bio 225 exam 3: digestion

why do we eat?

• psychological reasons

  • eating is:..

    • pleasurable

    • connected to other emotions

    • social

    • sensory appeal

• nutrition/digestion (our focus)

  • 1st law of thermodynamics - conversation of energy

    • energy in = energy out

  • 2nd law of themodynamics - entropy

    • disorder (death) within an isolated system always increase without the input of energy

• we eat to be alive:

  • maintain order (health) within our body

  • grow (growth/development)

  • reproduce (reproduction)

where do ingested calories go?

• gross energy

  • feces → indigestible energy

• digestible energy

  • urine → unmetabolizable energy

• metabolizable energy

  • heat → specific dynamic action (SDA)

• net energy

digestible (absorbable) nutrients - caloric

• lipids (fats)

• carbs

• proteins

lipids (fats)

• ~9kCal/gram; C, H

• glycerol with up to 3 fatty acid side chains

• can be saturated, mono-unsaturated, or poly-unsaturted

• kCal source/hormones/cell membranes

carbs

• ~4kCal/gram 1C:2H:1O

• composed of chains of monosaccharides

• main kCal source/structural/membrane_components/kCal storage

proteins

• ~4kCal/gram; H, C, O, N, S

• composed of chains of amino acids

• most abundant in the human body

• structural, enzymatic, antibodies, hormones, membrane components

fatty acids (FAs)

• saturated FA: no double bonds

• unsaturated FA: 1 or more double bonds

• 2 classes of “essential” FAs

  • omega-3 fatty acid: found in flax seed oil, fatty fish, chia, walnut

  • omega-6 fatty acid: found in vegetable oils, nuts, seeds

carbs

• typically from plant sources

  • glycogen source from meat, but minimal

• fiber is indigestible (cellulose/chitin)

• some starch is resistant to digestion

  • can be degraded by the microbiome

proteins

• linear chain of amino acids

• sources include meat (complete) and vegetable sources

• 20 total amino acids (in humans)

  • 9 “essential” AA

• serve structural, enzymatic, hormonal, etc. functions within humans

digestible (absorbable) nutrients - non-caloric

• vitamins

• minerals

• water

vitamins

• organic molecules which function as co-enzymes and other important metabolic functions

• often not just one form - vitamers

• generally required in small amounts

• 13 essential vitamins

• grouped into fat soluble and water soluble

• deficiency and toxicity is a problem

• requirements are highly debated

minerals

• inorganic compounds/elements

• required in small amounts, except calcium and phosphorus

• many important functions in metabolism

• basic “organic” minerals: C, H, O, N (not dietary minerals)

• quantity minerals: Na, Mg, P, S, Cl, K, Ca

• trace minerals: Mn, Fe, Co, Cu, Zn, Se, Mo, I

water

• most abundant inorganic molecule

• humans = 60-75% water

• necessary to control pH, BP, enzymatic reactions, etc.

metabolism (2 types)

• metabolism: the sum process of molecular synthesis and breakdown in an organism

• anabolism: synthesis of biomolecules from component molecules or elements

  • occurs during energy storage

  • requires energy input

• catabolism: breakdown of biomolecules

  • occurs during digestion

  • releases stored energy

energy balance: excess energy

• if food take > energy expenditure: anabolism

• 1st: muscle + liver store energy as glycogen (polymer of glucose)

• 2nd: if glycogen stores are full, excess energy converted to fat

energy balance: depleted energy

• if food intake < energy expenditure: catabolism

• 1st: liver glycogen is broken down into glucose

• 2nd: if liver glycogen is depleted, glucose is taken from muscle and fat

digestive system fuctions

• nutrient sensing

• mechanical digestion

• chemical digestion

• absorption

• waste removal

feeding and digestion

• animals cannot use macromolecules like proteins, fats, and carbs in complex form because:

  • polymers are too large to pass through cell membranes

  • the macromolecules that make up an animal are not identical to those of its food

