1. How does food change in consistency and form?
Food change in consistency and form for the efficient absorption of nutrients from food through digestion, both mechanical and chemical.
Mouth
- Mechanical Digestion - Mastication or chewing food with the aid of the manipulation by the tongue, grounded by the teeth, and mixed with saliva.
- Chemical Digestion - Salivary amylase is secreted by salivary glands which breaks down starch into smaller molecules.
- Net Result: Food becomes bolus (a soft, flexible, easily swallowed mass).
Stomach
- Mechanical Digestion - Propulsion and retropulsion are peristaltic movements that churn bolus into smaller pieces. Propulsion refers to moving gastric contents from body to stomach down into the antrum, while retropulsion refers to forcing food particles back to the stomach when food particles are too large to fit the pyloric sphincter. These process repeat until food is small enough to pass the pyloric sphincter.
- Chemical Digestion - Chemical digestion in the stomach is facilitated by three enzymes which are lingual lipase, pepsin, and gastric lipase. Lingual lipase is an enzyme of saliva that breaks down dietary triglycerides only after the food is swallowed as it is activated by the acidic environment of the stomach. Pepsin is an enzyme of the stomach secreted by chief cells which digest proteins in ingested food. Gastric lipase is also an enzyme of the stomach which breaks down triglycerides.
- Net Result: Bolus becomes chyme (soupy liquid as a result of mixture of partially digested food particles and gastric juices).
Small Intestine
- Mechanical Digestion - Segmentations and migrating motility complex (MMC) are responsible for mechanical digestion in the small intestine. Segmentations refer to contractions that mix chyme with digestive juices and bring food particles into contact with mucosa for efficient absorption. Migrating motility complex (MMC) refers to a type of peristalsis that pushes chyme forward along the small intestine.
- Chemical Digestion - Further digestion of food is a collective effort of pancreatic juice, bile, and intestinal juice in the small intestine.
- Net Result: Carbohydrates, proteins, and lipids in the chyme are broken down into simpler forms for easier absorption of nutrients.
2. How could the body absorb the nutrients from the foods we eat?
After all the phases of mechanical and chemical digestion, nutrients from food are absorbed by the mucosa in the small intestine through diffusion, facilitated diffusion, osmosis, and active transport.
- Diffusion - Dietary lipids diffuse into intestinal cells because their concentration is higher in the lumen than in the cells.
- Facilitated Diffusion - Fructose is absorbed into intestinal cells through transport proteins that help it cross the membrane.
- Osmosis - Water is absorbed in the small intestine and colon, following the concentration gradients created by dissolved nutrients and electrolytes.
- Active Transport - Glucose, galactose, and amino acids are absorbed into intestinal cells through sodium-potassium pump, where ATP pumps sodium ions out of the cells to maintain the gradient necessary for nutrient uptake.