Biol 115 Lecture 4:
Digestive System and Nutrition

T. Irving 01/07/97; revised A.Howard 01/27/00

Membranes & epithelial cells divide "inside" from "outside"

digestive tract is a term for the complete path from the mouth to the anus

can be considered in some sense to be outside the body

job reduce complex foodstuffs ---> small molecules

Amino acids, glucose, fatty acids

that can cross cell membranes

Liver and pancreas are exocrine glands that secrete digestive enzymes

enzymes are specific to pH and temperature

Proper nutrition is essential to supply energy

and essential AA’s, fatty acids, vitamins and minerals

Mastication of food. Limited digestion of starch

swallowing reflex

peristalsis

Stores food and grinds it up

very muscular walls

Alcohol is absorbed here but not other food substances

Mucus protects stomach from HCL

Ulcers - heliobacter pylori

gastric glands produce HCL + pepsin digest proteins

food becomes "chyme"

pyloric valve meters out chyme to small intestine

esophageal sphincter prevents backflow into esophagous

malfunctions -> heartburn

gastrin is a hormone (produced gastric glands) that is released into the bloodstream if protein is present in stomach

gastric glands release HCL and pepsin

6 m long

location where most digestion and absorption of nutrients occurs

pancreas and gall bladder are exocrine glands emptying into duodenum

Contain NaHCO₃ ( a weak base) to neutralize Hcl and create a basic environment

"brush border membranes"

microvilli increase surface area for absorption

Active transport is used for maximum absorption

lipids broken down to glycerol and fatty acids which cross epithlieal membraneand coverted to lipoprotein droplets which are then concentrated in the lacteals

Secretin stimulated when acidic chyme present duodenum

Increase Secretin levels in blood --> increased releaseby pancreas of NaHCO₃

Cholecystokinin (CKK)

stimulated presence of fat

elevated CCK levels blood -> increased production pancreatic juice and bile (gall bladder)

about 1.6 m long, 6.5 cm diameter vs 1.5 (small intestine)

absorbs water, salts. some vitamins

concentrates remaining material into feces

indigestible remains

bile pigments

bacteria thought predominantly e.coli but now known to be 99% facultative anaerobes

bacteria are symbiotic

breakdown some indigestible products

produce some vitamins and make some minerals available in forms that can be absorbed.

feces concentrated and stored temporailiy in rectum and expelled through anus

Most often caused by infection of lower tract

Usually "self limiting" since water is not absorbed by the large intestine and peristalsis expels the offending organisms

sooner ot later things are back to normal

Often caused by nervous stimulation

"irritable bowel syndrome"

Overly civilized people ignore desire to defecate

Feces become dry and hard

Fiber and water can help pervent this

If must take laxative "bulk laxatives"best

these supply large masses of cellulose mimic the effect of natural fiber

also good paradoxically for irritable bowel syndrome

other common laxatives work as lubricants, osmotic agents, or irritants.

Chronic constipation can lead to hemorrhoids.

Produces pancreatic juice

contains enzymes to digest, CHO, proteins, and fats

empties into duodenum

level controlled by CCK

Produces bile

contains bile pigments bilirubin and biliverder???

pigments derive from Hemoglobin, waste products from dead blood cells

Also contain bile salts

Derived from cholesterol

job is to emulsify fats i.e. make soluble

necessary forpancreatic

digestive enzymes to work

Hepatic Portal System

monitors incoming blood from intestine

glucose & glycogen ??

Gets rid of nasties in blood

process called detoxification

Produces urea from the breakdown of amino acids

amino acids --> glucose + amino groups

amino groups --> urea which is removed in kidneys

liver also produces plasma proteins from amino acids

liver interconvert glucose --> glycogen

glycogen -->glucose

when you eat, glucose rises, pancreas secretes insulin

liver produces mainly glycogen from glucose

Between meals glucose levels in blood lower, insulin production goes down

example of a negative feedback system

Food pyramid = ideal diet

carbohydrates on the bottom, fats on the top

A balanced diet contains all necessary nutirents in the right proportions

found in breads pastas, tubers, grains

Carbohydrates are needed to maintain blood glucose levels

Complex carbohydrates generally better than simple (e.g. sugar) because they are typically complexed with other nutrients including fiber - nondigestabale plant material

A certain amount of fiber important for correct working of the digestive system

foods rich in protein are meat fish poultry dry beans eggs, nuts, soy products

proteins are digested to amino acids which are normally used to build tissues and soluble proteins and not as an energy source

it is important to have a "complete" source of protein in diet

i.e. one that contains all 20 amino acids. Body is unable to synthesise 9 of these

Fats are abundant i butter, margarine, oils as well in red meat, fish and poultry

fatty acids absorbed in the small intestine are packed as lipoproteins and enter tha lacteals of the lymphatic system & hence can move through cirulatory system to all parts of body

linoleic acid is an "essential" fatty acid i.e. must be provided in diet

liver can synthesise others

fat has highest energy content

9 Kcal/gram vs 4 Kcal/gram for CHO

Nationally we eat too much fat, typical diet 40% of calories from fat recommended reduce to 30% or less.

High fat diets may increase risk of cancer

vitamins are generally small organic compounds that are "essential" for metabolism.

Many of these form portions of coenzymes eg. niacin forms part of NAD

Riboflavin part of FAD

Best way to get enough vitamins is to eat a wide variety of fruits and vegetables

while C & A may prevent cancer

"Megadoses" of vitamins are discouraged

vitamin C --> kidney stone, oxalic acid (a toxin)

Vitamin A in excess --> hair loss, bone & joint pains

vitamin D in excess --> excess Ca in blood, retard growth in children

Some"macro minerals" are required in relatively large amounts i.e greater than 100 mg/day others are "trace" elements i.e. less than 20 mg/day

Macrominerals include Na, P, K, Mg, Ca, Cl

Microminerals Zn, Fe, F, Cu, I

more are being discovered all the time eg.

Se, Cr, Ni, V, & even As

These are usually involved in the active sites of enzymes

"metalloproteins" and are needed in very small amounts

Tables 4.6 & 4.7 summarize roles of viamins and minerals required for adequate nutrition

Osteoporosis - weekens bones older people

drink lots of milk when you’re young!

post menopausal women largely too late.

Recommended daily allowance of Na 500mg.

average American eats 4,000 - 7,000mg

sources typically 1/3 natural, 1/3 added processing. 1/3 from the salt shaker

Major bad effect of salt is hypertension

doesn’t effect everybody but you won’t find out until its too late!

20% above ideal weight

caused by endocrinal, metabolic, and social factors

can coexist with obesity or anorexia nervosa

Most common in young women who gorge themsleves and then purge by inducing vomiting or using laxatives

Usually associated with depression

Not easy to treat

individuals are extremely thin but still "feel fat" and continue to diet

can cause menstruation to cease if body weight gets too low.

Eating disorders can have complex psychological causes. Effected individuals shouls seek medical attention since potentially dangerous.