Feeding & Digestion

Question 1

Human Digestive System

Function

Answer 1

Digestive system in human composed of gastrointestinal tract (GIT) and accessory organs
Function:
• Breakdown food into smaller molecules that can cross plasma membranes
• Storage food
• Absorption of food molecules (nutrients)
• Excretes waste (feces, defecation)

GI tract (alimentary canal): Tube from mouth to anus --Mouth, Pharynx, Esophagus, Stomach, Small & Large intestine 
Accessory Digestive Organs (mechanical/chemical digestion): Teeth, Tongue, Salivary glands, Liver, Gallbladder, Pancreas

Question 2

Layers of GI tract (4 layers)

Answer 2

From the deepest to the most superficial: Mucosa: innermost layer of GI wall that is surrounding lumen

• responsible for protection, secretion and absorption
• consists of epithelium, lamina propria (connective tissue), muscularis mucosae (thin layer of smooth muscle)

Submucosa: areolar connective tissue – binds mucosa to muscularis

Muscularis: skeletal muscle (voluntary swallowing and control of defecation) and smooth muscle (break down food and mix it with digestive secretions)

Serosa: serous membrane of areolar connective tissue and simple squamous epithelium

Question 3

Enteric nervous system

Answer 3

About 100 million neurons form 2 nerve plexus throughout GI tract controls:
o GI tract motility
o secretion of secretory cells of mucosal epithelium

Sympathetic nerves decrease GI secretion and motility
Sympathetic: Inhibits saliva, gut motility and causes vasoconstriction (several splanchnic nerves)

Parasympathetic nerves increase GI secretion and motility
Parasympathetic: Stim saliva, gut motility and causes vasodilation (mostly via vagus nerve)

Question 4

Hormonal regulation of gut physiology

Answer 4

Secretin: hormone secreted by duodenum that serves to regulate its acidity
Cholecystokinin (CCK): secreted by small intestine in response to fatty acids
- Stim gallbladder to release stored bile into intestine
Gastric inhibitory polypeptide (GIP): produced by intestine, inhibits GI motility and secretion of acid, induces insulin secretion
Gastrin: produced by stomach, stim secretion of HCl & gastric juice and aids in gastric motility

Question 5

Digestive Enzymes

Answer 5

Digestion is carried out by hydrolytic enzymes in water
Each enzyme is specific in the type of chemical bond it can break –species can digest only those ingested molecules for which it has enzymes that hydrolyze specific type of bond in the molecules

EX: chitinase (required to digest chitin (polysaccharide) to disaccharide subunits) – rodents/bats synthesize chitinase, but humans/rabbits do not

Nutritional value of food depends on digestive enzymes an animal possesses & chemical composition of the food

Question 6

Mouth

Answer 6

Where food is taken in and where digestion begins

Oral cavity: chamber contains teeth, tongue and receives secretions from salivary glands
Teeth break food into smaller bits

Question 7

Swallowing Reflex

Answer 7

Swallowing prevents entrance of food or other materials into lower respiratory tract
High degree of coordination b/w respiration and swallowing is required to maintain adequate ventilation
Swallowing movements are produced by central pattern generator (rhythmic swallowing pattern) located in medulla oblongata

1) Pharyngoesophageal sphincter muscle contracts, closing esophagus, epiglottis is up and glottis is open to let air into lungs
2) Swallowing reflex begins when bolus reaches pharynx
3) Elevation of soft palate prevents food bolus from entering nasal passages
4) Pressure of tongue seals back of mouth and prevents bolus from backing up
5) Larynx move upward, pushing glottis against epiglottis to prevent bolus from entering airway
6) Pharyngoesophageal sphincter muscle relaxes, permitting bolus entering esophagus

Question 8

Peristalsis

Answer 8

Involuntary constriction and relaxation of GI tract muscles
Coordinated contractions produce peristaltic waves that move digestive contents from mouth to anus

1) Circular layer of muscularis contracts in wave, constricting gut and pushing digestive contents onward
2) Longitudinal layers contracts, shortening and expanding gut and making space for contents to advance
3) Partially processed food (chyme) enters small intestine

Question 9

Pharynx & Esophagus

Answer 9

Pharynx:
Skeletal muscles
Lined by mucous membrane
Drive food into the esophagus

Esophagus:
Muscular tube lies posterior to trachea
Secretes mucus and transports food through cardiac sphincter

