Final answer:
Monosaccharides must cross the borders of the small intestine's epithelial cells and the capillaries' endothelial cells to enter the bloodstream, using active transport and facilitated diffusion.
Step-by-step explanation:
The two borders that monosaccharides must cross to enter the bloodstream during carbohydrate digestion are the epithelial cells of the small intestine and the endothelial cells of the capillaries within the intestinal villi. First, monosaccharides like glucose and galactose are transported into the epithelial cells via secondary active transport alongside sodium ions. Fructose, another monosaccharide, is absorbed by facilitated diffusion. After entering the epithelial cells, monosaccharides then use facilitated diffusion again to move out of these cells and into the intercellular clefts, eventually reaching the capillaries. Nutrients including these simple sugars then enter the bloodstream through the blood vessels located at the core of the microvilli in the small intestine.