4.1 cell communication, 4.2 introduction to signal transduction, 4.3 , 4.4, 4.5

• in single celled organisms, cell signaling influences how the cell responds to its environment

• in multicellular organisms, cell signaling coordinates the activates within individual cells that support the function of the organism as a whole

• have been suggested that they evolved first in prokaryotes then adopted by eukaryotes (evolution)

• ligand (stimulus) - signaling molecule that binds to a specific site on a target protein, typically a receptor. this reception can trigger a response, such as a change in the receptor’s shape, which leads to cellular signaling pathways. ligands for different functions have different shapes, otherwise there would mis-signaling.

  • hormones, neurotransmitters, etc

• receptor - protein molecule located on the surface of a cell or within a cell that binds to specific ligands, such as hormones, neurotransmitters, or other signaling molecules. when a ligand binds to its receptor, it typically causes a change in the receptor's shape or activity, initiating a cellular response

  • ion channel receptors, G-protein coupled receptors, and intracellular receptors.

• reception - binding between signaling molecule (ligand) and receptor is highly specific, a shape change in a receptor is often the initial transduction of the signal

• no distance signals - cell to cell communication

  • plant cells have these little holes called plasmodesmata where they can send messages from cell to cell

  • post it note

• local/short distance signals - cells send our local regulators, often used to communicate w/cell that is the same type

  • a neuron connected to another neuron through a synapse, you can send a neurotransmitter to make sure that message gets across

  • email

• long distance signals - targets cell that is not in the same area, often used to signal cells of a diff type

  • hormone, human growth hormone

  • you can also send to multiple cells rather than one cell

  • facebook status post

• intracellular signaling - inside the cell

• intercellular signaling - outside the cell

• intracellular receptors - located inside a cell, their ligands are small or nonpolar and can diffuse across the membrane

• membrane receptors - located on the cell surface have large or polar ligands that cannot diffuse thru membrane (GPCR or ligand gated ion channels)

  • G-protein coupled receptor

    • G-proteins in their inactive form are bonded to GDP. when GDP phosphorylates into GTP (caused by ligand binding), G proteins are activated

• paracrine - nearby, close but not connected

• autocrine - self, a cell secretes its own molecule that binds to itself

• juxtacrine - besides

• endocrine - far away, two different body parts, signals carried through the bloodstream

why cells communicate: some unicellular examples

• quorum sensing - is the ability to detect and to respond to cell population density by gene regulation

• yeast mating - only occurs between haploids, which can be either the a or alpha mating type and thus display simple sexual differentiation

• apoptosis - the death of cells which occurs as a normal and controlled part of an organism’s growth or development

• when cells release ligands to send a message, and the message is received, a whole host of events take place inside the cell. the ultimate goal is a response:

  • a gene turned on

  • a protein is manufactured

  • an enzyme is activated

  • the cell divides or dies

• the 3 phases of signal reception

  1. reception

     • at the cell wall/membrane

     • binding between signaling molecule (ligand)

  2. transduction

     • convert signal to a form that can bring about a cellular response

     • often means the receptor will change shape

     • signal transduction pathway

     • amplify original signal

  3. response

second messengers - internal signaling molecules released due to external (“first”) signals. trigger sub response pathways

• cyclic AMP: a typical second messenger that affects metabolism

  • the polar ligand, epinephrine (also known as adrenaline), attaches to the G protein coupled receptor and causes it to change shape. this change in shape activates the G protein inside the cell which is made of three subunits, alpha beta and gamma. the G protein becomes activated when the alpha subunit phosphorylates ADP (GDP) into ATP (GTP) causing the alpha subunit to detach. the alpha subunit activates the enzyme adenylyl cyclase which converts ATP into cyclic AMP or cAMP (this is the amplification process) the overall response varies depending on the target cell, and to terminate the signal to turn it off, the G-protein hydrolyzes GTP back to GDP, making the alpha subunit inactive again.

  • the transduction of this process:

    • inactive G-protein → active

    • inactive adenylyl cyclase → active

    • ATP → cAMP

    • inactive protein kinase A → active

    • inactive phosphorylase kindas → active

    • inactive glycogen phosphorylase → active

• calcium ions

protein phosphorylation and dephosphorylation

• many signals transduction pathways include phosphorylation cascades, in which series of protein kinases each add a phosphate group to the next one in line, activating it

• phosphatase enzymes soon remove the phosphates

organisms use feedback mechanisms to maintain their internal environments + respond to environmental changes

• external + internal cell environments r constantly changing

• two types of feedback mechanisms—

  • negative feedback - maintains homeostasis for a particular condition by regulating physiological processes

    • basically reverses stuff

    • operates at molecular + cellular

      basically how negative feedback cycles

    • ex: blood sugar feedback loop

  • positive feedback - amplifies processes (keeps going until the wanted condition is reached)

    • ex: giving birth (hormones are continuously secreted until baby is born)