Digestive system

V. Digestion

A. Functional Overview

1. Nutritional needs

- All animals are heterotrophs: They must obtain energy and nourishment from molecules made by other organisms.
- Basic nutrients = carbohydrates, lipids, proteins, minerals, vitamins. (underlined can serve as energy sources).
- Food is used for two purposes: fuel (to provide energy for cellular metabolism) and as a source of building blocks (monomers for cellular biosynthesis)

2. Food processing

- ingestion, digestion, absorption, elimination. We will be concentrating on digestion, absorption.

3. Digestion = hydrolysis - breakdown of macromolecules by the addition of water to form monomers; enzymes are called hydrolases.

----- + H2O -->  -----OH + H-


Carbohydrates (polysaccarides)     monosaccharides

Proteins                           amino acids

Nucleic acids                      nucleotides 

                                   (rem. phosphate -> nucleosides)

Lipids                             glycerol & fatty acids

4. Organismic strategies

a. No digestive system: sponges: intracellular digestion
b. Incomplete digestive system: single chamber; extracell. digest. [hydra, jellyfish: one opening; planaria (flatworms): extensive branching of gut.]
c. Complete digestive system: sequential, with specialized regions; two openings: mouth, anus; circ. system for distribution. [earthworm, humans]

C. Human Digestive System (example of complete system)

1. Mechanical breakdown

- biting, chewing, grinding, "emulsification"
Increases surface area for enzymatic attack

2. Chemical breakdown

- takes place extracellularly in gut "lumen"
- takes place in on an assembly line (continual, sequential processing) in which food is moved through system & mixed by peristalsis

3. Pathway [Fig. 42.4]

- mouth --> pharynx --> esophagus --> stomach --> small intestine --> large intestine --> rectum --> anus

4. Five major secretory organs

- salivary glands, stomach, liver (+ gallbladder), pancreas, small intestine

- Salivary glands

- secretes approx. 1 liter saliva / day; salivary amylase begins breakdown of carbohydrates (CHOs)

- Stomach

- secretes gastric juice; parietal cells in gastric glands secrete HCl; chief cells in gastric glands secrete pepsinogen which is converted to pepsin at low pH.
- HCl activates pepsinogen and kills most foreign bacteria ingested with food.
- pepsin hydrolyses protein into short polypeptides.

- Liver

- scretes bile which is stored in gallbladder; detergent-like action (= emulsification) disperses fat into small droplets for enzymatic attack by lipases.
[Note: bile contains no digestive enzymes]

- Pancreas

- secretes sodium bicarbonate and a number of digestive enzymes (hydrolases)
- bicarbonate neutralizes acidity -- pancr. enzymes active at pH 7 - 8.
- pancreatic hydrolases: trypsin, chymotrypsin (digest polypeptides to dipeptides); lipase (digests fats); pancreatic amylase (breakdown of CHOs to disaccharides); ribonuclease, deoxyribonuclease (degrades RNA, DNA into free nucleotides).
[Note: separate tissues in pancreas secrete insulin, glucagon into blood]

- Small intestine

- secretes numerous enzymes which complete breakdown to monomers [example: lactase splits lactose (a disaccharide) into glucose and galactose (monosaccharides)]
- most chemical digestion takes place in the initial portion of the small intestine called the duodenum
- absorption of products takes place along the rest of the small intestine.

5. Absorption of monomers

- takes place across wall of small intestine
- Structure of small intestine provides large surface area
[Fig. 42.8]

- long, folded tube (approx. 6 meters)
- villi = finger-like projections of the gut epithelium into lumen
- microvilli = microscopic projections of cell membrane into lumen (= brush border)

- Mechanisms [Fig. 42.10, part 1]

- facilitated diffusion (selective transport proteins; high conc. --> low conc.; example: fructose)
- active transport (selective, requires energy (ATP) examples: glucose, amino acids)

- Uptake by circulatory systems

- intestinal villi contain blood capillaries, lymph vessels [Fig. 42.9]
- blood capillaries take up monomers (= glucose, amino acids, nucleosides) and short chain fatty acids --> transport in blood from intestines to liver (via hepatic portal system)
- lymph vessels (lacteals) take up triglycerides (chylomicrons) and later empties them into the blood

6. How fats are handled [Fig. 42.10, part 2]

- Conditions in the intestine do not yield complete hydrolysis of lipids. Products include monoglycerides and some diglycerides as well as glycerol and free fatty acids.
- Inside intestinal cells, triglycerides are resythesized from gycerol and fatty acids and packaged into chylomicrons which are then transported by lymph system and dumped into blood. Only short-chain fatty acids are directly absorbed from small intestine into blood.

7. Large intestine (colon)

- approx. 90% of water in chyme is absorbed in the small intestine; the remaining 10% is absorbed in the large intestine along with sodium
- bacteria in colon (E. coli) produce vitamin K and some B vitamins.
- approx. 1/3 of feces is bacteria; remainder is undigestable material -- particularly cellulose

D. Requirements for Balanced Diet

1. Calories -- Energy input = Energy output --> no weight gain. Energy input > Energy output --> weight gain.
2. "Essential" amino acids -- 8 out of 20 cannot be sythesized to meet body's need. [Most plant proteins are "incomplete" and lack one or more essential a.a.s - typically lysine, tryptophan, threonine. "Complete" proteins found in eggs, milk, meat, fish.]
3. "Essential" fatty acids -- some fatty acids cannot be synthesized by the body; linoleic acid is an example
4. Vitamins -- organic compounds required in small amounts; body cannot synthesize. Fat-soluble = A, D, E, K; can accumulate to toxic levels. Water-soluble = B, C vitamins.
5. Minerals -- Ca, Na, K, Mg, Cl, Sulfur, Phosphorus