Digestive System - THSS Biology 12 · Digestive System 5/30/06 Human Biology 1 Digestive System A. Def’n - Digestion is the rendering of parts of organisms into small molecules,

Digestive System 5/30/06

Human Biology 1 Digestive System

A. Def’n - Digestion is the rendering of parts of organisms into

small molecules, which can be metabolized by:

Physical Digestion - Physical, or mechanical, digestion involves

physically breaking the food into smaller pieces without chemically changing it into different molecules.

- Physical digestion helps to facilitate the process of

chemical digestion because the food is physically broken down into smaller pieces, more surface area is thus exposed for the enzymes to carry out the process of chemical digestion. With more surface area exposed, chemical digestion is much more efficient. That is, more food can be broken down chemically in a shorter time.

e.g., - Mouth – teeth - Stomach – HCl - Small intestine – Bile (emulsification)

Chemical Digestion

- Chemical digestion is the breaking down of the large molecules, such as proteins, starch and fats, into smaller soluble molecules for easy absorption by the body.

i.e., via hydrolysis assisted by enzyme activity

Lipid + water –-Lipase (enzyme) glycerol + fatty acids Starch (amylose) + water –-Amylase maltose

• proteins are broken up into short polypeptides by acids and then cleaved into individual amino acids by specific enzymes

• complex carbohydrates are broken down by

enzymes ultimately into monosaccharides • lipids are enzymatically reduced to glycerol and fatty

acids

Notes

95% of ingested food is made available for the body’s use. Basal Metabolic Rate (BMR): female = 1300-1500 kcal/day male = 1600-1800 kcal/day

Review the Key Enzymes Table (p12), for a more detailed list of enzymes and products.



- humans lack the essential organic substances required to synthesize many necessary compounds and must obtain these organic substances, e.g., certain vitamins, essential a.a., essential fatty acids, from their diet.

- a number of trace elements must also be present B. Pathways: Into the breach

1. Mouth

a. Teeth

incisors (rip & shred), Carnivore canines (hold & tear), premolars (grind) Herbivore molars (grind)

= Omnivore - the number of incisors, canines, premolars, and molars in each quadrant of the mouth

of a normal adult is 2-1-2-3 b. Saliva

- secreted in the mouth to moisten food, aiding its journey into the digestive system — lubricates

- Salivary gland secretes amylase, an enzyme that

converts starches into disaccharides

Essential Amino acids (human), must be eaten:

1. tryptophan, 2. methionine, 3. valine, 4. threonine, 5. phenyalanine, 6. leucine, 7. isoleucine 8. lycine

Essential fatty acids: Animals are able to fabricate these essential fatty acids (but we cannot).

1. Linoleic acid (an omega-6) 2. alpha-linolenic acid (an omega-3)

Essential vitamins: The fat-soluble (A,D,E,and K) and the water-soluble (C, B's, niacin, biotin, folic acid).



- Mucus is a "slimy" material that coats many

epithelial surfaces and is secreted into fluids such as saliva. It is composed chiefly of mucins and inorganic salts suspended in water. - Mucus adheres to many epithelial surfaces,

where it serves as a diffusion barrier against contact with noxious substances (e.g. gastric acid, smoke) and as a lubricant to minimize shear stresses; such mucus coatings are particularly prominent on the epithelia of the respiratory, gastrointestinal and genital tracts.

- Mucus is also an abundant and important component of saliva, giving it virtually unparalleled lubricating properties - Mucins are glycoproteins

- Tooth health

- Antibacterial - Wash the tooth surface, to clear bacteria - Buffer - Control demineralization and mineralization

Bolus (food ball) - Mucins help hold chewed food together

2. Swallowing (Deglutition)

Source: http://www.dysphagiaonline.com/en/1patient/01_What_is_dysphagia.jsp

Uvula - A fleshy, pendulous portion of the soft palate that blocks the nasopharynx during swallowing.

Asides: (just for fun) Anosmia – inability to smell Ageusia – inability to taste



- Once past the mouth:

2. Pressure from the bolus on the pharynx stimulates

a swallowing response. - muscular contractions in the pharynx.

2. The palate seals off the nasal cavity, preventing food from entering it.

- The soft palate closes off the nasopharynx. The vocal cords in the larynx are moved up and towards the front of the throat thus closing it off to the passage of food.

- This is extremely important in preventing food from entering the airway.

3. This response stops breathing and seals off the

trachea. - Another effect of the process is to widen the

opening of the oesophagus thus making the passage of the bolus along the alimentary canal easier.

- Talking and eating at the same time can lead to a Café

Coronary – choking.

