Autumn.wmf (12088 bytes) Concepts of Biology (BIOL116) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321;;

Digestion/Nutrition can perhaps even view an animal as nothing more than a group of cells clustered around a gastro-intestinal tract, differentiated for and dedicated to the task of keeping that gut full.

Wayne Becker - University of Wisconsin
"What it means to be a plant" - unpublished course notes 

I.  Animals are heterotrophs.
    In other words, they require food (organic compound-rich materials) from the environment.  Heterotrophs cannot synthesize their own organic compounds (carbohydrates, proteins, fats) from simple, inorganic materials.  In contrast, autotrophs like plants (photosynthetic autotrophs) and some weird bacteria (chemoautotrophs) can use materials like water, some mineral ions, and carbon dioxide to produce all the materials they require.

A.  Animal diets vary

B.  The methods of obtaining food varies:

C.  Animal form & function are related to the specific diet 
    The evolution of animal structures has been greatly influenced by diet. 
For example, consider tooth-type or gut length in herbivores vs. carnivores vs. omnivores.  Obviously herbivores have nice flat teeth for grinding up tough plants.  The gut length of herbivores is typically much longer than that of carnivores because of the increased time needed for digestion of plant materials.

 D.   Animals are essentially motile "guts".  (check out the opening quote)

Recall our comparison of plants and animals (check out the plant way of life notes):   

  Plant Animal
form of nutrient inorganic organic
nutrient "complexity" simple complex, pre-fabricated
nutrient type CO2, water, minerals carbohydrates, lipids, proteins
nutrient concentration in environment dilute concentrated
localization omnipotent localized

Some conclusions we can make from this quick comparison are that: 

  1. Animals are motile largely because of the need to find concentrated, pre-fabricated, localized food in the environment.  Thus, the shape of the animal must be adapted for movement (minimal S/V ratio);  

  2. Because plants are autotrophic and their raw materials are essentially everywhere, there was no evolutionary pressure for motility in plants.  However, the big problem that plants face is how to absorb the dilute, omnipotent nutrients; hence their large S/V ratio, i.e., dendritic growth;   

  3. Animals needed a gut/digestive tract to process and remove foods/wastes.  Because animals are non-selective, like a child at the supper table whose eyes are too big for his/her stomach, there is lot of waste.  However, this would be a drag to carry around, so viola, the evolution of a complete digestive tract;   

  4. Plants feed like a "Marine" - they only absorb what they use so there is little waste - plants dispose wastes in the central vacuoles (this is too bulky for animals).  From a waste perspective, humans begin life �plant-like� and become more �animal-like�.  In other words, infants �eat� breast milk which is highly nutritious and digestible.  Thus, there is little leftover =  innocuous diapers.  As they eat more solid food with less digestible bits, the pleasantness of diaper changing rapidly fades.

Some Predictions:  If these hypotheses are true, then we expect that:

  1. non-motile animals will occur in habitats that enable them to act "plant-like.  In other words, they will be aquatic where food, especially for filter feeders, is essentially omnipotent.  And, this is true.

  2. non-motile animals will be more-or-less dendritic to efficiently harvest the dilute nutrients about.  They are, just check out a sea fan or coral.

 II.  The general sequence of nutrition processing

ingestion (oral cavity) digestion (stomach, mostly small intestine) absorption (small intestine) elimination (large intestive)   

A.  Primitive to advanced.
    Intracellular digestion (i.e., amoeba, phagocytosis) mouth/anus same orifice (hydra, flatworm) separate mouth & anus (complex invertebrates, vertebrates)

B.  General gut structure.

III.  Meet Square "Dude"  - a diagrammatic representation of the digestive system (on board during lecture).  We will travel through "square dude" examining individual components.

IV.  Oral Cavity
    Site of the initial stage of the digestion process. 

A.  Site of both mechanical (teeth/chewing) and chemical digestion

B.  Saliva 
    Produced by the salivary glands (parotid, sublingual, submandibular).  Contains amylase (degrades starch into maltose; initial phase of starch digestion; may be important for prevent starch accumulation around teeth - an organic toothbrush), bicarbonate (buffers between 6.5-7.5, prevents tooth decay from acids) and mucin (glycoprotein, helps in bolus formation; protects and lubricates).   Saliva also has anti-microbial properties.  

C.  Tongue - manipulates food into bolus  

V.  Pharynx (throat)
    Food presses against soft palate initiates swallowing reflex.  The esophageal (hypopharyngeal) SPHINCTER normally is closed.

