In the small intestine those acidic contents from the low pH of the stomach get passed here (chyme). It’s regulated at the pyloric sphincter. Pancreatic juices get secreted here which have those enzymes for each macronutrient that breaks it down to individual molecules so that it can move on. For carbs we have pancreatic amylase breaking carbs down and brush border enzymes lactase sucrase and maltase which break them down even further from disaccharides into monosaccharides. If the disaccharides can’t be broken down they can’t be absorbed! They’ll remain in the gut and create osmotic pressure which makes you bloated. They need to be broken into individuals. Their transport across the membrane is by secondary active transport and facilitated diffusion. They are also broken down prior to transport. For protein we have those proteases. Pepsin cleaves the protein, trypsin and chymotrypsin cleave them more, and FINALLY and carboxypeptidase cleaves then even more and makes them individual amino acids. Transport here is also through secondary active transport and facilitated diffusion. Amino acids then get secreted into the interstitial fluid and enter the bloodstream. Last- lipids are cleaved by the lipases to become individual fatty acids. This starts with emulsification. The globule of fat is emulsified by bile salts to be able to enter aqueous solutions. Otherwise, without these bile salts, it cannot enter. The bile salts act like detergent and break down the globule into smaller pieces. NOW, lipases can break them down into their individual forms. Once they’re individual fatty acids, they get packaged in these molecules called micelles, which diffuse into enterocytes. After this, the micelle’s get packaged into molecules called chylomicrons. So it’s like a box filled with something going into another box. So now the individual fatty acids packaged in the chylomicrons get secreted into the interstitial space and travel in lymphatics to the bloodstream because they’re too large.
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