Cell Signaling

• Action potentials are propagated along the axon via inward flow of sodium ($Na$). Calcium ions ($Ca^{2+}$) are involved in signal propagation at the synaptic cleft.
• Cell signaling involves signal perception and transduction, converting signals into cellular responses.
• Signaling can be local (direct contact, diffusion) or distant (hormonal signaling via vasculature).

Animal vs. Plant Cells

• Animal cells can move; plant cells are stationary due to cell walls.
• Animal cells use gap junctions for direct molecular movement.
• Plant cells use plasmodesmata for intercellular movement through pores in cell walls.

Local Signaling

• Animal cells communicate via direct contact (e.g., immune response).
• Paracrine signaling involves local diffusion of molecules (e.g., cytokines).
• Neurotransmitters facilitate local diffusion across synaptic clefts.

Distant Signaling

• Hormonal signaling uses vasculature (blood vessels in animals, xylem and phloem in plants) to transport molecules.
• Example in animals: osmoreceptors in the hypothalamus detect changes in blood osmolarity and stimulate the pituitary gland to release antidiuretic hormone (ADH), which affects water reabsorption in the collecting duct of the kidney.
• ADH increases water reabsorption by increasing aquaporin channels and stimulates thirst.

Plant Cell Signaling

• Xylem transports water, phloem transports sugars.
• Guard cells regulate gas exchange by opening and closing stomata.
• Abscisic acid (ABA) is released under water stress, causing stomata to close.
• Proton pumps create an electrochemical gradient.
• Chloride-proton symporters pump chloride into guard cells, followed by potassium influx, increasing turgor pressure for stomata opening.
• Under water stress, ABA binds to receptors on guard cells, leading to calcium influx.
• Calcium influx activates outward flowing chloride and potassium channels, deflating the guard cells and closing the stomata.