cell signaling
Extracellular signals can act over short or long distance
细胞本身是天然排斥外界信息/遗传物质的（例如外界DNA）。这也就是为什么病毒能成为病毒。
Signaling through cell-cell contact: cytoneme
三类细胞表面受体：G蛋白偶联受体、酶联受体、离子通道受体
Extracellular signal molecules bind to specific receptors
细胞表面结合小分子，大概是nmol级；基因组中的某个基因若只有一个拷贝，大概其翻译产物只有pmol级。
Each cell is programmed to respond to specific combinations of extracellular signals
- Given some signals ⇒ 能够有fixed responding patterns
- One signal many responses: the physiological state of receiving cell
Intracellular signals must be specific & robust
systems/quantitative thinking
- Response timing
- sensitivity
- dynamic range
- persistance
- signal processing
- integration
- coordination