Final answer:
Insulin is needed to facilitate the entry of carbohydrates, protein, and fat into cells. It promotes glucose uptake, storage of glucose as glycogen, and protein synthesis.
Step-by-step explanation:
Insulin is needed to support carbohydrate, protein, and fat metabolism, primarily to facilitate the entry of these substances into cells. When the blood glucose levels rise, the pancreas releases insulin, which allows glucose to be taken up by liver hepatocytes, adipose cells, and muscle cells. Inside these cells, glucose is converted into glucose-6-phosphate and stored as glycogen. Insulin also promotes protein synthesis in muscles.