Here's why:
* Mechanical Digestion: This involves physically breaking down food into smaller pieces. This happens through chewing (mouth) and churning (stomach). While this changes the size of the food, it doesn't alter its chemical composition.
* Chemical Digestion: This involves enzymes breaking down large food molecules (like carbohydrates, proteins, and fats) into smaller molecules that the body can absorb. This is a chemical change because the bonds within the food molecules are broken and reformed.
Examples of chemical digestion:
* Mouth: Salivary amylase begins breaking down starches.
* Stomach: Pepsin breaks down proteins.
* Small Intestine: Pancreatic enzymes (amylase, lipase, protease) break down carbohydrates, fats, and proteins.
So, while the entire digestive process is essential for breaking down food, it's the chemical digestion stage that specifically involves a chemical change in the food's composition.
