The nutrient primarily responsible for growth and repair in living organisms is protein. Proteins are made up of amino acids, which are essential for building and repairing tissues, synthesizing enzymes and hormones, and supporting various bodily functions. Adequate protein intake is crucial during periods of growth, like childhood, adolescence, and recovery from injury. Sources of protein include both animal (meat, dairy) and plant (beans, lentils) products, emphasizing the importance of a balanced diet.