Positive and negative feedback loops Side notes: Maintaining constant protein expression throughout the lifetime is often based on _____ • Positive feedback loop • Negative feedback loop Ion channel linked receptors are mostly located on… • At synapses on presynaptic neurons • At synapses on postsynaptic neurons • On the axons • Both B and C Negative feedback results in oscillation of an effector concentration Phosphorylation of effector kinase E by signaling kinase S results in its activation Active E kinase activates an inhibitory phosphatase I by adding a phosphate (with a delay) Phosphatase starts to inactivate the E kinase by removing phosphate E kinase activity drops E kinase stops activating new phosphatase molecules, and the old ones gradually degrade S kinase keeps activating new E kinase molecule E kinase activates the phosphatase Phosphatase inhibits E kinase The cycle repeats while the S kinase is present Oscillation cycle Growth of grass results in activation of caterpillars that eat it with a delay When all grass is eaten, caterpillars also die Grass is growing again As a result, amount of grass oscillates Cell-surface receptor proteins Three classes of cell-surface receptors Ion-channel-coupled receptors G-protein-coupled receptors Enzyme coupled receptors Ion-channel-coupled receptors Ion channel-linked receptors = ligand-gated ion channels = ionotropic receptors Located at synapses Signal molecule here is a neurotransmitter, it binds and opens the channel Ions, such as Na+ and K+ can pass through the channel