Question 10

Stomach

Answer 10

Thick-walled, J-shaped organ that lies on the left side of the body
Food mixed w stomach acid and enzymes to break it down into smaller pieces (combo of food and stomach juices=chyme)
Stomach serves as a reservoir of food before release into small intestine

Question 11

***Stomach secretes…

Answer 11

Gastric juice: contains HCl, which causes stomach to have a high acidity with pH of ~2, and kills most bacteria present in food
Pepsin: begins digestion of proteins
Intrinsic factor: aids absorption of vitamin B12
Gastric lipase: aids digestion of triglycerides
Gastrin & histamine hormone: stim gastric acid release
Somatostatin hormone (by stomach and intestine): reduce gastric secretion, gastrointestinal motility

Question 12

***Origin of Stomach enzymes

Answer 12

Mucous neck cell: secretes mucus (protects lining), bicarbonate
Parietal cells: secretes gastric acid, intrinsic factor (Ca2+ absorption)
Enterochromaffin-like cells: secretes histamine (stimulates acid)
Chief cells: secretes pepsin(ogen), gastric lipase
D cells: secretes somatostatin (inhibits acid)
G cells: secretes gastrin (stimulates acid)

Question 13

Ulcers

Answer 13

Although HCl does not digest food, it does break down connective tissue of meat and activate pepsin
Mucus protects wall from effects of HCl
If HCl penetrates mucus, wall begins to break down and ulcer forms

Most ulcers due to:
• bacterial infection - impairs ability of epithelial cells in wall to produce protective mucus
• long-term use of nonsteroidal anti-inflammatory drugs such as ibuprofen (Advil, Aspirin)

Stress and spicy foods do NOT cause peptic ulcers

Question 14

Small Intestine

Answer 14

Has small diameter (2.5 cm), 6 meter long, slightly basic pH
First 25 cm of small intestine is called the duodenum, second called Jejunum (2.2-2.4m), third Ileum (3.3-3.6 m)
Small intestine connection to large intestine via Ileocecal sphincter

Question 15

The wall of the intestine contains:

Answer 15

Permanent ridges circular folds (enhance absorption by increasing surface area, cause chyme to spiral rather than move in a straight-line)
Projections called villi (sing. villus)
Outer layer of each cell of each villus has thousands of microvilli
Lacteal: lymphatic capillaries

Question 16

Four major types of epithelial cells:

Answer 16

1. Enterocytes: comprising >80% of all small intestinal epithelial cells (for absorption)
2. Goblet cells: producing various glycoprotein constituent of mucus (mucin) and peptides needed for epithelial growth and repair
3. Entero-endocrine cells: export peptide hormones such as GLP-1 & 2, CCK, serotonin
4. Paneth cells: secrete antimicrobial cryptidins or defensins, digestive enzymes, and growth factors

Question 17

Duodenum & Pancreas

Answer 17

Duodenum: digestive juices from pancreas and liver add enzymes and digestive aids to the food mass
Pancreatic juice: digestive enzymes & bicarbonate ions (neutralize acidity)

Question 18

Liver & Gallbladder

Answer 18

Liver secretion (bile) emulsifies fats and forms micelle (small aggregates of mixed lipids and bile acids) 
Bile salts & acids: help break down fats, aid digestion, absorb important vitamins (vitamins A and D), and eliminate toxins & cholesterol, bilirubin (orange-yellow pigment that occurs normally when RBCs break down), additional bicarbonate ions 
Gallbladder stores bile

Question 19

Hepatic portal vein & Liver functions

Answer 19

Hepatic portal vein: carries blood and nutrients from stomach, spleen, intestines and gall bladder to the liver

In the liver:
• Excess glucose converted into glycogen
• Synthesizes lipoproteins
• Detoxifies ethyl alcohol and other toxic molecules
• Inactivates steroid hormones

All blood from gut drains into liver via the hepatic blood portal system (lymph vessels drain into vena cava):
• Sugars & aa processed for storage in liver
• Lipids mostly go to adipose cells first for storage

Question 20

Midgut

Answer 20

Consists of: Distal half of duodenum, Jejunum, Ileum, Cecum, Ascending colon, Proximal half of transverse colon