• the process of feeding and digestion helps solve these problems

2 steps of digestion

1. mechanical fragmentation

   1. chewing, mashing, physically breaking food into smaller pieces

   2. increases the surface area exposed to digestive enzymes

2. enzymatic digestion

   1. hydrolysis of complex macromolecules into monomers (ex: proteins → amino acids)

mammalian digestive system

• mouth → pharynx → esophagus → stomach → intestine → anus

• compartmentalized system - allows ingestion of additional meals before digestion of earlier meals

the mouth

• mechanical digestion

  • mastication (chewing)

  • pushing of food against the rough hard palate

  • movement/squeezing motion of the esophagus (peristalsis)

• saliva (exocrine salivary glands)

  • lubricates food

  • contains salivary amylase

  • has antimicorbial properties

  • controlled by nervous signals

    • parasympathetic: stimulates salivation

    • sympathetic: inhibits salivation

the stomach

• sphincters at the top and bottom

• very muscular

• surface is composed of columnar epithelial cells

  • tight junctions: prevent leakage across epithelium

  • mucous neck cells: secrete mucus

  • parietal cells: secrete hydrochloric acid (HCl)

  • chief cells: secrete pepsinogen

  • enteroendocrine cells: secrete hormones (secondary endocrine function)

pepsin production in the stomach

• pepsinogen = precursor of digestive enzyme pepsin

• HCl from parietal cells cleaves pepsinogen to active form

the small intestine

• most important site of digestion

• divided into 3 sections:

  • most digestion occurs in the duodenum

• very long (up to 6m in humans)

  • inner surface as large as a tennis court

• lined with villi and microvilli

  • maximize absorptive area

villi

• small intestine cells:

  • enterocytes: absorptive cells within the microvilli

  • goblet cells: secrete mucus

  • enteroendocrine cells (G cells): secrete hormones (gastrin)

  • paneth cells: secrete antimicrobial molecules

• in most animals, nutrients are hydrolyzed in the lumen of the GI tract

• nutrients taken up by enterocytes

the liver (exocrine function)

• secretes bile

  • solution of digestive chemicals and liver waste products

  • produced in the liver and stored in the gallbladder

  • 22% phospholipids (aid in the uptake of lipids)

  • rest is mostly bile slats (emulsify fats)

• bile duct connects to duodenum

pancreas (exocrine function)

• secretes to duodenum:

  • digestive zymogens (inactive, precursor enzymes)

  • bicarbonate-rich, alkaline solution

    • neutralizes HCl in gastric juices

  • amylase - carbohydrate hydrolysis

  • nucleases

  • lipase

• slide 34

duodenum

• receives bile from liver and enzymes and zymogens (inactive proenzymes) from the enzyme

• duodenum produces the enzyme enterokinase

  • enterokinase will cleave trypsinogen (zymogen) into trypsin (active protease)

  • trypsin will cleave other digestive zymogens into their active forms

advantages of zymogens

• prevent unwanted protein degradation

• enable spatial and temporal regulation of proteolytic activity

• allows the body to control when and where an enzyme is active

• can be stored or transported safely

• unregulated protease activation → pancreatitis (pancreas self-figestion)

duodenum

produces:

• gastic inhibitory peptide (GIP): decreases gastric emptying, produced by the K cells also stimulated insulin release

• cholecystokinin (CCK): activated by high fat/protein chyme, stimulates fat digestion, bile release, and is an appetite suppresant

the large intestine (colon)

• absorbs water and ions (electrolytes)

• fairly long (1.5m)

• regions:

  • cecum (appendix)

  • ascending colon

  • transverse colon

  • descending colon

  • pelvic

• site of most of the gut microbiome

• store and pass insoluble starches and undigested food

  • final destination is the rectum for storage as feces

carbohydrate digestion

• mouth → partial breakdown into oligosaccharides

• small intestine → breakdown into monosaccharides (glucose, fructose, galactose)