Source: http://greenfield.fortunecity.com/rattler/46/upali4.htm

Taste Buds on the tongue detect sweet, salt, sour, bitter and umami (essentially the taste of protein rich food, and MSG) — and possibly fat (or more precisely the texture or feeling of fat) The Olfactory bulb (one of two clusters of olfactory neurons at the base of the brain one on the left and one on the right) in the nose would seem to detect all other flavours… Chemesthesis - sensations that arise when chemical compounds activate receptor mechanisms for other senses, usually those involved in pain, touch, and thermal perception in the eye, nose, mouth and throat. The burn from chili pepper, the cooling from the menthol in mouthwash, and the stinging of carbonation. Gustation – the sense of taste, all of the above



3. Esophagus (Oesphagus) - Peristalsis

- rhythmic contractions of the esophagus (also stomach, small intestine, and large intestine) - moves the food (bolus) forward - increases SA, by stirring, folding, mixing and

agitating C. At the Stomach - 1/2L empty, stretchable to ~2L - the upper epithelial surface of the stomach is dotted

with deep depressions called gastric pits, the source of the stomach’s “digestive juices”

- the gastric pits contain two types of secreting cells:

1. Parietal cells - secretes concentrated HCl - ~ 2L a day

2. Chief cells - which secrete pepsinogen

- within the stomach HCl converts pepsinogen into the enzyme pepsin

- HCl breaks up connective tissue and proteins into

molecular fragments, - which are further digested by pepsin into short

polypeptides.

Protein –-pepsin short strands of a.a. (≠ a.a.)

- Carbohydrates and fats are not “chemically” digested, only proteins are “chemically” digested within the stomach

- Mucus

- epithelial cells (goblets cells in the gastric pits) in the stomach also secrete mucus - which lubricates the stomach wall and

facilitates the passage of food, - protects stomach wall from abrasion by

food, - and protects the walls from autodigestion by

the gastric juices

- failure to do this may result in ulcers - cause: bacterial infection

…still the cells lining the stomach are replaced every 2-3 days.

Source: http://www.sirinet.net/~jgjohnso/modperistalsis.jpg



(heliobacterium – Heliobacter pylori), possibly acerbated by stress and diet

- treatment: antibiotic regime N.B. The Parasympathetic nervous system regulates

“normal” or “at rest” body activity slowing down the heart beat and respiration, but also stimulates digestive activity when food is present. Similarly when body activity increases during periods of stress the Symapthetic nervous system, decreases digestive activity (an increases heart rate, respiration…) to ensure sufficient blood for muscles, heart etc.

Chyme (as food exits stomach = a pasty material) D. The Small Intestines

- food passes from the stomach to the small intestines: the duodenum, the jejunum, and the ileum.

- where pH is neutralized, by the presence of bicarbonate secreted by the pancreas

- and a variety of enzymes, many synthesized in the pancreas and small intestine, act to complete digestion.

- Most digestion occurs in the duodenum.

- The duodenum received its name from being about equal in length to the breadth of twelve fingers (25cm.). It is the shortest, the widest, and the most fixed part of the small intestine. Its course presents a remarkable curve, somewhat of the shape of an imperfect circle or letter “C”, so that its termination is not far removed from its starting-point.

- Liver produces bile salts, which are stored in

the gall bladder, which are then directed to the small intestines to emulsify fats - to increase the lipids SA for the enzyme

lipase to act upon

Hemochromatosis: • a common genetic disorder • inability to excrete excess

iron….



Bile is a complex fluid containing water, electrolytes and a battery of organic molecules including bile acids, cholesterol, phospholipids and bilirubin that flows through the biliary tract into the small intestine.

There are two fundamentally important functions of bile in all species:

• Bile contains bile acids, which are critical for digestion and absorption of fats and fat-soluble vitamins in the small intestine.�

• Many waste products are eliminated from the

body by secretion into bile and elimination in feces. • Bile acids are lipid carriers and are able to solubilize

many lipids by forming micelles - aggregates of lipids such as fatty acids, cholesterol and monoglycerides - that remain suspended in water.

• Hepatic synthesis of bile acids accounts for the

majority of cholesterol breakdown in the body. In humans, roughly 500 mg of cholesterol are converted to bile acids and eliminated in bile every day.

The gall bladder stores and concentrates bile during the fasting state. Typically, bile is concentrated five-fold in the gall bladder by absorption of water and small electrolytes - virtually all of the the organic molecules are retained.

