Introduction to Organismal Biology (BIOL221) - Dr. S.G. Saupe; Biology Department, College of St. Benedict/St. John's University, Collegeville, MN 56321; ssaupe@csbsju.edu; http://www.employees.csbsju.edu/ssaupe/

Digestion/Nutrition 

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

• herbivores

• carnivores

• omnivores

• detriviores

B.  The methods of obtaining food varies:

• suspension-feeders -  whales, clams, and hydra

• fluid-feeders -  aphids

• bulk-feeders - most vertebrates

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):   

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

• The gut is a series of chambers separated by SPHINCTERS

• four layers - (a) mucosa - secretion and absorption; (b) submucosa - blood and lymph; muscle layer - inner circular, outer longitudinal; and (d) serosa - outer, protective & support layer

• peristalsis moves materials through the gut.   Consider the problem of moving a golf ball through a garden hose.  You can either:  (a) increase the pressure behind the golf ball; or (b) decrease the pressure in front of the ball (suck on the hose); or (c) squeeze the hose just behind the ball.  Obviously, the latter is the model for peristalsis; smooth muscles contract (involuntarily) in a rhythmic wave behind the bolus moving it along through the digestive tract.

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

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

• stores and digests food

• acidic pH (2-3)

• elastic walls

• folds

• holds about 2 liters

• some absorption occurs in stomach (i.e., alcohol)

• the opening to the stomach is the cardiac orifice (SPHINCTER) which is normally closed but opens upon arrival of bolus.  Back flow through the SPHINCTER is heartburn.

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 (parietal) lining stomach; initiates hydrolysis of foods; kills bacteria
   

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 mucous (goblet) 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.  The latter 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.