   polypeptides into amino
                                                                                     Mucus lubricates
                                   Maltase breaks maltose
                                     into simple sugars

               Intestinal Glands    Sucrase breks sucrose
                                      into simple sugars

                                    Lactase breaks lactose
                                      into simple sugars
                                                                                        Mucus to
    Large                                                            Water is
                Mucus glands                 None                                        lubricate
  Intestine                                                        reabsorbed
Biology 2201                                        Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                         Topic: Human Digestive System
                                                                               Page 12 of 13

                                    Digestive Disorders

Problems with the digestive system can range from minor inconveniences to a major
impediment to a normal life.

We will discuss the following 6 digestive disorders.

     1.      Ulcers
     2.      Gall Stones
     3.      Ileitis
     4.      Colitis
     5.      Anorexia Nervosa
     6.      Bulimia Nervosa

1.        Ulcers

     •    Slow – healing sores in the linings of the stomach and intestines.
             – Causes:
                   • Acids
                          – Causes breakdown of mucus layer of stomach and
                   • Heliobacter pylori
                          – Acid resistant bacteria that causes mucus production to stop.
                   • Lifestyle
                          – Stress, smoking, alcohol consumption
          • Cures/Treatments
                – Medications to reduce acid production
                – Lifestyle change

2.        Gall Stones

     •    Hard masses that form in the gall bladder.
                  • Cholesterol in the bile “precipitates” out as a solid forming crystals.
             – Causes
                  • Obesity
                  • Alcohol
                  • Heredity

             –   Treatments
                    • Ultrasound (busts up the gall stone so it can pass)
                    • Diet Changes (lower fat content diet – reduce cholesterol)
                    • Surgery (Removal of gall bladder).
Biology 2201                                           Unit 3 – Maintaining Dynamic Equilibrium I
Notes                                                            Topic: Human Digestive System
                                                                                  Page 13 of 13

3.       Ileitis

     •   Inflammation of the Ileum causing much pain.
             – Inflammation causes intestine to empty often causing diarrhea.
             – Causes
                   • Relatively unknown
                   • Does run in families

             –  Treatments/Cures
                   • No cure
                   • Treatment includes drugs to reduce swelling
                   • Surgical removal of affected area
4.       Colitis

     •   Inflammation and ulceration of the lining of the Colon.
             – Affects the innermost lining of the colon.
             – Causes bleeding, diarrhea and abdominal pain.

             –     Causes
                     • Relatively unknown
                     • Does run in families

             –     Treatments/Cures
                      • No cure
                      • Treatment includes drugs to reduce swelling
                      • Surgical removal of affected area

5.       Anorexia Nervosa

             •     Eating disorder where person has a fear of gaining weight and they go on
                   a very restrictive diet
             •     Person often loses weight and has a lack of nutrients to the cells of body.
             •     Many complications such as constipation, heart failure, organ failure can

6.       Bulimia Nervosa

             •     Eating disorder where a person has episodes of binge eating followed by
                   “purging” by vomiting or taking laxatives.
             •     Very dangerous because it affects several organs
             •     Damage to heart, kidneys, esophagus and teeth are common.

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