4.1 - Cell Communication

• The ability of a cell to communicate with its environment and with itself - key to maintaining homeostasis and surviving.

• Survival depends on receiving and processing information from outside the cell—information about changing environmental conditionals, availability of nutrients, or any other information vital to surviving.

• Cells - developed a variety of signaling mechanisms to accomplish the transmission of important biological information.

• Cell signaling involves: ligand, a signaling molecule, a receptor protein to which the ligand binds.

• The receptor can be located on the cell membrane for hydrophilic ligands that can’t cross the membrane, or it can be located inside the cell for hydrophobic ligands that are small enough to cross the membrane.

  Overview of cell signaling

  

4.2 - Cell Signalling

• Direct cell-to-cell signaling **involves the direct physical contact between cells during communication. Gap junctions in animals and plasmodesmata in plants are tiny channels that directly connect to neighboring cells, which allow the cells to transfer signaling molecules that transmit their current state of homeostasis with one another.

• Paracrine signaling **involves cells close to one another but not in direct contact. A cell releases a short-lived signal into a localized area that will induce changes in a nearby cell. Paracrine signals can diffuse only over relatively short distances.

• Endocrine signaling **involves cells far apart in which a longer-lasting signal, called a hormone, is released into the extracellular fluid and travels widely throughout the organisms to target cells. Protein hormones are large molecules that must bind to receptors on the cell membrane. Steroid hormones are lipid-soluble molecules that are able to pass through the cell membrane and attach to an intracellular receptor. One example of endocrine signaling involves the release of human growth hormone (HGH) from the pituitary gland into the bloodstream, which targets bone and muscle cells to trigger growth.

• Synaptic signaling **involves a specialized nerve cell, a neuron, and its target cell. This association is called a chemical synapse and involves the release of neurotransmitters from the neuron into the synaptic gap to target the target cell.

• Autocrine signaling **occurs when a cell sends a signal to itself by secreting something that in turn binds to specific receptors on its own membrane. This plays an important role in cell development and the immune system.

4.3 - Signal Transduction Pathway

• When a ligand binds to a receptor on a cell, the cell relays the message through a series of reactions to elicit a cellular response - signal transduction pathway.

• The binding of the ligand to the receptor generally causes the receptor to change shape, resulting in an activation of an enzyme or binding of other molecules.

• Starts a signaling cascade that can amplify the signal through a series of reactions that leads to a cellular response, resulting in a change to a cell’s behavior or characteristics.

4.4 - Phosphorylation

• The signal transduction pathway - requires activating or inactivating proteins - via the addition of a phosphate group - phosphorylation.

• The phosphorylation of proteins (addition of phosphate groups) - catalyzed by enzymes - kinases.

• Different types of kinases target different proteins in the cell.

• The dephosphorylation of proteins (removal of phosphate groups) - catalyzed by enzymes - phosphatases.

• Many proteins - activated when phosphorylated; deactivated when dephosphorylated - way for cells to turn on and off various important cellular pathways.