"They mostly break down carbohydrates and fats. Once a protein source reaches your stomach, hydrochloric acid and enzymes called proteases break it down into smaller chains of amino acids. Amino acids are joined together by peptides, which are broken by proteases" - Protein Digestion: Enzymes, Absorption, and Ways to Improve
"Dietary protein is a vital source of amino acids. Proteins ingested in the diet are digested into amino acids or small peptides that can be absorbed by the intestine and transported in the blood." - Proteins Are Degraded to Amino Acids - Biochemistry - NCBI
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The answer to this question is the term rhizome. A rhizome is a stem of a plant that is growing underground and is growing horizontally. It is also known as rootstocks. The rhizome's function in the plant is that it is used to store starches and protein that enables the plant to grow underground.