OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

How are proteins primarily broken down in the body?

1. By bacteria in the large intestine

2. By acids in the stomach

3. By enzymes in the stomach and small intestine

4. By the liver

The correct answer is: By enzymes in the stomach and small intestine

Proteins are primarily broken down in the body through the action of enzymes in the stomach and small intestine. This process begins in the stomach, where the enzyme pepsin, activated by the acidic environment created by stomach acid, starts to break down the protein chains into smaller peptides. The stomach acid also helps denature proteins, making them more accessible for enzymatic action. Once the partially digested proteins enter the small intestine, they encounter additional enzymes, such as pancreatic proteases (like trypsin and chymotrypsin) released by the pancreas. These enzymes further digest the peptides into even smaller fragments and ultimately into amino acids. The amino acids can then be absorbed through the intestinal wall into the bloodstream for use by the body. This enzymatic breakdown is critical because proteins are large molecules made up of long chains of amino acids. The body cannot directly use proteins in their native form; they must be broken down into their constituent amino acids for absorption and utilization in various biological functions, such as building and repairing tissues, making hormones, and supporting immune function. Other processes, such as bacterial action in the large intestine or liver functions, do play roles in digestion and metabolism, but they are not the primary means by which proteins are broken down