• some carbs (cellulose) cannot be digested - dietary fiber

carbohydrate digestion: fructose absorption

• passive

• facilitated diffusion by GLUT5 transporter located in apical and basolateral side

• concentration gradient dependent

carbohydrate digestion: glucose absorption

• active

• involves secondary active transport by Na+/glucose symporter (SGLT1) on apical side

• energy dependent

glucose absorption steps

1. Na+/K+ pump uses ATP to set up strong Na+ concentration gradient

2. transport of Na+ down its concentration gradient provides energy to move glucose against its concentration gradient (secondary active transport)

3. glucose moves out of the cell through facilitated diffusion

protein digestion

• digestion by proteases

  • stomach - pepsin

  • pancreas

    • trypsin

    • chymotrypsin

    • carboxypeptidases

    • dipeptidases

• broken down into amino acids by peptides bonds

types of protein digestion

• pepsin, trypsin, and chymotrypsin (PT & C)

  • endopeptidases - cleave in the middle of a long peptide chain

  • pepsin = acidic environment

  • trypsin = basic environment

• dipeptidase (DP)

  • separates two peptides

• carboxypeptidases (CP)

  • removes single aa’s from C-terminus

• aminopeptidases (AP)

  • removes single aa’s from N-terminus

amino acids as fuel sources

• amino acids, small peptides, and vitamins are absorbed by enterocytes in the small intestine by active transport

• absorbed amino acids enter bloodstream and go to liver

• can be used for protein synthesis or as energy

nucleic acid digestion

• broken down in small intestine by hydrolytic nucleases secreted from pancreas

• sugars, nitrogen bases, and phosphates absorbed by epithelium

lipid digestion

• occurs solely in small intestine

• bile salts

  • secreted by the gallbladder

  • emulsify large fat droplet → increases surface area of fat droplet exposed to lipase

• lipase

  • secreted by pancreas

  • cleaves ester bond within fat molecule

lipid digestion steps

in intestinal lumen:

• bile salts emulsify fat gobules

• lipase breaks down into smaller fatty acids and monoglycerides

• form water-soluble micelles which deliver lipids to enterocytes

in intestinal epithelial cells:

• re-esterified into triglycerides

• mixed with cholesterol and coated with apolipoproteins to form chylomicrons

• exocytosed out of epithelial cells into lacteal

lipid transportation steps

1. chylomicrons move through the lymphatic system until they enter the bloodstream through the thoracic duct

   1. in bloodstream, lipoprotein lipase (LPL) removes some triglycerides, the remaining will go to liver

2. liver breaks down remaining chylomicrons to synthesize VLDL (very-low-density lipoproteins) for transport in bloodstream

3. in blood stream, triglycerides are removed from the VLDL for use in tissues → increases lipoprotein density

   1. VLDL → IDL → LDL

4. LDL carries cholesterol to tissues for uptake

5. HDL collects cholesterol from tissues for recycling in liver

lipid transportation: intestinal exit routes

• free fatty acids and glycerol pass directly through the enterocyte into the blood (passive diffusion)

• chylomicrons are too large to enter capillaries

  • they bypass the blood vessels and enter the lacteal (lymphatic system)

• som short- and medium-chain fatty acids are delivered to the liver via the hepatic portal vein (direction connection from intestine → liver)

lipid transportation

• lipid transport molecules differ based on diameter and ratio of protein:triglycerides

• smaller = more dense

• higher protein = more dense

hormonal regulation of digestion

• the wall of the stomach and duodenum secrete hormones that regulate digestion

• stomach - gastrin

• small intestine:

  • secretin

  • gastric inhibitory polypeptide (GIP)

  • cholecystokinin (CCK)

  • peptide YY (PYY)

• leptin and insulin

digestive motility

• regulated by sympathetic, parasympathetic, and enteric nervous system

• occurs via