Physical changes:

1. Mastication: The process of chewing food breaks it down into smaller pieces, increasing the surface area for digestive enzymes to work on.

2. Mixing and churning: The stomach and intestines use muscle contractions to mix and churn the food, further breaking it down and mixing it with digestive juices.

3. Absorption: Once food is broken down into small enough molecules, it is absorbed through the walls of the digestive tract into the bloodstream.

Chemical changes:

1. Enzymatic reactions: Digestive enzymes, such as amylase, protease, and lipase, break down complex molecules such as carbohydrates, proteins, and fats into simpler molecules that can be absorbed.

2. Hydrolysis: Many digestive enzymes use water to break down molecules. For example, amylase breaks down carbohydrates by adding water to the bonds between sugar molecules.

3. Acid secretion: The stomach secretes hydrochloric acid, which creates an acidic environment that kills bacteria and helps break down proteins.

4. Bile production: The liver produces bile, which helps break down fats into smaller droplets that can be digested by lipase.

So, digestion of food involves both physical changes (breaking down food into smaller pieces) and chemical changes (breaking down complex molecules into simpler ones) to make them suitable for absorption and utilization by the body.