In vertebrates, midgut is most important site of digestion and absorption b/c:
• Apical membranes of midgut epithelial cells are richly populated by membrane-associated digestive enzyme proteins and by transporter proteins
• Pancreatic and biliary secretions enter midgut
• Animal’s digestive and absorptive capabilities are major determinants of nutritional value of foods b/c ingested organic compounds can be used only to the extent that they can be digested and absorbed

Question 21

***Factors Which Aid Nutrient Absorption

Answer 21

Large surface area (villi, microvilli) increase rate of absorption
Churning actions expose more nutrients to mucosa
Reabsorption of large volume of water aids nutrient absorption (90% in small intestine & the rest in large intestine)
Bile necessary for absorption of fats and fat-soluble vitamins
Intrinsic factor secreted by stomach wall necessary for absorption of vitamin B12

Question 22

The absorption of relatively simple hydrophilic compounds

monosaccharides, amino acids, and water-soluble vitamins

Answer 22

Absorption usually requires transporter proteins in the cell membranes
It occurs by either facilitated diffusion or secondary active transport or co-transport

Question 23

Mech of Absorption of Monosaccharides

Answer 23

4 membrane transporter proteins absorb carbohydrate monomers into bloodstream: SGLT1, Na+/K+-ATPase, GLUT2, and GLUT5

Enterocytes turn over rapidly in midgut epithelium (3-4 days); thus rate of transporter proteins has equally rapid turnover

GLUT5: mediates facilitated diffusion of fructose INTO cell from gut lumen
SGLT1: mediates secondary active transport of glucose/galactose INTO cell from gut lumen. Energy source for transport is Na+ electrochemical gradient (Na+ move from high to low conc into the cell).
GLUT2: mediates facilitated diffusion of glucose, fructose, galactose OUT of cell across basolateral membrane. After sugars exit, go into blood capillaries and carried throughout body.
Na/K-ATPase: uses ATP to produce Na electrochemical gradient (used as SGLT1 energy source)

Question 24

The absorption of the hydrophobic fatty acids and monoacylglycerols

Answer 24

Lipids transported from enterocyte into blood by different mech to monosaccharides/aa
Once inside enterocyte, fatty acids and monoglyceride are transported into ER (where used to synthesize triglycerides)
Then transported to Golgi — triglyceride is packaged w cholesterol, phospholipids and small amount of protein into particles called chylomicrons (lipoproteins)
Chylomicrons in Golgi=exocytotic vesicles=vesicles fuse w plasma membrane and undergo exocytosis dumping chylomicrons into space outside cells

Transport of lipids into circulation also diff from what occurs with sugars/aa
Instead of being absorbed directly into capillary blood, chylomicrons transported first into lymphatic vessel penetrates into each villus (central lacteal)

Question 25

Absorptive State

Answer 25

Emphasis on storage & synthesis
1. Nutrients enter blood from intestine
2. Stim insulin release by pancreas –increase uptake of glucose, reducing blood glucose conc
3.
A) liver cells convert excess glucose to glycogen, aa into ketone bodies
B) muscle cells convert excess glucose to glycogen, aa used to synthesize actin and myosin
C) adipose cells store excess lipids, increasing fat reserves

Question 26

Post-Absorptive State

Answer 26

Stored energy is released back to blood, Emphasis is on producing glucose
1. No nutrients enter blood
2. Sugar conc in blood drop
3.
A) glycogenolysis release glucose into blood, ketone oxidation release lipids and glucose used to generate ATP
B) muscle cells release glucose into blood, catabolized aa from muscle pr can also generate ATP after ketogenesis/ketone oxidation
C) adipose cells release stored lipid used to generate glucose, ketone bodies, ATP

Question 27

Large Intestine & Colon

Answer 27

Large Intestine: Cecum, appendix, colon, rectum

• Secretes mucus and bicarbonate ions
• Absorbs water and other ions
• At end of large intestine, undigested remnants (feces) are expelled from anus

Colon:

• Absorption of water, vitamins and electrolytes
• Serves as a storage area

Question 28

2 kinds of mechanical activity contractions

Answer 28

Haustral Churning (contraction of wall of colon):
Occurs every 30 minutes
Slow segmenting movements
Move food to adjacent haustra

