Undigested Food Components in Humans

Introduction:
In the process of digestion, food is broken down into smaller molecules that can be absorbed by the body. However, not all food components are completely digested and some reach the stomach undigested. Among the given options, the pair of food components that reach the stomach totally undigested in humans is fat and cellulose.

Explanation:
1. Starch:
- Starch is a complex carbohydrate found in many food sources such as rice, potatoes, and bread.
- The digestion of starch begins in the mouth with the action of an enzyme called amylase, which breaks down starch into smaller sugar molecules.
- The process continues in the stomach where amylase is inactivated by the acidic environment.
- However, in the small intestine, pancreatic amylase is secreted and completes the digestion of starch into maltose, a disaccharide.

2. Cellulose:
- Cellulose is a type of dietary fiber found in the cell walls of plants.
- Humans lack the necessary enzymes, such as cellulase, to break down cellulose.
- Therefore, cellulose remains undigested as it passes through the digestive system.
- Cellulose provides bulk to the feces and helps in the movement of food through the intestines.

3. Protein:
- Proteins are complex molecules made up of amino acids.
- The digestion of proteins starts in the stomach with the action of pepsin, an enzyme that breaks down proteins into smaller peptide fragments.
- In the small intestine, pancreatic proteases further break down proteins into individual amino acids.
- These amino acids are then absorbed into the bloodstream for various physiological functions.

4. Fat:
- Fats are a type of nutrient called lipids and are composed of fatty acids and glycerol.
- The digestion of fats starts in the small intestine with the action of bile salts, which emulsify fats into smaller droplets.
- Pancreatic lipase then breaks down these droplets into fatty acids and glycerol.
- These smaller molecules are absorbed into the intestinal cells and eventually enter the bloodstream.

Conclusion:
Among the given options, fat and cellulose are the food components that reach the stomach totally undigested in humans. While cellulose cannot be broken down by human enzymes, fat remains undigested in the stomach due to the lack of sufficient digestive enzymes and the acidic environment. It is important to note that although cellulose remains undigested, it plays a crucial role in providing dietary fiber and promoting healthy digestion.