1. How does the food change in consistency and form?
- The mechanical and chemical breakdown of food in the various stages of the digestive system causes the change in consistency and form
1. Mouth
- The mouth is the first part of the alimentary canal where food changes in form. The tongue and teeth work to manipulate and grind the food, while also mixing it with saliva during mastication or chewing, which forms a soft, flexible, and easily swallowed mass called bolus.
- Saliva released by salivary glands contains enzymes (salivary amylase and lingual lipase) that contribute to the chemical breakdown of starch and dietary triglycerides.
2. Stomach
- The contents of the stomach move through it by peristaltic waves in the process known as propulsion. If the contents are still too large to fit through the pyloric sphincter, they are forced back to the body of the stomach for another round of propulsion, which further exposes the gastric contents to gastric juice. Gastric juice contains HCl, which partially denatures proteins. Pepsinogen is an enzyme activated by HCl and converted into pepsin, which breaks down protein chains into smaller peptide fragments. Gastric lipase is another enzyme that breaks down triglycerides into fatty acids and monoglycerides. Bolus eventually becomes a soupy liquid called chyme.
3. Small intestine
- In the small intestine, localized mixing contractions which mix chyme with digestive juices. The enzymes in the intestinal juice of the small intestine along with pancreatic juice, and bile complete the digestion of carbohydrates, proteins, and lipids.
4. Large intestine
- In the large intestine, peristalsis and haustral churning move the chyme through the large intestine. Bacteria prepared chyme for elimination, breaking down any remaining molecules. After 3-10 hours in the large intestine, chyme is now a solid or semisolid called feces.
2. How could the body absorb the nutrients from the foods we eat
- Mucous cells of the stomach absorb some water, ions, short-chain fatty acids, some drugs, and alcohol
- The majority (90%) of absorption of nutrients occurs in the small intestine due to the absorptive epithelial cells lining the mucosa. Absorption occurs via facilitated diffusion, simple diffusion, active transport, and osmosis.
- Facilitated diffusion: fructose
- Active transport: glucose, galactose, amino acids, vitamin B12
- Diffusion: dietary lipids, fat-soluble vitamins A, D, E, K, most water-soluble vitamins such as most B vitamins and vitamin C
- Osmosis: water
- The large intestine absorbs water, ions including sodium and chloride, and some vitamins