Mass Peristalsis: 3-4 times a day, during or just after eating (activated by gastrocolic reflex)
Large section of colon contracts moving contents towards rectum

Question 29

Responses to Eating

Answer 29

First, chewing and saliva secretion: voluntary swallowing to pharynx & involuntary waves of peristatic muscular contraction to pass from pharynx to esophagus
- 2 sphincters at the 2 ends of esophagus are kept closed except when food is passing

In stomach, upper part serves as storage region for food waiting for processing (when food arrive, the muscles of this part relax=expand=accommodate food)
Presence of food in stomach exerts mechanical/chemical effects on G-endocrine cells
• stimulated G-cells release gastrin–gastrin reaches other stomach cells which stim:
o acid-secretion cells (parietal cells) & pepsinogen-secreting cells
o stim muscles in non-storage region=begin peristaltic contractions=mix and breaks up food at the same time=acid & pepsins initiate digestion

Question 30

Additional controls involved in modulating stomach functions include:

Answer 30

Gastrin: stim gastrin receptors in acid-secreting cells
Acetylcholine: stim acid-secreting cells via cholinergic receptors
Histamine (by paracrine cells): stim histamine receptors, act as a paracrine agent and stim acid-secreting cells
Smell & taste of food: stim signals of parasympathetic system to be passed from brain to the acid-secreting cells

Question 31

3 classes of cells secrete 3 polypeptide hormones in upper midgut in response to exposure to nutrient molecules and acidity

Answer 31

Secretin, CCK, GIP
o CCK stim pancreas and biliary system of liver to secrete bile
o Secretin stim pancreas to secrete pancreatic enzymes, also stim HCO3- secretion=neutralizes acid from the stomach

Midgut hormones affect the stomach–promote retention of food
• All 3 inhibit: Gastric-acid secretion, Gastric emptying, Muscle contraction in storage part

Segmentation of midgut:
Pushes midgut content back and forth=mix food products with intraluminal digestive enzymes and bile=ensure contact of all digestion material with walls of midgut and membrane-associative digestive enzymes and absorption transporters
Exhibits progressive peristaltic contractions=move material toward hindgut

Question 32

Hunger & Satiation

Answer 32

2 brain regions (hypothalamus and hindbrain) are preeminent control centers

Ghrelin secreted by endocrine cells in mucosal lining of stomach and upper intestine stim hunger

Signals sent from stomach exert effects not dependent on food composition, but instead quantity (volume) of food
–>As quantity increases, signals from stomach promote sense of satiation and terminate eating
Signals sent from intestines and pancreas depend on composition of meal and can exert complex composition-dependent effects on satiation and meal termination

5 peptides directly & indirectly promote satiation and meal termination are secreted after meal:
• CCK: secreted by endocrine cells in upper midgut epithelium
• Peptide YY and glucagon-like peptide-1: secreted by endocrine cells in lower midgut epithelium and hindgut
• Glucagon and amylin: secreted by the pancreas
• White adipose tissue: secretes leptin
- secreted in proportion to amount of stored fat in the body
- leptin tends to inhibit feeding in rodents

Question 33

Pythons fasting

Answer 33

Pythons fast and wait for food to come to them, they eat huge size preys (need massive effort of digestion and absorption), or they stay with no food for weeks –but they deconstruct their gastrointestinal systems b/w meals
In 24 hrs after feeding, they double mass of their midgut (by growth of new gut epithelium) & synthesize transporter proteins

Question 34

Python & Midgut Genomics Study

Gene Frizzled-4
Gene TCF7L2

Answer 34

Midgut Genomics: massive upregulation/downregulation of midgut genes after food consumption in fasted pythons

After group of Burmese pythons was fasted for 30 days:
• each python was fed a rat at time 0
• gene expression measured in midgut tissue b/f feeding and at 6h and 12h after feeding

Gene Frizzled-4 was upregulated and Gene TCF7L2 was downregulated
• Gene Frizzled-4 codes for key receptor protein in signaling system for growth and differentiation
• TCF7L2 gene codes for protein that can inhibit targets of the same signaling system– involved in signaling of tissue growth AND control of apoptosis and synthesis of proteins involved in microvillus growth, digestion and absorption

This research indicates massive growth of midgut and its function after a meal are controlled by genome wide regulation