All source of energy because, unlike glucose, the concentration

   All monosaccharides are transported into the intestinal cells via protein transporters (Silverthorn, 2016). In a form of secondary active transport, glucose is transported across the apical cell membrane using Sodium Glucose Co-Transporters (SGLT-1/2) (Silverthorn, 2016). The free energy released when sodium is pumped into the into cell through the transport protein down its concentration gradient allows for the transport of glucose into the cell along with it (Silverthorn, 2016) (Thompson et al., 2013). In order to maintain the concentration gradient of sodium (more outside of the cell than inside) that allows for this process to work, an Na+/K+ pump then pumps the sodium back out of the cell (Thompson et al., 2013). Fructose, while also being transported across the cell membrane using a transport protein, does not require a source of energy because, unlike glucose, the concentration of the fructose in the intestinal lumen is higher than in the intestinal cells, and thus fructose can just flow into the cell via facilitated diffusion using Glucose Transporter 5 (GLUT5) proteins (Silverthorn, 2016) (Thompson et al., 2013). Once absorbed, the monosaccharides are then able to be transported across the basolateral membrane via Glucose Transporter 2 (GLUT2) transporters to be taken up by the capillaries for transport in the bloodstream (Silverthorn, 2016).   Epithelial cells in the mucosal layer of gastrointestinal tract structures are responsible for absorbing nutrients from the GI lumen (Silverthorn, 2016). Most nutrient absorption occurs in the small intestines (Thompson et al., 2013).    Increased surface area allows for increased absorption across the epithelial layers of both the stomach and small intestines by increasing the number of epithelial cells (the total area of the lumen) that come into contact with nutrients in the gastrointestinal tract in order to absorb them (Thompson et al., 2013).?   In the stomach, the mucosal epithelium is crinkled up into ridges called rugae (Silverthorn, 2016). Similar epithelial folds, called plicae, are found in the small intestines, but the mucosal epithelium of the small intestines also does more to increase surface area by wrinkling further into finger-like projections called villi, each made up of many epithelial cells termed enterocytes (Thompson et al., 2013). Each enterocyte is further covered with even more tiny projections called microvilli, creating the brush border (Thompson et al., 2013). Surface area of both the stomach and small intestines is further increased via columnar invaginations in their mucosal epithelium, called gastric glands and crypts, respectively (Silverthorn, 2016). Through these various forms of folding, the overall surface area of the small intestines is increased by around 500 times to ~250m2 (Thompson et al., 2013) (Silverthorn, 2016).     Transported through the watery gut to them by amphipathic micelles, enterocytes of the mucosal epithelium of the small intestine absorb lipids broken down into monoglycerides and free fatty acids, which are absorbed via diffusion, and cholesterol, which is moved across the membrane by transport proteins (Silverthorn, 2016) (Thompson et al., 2013).