Digestive System (4 step process) ? 

Indigestion, Digestion, Absorption, Elimination (egestion) 

1. Ingestion: placing food into the mouth 
2. Digestion (Mechanical and chemical breakdown) mastication + mix bolus with water, acids, bile and enzymes in the stomach and intestine to break down complex molecules into simple structures,
3. Absorption: of nutrients from the digestive system to the circulatory and lymphatic capillaries through osmosis, active transport, and diffusion, and
4. Egestion: Removal of undigested materials from the digestive tract through defecation.

the cephalic phase - saliva is produced in the mouth and digestive enzymesare produced in the stomach.

Mechanical and chemical digestion begin in the mouth where food is chewed, and mixed with saliva to begin enzymatic processing of starches. The stomach continues to break food down mechanically and chemically through churning and mixing with both acids and enzymes. 

Absorption occurs in the stomach and gastrointestinal tract, and the process finishes withdefecation.^[1]

Think about food: Cephalic 

Enzymes needed for Carbohydrate digestion?

Chemical Pathway of Carb digestion? 

need amylase and dissaccharidases

starch -> broken down to small units ->broken down to sugar outside-> into cell-> sugar absorb by cell -> blood supply

1. Salivary amylase (phyalin) (mouth) 
2. pancreatic amylase (pancreas)
   • product: usually maltose- glucose + glucose dimer   

disaccharidases (breaks the dimers)

1. maltase (brush border enzyme)  (disaccharidases)
2. sucrase (disaccharidases)
3. lactase (disaccharidases)

• made in : pancreas 
• used in: small intestine 
• breaks: starch -> maltose

• made in : salivary glands 
• used in: mouth 
• breaks: starch -> maltose
• more or less to protect our teeth. 

• made in: intestinal glands 
• used in: small intestine 
• breaks: disaccharide maltose -> 2 glucose 
• brush border enzyme

• made in: intestinal gland 
• used in: small intestine 
• breaks: lactose -> glucose and galactose

misc: as a child, the brush border will make lactase. This enzyme is no longer active, lactase, as a teen for some. Then you are lactose enzyme. You cannot absorb lactose so it goes thorugh.

 enzymes that break down disaccharides into monosaccharides.

example:

• Lactase (breaks down lactose into glucose and galactose)
• Maltase (breaks down maltose into 2 glucoses)

•  disaccharide = two units of glucose joined with an α(1→4) linkage. 
• Saliva contains salivary amylase
  • hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose.

Enzymes needed for protein digestion?

Chemical Pathway of Protein digestion? 

amino acids are broken down by proteases

protein --> hormone CCK --> enterokinase enzyme -> activate zymogens 

amino acids are broken down by proteases

 (separated bc don't want them to be digested 

1. pepsinogen ->  pepsin (stomach) (attack from middle)
2. trypsinogen ->  trypsin (pancrease)(attack from middle)
3. chymotrysinogen -> chymotrypsin(pancrease)(attack from middle)
4. procrboxypeptidase ->  carboxypeptidase (pancrease) (attack from end)
5. proamino peptidases >  aminopeptidase (pancreas) (attack from end)
6. dipeptidase (generic amino acid) (attack from middle)
7. enterokinase (small intestine) 

made in: gastric glands (chief cells) 

used in: stomach 

function: hydrolyze specific peptide bond (which subsequently will go into small intestine)

• made in: pancreas - carboxypeptidases are initially produced in an inactive form;
  • ex) the inactive zymogen form, (pro-carboxypeptidase A), is converted to its active form - carboxypeptidase A, by enterkinase 
  • why? mechanism ensures that the cells wherein pro-carboxypeptidase A is produced are not themselves digested.
• used in: small intestine 
• function: hydrolyze terminal peptide at carboxyl terminal
  •  involved in catabolism; they help to mature proteins or regulate biological processes

• made in: intestinal glands 
• used in: small intestine 
• function: hydrolyze peptide bond at amino terminal size
•  a zinc-dependent enzyme .

• made in: intestinal glands
• secreted by: enterocytes onto the brush border of the villi
• used in: small intestine 
• function: hydrolyze PAIRS of amino acids (dipeptides)

Enterocytes, or intestinal absorptive cells, are simple columnar epithelial cells found in the small intestines and colon. 

What's needed for Lipid digestion? 

Chemical Pathway for Lipid digestion? 

fat: 3 C backbone attached to three fatty acids

      food and building block 

mix fat with water and bile and lipases -> attack and cut out two fatty acids à monoglyeride and two fatty acids à get them in cellà reassembled to trigycerlide -> chylo micron -> lymph villi (vessel called lacteal)--> blood supply -> break down to monoglycerid and fatty acide --> adipose, liver or acetyl CoA (cellular respiration ) 

1) chyme --> duodenum secretes CCK into the blood --> stimulate secretion of pancreatic enzyme and bile

2) bile-->NOT enzyme, made in liver, emulsify fat in small intestine

3) lipase -->  IS enzyme, made in pancreas, hydrolyze lipids in small intestine

same concept as proteins. 

1. 5’ or 3’ exonucleases  (chew from opposite ends of RNA or DNA)
2. endonuclease (just break the million long polymer into smaller short polymers).

all enzymes in digestive systems are... 

water based.

(Problem: fats. Fats don’t want to mix with water. So need help from other secreion)

GI Tract

What is it?

Goal? 

• tube from mouth to anus 
• anything inside the tube is essentially outside the body 

1. Chewing: to give surface area for enzymes to work
2. Churning: stomach breaking down food
3. Segmentation: in small intestine, like kneading. 

