1. How does food change in consistency and form?
Food undergoes changes in its consistency and form during digestion to facilitate the absorption of nutrients. This process involves both mechanical and chemical digestion.
Mouth
- Mechanical Digestion: The food is broken down through chewing, with the teeth grinding the food while the tongue helps to mix it with saliva.
- Chemical Digestion: Salivary glands secrete amylase, which begins breaking down starches into smaller sugar molecules.
- Result: The food becomes a soft, flexible mass called a bolus, making it easier to swallow.
Stomach
- Mechanical Digestion: Peristaltic movements, including propulsion and retropulsion, help to break down the bolus further. Propulsion moves food down the stomach into the antrum, while retropulsion pushes food particles back when they are too large to pass through the pyloric sphincter. These movements continue until the food is small enough to pass through.
- Chemical Digestion: Three key enzymes work to break down food in the stomach: lingual lipase (which breaks down fats), pepsin (which digests proteins), and gastric lipase (which also helps digest fats). Lingual lipase is activated by stomach acids after swallowing.
- Result: The bolus is transformed into chyme, a soupy mixture of partially digested food and gastric juices.
Small Intestine
- Mechanical Digestion: In the small intestine, segmentation contractions mix chyme with digestive juices, helping to bring food particles into contact with the mucosa for better absorption. The migrating motility complex (MMC) propels the chyme forward.
- Chemical Digestion: The digestion of food in the small intestine involves the combined actions of pancreatic juice, bile, and intestinal enzymes.
- Result: Carbohydrates, proteins, and lipids are broken down into simpler forms, making it easier for the body to absorb essential nutrients.
2. How does the body absorb nutrients from food?
Once food has been broken down through mechanical and chemical digestion, nutrients are absorbed through the mucosa of the small intestine. This process involves several methods of transport, including diffusion, facilitated diffusion, osmosis, and active transport:
- Diffusion: Lipids pass into the intestinal cells by simple diffusion, as their concentration is higher in the lumen of the intestine than inside the cells.
- Facilitated Diffusion: Fructose is absorbed into the intestinal cells via transport proteins that assist its movement across the membrane.
- Osmosis: Water is absorbed in both 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 the intestinal cells through active transport, specifically the sodium-potassium pump, which uses ATP to maintain the concentration gradient for nutrient uptake.