Chapter 41Chapter 41Digestion and Human NutritionA Summary

AP BiologySpring 2011

The Nature of Digestive The Nature of Digestive Systems Systems A digestive system

mechanically and chemically reduces food particles and molecules small enough to be absorbed into the internal environment

Incomplete and Complete Incomplete and Complete SystemsSystemsIncomplete System: has one

opening ◦Food enters and waste leaves

through the same opening ◦Digestive produces are absorbed

directly to the needy tissues ◦Ex. Flatworm

Incomplete and Complete Incomplete and Complete SystemsSystemsComplete Systems: tube with

two openings, allowing food to move in one direction through the lumen

Incomplete and Complete Incomplete and Complete SystemsSystemsComplete system performs 5 tasks:

1. Mechanical processing and motility is the breaking up, mixing, and transporting of food material

2. Secretion is the release of needed enzymes and other substances into the lumen

3. Digestion is the chemical breakdown of food matter to molecules small enough to cross the gut lining

4. Absorption is the passage of digested nutrients into the blood or lymph

5. Elimination is the expulsion of undigested and unabsorbed residues at the end of the gut

Correlations with Feeding Correlations with Feeding Behavior Behavior The digestive system is an

internal space or tube with specialized regions for food intake, transport, processing, and storage

Correlations with Feeding Correlations with Feeding Behavior Behavior Regional specializations correlated

with feeding behavior Birds store meals in a stretchable

crop and grind the food in a gizzard Ruminants can eat grass almost

continuously and have multiple stomachs to digest cellulose

Carnivores have shorter intestines compared to ruminants, with expandable stomachs to accept a lot of food in a single sitting

Overview of Human Overview of Human Digestion SystemDigestion SystemThe human digestive system is a

tube with two openings and many specialized regions

Its overall extended length is 6.5 to 9 meters, comprising of mouth, pharynx, esophagus, stomach, small intestine, colon, rectum, and anus

Accessory glands include salivary glands, liver (with gallbladder), and pancreas

Overview of Human Overview of Human Digestion SystemDigestion SystemAll along tube, mucus coated

epithelium for protection and promotes diffusion

Mouth: food partially processed ◦Tongue- taste buds (chemoreceptor)◦Muscles help position food, swallow,

and speech

Overview of Human Overview of Human Digestion SystemDigestion SystemPharynx: tubular entrance to

esophagus and tracheaEpiglottis: keeps food from

entering tracheaEsophagus: muscular tube Sphincter between esophagus

and stomach (ring of smooth muscle)

Overview of Human Overview of Human Digestion SystemDigestion SystemStomach: breaks apart, mixes,

propels food onward Small intestine: breaks apart,

mixes, propels food onward; absorption; digestive enzymes and other secretions enter gut lumen and go to work

Overview of Human Overview of Human Digestion SystemDigestion SystemLarge intestine: (colon) water

and ions are absorbed which compacts the undigested residues

Rectum: wastes are briefly stored before being expelled through the anus

Anus: terminal opening

Overview of Human Overview of Human Digestion SystemDigestion System

Prepping Food in the Prepping Food in the MouthMouth32 teeth in

humans interact with saliva to mechanically and chemically prepare food in the mouth

Prepping Food in the Prepping Food in the MouthMouthTeeth are hardened jaw

appendages with an enamel coat of dentin and calcium with a pulpy matrix ◦Incisors bite off chunks, canines tear,

and premolars and molars grind foodSaliva (from salivary glands)

contains salivary amylase to begin carbohydrate digestion, bicarbonate to neutralize acids, and mucins to lubricate

Food Breakdown in the Food Breakdown in the StomachStomachStomach is a muscular sac that

stores and mixes food, secretes substances that dissolve and degrade food, and controls the rate at which food enters the small intestine

Food Breakdown in the Food Breakdown in the StomachStomachGastric fluid: includes

hydrochloric acid, pepsinogens, and mucus ◦HCl dissolves bits of food to form a

soupy chyme, also converts pepsinogen (inactive) to pepsin (active)

◦Pepsin begins digestion of proteins

Food Breakdown in the Food Breakdown in the StomachStomachMucus and bicarbonate ions protect the

stomach liningIf these are blocked, hydrogen ions

stimulate release of histamine, which

in turn stimulates release of more HCl, which may result in a peptic

ulcer

Food Breakdown in the Food Breakdown in the StomachStomachPeristaltic contractions churn

the chyme and keep the sphincter of the stomach’s exit closed

Small amounts are released at regular intervals into the small intestine

http://www.youtube.com/watch?v=hpS5kMn_B0I&NR=1

Digestion in the Small Digestion in the Small Intestine Intestine 3 regions of SI