1. begins in the stomach as a recently swallowed meal --> nutrient-rich broth known 
2. result of mixing and enzyme action
3. occasional backflow of acid chyme from the stomach into the lower esophagus causes heartburn.

1. major organ of digestion and absorption (5-6 hours). 

• enzymatic hydrolysis of food macromolecules
•  most of the absorption of nutrients into the blood

Exocrine and endocrine (hormone)

1. exocrine: produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate that buffers the acidity of the chyme from the stomach.
2. exocrine: accessory glands include the salivary glands, the pancreas, the liver, and the gallbladder.

The terminal portion of the colon is called the rectum, where feces are stored until they can be eliminated.

• Between the rectum and the anus are two sphincters, one involuntary and one voluntary.
• Once or more each day, strong contractions of the colon create an urge to defecate.

The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a bolus.

During swallowing, the tongue pushes a bolus back into the oral cavity and into the pharynx.

layers of stomach wall*

surface of stomach wall?* 

first three three layers are perendicular and oblique  

1. mucosa
2. submucosa
3. muscularis 
4. serosa (simple cubodial ephithelial) 

The oral cavity, pharynx, and esophagus initiate food processing.

The pharynx, also called the throat, is a junction that opens to both the esophagus and the trachea (windpipe).

The esophagus conducts food from the pharynx down to the stomach by peristalsis.

When we swallow, the top of the windpipe moves up so that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis.

When not swallowing, the esophageal sphincter muscles are contracted, the epiglottis is up, and the glottis is open, allowing airflow to the lungs.

When a food bolus reaches the pharynx, the larynx moves upward and the epiglottis tips over the glottis, closing off the trachea.

The oral cavity, pharynx, and esophagus initiate food processing.

After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested.

The esophagus conducts food from the pharynx down to the stomach by peristalsis.

• The muscles at the very top of the esophagus are striated and, therefore, under voluntary control.
• Involuntary waves of contraction by smooth muscles in the rest of the esophagus then take over.

• villi: fingerlike projections on large circular folds in the lining [of the small intestines] 
•  epithelial cell of a villus has many microscopic appendages called microvilli 
• basis of the term “brush border” for the intestinal epithelium.

proteases – enzymes that help break down protein. Body produce these in an inactive form: zymogen. “–OGEN” à notate inactive form. 

§  bicarbonate is brought by pancreas to increase the chyme and neutralize it and make the pH at 8. 

bulk of bulk of feces ...

heart pump blood to arteries à capillaries à veins à to another system organ/tissue and then à back to heart 

instead of normal circulation:

heart pump blood to arteries à capillaries à veinsà back to heart.

mucosa – a simple columnar ephitelial. These cells are specialized it the small intestine with microvilli. This covers from mouth to anus. This invaginates a lot of glands. Will secrete mucus 

muscularis* 

a region of muscle in many organs in the vertebrate body

most organs have  smooth system (2 layers) 

stomach 3 layers are perpendicular and oblique (need three for the mechanical griding- contract in three direction)

1) goblet cells 

• functon: produce mucus - basic that allows it to protect cells from the acidity.
• where: in the epithelial 

2) parietal cells

• function: make HCl

3) chief cells

• function: produce pepsinogen 
  • (pepsinogen is inactive until it reaches the lumen of the stomach).
  •  Activates when the pH is low but also activates itself when pH is low.

** everything above is used to go OUTSIDE the body - mucos, HCL, pepsongin

4) enteroendocrine cells: "within the body" hormones- used inside the body

• function: make hormones 
• function is in the CNS, not in the stomach ex) what tells you if you're hungry

impt hormone made in gastric pit by enteroendocrine

gastrin – stimulate gastric juice.

• protein enters, > lower pH >release gastrin -> release HCl 
• Gastrin stimulates itself- positive feed back system.
• When acidity gets too high, it shuts down.
• (happens during gastrin phase)

glarelin – important for growing –stimulate feeding.

obestatin – stop eating 

obestatin and glarelin are made by same protein- cut one protein into two. You can produce one without the other. You can use one without the other. 

Cephalic phase: sight, smell and etc (pavolian). Intestine gets ready, saliva and enzyme gets ready

gastric phase: change in acidity by the actual food itself

intestinal phase: continue the stimulatory cycle but also the regulator to slow everything down. 

Crypt/ intestinal gland: intestinal juice (mucous where produced)

A lot of mitosis taking place here.

Make the cells that will replace the dying ones 

Secretin 

CCK

Enterogaston 

Nueropeptide Y 

Peptide YY

Leptin

Impt Hormones in Small Intestine 

1) secretin 

signal: low pH 

product: bicarbonate

2) CCK 

signal: lipid and peptides

product: target pancrease of enzyme  and ask gall bladder for bile

3) enterogaston 

signal: lipid

product: slow down stomach

bilireubin* 

pathway to get rid of waste

ex) bruise / hemoglobin: when it starts to break down, pigement gets green/yellow. this color represents bilireubin. liver gets rid of this and puts it in the bile. If this builds up, you get jaunice.

• made in liver
• made up of phospholipids (cholesterol), neutral fats, billireubin
• act as an emulsfier to break fat up to small droplets 

• rarely a whole protein will be engulfed to an adult but can happen in an infant.
• breast milk is supposed to come through the epithetlium and enter the blood and work in the bloodstream (immune) 

• basket of protein that carry cholestrol.

• One has more protein than lilpid (HDL),
• HDL is a good form of cholesterol
• HDL cleans

• one has more lipid than protein (LDL- low density).
• LDL deposts

• no such thing of good or bad choletsterol. depends on how it's carried to the liver.