Secretion into bile is a major route for eliminating cholesterol. Free cholesterol is virtually insoluble in aqueous solutions, but in bile, it is made soluble by bile acids and lipids like lethicin. Gallstones, most of which are composed predominantly of cholesterol, result from

Adult humans produce 400 to 800 ml of bile daily. Bile: - Emulsification of lipids

o Increased SA for lipase activity

o “physical” digestion - Transport of lipids in an

aqueous environment - Critical for transport and

absorption of the fat-soluble vitamins

- Cholesterol Homeostasis



processes that allow cholesterol to precipitate from solution in bile.

Source: http://arbl.cvmbs.colostate.edu/hbooks/pathphys/digestion/liver/bile.htm - Most absorption of nutrients and water occur in

the Jejunum and Ileum - The remainder of the small intestine (~ 6 m)

- ~3m while alive, and muscles contracted - The wider Jejunum occupies the upper two-

fifths and the Ileum the lower three-fifths. - There is no morphological line of distinction

between the two, and the division is arbitrary; but at the same time the character of the intestine gradually undergoes a change, so that a portion of the bowel taken from these two situations would present characteristic and marked differences.

a. Absorption of Nutrients

- The products of digestion are absorbed across the heavily folded walls of the small intestine,

- which possess numerous villi,

- ~ 30 per mm2, ~ 1 mm long

- the membranes of the epithelial cells have microscopic projections, called microvilli

- produce a surface layer called the brush border - and so achieves a very great absorptive surface

area.

Surface area of the small intestine is ~ 250-300m2 (about the size of a tennis court 260m2). Source: Campbell & Reece, Biology, 6e

Question: The average length of the small intestine, when fully extended after death, is ~6m (to 7m), with a diameter of ~2.5cm. Because of folds, villi, and microvilli, the surface area is 25,000cm2. What percentage of increase is attributed to the folding, villi, and microvilli in the small intestine? Answer: The small intestine is essentially a long cylinder and to calculate the surface area of a cylinder you multiply the length by the circumference, which in this case is: 600 cm x.2.5π = 4,710cm2 Now, to calculate the percentage of increase attributed to the folding, villi, and microvilli subtract the calculated surface area from the total surface area and divide by the calculated surface area: 25,000 – 4,710 / 4,710= 4.3x larger (~5x or ~80% more surface area) Aside: The nature of the chyme also increases the surface area for absorption even further.



1. Amino acids and sugars are transported by specific transmembrane channels directly to the blood stream, via the hepatic portal vein into the liver - toxic nitrogenous wastes (ammonia) left over from

the breakdown of amino acids are converted to (the less toxic) urea and released in the blood stream for the kidneys to remove. (excretion)

- Excess amino acids are converted to urea

(excreted in urine); excess glucose formed from this process (deamnification) is converted to glycogen.

2. Simple fats, which are lipid soluble, pass readily

across the membranes of the villi into the lacteals, lymph vessel draining the villi.

- eventually passing through the lymph system to drain into the subclavian vein superior vena cava heart

3. Glucose and other metabolic products of digestion

do not enter the general circulation directly, but instead flow to the liver, via the hepatic portal vein - the liver removes and stores any excess metabolic

products and maintains blood glucose levels within narrow bounds. (homeostasis)

Liver: • bile production

o excretion of bilirubin, cholesterol, hormones, and drugs

• fat, carbohydrate storage • regulation blood glucose

o glucose to glycogen (glycogenesis)

• synthesis blood (plasma) proteins o prothrombin, fibrinogen,

albumin, hemoglobin (globulins)

• storage iron, vitamins • conversion of ammonia >> urea • detoxification of blood The liver is among the few internal human organs capable of natural regeneration of lost tissue; as little as 25% of remaining liver can regenerate into a whole liver again.



- excess glucose is converted to glycogen by the

liver. - excess glycogen is stored as fat.

- associated hormone: insulin

- lowers blood glucose levels by causing the muscles and liver to take up and store glucose as glycogen

E. The large Intestines: Colon (Bowels) - extends (~1.5m) from the end of the ileum to the anus. - the large intestines have little digestive or (nutrient)

absorptive activity; - it functions principally to compact the

undigested wastes that is left over from digestion for easier elimination/defecation

- bacterium E. coli aid in this process by breaking

down & softening further undigested material - E. coli also synthesizes vitamin K, needed by

the liver to synthesize prothrombin, a necessary enzyme of blood clotting

- colon also absorbs water - “Principal function” on exam

- it receives approximately 10 liters of water per day. 1.5 liters is from food and 8.5 liters is from secretions into the gut. 95% of this water is reabsorbed

- if water is not absorbed, diarrhea can result, causing dehydration and ion loss.

- excretes iron and calcium salts - absorbs sodium and other ions

- the last 20 cm is the rectum.

