Individual cells in a multicellular organism respond to external stimuli, just as prokaryotes. However, the response may be delayed through a series of cell-cell communication. Signaling Pathways Steroid hormones are small lipid-soluble molecules that can pass directly through cell membranes. After entering the target cell, the steroid hormone forms a complex with a receptor protein. Biosynthesis of testosterone occurs in the testes as a result of primary sex determination.
Peptide hormones are proteins that are used for cell-cell communication.
Peptide hormones elicit signal transduction pathways that are initiated by binding to a receptor protein in the cell membrane of the target tissue. Hormone-receptor complex activates a cytoplasmic signal that is transferred to the nucleus. Insulin is a peptide hormone that is synthesized in the b -cells of the pancreas. Synthesis and secretion of insulin is stimulated by high blood glucose, circulating insulin binds to insulin receptor in cell membrane resulting in the synthesis of target genes, proteins involved in glucose uptake in all cells, suppression of glycogenolysis in the liver, glycogen synthesis in muscle.