◦Duodenum ◦Jejunum◦Ileum

Digestion in the Small Digestion in the Small Intestine Intestine Secretions from the liver,

gallbladder, and pancreas enter via a common duct ◦Bile (stored in gallbladder) is a secretion

consisting of bile salts, pigments, cholesterol, and lecithin

◦Bile salts speed up fat digestion and emulsification

◦Triglycerides tend to form large globules, but when smaller fat droplets become coated with bile salts, the negative charges on the droplets repel and cause them to stay separated

Digestion in the Small Digestion in the Small IntestineIntestine

Digestion in the Small Digestion in the Small Intestine Intestine Contractions known as segmentation mix the chyme and are responsible for coating the fat droplets with bile salts

Controls Over Digestion Controls Over Digestion Distention of the gut wall after a

meal stimulates mechanoreceptors and their neurons, which respond with muscle action and enzyme secretion

Controls Over Digestion Controls Over Digestion 4 gastrointestinal hormones play

roles1. Gastrin: produced by stomach lining,

stimulates secretion of acids into the stomach

2. Cholecytokinin (CCK): enhances the actions of secretion and stimulates gallbladder contractions

3. Secretin: from intestinal lining stimulates insulin secretions from pancreas

4. GIP (glucose insulinotropic peptide): released in the presence of glucose and fat, stimulates insulin secretion

Absorption From the Small Absorption From the Small Intestine Intestine

From structure to function◦Lining of small intestine, mucosa, is

not smooth but highly folded ◦Absorptive surface area increased by

fingerlike projections of intestine lining called villi Cells of villi bear even smaller microvilli

◦Different cell types secrete mucus, hormones, bicarbonate, or lysozyme

◦Brush border cells are covered with as many as 1,700 microvilli on their free surface in order to facilitate nutrient absorption

Absorption From the Small Absorption From the Small Intestine Intestine

Absorption From the Small Absorption From the Small Intestine Intestine Water and Solute Absorption

◦Absorption of water depends on solute concentration gradients across lining of small intestine

◦Cotransporters passively shuttle Na+ from lumen into epithelial cells, along with simple sugars and amino acids

Absorption From the Small Absorption From the Small Intestine Intestine

Water and Solute Absorption◦An Na+ concentration

gradient results, which sets up an osmotic gradient that attracts water from the lumen, into or between interstitial cells, and into blood capillaries in the villus

Absorption From the Small Absorption From the Small Intestine Intestine Water and Solute Absorption

◦Brush border cells have several kinds of sodium-dependent transporters For example, SGLUT-1 transports glucose

and sodium into cells together SGLUT-2 helps glucose, galactose, and

fructose diffuse out of brush border cells and into interstitial fluid

Absorption From the Small Absorption From the Small Intestine Intestine Fat Absorption

◦Fatty acids and monoglycerides diffuse across the bilayer of brush border cells aided by the bile salts

◦First products combine with bile salts to form micelles

Absorption From the Small Absorption From the Small Intestine Intestine Fat Absorption

◦Diffusion gradients favor movement of products out of micelles and into the epithelial cells of the mucosa

◦Triglycerides combine to form chylomicrons that leave the cell by exocytosis to enter the lymph vessels mainly

Fat Absorption in SIFat Absorption in SI

Large IntestineLarge IntestineLarge intestine: stores and

concentrates feces (undigested and unabsorbed material, water, and bacteria)◦Begins at cup shaped pouch at its

junction with the small intestine (appendix attached here)

◦Draped across lower abdomen, ends in a rectum (feces storage) that opens to the outside through the anus

Large IntestineLarge Intestine

Colon FunctionColon FunctionSodium actively transported out of

colon Sodium concentration drops,

water concentration increases, sets up a gradient that results in water moving out by osmosis

Fiber (bulk) in diet is important in moving material in feces through the large intestine at the proper speed

Colon Malfunction Colon Malfunction Several factors, including stress

and a low-bulk diet, can delay defecation, resulting in constipation

Fecal material lodged in appendix can lead to complications of appendicitis

Colon Malfunction Colon Malfunction Some people are genetically

predisposed to develop colon polyps, which start as benign growths, but may later become cancerous ◦Colon cancer is highly curable when

detected early enough◦Diagnosis can come from a colonoscopy

and a virtual colonoscopy, which uses X-rays and a computer to generate an image of the colon

What Happens to Absorbed What Happens to Absorbed Organic Compounds Organic Compounds Nutrient molecules are shuffled

and reshuffled once they have been absorbed ◦All cells continually recycle some

carbohydrates, lipids, and proteins by breaking them apart

◦Cells use the products as energy sources and building blocks

What Happens to Absorbed What Happens to Absorbed Organic Compounds Organic Compounds When you eat, excess

carbohydrates and other organic compounds are converted to fat for storage in adipose tissue or converted to glycogen in the liver and muscle tissue

