BIO 304 · Human Anatomy & Physiology
Digestion & Absorption
Digestive System · Module 14
A reference for the Digestion & Absorption video. Macromolecules in food are too big to absorb. Digestion breaks them into small enough units; absorption moves those units across the intestinal wall into blood or lymph.
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- Trace carbohydrate, protein, and lipid digestion from start to absorbable units.
- Identify the major absorptive sites for each nutrient class.
- Describe how fat-soluble nutrients are absorbed differently from water-soluble ones.
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Digestion
Carbohydrates
- Starch (mouth)salivary amylase begins breakdown
- Starch (small intestine)pancreatic amylase → disaccharides
- Brush border enzymesmaltase, sucrase, lactase → monosaccharides (glucose, fructose, galactose)
- Absorbable unitmonosaccharides only
Proteins
- StomachHCl denatures; pepsin breaks into polypeptides
- Small intestinepancreatic trypsin, chymotrypsin, carboxypeptidase → small peptides + amino acids
- Brush borderaminopeptidases, dipeptidases finish the job
- Absorbable unitamino acids (some di- and tri-peptides absorbed too)
Lipids
- Mouth/stomachlingual + gastric lipase begin (minor in adults)
- Small intestinebile emulsifies; pancreatic lipase + colipase → fatty acids + monoglycerides
- Micellesbile salt-coated droplets that ferry products to brush border
- Absorbable unitfree fatty acids + monoglycerides (re-esterified inside enterocyte)
Nucleic acids
- Pancreatic nucleasescleave DNA/RNA to nucleotides
- Brush borderfurther break to bases + sugars + phosphates
Absorption
Carbohydrate absorption
- Glucose & galactosesecondary active transport with Na+ (SGLT1) into enterocyte
- Fructosefacilitated diffusion (GLUT5)
- Exit basolateralGLUT2 into capillary; to portal vein → liver
Protein absorption
- Amino acidsmultiple carriers; Na+-dependent and independent
- Di-/tri-peptidesabsorbed by PEPT1 then hydrolyzed inside enterocyte
- Exit basolateralinto capillary → portal vein → liver
Lipid absorption
- Diffuse into enterocytemonoglycerides + free fatty acids from micelle
- Reassembledinto triglycerides inside the enterocyte
- ChylomicronTG + cholesterol + phospholipid + protein shell
- Exitinto lac#0B1530 (lymphatic), not capillary — bypasses liver until thoracic duct
- Bile salts recycledreabsorbed in terminal ileum; enterohepatic circulation
Vitamins, water, electrolytes
- Water-soluble vitaminsabsorbed in small intestine with their cofactor systems
- Fat-soluble vitamins (ADEK)travel with chylomicrons; need bile to absorb
- Vitamin B12binds intrinsic factor (from stomach parietal cells); absorbed in terminal ileum
- Watermostly small intestine; remainder in colon; ~9L/day total handled
- Na+, Cl-absorbed actively in small intestine and colon
- K+mostly absorbed in small intestine
- Ca²⁺small intestine; vitamin D required for active transport
- Ironduodenum (heme & non-heme paths)
Common hormones in digestion
- Gastrinstomach G cells; stimulates HCl and motility
- Secretinduodenum response to acid; signals pancreas to release HCO₃-
- CCKduodenum response to fat & protein; signals gallbladder + pancreas
- GIPincretin; stimulates insulin release
- Motilinbetween meals; migrating motor complex
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