the churning action mixes it with stomach acid which inactivates the enzyme and activates lingual lipase. Hydrochloric acid is secreted into the stomach. The acidic environment kills many microbes in food. HCl also partially denatures proteins and stimulates the secretion of hormones that promote flow of bile and pancreatic juice. The proteins in the meatballs will be denatured by HCl and digested by pepsin. Pepsin is an enzyme secreted by chief cells in the stomach. Pepsin break certain peptide bonds between amino acids. This breaks down proteins into smaller peptide fragments. Pepsin is most effective at a pH of 2 and becomes inactive at a higher pH. Gastric lipase is another stomach enzyme that splits triglycerides into monoglycerides. The mucous cells of the stomach absorb some water, ions, and short-chain fatty acids. Most digestion and absorption of nutrients occur in the small intestine. The small intestine contains absorptive cells on its walls that secrete brush-border enzymes. These enzymes include four carbohydrate-digesting enzymes called alpha-dextrinase, maltase, sucrase, and lactase; protein-digesting enzymes called peptidases (aminopeptidase and dipeptidase. Segmentation is the contraction of certain portions on the small intestine Assignment 3 :: Biology 235: Human Anatomy and Physiology
that bring the food in contact with the mucosa for absorption. After most of the meal is absorbed, segmentation stops and peristalsis begins to push chyme down into the ileum. Alpha-dextrinase further breaks down the remaining alpha-dextrins from the partially digested starch. Sucrose, lactose, and maltose are broken down by sucrase, lactase, and maltase respectively in the small intestine. The resulting monosaccharides are then absorbed through the small intestinal walls. Proteins are digested in the small intestine by aminopeptidase which cleaves off the amino acid at the end of a peptide, and dipeptidase splits dipeptides into single amino acids. The monomers of carbohydrates, lipids, and proteins are absorbed into the underlying blood and lymphatic vessel through the epithelial cells lining the mucosa. Fructose is absorbed via facilitated diffusion; glucose and galactose are transported via secondary active transport in the villi. Most proteins are absorbed as amino acids via active transport in the duodenum and jejunum. All lipids are absorbed through simple diffusion. Electrolytes are actively transported out of absorptive cells through sodium-potassium pumps after they have moved into absorptive cells via diffusion and secondary active transport. Vitamin C and B complexes are absorbed via simple diffusion as they are water soluble. Yet, Vitamin B 12 combines with intrinsic factor and is absorbed in the ileum via active transport. Water is absorbed via osmosis from the lumen of the intestines through absorptive cells and into blood capillaries. The absorption of water depends on the movement of electrolytes and nutrients to maintain osmotic pressure in the blood as it can move across the intestinal mucosa in both directions.
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