Cell Structure and Signaling by Chemical Messengers

• The cell is the basic unit of living organisms.
• Unique features of each cell type define tissue specificity and function.
• Despite the variety of cell types, cells share many common features, which include a plasma membrane and intracellular organelles.
• In eukaryotes, the intracellular organelles consist of lysosomes, the nucleus, ribosomes, the endoplasmic reticulum, the Golgi apparatus, mitochondria, peroxisomes, and the cytoplasm. Some cells may lack one or more of these internal organelles.
• In order to integrate cellular function with the needs of the organism, cells communicate with each other via chemical messengers. Chemical messengers include neurotransmitters (for the nervous system), hormones (for the endocrine system), cytokines (for the immune system), retinoids, eicosanoids, and growth factors.
• Chemical messengers transmit their signals by binding to receptors on target cells. When a messenger binds to a receptor, a signal transduction pathway is activated and generates second messengers within the cell.
• Receptors can be either plasma membrane proteins or intracellular binding G-proteins.
  Intracellular receptors act primarily as transcription factors, which regulate gene expression in response to a signal being released.
• Plasma membrane receptors fall into different classes, such as ion channel receptors, tyrosine kinase receptors, tyrosine kinase–associated receptors, serine-threonine kinase receptors, or G-protein–coupled receptors (GPCR).