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Biology 2108 Lecture
Physiology:
Digestive System 

 
How does the body convert large, chemically complex foods to a size that can be passed through a cell membrane (yet still large enough to contain useable chemical energy)?

organic carbon molecules + O2 -> CO2  + H2O + useable energy



But first, let us consider other organisms:

Plants:


Do plants respire?

Where do they get organic carbon for energy metabolism?

Are CO2, light, and water alone sufficient for maintenance and growth?


Other animals:


Why can sponges get away without
a digestive system?







Why are herbivore digestive systems so long?







Humans:

The "dis-ASSEMBLY LINE" (21 to 30 ft. in length):

    


What regulates the direction and speed of this dis-assembly line?

   







             

The cause of ulcers has been re-examined in recent years, and now the bacterium Helicobacter pylori has been implicated.  Infection by H. pyloris is linked to stomach cancer.  The prevalence of infection is lowest in developed nations (why?) and the incidence of ulcers and stomach cancers have been on the decline.  However, the incidence of acid reflux disease and adenocarcinoma of the esophagus is on the rise, suggesting that H. pyloris protects the esophagus (coevolution between host and parasite has given H. pyloris the ability to regulate stomach acidity).




Bile emulsifies fats.  How does this aid digestion?



Absorption of food molecules into the circulatory system:


Why is the lining of the small intestine so folded?
What do the presence of capillaries indicate?

Glucose, vitamins, and amino acids require active transport into epithelial cells and diffuse into blood.  Fructose and fatty acids cross by diffusion.  Fatty acids and glycerol recombine and are coated with protein  cholesterol to become chylomicrons which are water soluble and are moved to the lymphatic system.


Interconversion of food molecules upon entering the circulatory system:

Do you have to eat a diet perfectly balanced according to the metabolic needs of cells for the types of molecules discussed above?

Why do the veins leaving the villi of the small intestine collect into the hepatic portal vein and pass through the liver before joining the main branch of the circulatory system?  What would happen after your breakfast of coke and a candy bar if not?

The liver aids in the conversion of molecules from on class to another.


amino acids simple sugars fats

    


  glycogen



   


For example, after meal, high sugar in blood results in release of insulin from the pancreas resulting in:
  1. liver and other cells convert glucose to amino acids, fats, glycogen
  2. stimulate cells to use glucose as energy source by facilitating its entry into cells.
Opposite occurs when blood sugar is low; several hormones involved, e.g. glucagon (from the pancreas):
  1. breakdown of glycogen by liver
  2. release of glucose to blood.




Can the body manufacture all the types of molecules required by the body?

Water soluble vitatmins, fat soluble vitamins, and essential amino acids are required in the human diet.  If other organisms can manufacture these molecules, why can't humans?




Certain minerals can also become scarce in human diets.

Much starvation is one of poor quality (e.g. iron deficiency) not quantity, resulting in a weakening of the immune system and susceptibility to disease


Can a nation be 'over-nourished'?  Does a nation have to be wealthy?
What is the evolutionary explanation for this?






 


The movement of peoples from areas in which they evolved is also important in explaining susceptibility to disorders associated with nutrient deficiencies.  Sunlight is required for production of vitamin D, though UV light destroys folic acid (or folate, one of the B vitamins).  Skin pigment blocks UV light so how would migration of peoples differing in skin color to different latitudes affect their health?




Not all ingested material enters the circulatory system and instead is passed to the large intestine   
  



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