What Happens to Absorbed What Happens to Absorbed Organic Compounds Organic Compounds Between meals, glucose levels

are maintained by breakdown of glycogen reserves in the liver and amino acids are converted to glucose

Adipose cells degrade fats to glycerol and fatty acids, all which enter blood stream

What Happens to Absorbed What Happens to Absorbed Organic Compounds Organic Compounds Liver stores, converts, and

maintains the concentrations of required organic compounds in the blood◦Inactivates most hormones, sending

them to kidneys for excretion and it removes worn-out RBC and detoxifies chemicals

Human Nutritional Human Nutritional Requirements Requirements

USDA Dietary Recommendations◦Nutritional guidelines have

replaced “food pyramid” ◦Diet recommends lowering

intake of refined grains, saturated fats, trans-fatty acids, added sugars or caloric sweetners, and no more than one teaspoon of full salt daily

Human Nutritional Human Nutritional Requirements Requirements

USDA Dietary RecommendationsMore fruits and

vegetables with high potassium and fiber content are encouraged with fat-free or low-fat milk products and whole grains; 55% of daily caloric intake is to come from carbohydrates

Human Nutritional Human Nutritional Requirements Requirements Carbohydrate-Insulin Connection

◦Refined sugars and starches have high-glycemic index

◦Cause a surge in insulin for fast uptake of ingested sugar, which later leads to increased hunger

◦Excessive intake of high fructose corn syrup is concern because fructose does not stimulate same release of leptin from adipose cells as glucose does, leading to decrease in stomach’s ghrelin output Calories are the same, hunger remains

GhrelinGhrelinGhrelin is a hormone produced mainly by

P/D1 cells lining the fundus of the human stomach and epsilon cells of the pancreas that stimulates hunger. Ghrelin levels increase before meals and decrease after meals. It is considered the counterpart of the hormone leptin, produced by adipose tissue, which induces satiation when present at higher levels. In some bariatric procedures, the level of ghrelin is reduced in patients, thus causing satiation before it would normally occur.

Human Nutritional Human Nutritional Requirements Requirements Good fat, bad fat

◦Phospholipids and cholesterol are important components of membranes

◦Fats are energy reserves and provide insulating cushioning

◦Body needs very little polyunsaturated fat to supply essential fatty acids, those not made by the body itself

Human Nutritional Human Nutritional Requirements Requirements

Body building proteins◦Of the 20 amino acids in

proteins, 8 are essential◦Most proteins in animal

tissues are complete; their amino acid ratios match human nutritional needs

◦Nearly all plant proteins are incomplete, they lack one or more amino acids that are essential for humans

Human Nutritional Human Nutritional Requirements Requirements Alternative Diets

◦Mediterranean diet emphasizes grains, fruits, and vegetables and limits fat to olive oil

◦Low-carb diets, which do not result in weight loss but may have undesirable side effects such as ketone production Low-carb dieters are advised to select

lean cuts of meats and fish over fattier, high protein foods

Vitamins and MineralsVitamins and MineralsHumans need small amounts of

at least 13 organic molecules, called vitamins, to assist in cellular metabolism

Inorganic substances, called minerals (Ca, Mg, K, and Fe for ex.) are also needed

Vitamins and MineralsVitamins and MineralsBalanced diet will normally meet all

requirements for these substances for people who are in good health

Excessive intake is at the least wasteful and at worst harmful

Some studies show supplementing vitamins A,C and E may counter effects of aging, protecting immune system by inactivating free radicals

Weight Questions, Weight Questions, Tantalizing AnswersTantalizing AnswersBody mass index (BMI): ratio of

weight and height, is a good way to measure ideal or overweight conditions

Score of 25 to 29.9 are considered overweight, score of 30 or more indicates obesity

Weight Questions, Weight Questions, Tantalizing AnswersTantalizing AnswersObesity is an overabundance of

fat in adipose tissue that may lead to health problems

To achieve and maintain acceptable weight, calorie intake and utilization must be balanced, and exercise

Weight Questions, Weight Questions, Tantalizing AnswersTantalizing AnswersPortions at restaurants are

becoming more and more super sized

Many correlate bigger portions with problems of obesity in America ◦Daily caloric increase is filled more

often with fast food ◦An extra 10 kilocalories per day

translates into a gain of one pound per year

Weight Questions, Weight Questions, Tantalizing AnswersTantalizing AnswersGenes may have an influence on

whether or not we gain weightStudy of 12 twins found that genetic

differences affected the response each set had to overfeeding or to losing weight

Glandular cells in lining of stomach and SI release an appetite-affecting hormone (PYY3-36) after a meal where it acts in the brain to suppress appetite

ReviewReviewhttp://www.youtube.com/watch?v

=hpS5kMn_B0I&NR=1