4.5 Synthesis of proteins Proteins can be synthesized by both plant and animal cells if the plant has available nitrogen or the animal has amino acids. Without being able to synthesize proteins, neither plants nor animals can survive and grow. Plants obtain essential nitrogen from the soil. The nitrogen is provided by chemical fertilizers or is released from nitrogen- containing organic fertilizer by the action of bacteria. Some plants such as legumes have small nodules in bacteria on their roots are capable of fixing, or trapping atmospheric nitrogen. Animals obtain most of their nitrogen in the form of amino acids from either plant or other animal sources. Animals are also able to synthesize a few amino acids in their gastrointestinal tract, using dieting nitrogen.
Protein synthesis involves the formation of long chains of amino acids called peptide chains. These chains are so named because the chemical bond that holds two amino acids together is called a peptide bond. In forming the chain, hydrogen from the amino group of one amino acid joins with the OH from a carboxyl group of the adjoining amino acid. One molecule of water is formed and the remaining carbon and NH are joined through a peptide bond. When the peptide bond breaks, water must be added before the amino acid can be split apart either by acid or by digestive enzymes. When two amino acids are bonded together the resulting structure is a dipeptide . As additional structure are added to the chain a polypeptide is formed 4.6 Digestion and absorption of proteins Mouth There is no chemical or enzymatic digestion of protein in the mouth. There is chewing that reduces food particles size and also lubrication of food with saliva in preparation for digestion. Stomach Chemical digestion of proteins begins in the stomach, where hydrochloric acid is released and this lowers the pH of the food mass so that the inactive protein splitting enzymes pepsinogen can be converted to active pepsin . This allows hydrochloric acid to enter the cells to denature the proteins so that the protein spitting enzymes can act it on. Small intestines The majority of protein digestion occurs in the small intestine. Bicarbonate from pancreatic juice neutralizes acidity in the food mains to allow intestinal enzymes to digest the denatured proteins. Pancreatic proteases reduce polypeptides to shorter chains, tripeptides, dipetides, and amino acids. The enzymes trypsin and chymotrypsin act to break peptides bonds between specific amino acids. Carboxypeptidase breaks off amino acids from the acid (carboxyl) end of the polypeptides. Enzymes located on the surface of the cells that line the small intestine complete the digestion. Amino peptidase splits
amino acids from the amino ends of short peptides and dipeptidase reduces dipeptides to amino acids.
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