Unlike plants that are able to make their own food via photosynthesis, animals must rely on other organisms as sources of food -- animals are heterotrophic.
Herbivores -- eat plants (cows) ; Carnivores -- eat other animals (e.g. lion); Omnivores -- eat both plants and animals (e.g. humans, bears).
Digestive System -- system for securing and processing food -- we will primarily limit our discussion to the human digestive system.
Generally, animals have some sort of digestive tract = gut.
1. incomplete digestive system -- typical of primitive animals--digestive system without specializeds -- part single opening (the mouth) serves as entrance for food and exit for undigested material -- food partially digested extracellularly in a gastrovascular cavity -- digestion completed intracellularly.
Figure 36.3a. Digestive system of a planarian.
2. complete digestive system -- found in higher animals (e.g. humans) -- separate openings for food intake (mouth) and excretion (anus) -- digestion done extracellularly in the intestine -- specialization of parts (e.g. crop, gizzard, stomach, small intestine etc.).
Figure 36.3b. Digestive system of a frog.
Mouth --
1. teeth -- 32 teeth in humans -- four types with different functions.
a. incisors -- chisel-shaped for biting.
b. canine-- pointed for tearing
c. premolars -- for grinding.
d. molars -- for crushing.
2. salivary glands -- moisten food and secretes salivary amylase, an enzyme that begins breaking starches into smaller disaccharide and monosaccharide units.
Figure 36.7. Human dentition.
Trachea -- tube (windpipe) to lungs -- covered by flap of skin called epiglottis -- prevents food from being aspirated (usually).
Esophagus -- muscular tube connecting mouth and the stomach -- food passes down esophagus due to rhythmical contractions of muscles called peristalsis
Stomach -- sac-like structure that stores partially digested food and secretes gastric juice -- gastric juice contains HCl and the digestive enzyme pepsin --
1. HCl -- pH of stomach about 2.0 -- kills much of bacteria that may be ingested along with food -- stops activity of salivary amylase, but promotes pepsin activity.
2. pepsin -- hydrolytic enzyme that breaks proteins down to peptides.
What leaves stomach is a thick, soupy material called chyme.
Small intestine -- tube leading away from stomach -- small intestine receives secretions from two accessory glands: the liver (bile) and the pancreas (pancreatic juice).
1. liver -- produces bile and sends it to gall bladder, which, secretes the bile into the small intestine -- bile important to digestion of fats.
2. pancreas -- secretes hormone insulin into blood and pancreatic juices into the s.i. -- pancreatic juices contains sodium bicarbonate -- makes chyme slightly basic -- also contains enzymes which are involved in digestion of carbohydrates (pancreatic amylase), protein (e.g. trypsin) and fats (lipase).
3. villi -- small, highly vascularized projections of cells lining the small intestine. -- secrete intestinal juices containing peptidases (peptides to amino acids) and maltase (maltose to glucose) -- sugars and amino acids absorbed (active transport) by villi and enter bloodstream where they are transported to the cells of the body.
Liver -- accessory organ with many functions -- some related to digestion -- functions:
1. detoxify blood -- remove and metabolize poisonous substances (e.g. alcohol).
2. destroy old RBC -- hemoglobin converted to bile -- bile stored in gall bladder and used in digestion of fats.
3. stores glucose as glycogen -- converts glycogen to glucose to keep blood sugar concentration in blood constant --
4. produces urea from amino groups and ammonia -- when glycogen runs out, glucose can be produced through the deaminization of amino acids -- amino groups converted to urea in liver.
Large Intestine -- final structure in digestive tract --two important functions:
1. water reabsorption -- water removed and put back into blood -- about 95% of water is reabsorbed-- failure to reabsorb water leads to diarrhea -- can cause dehydration and even death in small children.
2. ion regulation -- sodium reabsorbed and metallic ions excreted.
Digestive wastes leave body through the rectum -- wastes about 75% water and 25% solid material (e.g. 1/3 bacteria, "roughage", dead intestinal cells, fats).
Figure 36.6. The human digestive system.
Now, let's talk about nutrition -- cells require particular types of molecules in order to function properly -- these molecules come from food -- human nutritional requirements can be can be placed into 4 categories:
1. energy sources
2. organic precursors
3. vitamins
4. minerals
Energy Sources -- substances that can be oxidized to produce ATP.
a. carbohydrates -- quickest, most readily available source of energy -- sugars, starches, meat (glycogen).
b. fats.
c. proteins.
