Which one of the following pairs of food components starts its digestion only when it reaches the stomach ?

Options

A. Starch and cellulose
B. Protein and casein
C. Starch and fat
D. Fat and cellulose

Answer

To solve the question "Which one of the following pairs of food components starts its digestion only when it reaches the stomach?", we will analyze the digestion process of different food components.

### Step-by-Step Solution:

1. **Understanding Digestion in the Mouth:**
- In the mouth, the enzyme salivary amylase begins the digestion of starch. Approximately 30% of starch is broken down into maltose and limit dextrin in the mouth.
- Proteins do not start their digestion in the mouth as there are no enzymes present to break them down.

2. **Food Movement to the Stomach:**
- After the mouth, the food bolus travels down the esophagus and enters the stomach.

3. **Digestion in the Stomach:**
- In the stomach, the enzyme pepsinogen is activated by hydrochloric acid (HCl) to form pepsin, which begins the digestion of proteins.
- Proteins are broken down into smaller peptides and peptones in the stomach.

4. **Casein Digestion:**
- Casein, a milk protein, is also digested in the stomach. It is converted into calcium paracaseinate by the enzyme renin (activated from pro-renin by HCl).

5. **Evaluating the Options:**
- **Option 1:** Cellulose - This is not digested in the human system as humans lack the enzyme cellulase.
- **Option 2:** Proteins and Casein - Both start their digestion in the stomach, making this option likely correct.
- **Option 3:** Starch - This begins digestion in the mouth, so it is incorrect.
- **Option 4:** Fats - Fat digestion starts in the intestine, not the stomach.

6. **Conclusion:**
- The only pair of food components that starts its digestion only when it reaches the stomach is **Proteins and Casein**. Therefore, the correct answer is **Option 2**.

### Final Answer:
**Option 2: Proteins and Casein**