other cells what identity they need to take on. It also happens in synaptic signaling between two nerve cells. As mentioned in the previous chapter, neurotransmitters cross the gap between cells to send signals between the cells. The neurotransmitter is either degraded or taken back up into the sending cell so another signal can be sent. Autocrine signaling involves a cell signaling itself, releasing a ligand that binds to its own receptors. It is important during development as cells take on their identity. It is important in cancer and in cancer metastases. Signals can be both autocrine and paracrine at the same time. Endocrine signaling happens over the longest distances. It makes use of the circulatory system in order to pass a signal from one place to another. The signal goes through the bloodstream in order to get from one place to another in the organism. The target cell can be anywhere in the organism’s body. The cell signals in such cases are referred to as hormones. Signaling can happen through direct cell to cell contact. This happens through connections called plasmodesmata (in plants) and gap junctions (in animals). These allow for intracellular mediators to diffuse between the cells so both cells have the same signal. This especially connects things like ions but will not allow larger molecules to pass between the cells. Plasmodesmata connect practically every plant cell together in the organism. When cell signaling happens, it starts with a ligand and a receptor. A receptor can match just a single or a few ligands to it in a sort of lock-and-key mechanism. When the ligand binds the receptor, it changes the shape or activity of the receptor so that there is an intracellular change.
RECEPTORS There are two types of receptors. There can be intracellular receptors inside the cell (nucleus or cytoplasm) and cell surface receptors (on the plasma membrane). Most intracellular receptors are small hydrophobic molecules that can cross the plasma membrane. These include the different steroid hormones, like sex hormones. These intracellular receptors often induce a change in gene activity, regulating transcription. 166
