Digestion is the process that turns food into nutrients your cells can use for energy, growth, and repair. It matters because every body system depends on a steady supply of water, vitamins, minerals, amino acids, fatty acids, and glucose. The digestive system uses organs, muscles, enzymes, and helpful microbes to break food down and move it through the body.
Understanding digestion connects biology, chemistry, nutrition, and health decisions.
Key Facts
- Digestion has two main parts: mechanical digestion physically breaks food apart, and chemical digestion uses enzymes and acids to break molecules apart.
- In the mouth, chewing increases surface area and saliva begins starch digestion with the enzyme amylase.
- The stomach mixes food with acid and pepsin, which helps begin protein digestion at a low pH.
- Most nutrient absorption happens in the small intestine, where villi and microvilli greatly increase surface area.
- Carbohydrates are broken into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol.
- Energy from food is measured in Calories: 1 Calorie = 1 kilocalorie = 1000 calories.
Vocabulary
- Enzyme
- An enzyme is a protein that speeds up a chemical reaction, such as breaking food molecules into smaller parts.
- Peristalsis
- Peristalsis is the wave-like muscle movement that pushes food through the digestive tract.
- Villi
- Villi are tiny finger-like structures in the small intestine that increase surface area for nutrient absorption.
- Bile
- Bile is a fluid made by the liver and stored in the gallbladder that helps break large fat droplets into smaller droplets.
- Microbiome
- The microbiome is the community of helpful bacteria and other microbes that live in the digestive tract and support health.
Common Mistakes to Avoid
- Thinking digestion only happens in the stomach is wrong because digestion begins in the mouth and most absorption happens in the small intestine.
- Confusing digestion with absorption is wrong because digestion breaks food into smaller molecules, while absorption moves those molecules into the blood or lymph.
- Assuming enzymes are used up during digestion is wrong because enzymes help reactions happen faster and can be reused many times.
- Thinking all bacteria in the gut are harmful is wrong because many gut microbes help digest fiber, make useful compounds, and support immune function.
Practice Questions
- 1 A snack contains 30 g of carbohydrate, 8 g of protein, and 10 g of fat. Using 4 Calories per gram for carbohydrate and protein and 9 Calories per gram for fat, how many Calories are in the snack?
- 2 A piece of food takes 6 seconds to travel 18 cm through part of the esophagus during peristalsis. What is its average speed in cm/s?
- 3 Explain why the small intestine has many folds, villi, and microvilli instead of being a smooth tube.