absorption

Meaning: Absorption is the movement of digested nutrients, water, vitamins, and ions from the lumen of the GI tract across the epithelium into blood or lymph. Most occurs in the small intestine, where folds, villi, and microvilli greatly increase surface area. Glucose and galactose enter enterocytes by secondary active transport coupled to Na⁺; fructose uses facilitated diffusion. Monosaccharides and amino acids enter capillaries and travel through the hepatic portal vein to the liver. Long-chain fatty acids and monoglycerides, carried in bile salt micelles, enter enterocytes, are rebuilt into triglycerides, packaged as chylomicrons, and enter lacteals (lymph). The large intestine mainly absorbs water and ions. In spectroscopy, absorption has a different sense: a molecule taking up photons.

On the MCAT: Passages ask you to trace a nutrient's route (lumen → enterocyte → capillary or lacteal) or to predict the effect of a change. Blunted villi reduce surface area and lower absorption, and a lack of bile salts impairs fat absorption in particular. Water follows absorbed solutes by osmosis.

Common confusion: Digestion breaks molecules down; absorption moves them across the epithelium. Long-chain fats do not take the portal route used by sugars and amino acids. They enter lacteals and reach the blood through lymph.