Biology - Chapter 11 Notes (Cell Communication)

Cell Communication External Signals are Converted to Responses Within the Cell Cell signaling – The ability of a cell to receive, process, and transmit signals with its environment and itself Evolution of cell signaling –> In the 1970s, scientists discovered that bacteria were capable of signaling to each other Quorum sensing – Sensing the concentration of bacteria cells allows bacteria to monitor their own local cell density and coordinate the behavior of all cells in a population for activities that require a given density Example: Biofilm – an aggregation of bacterial cells adhered to a surface - only attaches to substrate after the biofilm has reached a certain density Local and long-distance signaling Multicellular organisms also communicate by using signaling molecules. Sometimes, the signal needs to go to the adjacent molecule and sometimes it needs to go far away. 2 Types of Direct Contact Between Cells: 1) Cell junctions a. Directly connects the cytoplasms of adjacent cells 2) Cell to cell recognition a. Communication between cells by interaction between molecules protruding from their surfaces Paracrine signaling – A signaling cell acts on nearby target cells by secreting molecules of a local regulator; local signaling in animals Growth factors – Compounds that stimulate nearby target cells to grow and divide; local regulators in animals Synaptic signaling – A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell; occurs in the animal’s nervous system Synapse – The narrow space between the nerve cell and its target cell Hormones – Long-distance signaling molecules that act on specific target cells, changing their functioning Endocrine signaling – Specialized endocrine cells release hormones that travel through the circulatory system to other parts of the body; they reach target cells that recognize and respond to them Plant hormones – Travel in plant vessels but more often reach their targets by moving through cells or by diffusing through the air as a gas 3 Stages of Cell Signaling: 1) Reception - The target cell’s detection of a signaling molecule coming from outside the cell 2) Transduction – Converts the signal to a form, by a signal transduction pathway, that can bring about a specific cellular response a. Signal transduction pathway – A series of steps linking a mechanical, chemical, or electrical stimulus to a specific cellular response 3) Response – A specific cellular response Signal Reception: A Signaling Molecule Binds to a Receptor, causing it to Change Shape Ligand- A molecule that binds to a larger molecule Conformational change – Causes a molecule to change its shape Reception – The first step of a signaling pathway in which a signaling molecule is detected by a receptor molecule on or in the cell Intracellular receptors - Found in either the cytoplasm or the nucleus of the target cells Relay molecules – Protein molecules that relay a signal from receptor to response 3 Types of Cell-surface Transmembrane Receptors: 1) G protein-coupled receptors (GPCRs) - A cell-surface transmembrane receptor that works with the help of a G-protein (A protein that binds the energy-rich molecule GTP) 2) Receptor tyrosine kinases – Belong to a major class of plasma membrane receptors characterized by having enzymatic activity a. Protein kinase – An enzyme that transfers phosphate groups from ATP to a protein 3) Ion channel receptors a. Ligand-gated ion channel – A type of membrane channel receptor containing a region that can act as a Active protein kinase I Inactive protein kinase II -> Active protein kinase II (ATP -> ADP) Inactive protein -> Active protein (cellular response) (ATP -> ADP) OR (Test/Easier Version) Receptor -> Molecule 1 -> tells 3 other molecules which tells 3 other molecules (amplifies the signal) Eventually, one molecule will give a cellular response Phosphorylation cascade – A series of chemical reactions during cell signaling mediated by enzymes in which each kinase phosphorylates and activates another Protein phosphatases – Enzymes that can rapidly remove phosphate groups from proteins, a process called dephosphorylation Secondary messengers – A molecule that relays messages in a cell from a receptor to a target where an action within the cell takes place Small molecules and Ions as Second Messengers Not all components are proteins Cyclic AMP Cyclic AMP – Cyclic adenosine monophosphate Adenylyl cyclase – An enzyme embedded in the plasma membrane that converts ATP to cAMP in response to an extracellular signal Cyclic GMP – A molecule produced by a muscle cell in response to the release of NO gas by a neighboring cell The pathways leading to calcium release involve two other secondary messengers: 1) Inositol trisphosphate (IP3) 2) Diacylglycerol (DAG) *These two messengers are produced by cleavage of a certain kind of phospholipid in the plasma membrane* Cellular Response: Cell Signaling Leads to Regulation of Transcription or Cytoplasmic Activities. DNA -> RNA -> Proteins 1) Transcription - DNA -> RNA 2) Translation - RNA -> Proteins Regulation of the Response 1) Signaling pathways amplify the cell’s response to a single signaling event 2) The many steps in a multistep pathway provide control points at which the cell’s response can be further regulated 3) The overall efficiency of the response is enhanced by the presence of proteins known as scaffolding proteins 4) A crucial point in regulating the response is the termination of the signal *Different kinds of cells have different collections of proteins* Scaffolding proteins – Large relay proteins to which several other relay proteins are simultaneously attached Apoptosis Requires Integration of Multiple Cell-Signaling Pathways Apoptosis – Programmed cell death that is brought out by activation of enzymes which break down chemical components of the cell (