Organic Precursors -- food molecules that provide "building blocks".
a. fats -- constituents of cell membranes and hormones -- e.g. phospholipids.
b. protein (amino acids) -- amino acids needed by cells to manufacture own proteins for structure, enzymes, hormones, etc.
Recall that there are 20 different amino acids -- our bodies can make some of these from scratch, but 8 must be obtained from food -- these are the 8 essential amino acids.
Proteins from animals contain all 8 -- complete protein.
Proteins from plant sources often lack 1 or more essential a.a. -- incomplete protein.
Strict vegetarians must carefully plan what fruits and vegetables they are eating to insure that they are getting all 8 essential a.a.'s at the same time.
Kwashiorkor -- Swahili word for a nutritional disorder stemming from protein deficiency -- protruding stomach, stunted growth, listlessness, retardation -- may be fatal -- affects 50% of children in some developing countries -- can be prevented by ca. 1 oz. protein/day for a 50 lbs. child.
Vitamins -- required in small quantities -- e.g. a few billionths of body wt. per day -- these are organic molecules that are frequently components of coenzymes that are needed in metabolic pathways -- plants synthesize vitamins, but animals have largely lost the ability to make them --13 different vitamins needed by humans.
1. Niacin (B vitamin) -- component of NAD and NADP.
2. Riboflavin (B vitamin) -- component of FAD.
3. Vitamin D -- deficiency results in rickets -- prevents normal calcium absorption and deposition -- deformed extremities.
4. Vitamin C -- deficiency results in scurvy -- bleeding gums, poor healing, loss of teeth.
Most healthy people who eat a balanced diet get enough vitamins and vitamin supplements are not usually needed.
Megavitamin therapy -- should consult a doctor -- can be dangerous.
1. excessive vitamin A -- loss of hair, pains in joints, loss of appetite.
2. excessive vitamin C -- some evidence of toxicity and dependence.
Minerals -- usually ions (e.g. Na+, K+, Fe++) -- required in small amounts and are usually in abundant supply in food.
1. iron -- needed for cytochromes and hemoglobin -- deficiency results in anemia.
2. iodine -- component of thyroid hormone thyroxin -- deficiency can lead to goiter and mental retardation.
Another important nutritional disorder is malnutrition -- eating too little or too much of some things -- often self-inflicted and prevalent in affluent countries.
Obesity -- excess fat -- fat is required as a precursor and is also used for insulation -- excessive amounts are hazardous to health -- e.g. heart disease, diabetes, joint problems, atherosclerosis (fat in arteries) -- social stigmas -- 1 our of every 3 Americans is fatter than is healthy.
Americans spend ca. $30 billion per year on spas, tablets, books, etc. to lose weight -- check out magazine covers at the store!
Losing weight is big business -- as such one must be very careful about claims made by products and individuals -- let's look at a couple of examples.
Vegetarian diets -- already mentioned need to make sure all essential a.a.'s are being obtained -- a strict vegetarian diet w/o eggs or milk can be dangerous for children due to lack of B12 vitamin needed for RBC formation.
Zen Macrobiotic Diet -- extremely dangerous -- has nothing to do with religion Zen Buddhism -- recognizes 7 categories of food -- ultimate goal is to subsist entirely on the optimum food = brown rice -- rice is an incomplete protein -- has led to permanent kidney damage, scurvy, anemia.
Beware of diets that advocate eating unlimited amounts of one kind of food.
1. Zen Macro. -- nothing but brown rice.
2. Dr. Stillman's Quick Weight Loss Diet -- nothing but protein.
3. Dr. Atkins' Diet Revolution -- anything but carbohydrates.
Dr. Atkins' book talks about "fat mobilizing hormone" produced by human body which is secreted in absence of carbohydrates. to oxidize fat -- this is a hormone that is unknown to biochemists and the rest of the medical world.
Law suit filed by AMA against Dr. Atkins-- also the subject of investigation by U.S. Senate Committee on nutrition -- using fats predominantly as energy source can cause excess buildup of H+ ions (= acidosis) -- acidosis can lead to coma and even death -- elevated blood cholesterol and fat levels may also occur.
Take home message: diets are serious things and can be hazardous to your health -- competent medical supervision and advice should be sought if you are serious about losing weight.
Next time: Patterns of Reproduction.