Download Week 9 Topic: Nutrition and Digestion Reading: Chapter 30 Main

Notes
Week 9
Biology 103, Spring 2007
Dr. Karen Bledsoe
http://www.wou.edu/~bledsoek/
Reading: Chapter 30
Topic: Nutrition and Digestion
Main concepts:
• The food we consume is our source of chemical energy (from breaking chemical bonds, and measured in
units called calories), and our source of materials to construct our own tissues from.
• Lipids (fats, oils), carbohydrates (starch, sugars), and proteins can all be broken down and metabolized for
energy. Proteins and lipids are also used for tissue construction: lipids form cell membranes, are used to
make steroid hormones, and store energy, while proteins are used to make muscle filaments, membrane
proteins, hair, fingernails, enzymes, peptide hormones, and many other things. A few animals also use
structural carbohydrates: arthropods, for example, use the carbohydrate chitin in their exoskeletons.
• Lipids are composed of fatty acids and glycerine; proteins are made up of amino acids; complex
carbohydrates such as starch are made up of simple sugars such as glucose. Enzymes in the digestive
system break large polymers (lipids, proteins, starch) into their monomers (fatty acids, amino acids,
glucose), because only the small monomers can be absorbed by cells.
• Food also contains substances that are not energy sources, but are needed in various body processes:
minerals, vitamins, water.
• The processes of digestion:
• Ingestion: taking food in to a mouth or similar opening.
• Mechanical breakdown: teeth, gizzard, or other structure break the food into smaller pieces, making it
easier for enzymes to work on the food.
• Chemical breakdown: enzymes (and acid in the stomach) break down food chemically. In some animals,
such as grazing animals, microbes in the upper digestive system take part in the chemical breakdown of
food.
• Absorption: monomers released from food diffuse across the cells of the intestinal lining and are taken
into the bloodstream.
• Elimination: undigested material is expelled. No animal has such a wide range of enzymes that
everything in the food can be completely broken down.
• Compartmentalized digestion
• Many animals have multiple compartments in their digestive systems for breaking down food in different
ways.
• In humans, there are two main compartments:
• Acid digestion takes place in the stomach. Some enzymes, such as pepsin, require an acidic
environment. Protein digestion begins in the stomach.
• Alkaline digestion takes place in the upper small intestine. As chyme (partially digested food) moves
through a valve from the stomach to the upper small intestine, it is neutralized by sodium
bicarbonate from the pancreas. The pancreas releases enzymes that require an alkaline
environment. The liver produces bile, which is released from the gall bladder, and used to break
apart lipids. Cells lining the intestines also release enzymes that complete the digestive process.
• Absorption
• Absorption of nutrients takes place mostly in the small intestine.
• Some water is absorbed in the stomach and small intestine, but most is absorbed in the large intestine.
If there is a lot of undigested sugar in the chyme (from eating too much sugar, or from eating indigestible
sugars in dietetic candies), this created an osmotic gradient that tends to prevent the large intestine
from absorbing water. This is why eating these candies causes diarrhea.
• Hormones control the process
• Hormones are involved in the secretion of acid in the stomach.
• Several hormones produced by the stomach appear to be involved in appetite control.
Common misconceptions:
• Many people believe that all digestion occurs in the stomach. However, digestion occurs in the small
intestine as well.
Notes
Biology 103, Spring 2007
Dr. Karen Bledsoe
http://www.wou.edu/~bledsoek/
• The term “energy” is often misunderstood. Many people think of energy as a kind of substance. We use the
word “energy” when we describe how we feel, so it seems evident that caffeine gives us “energy” — but it
doesn’t in a scientific sense. Caffeine is a nervous stimulant, not a source of food energy. The only energy in
coffee, tea, or “energy” drinks is in the sugar that is added to them.
• Some people discuss “calories” as though “calorie” is some kind of substance. A calorie is a unit in a system
for measuring food energy, just as “degree” is a unit in a system for measuring heat energy. Therefore it’s a
contradiction in terms if an “energy” drink claims to be “low-cal” or “calorie-free.”
• A lot if diet literature suggests that “toxins” in our body cells make our metabolism sluggish, and therefore
the only way to lose fat is to “detoxify” the body. This is, in fact, nonsense. While there are metabolic “toxins”
produced by our cells, our liver and kidneys are designed to remove them. People who have a buildup of
metabolic toxins suffer from liver or kidney failure and belong in the hospital, not on a reducing diet. Various
juices, teas, and fruit concoctions designed to “detoxify” the body usually contain diuretics such as parsley
or dandelion. The “fat flushing” or “detoxifying” effects are in reality an increase in water loss, which can, if
done over extended periods, cause a person’s blood potassium to drop (and nothing will induce the body to
dispose of fat through the kidneys — we never see an oil slick in the toilet after urinating). Sorry, but the only
safe way to lose weight is to 1) consume fewer calories by eating moderately and sensibly and 2) use up
more energy through frequent, moderate exercise.
Reading notes:
• List the major nutrients (polymers) that animals need, some examples, and the smaller molecules
(monomers) that they are composed of.
• List major minerals and vitamins that humans need, and their roles in the boyd.
• Describe the processes of ingestion, mechanical breakdown, chemical breakdown, absorption, and
elimination in humans. The describe some differences seen in other animals.
• Describe examples of digestive specialization seen in cows, birds.
• Describe what digestive processes occur in the: mouth, esophagus, stomach, small intestine, large
intestine. What roles do the liver and pancreas play in digestion?
• Describe ways in which hormones influence digestion and demonstrate homeostasis.
• Describe how genes, the environment, and our own choices can influence our weight. If human genes
haven’t changed in the past 20 years, why are Americans heavier now than they were 20 years ago?
Useful websites:
• “Digestion” http://www.whfoods.com/genpage.php?tname=faq&dbid=16#digestion is an interactive feature
that allows you to follow food through the digestive system and see what happens to it along the way. There
is also text explaining how the system works and how to keep the digestive system healthy.
• “Absorption” http://www.kscience.co.uk/animations/duodenum.htm is an animation showing how nutrients
are absorbed in the small intestine and moved into the bloodstream.