Cell Communication Chapter 5

Multicellular Organisms Behave as a community – Cells talk Neighbors carry on private conversations Messages are sent over distances Phone Mail Public announcements are made Alarms are rung when there is danger

Signal Transduction Pathways Conversions of signals from one form to another in a cell Verbal instructions  written text  email  voice mail  action Eventually a response will occur due to the original signal

Signals in Cells Chemicals Hydrophilic – can’t enter cell membrane Proteins Amino acids Peptides Nucleotides Hydrophobic – can enter cell membrane Steroids - hormones Gasses

Communication Signal cell  Target cell Cell to cell recognition Cell Junctions Local Regulators Distance Regulators

Cell to cell recognition Glycoprotiens, glycolipids, and other molecules on plasma membranes serve as recognition markers. Important for immune response and embryonic development

Cell Junctions Plasmodesta – plant cells Gap Junctions – animal cells Allow communication between the cytoplasm of neighboring cells. Plasma membranes Plasmodesmata between plant cells Gap junctions between animal cells Figure 11.3 (a) Cell junctions. Both animals and plants have cell junctions that allow molecules to pass readily between adjacent cells without crossing plasma membranes.

Local Regulators Communicate and influence cells in close proximity Figure 11.3 (b) Cell-cell recognition. Two cells in an animal may communicate by interaction between molecules protruding from their surfaces. Communicate and influence cells in close proximity Growth factors – cause multiplication and growth of target cells Paracrine Signaling – simultaneous response by more than one cell Synaptic Signaling – Nervous system Neurotransmitters diffuse from one nerve cell to stimulate the next

diffuses across synapse (a) Paracrine signaling. A secreting cell acts on nearby target cells by discharging molecules of a local regulator (a growth factor, for example) into the extracellular fluid. (b) Synaptic signaling. A nerve cell releases neurotransmitter molecules into a synapse, stimulating the target cell. Local regulator diffuses through extracellular fluid Target cell Secretory vesicle Electrical signal along nerve cell triggers release of neurotransmitter Neurotransmitter diffuses across synapse is stimulated Local signaling Figure 11.4 A B

Long-distance signaling Distance Regulators Hormone travels in bloodstream to target cells (c) Hormonal signaling. Specialized endocrine cells secrete hormones into body fluids, often the blood. Hormones may reach virtually all body cells. Long-distance signaling Blood vessel Target cell Endocrine cell Figure 11.4 C Hormones Carried in the bloodstream of animals Endocrine System  Circulatory System Carried in the sap of plants

Cell Signaling 11_06SignalingOverview_A.swf EXTRACELLULAR FLUID Receptor Signal molecule Relay molecules in a signal transduction pathway Plasma membrane CYTOPLASM Activation of cellular response Figure 11.5 Reception Transduction Response 1. Reception – detection of message by a receptor protein on a target cell 2. Transduction – receptor changes and initiates a cascade of events 3. Response – activation of target

Reception Receptors – ligands – specific to signaling molecules Hormone (testosterone) EXTRACELLULAR FLUID Receptor protein DNA mRNA NUCLEUS CYTOPLASM Plasma membrane Hormone- receptor complex New protein Figure 11.6 Receptors – ligands – specific to signaling molecules Intracellular Receptors – cytoplasm or nucleus Usually steroid or hormone receptors - hydrophobic Plasma Membrane Receptors – proteins in the cell membrane Hydrophilic receptors

Plasma Membrane Receptors G-protein-linked receptor Receptor tyrosine kinase (kinase is an enzyme that phosphorylates other proteins to activate them) Ligand-gated ion channel

G-Protein-Linked Receptor Signal molecule attaches to receptor and activates G-protein is activated by the replacement of GDP with GTP G-protein activates an enzyme which triggers a cascade in the cell to the target. Used for vision and smell 60% of medicines activate G-proteins

Figure 11.7 G-protein-linked Activated Receptor Inctivate Plasma Membrane Enzyme G-protein (inactive) CYTOPLASM Cellular response Activated enzyme Activated Receptor Signal molecule Inctivate Segment that interacts with G proteins GDP GTP P i Signal-binding site Figure 11.7

Receptor Tyrosine Kinase Signals bind to receptors activating 2 kinases at a time – dimer ATP further activates the dimers Once activated relay proteins trigger cascades within the cell until the targets are reached. Used to trigger multiple reactions at once Cell growth and reproduction Abnormal tyrosine kinases are thought to cause some cancer

Figure 11.7 Signal-binding sitea Signal molecule Helix in the CYTOPLASM Tyrosines Signal molecule Helix in the Membrane Tyr Dimer Receptor tyrosine kinase proteins (inactive monomers) P Cellular response 1 Inactive relay proteins Activated relay proteins Cellular response 2 Activated tyrosine- kinase regions (unphosphorylated dimer) Fully activated receptor tyrosine-kinase (phosphorylated 6 ATP 6 ADP Figure 11.7

Ligand-Gated Ion Channels Cellular response Gate open Gate close Ligand-gated ion channel receptor Plasma Membrane Signal molecule (ligand) Figure 11.7 Gate closed Ions Signal molecule attached to ligand opening a gate for specific ions Once Ions enter specific reactions take place Na+ Ca+

Transduction 11_13SignalTransduction_A.swf Cascade Step 1 step 2  step 3  step 4  reaction Activated by phosphorylation ATP  ADP + P Protein Kinases Deactivated by dephosphorylation P removed Protein phosphatases

Second Messengers Small, non protein, water soluble, ions Cyclic AMP Levels regulate gene expression in bacteria Calcium Ions – Ca2+ Receptors on the outside of the membrane are the 1st messengers

Cyclic AMP Created from ATP from adenyl cyclase Used with G proteins 1st or 2nd messenger Figure 11.9 O –O N O P OH CH2 NH2 ATP Ch2 H2O HO Adenylyl cyclase Phoshodiesterase Pyrophosphate Cyclic AMP AMP i

ATP GTP cAMP Protein kinase A Cellular responses G-protein-linked receptor Adenylyl cyclase G protein First messenger (signal molecule such as epinephrine)

Ca2+ Most widely used 2nd messenger Increase in Ca2+  muscle contraction and cell division Causes greening in plants in response to sunlight Used with all 3 membrane receptors G-protein Tyrosine Kinase Gated Ion Channel EXTRACELLULAR FLUID Plasma membrane ATP CYTOSOL Ca2+ pump Endoplasmic reticulum (ER) Nucleus Mitochondrion Key High [Ca2+] Low [Ca2+]

Response Reception Transduction Response mRNA NUCLEUS Gene P Active transcription factor Inactive DNA Phosphorylation cascade CYTOPLASM Receptor Growth factor Figure 11.14 Signals are amplified as they are sent from messenger to messenger. Signals are specific to target cells and enzymes Not all cells respond to a signal

Concept Map & Summary Hydrophobic Signal Cell Signals Transduction Hydrophillic Response