- Feces is composed of approximately 75% water and 25% solids. - One-third of the solids is intestinal bacteria,

2/3’s is undigested materials

- The cecum is a pouch at the junction of the small

Good Bacteria & the Colon: • There are10x more bacteria in the

gut, than cells in your body • Up to 60% of feces is bacteria • More lymph tissue than anywhere

else in the body • Good bacteria keep bad bacteria

from getting a foot hold • Digest mucus, synthesis of

vitamins, e.g, K • Lactobacillus aids in lactose

digestion, as does yogurt’s acidophilus

• Probiotic – hype? • But they do like fibre…



intestine and large intestine. - In herbivorous mammals, it is large and

houses bacteria capable of digesting cellulose.

- In human ancestors, the cecum was larger but has been reduced by evolutionary change to form the appendix.

- Appendicitis is an infection.

- The appendix may swell and burst, leading to peritonitis (infection of the abdominal lining).

- Hypothesized functions for the appendix

include lymphatic, exocrine, endocrine, and neuromuscular. - while it is rich in infection-fighting

lymphoid cells, suggesting that it might play a role in the immune system.

- it appears to lack significant function - and exist primarily as a vestigial

remnant of the larger cellulose-digesting cecum found in our herbivorous ancestors.

Source: http://en.wikipedia.org/wiki/Vermiform_appendix http://www.talkorigins.org/faqs/vestiges/appendix.html http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20102/Bio%20102%20lectures/Digestive%20System/digestive%20system.htm How Fibre works:

- draws water into colon - swells - and initiates peristaltic contractions to remove watery wastes

- daily fibre requirement (?) – no definitive data, but a range between 25 - 30 grams/day is most recommended - How to tell? Are you regular, defecating 1-2

times in a 24h period. Key Hormones of Digestion (abridged – key function) HORMONE SOURCE STIMULUS ACTION Gastrin Pyloric section of

stomach Entry of food (bolus) into the stomach

Regulates secretion of HCl from parietal cells

Other Hormones: Grehlin – makes people want to eat Leptin – suppresses hunger (a lack of sleep lowers leptin levels, and increases hunger)



Cholecystokinin (CCK) cholecysto = gallbladder kinin = movement

Duodenum Arrival of food (lipid-rich chyme) in the small intestines

Stimulates gall bladder contraction, to release bile salts (to emulsify fats – physical digestion) to intestines. Stimulates secretion of digestive enzymes from pancreas

Secretin* Duodenum HCl entering the duodenum

Stimulates pancreas to secrete Sodium bicarbonate (NaHCO3), to neutralize stomach acids (Buffer)

Insulin Pancreas High blood glucose levels

Stimulates the conversion of glucose into glycogen, stored in the liver and muscle

Glucagon Pancreas Low blood glucose levels

Stimulates the conversion of glycogen into glucose

* Secretin was the first hormone to be identified, back in 1902. Key Digestive Enzyme (abridged) LOCATION

ENZYME

SUBSTRATE

DIGESTION PRODUCT

Salivary glands Amylase starch, glycogen maltose (disaccharides) Stomach Pepsin proteins short peptides

Peptidases short peptides amino acids Nucleases

(Nucleosidases) nucleoside pentose (5-carbon and a nitrogenous base (purines, pyrimidines)

(Nucleotidase) nucleotides nucleosides and phosphates(H3PO4)

(Disaccharidase) disaccharides monosaccharides Maltase Maltose glucose Sucrase Sucrose glucose and fructose

Small Intestines

Lactase Lactose glucose and galactose



Lipase triglycerides fatty acids, glycerol Trypsin, (Chymotrypsin)

proteins peptides

Nucleases (DNAse, RNAse)

DNA, RNA

nucleotides

Pancreas All fcn in the small intestine.

Amylase starch, glycogen maltose (disaccharides) Nucleases: RNA and DNA are hydrolyzed into nucleotides by ribonuclease and deoxyribonuclease, respectively. Phosphatases remove the phosphate group from a nucleotide, and nucleosidase decomposes the remnant of the nucleotide (a nucleoside, a sugar and a base) into a pentose (5-carbon sugar) and a nitrogenous base. Proteases: Pepsin and Trypsin.

- The pancreas secretes protein-digesting enzymes in an inactive form, e.g., trypsinogen, which is activated by enterokinase / enteropeptidase (from the small intestine) and converts trypsinogen into trypsin

Sketch Diagram of the digestive system

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Digestive System - THSS Biology 12 · Digestive System 5/30/06 Human Biology 1 Digestive System A. Def’n - Digestion is the rendering of parts of organisms into small molecules,