ACTIVITY 1. THE FOOD JOURNEY
1. How does the food change in consistency and form?
Once food is ingested, it goes through a series of processes in the body that can significantly alter its consistency and form. The food is first physically broken down into smaller particles by the teeth when they are being chewed. Salivary amylase then lubricates and dissolves food when it is combined with saliva, initiating the chemical breakdown process known as hydrolysis, or the enzyme assisted breakdown of food. Its goal is to make a bolus (sphere of digested food that can be swallowed). The stomach wall begins to expand as the bolus enters and fills it, causing the gastric juices to be produced as well. Subsequently, the stomach wall's three muscular layers contract and pound the food, physically disintegrating it while continually combining it with the gastric juice that contains enzymes to generate a semifluid chyme that resembles heavy cream. After thoroughly mixing the food, a rippling peristalsis begins and the food approaches the pyloric valve. Every contraction of the stomach muscle expels 3 ml or less of chyme into the small intestine on route to the pyloric sphincter, which hardly opens. Foods that reach the small intestine are practically deluged with bile from the liver and enzyme-rich pancreatic juice that is ducted in from the pancreas. Finally, the large intestine is where the body expels any undigested food or waste materials as feces.
2. How could the body absorb the nutrients from the foods we eat?
- Absorption of the following nutrients:
- Carbohydrates
- Monosaccharides, including glucose, galactose, and fructose, are easily absorbed carbohydrates. The small intestine effectively absorbs these carbohydrates at a rate of 120 grams per hour. Indigestible fibers are expelled in stools, but all regularly digested food carbohydrates are absorbed. Common protein carriers use secondary active transport to deliver glucose and galactose into epithelial cells, exiting through facilitated diffusion and entering capillaries. Fructose is absorbed and transported only through facilitated diffusion, joining forces with transport proteins after disaccharides are broken down.
- Proteins
- Proteins, composed of amino acids, make up 15-20% of total calories and are reduced during digestion. The stomach is where protein digestion starts because this is where pepsin and HCl break down proteins into smaller polypeptides. Peptide chains are broken down in the small intestine by brush border cells and pancreatic enzymes, releasing tiny molecules into the circulation. Since amino acids are the byproducts of protein breakdown, most proteins are absorbed by active transport systems, which are mostly found in the duodenum and jejunum. The small intestine is where 95–98% of protein is broken down and absorbed. They are converted into their amino acids once they reach the absorptive epithelial cells, after which they exit the cell and diffuse into the capillary blood.
- Carbohydrates
-
- Lipid
- Triglycerides are the most common dietary lipids, consisting of glycerol and three fatty acid chains. Triglycerides are broken down into free fatty acids and monoglycerides by lipases, which include lingual, gastric, and pancreatic lipases. The majority of lipid digestion takes place in the small intestine, where triglycerides are converted into short- and long-chain fatty acids by pancreatic lipase. Bile salts facilitate the absorption of finished products and hasten the digestion of lipids. Like monosaccharides and amino acids, short-chain fatty acids may directly enter enterocytes and are soluble in water. Enterocytes readily absorb short-chain fatty acids, which then flow into a villus's blood capillary. In the watery intestinal chyme, long-chain fatty acids and monoacylglycerides are more difficult to suspend. These fatty acids are enclosed in micelles by bile salts and lecithin; micelles are minuscule spheres with hydrophobic tails pointing inward and polar ends facing the aqueous environment. Micelles also contain fat-soluble vitamins and cholesterol. Lipids would stay on the chyme surface in the absence of micelles. Diffusion allows lipid materials to leave the micelle and be absorbed. Reincorporated into triglycerides, they are encased in a protein sheath and combined with cholesterol and phospholipids. Chylomicron, a water-soluble lipoprotein, is produced as a result.
- Lipid