A.  Epiglottis - seals entrance to trachea

B.  Heimlich maneuver

VI.  Esophagus.
    Leads from pharynx to stomach.  Striated muscle at top, thus voluntary contractions (i.e., swallowing).  Then, smooth muscle which result in involuntary contractions for moving food along (peristalsis).  

VII.  Stomach

A.  General.

B.  Gastric juices
   Produced by glands that line the epithelium.  Sunken in chambers with various types of cells.  Gastric fluid contains:

  1. HCl - produced by cells lining stomach; initiates hydrolysis of foods; kills bacteria; produced by the parietal cells

  2. Mucous -protects lining from autodigestion; lining replaced every 3 days or so; ulcers breakdown of lining of stomach which are caused by bacteria; produced by mucuous cells

  3. Enzymes like pepsinogen; produced in inactive form called a zymogen.  Pepsin is the active form, digests protein.  Pepsinogen is converted ("activated") to pepsin  by treatment with acid or pepsin.  This is a good example of positive feedback; produced by chief cells.

C.  Gastrin
    Hormone released by stomach cells into circulatory system; stimulates stomach wall to release HCl.  High conc of HCl inhibits release of gastrin (ex. of negative feedback).  This prevents the acid concentration from becoming too large).  Gastrin is released in response to stretch receptors in stomach and sights/smells of food.  

D.  Chyme
    Nutrient broth, result of stomach action  

E.  Hunger pangs.
    Churning of empty stomach (churns every 30 s or so).  Food lasts 2-6 hours in stomach.  

F.  Lining.

VIII.  Small intestine 

A.  General

 B.  Function.
    Action in the small intestine:  chyme   small intestine (duodenum) stimulates intestinal wall to:

  1. release secretin pancreas produce bicarbonate to neutralize pH  

  2. release CCK (choleocystokinin) which: (a) stimulates gall bladder to contract releasing bile (contains bile salts, etc to disperse lipids, like a detergent); and (b) stimulates pancreas to release digestive enzymes  

  3. release GIP (gastric inhibitory protein) (if food rich in fat) stomach slows peristalsis provide time for fat digestion

IX.  Pancreas/liver/gallbladder
    Collectively dump secretions (i.e., enzymes, bile) into small intestine to facilitate digestion, diffusion, and absorption .  Liver is particularly important for direction nutrient traffic of fuels, carries out gluconeogenesis.  The pancrease releases a variety of digestive enzymes include pancreatic amylase, lipase (digests triglycerides), trypsinogen (zymogen converted to trypsin by another enzyme, enterokinase (enteropeptidase), which is released by the intestine.

X.  Large intestine (colon)

XI.  Digestion 
Mostly occurs in duodenum

A.  Carbohydrates.    
Polysaccharides converted by salivary or pancreatic amylase to maltose and other disaccharides and sugar fragments.  Other enzymes like maltase, sucrase, lactase, which are found in epithelium of small intestine, convert the disaccharides to monosaccharides.  Monosaccharides are absorbed into blood stream. 

B.  Protein.
Polypeptides (proteins) are broken down by pepsin (stomach), trypsin (sm. intestine) and chymotrypsin (sm. intestine) into small fragments (peptides).  Hydrolysis by these enzymes occurs at specialized locations in the polypeptide.  Aminopeptidase and carboxypeptidase work from the ends of the peptide chain clipping off one amino acid at a time.   Trypsin, chymotrypsin, pepsin, and carboxypeptidase are released as zymogens.

C.  Fats (trigylcerides)
    Insoluble in water, so there is a problem with digestion that is occurring in a primarily aqueous environment.  Fat digestion can only occur at the surface of fat globules.  Bile salts helps emulsify fat into small globules for more efficient digestion (another good example of S/V ratios in action).  Lipase in the small intestine works at the surface of the globs and releases glycerol and 3 fatty acids.  These are absorbed, then reassembled into triglycerides inside intestinal cells.  Done by smooth ER.  The triglycerides are then packaged into membranes, transported to golgi then into vesicles into lymph into blood.

XII.  Absorption 
    Occurs in jejunum and ileum.   Absorption requires lots of surface area.  The human small intestine has a surface area of ca 300 m2, about the size of tennis court!  The length of intestine is related to diet - the more meat in the diet the diet the shorter the intestine because plant material is more resistant to digestion and needs a longer period in the digestive tract.  The inner lining of the intestine is folded into villi which increase S/V ratio.   And, the lining of each epithelial cell has brush-like projections on the surface (microvilli).  Both adaptations are designed for increasing s/v ratios.

XIII.  Time for supper!

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