C2.1 Chemical Signaling

• cells communicate through signaling molecules
• signaling molecules require receptors in order to communicate
• signaling molecules are known as chemical ligands
• The receptors for signaling molecules are the ligands binding site and are embedded in the membrane
• Once a ligand binds to a protein with an active site, it causes conformational changes and initiates a series of events such as gene expression or cellular responses
• ligands are specific to their receptors and remain changed (only the protein changes when they're atatched)
• the more ligands the greater the response in signaling
• Quorum sensing is a change in the behavior of a colony when its' population density reaches a certain threshold
• the more cells we have the more chemical messenger molecules
• in quorum sensing, when a certain density has been reached (threshold) enough molecules are available to cause a change in activity
• quorum sensing is an example of interdependence i.e. bioluminescence in Vibrio Fischeri and mutualism with the bobtail squid
• there are many chemical signaling molecules including: hormones, neurotransmitters, cytokines and calcium ions
• hormones come from endocrine glands
• endocrine glands are glands that secrete hormones directly into the blood stream
• exocrine glands secrete non-hormone substances through ducts
• in order to ensure accuracy, every hormone has their target cells that contain the receptor proteins needed for that hormone
• hormone signaling can have a long time effect and examples include glucose homeostasis (insulin and glucagon) and sex hormones (testosterone and estrogen)
• neurotransmitters don't travel in the blood, they travel between a small space called 'synaptic transmission' between two neurons (presynaptic neuron and postsynaptic neuron)
• the synapse is the gap between two neurons usually called a synaptic cleft
• neurotransmitters are rapid chemical mechanisms because they're very quickly released from the synapse
• examples of neurotransmitters are acetylcholine (which can move between motor neurons and muscle), dopamine and serotonin
• cytokines are proteins that are passed between nearby cells (cells in tissues communicate like this) i.e. they act like chemical messengers
• cytokines will be activated by a stimulus from the environment on the cell, allowing cells to release these cytokines which bind to receptors on nearby cells
• cytokine effects depend on the binding site e.g. interferons (enzymes that help with inflammation and immune cells) and erythropoietin's (which produce red blood cells)
• calcium ions are not biological molecules, they are ions
• hormones, neurotransmitters, and cytokines are examples of biological molecules
• calcium ions activate in muscle fibers - when muscles contract calcium ions will allow the myosin to bind to the actin (which allows muscles to slide inward and contract)
• calcium ions also activate in neurons - where neurotransmitters are released from the re-synapse which was initiated by the influx of calcium ions into the presynaptic neurons
• hormones and neurotransmitters evolved separately many times (this is known because they have many different forms and functions)
• hormones and neurotransmitters are both;

  1. small
  2. soluble
  3. have a specific shape compatible to a specific receptor
• hormones consist of: steroids (testosterone), amines (melatonin) and peptide (insulin). consisting of different chemical structures
• neurotransmitters consist of: amines (dopamine) , gases (nitrous oxide), amino acids (glutamate) or esters (acetylcholine). all have different chemical structures
• hormones and neurotransmitters have different structures and effects on molecules
• hormones can act over large distances as they're traveling through the blood to target cells
• neurotransmitters produce a localized effect because they only travel a small distance between two neurons
• if signaling molecules can enter a cell, its receptor proteins will be located inside the cell (intracellular receptor). This means the receptor must be covered in hydrophilic amino acids that have dissolved i.e. the surface of receptors and they're floating around in the cytoplasm i.e. steroids (please note the hormone itself is hydrophobic)
• if signaling molecules cannot enter the cell (insulin) its' receptor proteins will be embedded in the plasma membrane as transmembrane proteins (a part inside and outside the membrane i.e. hydrophilic and hydrophobic amino acids)
• there are two major types of transduction pathways
• transduction pathways are sequences of interactions that are initiated by the binding of the ligand to the receptor
• one of the transduction pathway types are the intracellular pathway; where the ligand enters the cell, binds to the receptor and that complex (ligand + receptor) enter the nucleus and regulate gene expression