Membrane Structure and Function PDF

The cell surface and the extracellular matrix A. Nature and composition of plasma membrane B. Functions and activities of cell membrane A. Cell adhesion B. Signal transduction C. Vacuole formation 1. Which plasma membrane component can be either found on its surface or embedded in the...

The cell surface and the extracellular matrix A. Nature and composition of plasma membrane B. Functions and activities of cell membrane A. Cell adhesion B. Signal transduction C. Vacuole formation 1. Which plasma membrane component can be either found on its surface or embedded in the membrane structure? A. protein B. carbohydrate C. cholesterol D. phospholipid 2. What is the primary function of carbohydrates attached to the exterior of cell membranes? A. identification of the cell B. strengthening the membrane C. flexibility of the membrane D. channels through membrane 3. Water moves via osmosis _________. A. throughout the cytoplasm B. from an area with a high concentration of solutes to a lower one C. from an area with a high concentration of water to one of lower concentration D. from an area with a low concentration of water to higher concentration 4. What problem is faced by organisms that live in freshwater? A. Their bodies tend to take in too much water. B. They have no way of controlling their tonicity. C. Only salt water poses problems for animals that live in it. D. Their bodies tend to lose too much water to their environment. 5. A plasmolyzed cell is caused by _________ A. hypotonic solution B. isotonic solution C. hypertonic solution D. D. both A and B 6. Facilitated diffusion differs from active transport in that facilitated diffusion A. expends no ATP. B. moves molecules from an area of higher concentration to one of lower concentration. C. does not require a carrier protein for transport. D. moves molecules in vesicles across a semipermeable membrane.  Overview: Life at the Edge The plasma membrane is the boundary that separates the living cell from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others Fig. 7-1  Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Amphipathic molecule Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Fig. 7-2 WATER Hydrophilic head Hydrophobic tail WATER Fig. 7-3 Phospholipid bilayer Hydrophobic regions Hydrophilic of protein regions of protein  Freeze-fracture studies of the plasma membrane supported the fluid mosaic model Freeze-fracture is a specialized preparation technique that splits a membrane along the middle of the phospholipid bilayer Fig. 7-4 TECHNIQUE RESULTS Extracellular layer Proteins Inside of extracellular layer Knife Plasma membrane Cytoplasmic layer Inside of cytoplasmic layer  The Fluidity of Membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally Rarely does a molecule flip- flop transversely across the membrane Unsaturated hydrocarbon tails enhance membrane fluidity, because kinks at the carbon-to-carbon double bonds hinder close packing of phospholipids. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  The Fluidity of Membranes Membranes solidify if the temperature decreases to a critical point. Critical temperature is lower in membranes with a greater concentration of unsaturated phospholipids.  The Fluidity of Membranes Cholesterol, found in plasma membranes of eukaryotes, modulates membrane fluidity by making the membrane: ❑Less fluid at warmer temperatures (e.g., 37°C body temperature) by restraining phospholipid movement. ❑ More fluid at lower temperatures by preventing close packing of phospholipids.  The Fluidity of Membranes Cells may alter membrane lipid concentration in response to changes in temperature. Many cold tolerant plants (e.g., winter wheat) increase the unsaturated phospholipid concentration in autumn, which prevents the plasma membranes from solidifying in winter. Why is it advantageous for the cell membrane to be fluid in nature? Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane? Fig. 7-5 Lateral movement Flip-flop (~107 times per second) (~ once per month) (a) Movement of phospholipids Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity Cholesterol (c) Cholesterol within the animal cell membrane Fig. 7-5a Lateral movement Flip-flop (107 times per second) ( once per month) (a) Movement of phospholipids  As temperatures cool, membranes switch from a fluid state to a solid state The temperature at which a membrane solidifies depends on the types of lipids Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids Membranes must be fluid to work properly; they are usually about as fluid as salad oil Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5b Fluid Viscous Unsaturated hydrocarbon Saturated hydro- tails with kinks carbon tails (b) Membrane fluidity  The steroid cholesterol has different effects on membrane fluidity at different temperatures At warm temperatures (such as 37°C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-5c Cholesterol (c) Cholesterol within the animal cell membrane Membrane Proteins and Their Functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-7 Fibers of extracellular matrix (ECM) Glyco- Carbohydrate protein Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Microfilaments Peripheral of cytoskeleton proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE  Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices  N-terminus EXTRACELLULAR Fig. 7-8 SIDE C-terminus CYTOPLASMIC  Helix SIDE The hydrophobic regions of an integral protein consist of one or more stretches of nonpolar amino acids, often coiled into alpha helices  Six major functions of membrane proteins: Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-9 Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM) Fig. 7-9ac Signaling molecule Enzymes Receptor ATP Signal transduction (a) Transport (b) Enzymatic activity (c) Signal transduction Fig. 7-9df Glyco- protein (d) Cell-cell recognition (e) Intercellular joining (f) Attachment to the cytoskeleton and extracellular matrix (ECM)  The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-10 ER 1 Transmembrane glycoproteins Secretory protein Glycolipid Golgi 2 apparatus Vesicle 3 Plasma membrane: Cytoplasmic face 4 Extracellular face Transmembrane Secreted glycoprotein protein Membrane glycolipid  Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly Polar molecules, such as sugars, do not cross the membrane easily Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel proteins called aquaporins facilitate the passage of water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Once the molecules become uniformly distributed, dynamic equilibrium exists. The equilibrium is said to be dynamic because molecules continue to move, but despite this change, there is no net change in concentration over time. Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-11a Molecules of dye Membrane (cross section) WATER Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute  Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-11b Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium (b) Diffusion of two solutes  Effects of Osmosis on Water Balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-12 Lower Higher Same concentration concentration concentration of sugar of solute (sugar) of sugar H2O Selectively permeable membrane Osmosis  Water Balance of Cells Without Walls Tonicity is the ability of a solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-13 Hypotonic solution Isotonic solution Hypertonic solution H2O H2O H2O H2O (a) Animal cell Lysed Normal Shriveled H2O H2O H2O H2O (b) Plant cell Turgid (normal) Flaccid Plasmolyzed  Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-14 50 µm Filling vacuole (a) A contractile vacuole fills with fluid that enters from a system of canals radiating throughout the cytoplasm. Contracting vacuole (b) When full, the vacuole and canals contract, expelling fluid from the cell.  Water Balance of Cells with Walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Facilitated Diffusion: Passive Transport Aided by Proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-15 EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Carrier protein Solute (b) A carrier protein  Carrier proteins undergo a subtle change in shape that translocate the solute- binding site across the membrane Some diseases are caused by malfunctions in specific transport systems, for example the kidney disease cystinuria Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Active transport uses energy to move solutes against their gradients Facilitated diffusion is still passive because the solute moves down its concentration gradient Some transport proteins, however, can move solutes against their concentration gradients Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  The Need for Energy in Active Transport Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Active transport is performed by specific proteins embedded in the membranes Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-16-1 EXTRACELLULAR [Na+] high FLUID [K+] low Na+ Na+ [Na+] low Na+ CYTOPLASM [K+] high 1 Cytoplasmic Na+ binds to the sodium-potassium pump. Fig. 7-16-2 Na+ Na+ Na+ ATP P ADP 2 Na+ binding stimulates phosphorylation by ATP. Fig. 7-16-3 Na+ Na+ Na+ P 3 Phosphorylation causes the protein to change its shape. Na+ is expelled to the outside. Fig. 7-16-4 P P 4 K+ binds on the extracellular side and triggers release of the phosphate group. Fig. 7-16-5 5 Loss of the phosphate restores the protein’s original shape. Fig. 7-16-6 K+ is released, and the cycle repeats. Fig. 7-16-7 EXTRACELLULAR [Na+] high Na+ FLUID [K+] low Na+ Na+ Na+ Na+ Na+ Na+ Na+ [Na+] low ATP Na+ P [K+] high P CYTOPLASM ADP 1 2 3 P P 6 5 4 Injecting a potassium solution into a person’s blood is lethal. This is how capital punishment and euthanasia subjects die. Why do you think a potassium solution injection is lethal? Fig. 7-17 Passive transport Active transport ATP Diffusion Facilitated diffusion  How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage difference across a membrane Voltage is created by differences in the distribution of positive and negative ions Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane: A chemical force (the ion’s concentration gradient) An electrical force (the effect of the membrane potential on the ion’s movement) Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  An electrogenic pump is a transport protein that generates voltage across a membrane The sodium-potassium pump is the major electrogenic pump of animal cells The main electrogenic pump of plants, fungi, and bacteria is a proton pump Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-18 – EXTRACELLULAR + FLUID ATP – + H+ H+ Proton pump H+ – + H+ H+ – + CYTOPLASM H+ – +  Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of another solute Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-19 – + ATP H+ H+ – + Proton pump H+ H+ – + H+ – H+ + H+ Diffusion of H+ Sucrose-H+ cotransporter H+ Sucrose – + – + Sucrose  Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Small molecules and water enter or leave the cell through the lipid bilayer or by transport proteins Large molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesicles Bulk transport requires energy Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Endocytosis In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis is a reversal of exocytosis, involving different proteins There are three types of endocytosis: Phagocytosis (“cellular eating”) Pinocytosis (“cellular drinking”) Receptor-mediated endocytosis Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export their products Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings  In phagocytosis a cell engulfs a particle in a vacuole The vacuole fuses with a lysosome to digest the particle Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-20 PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM 1 µm FLUID Pseudopodium Pseudopodiu m of amoeba “Food”or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM) PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit Coat and a coated protein vesicle formed during receptor- mediated endocytosis (TEMs) Plasma membrane 0.25 µm Fig. 7-20a PHAGOCYTOSIS EXTRACELLULAR CYTOPLASM 1 µm FLUID Pseudopodium Pseudopodium of amoeba “Food” or other particle Bacterium Food vacuole Food vacuole An amoeba engulfing a bacterium via phagocytosis (TEM)  In pinocytosis, molecules are taken up when extracellular fluid is “gulped” into tiny vesicles Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-20b PINOCYTOSIS 0.5 µm Plasma membrane Pinocytosis vesicles forming (arrows) in a cell lining a small blood vessel (TEM) Vesicle  In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Fig. 7-20c RECEPTOR-MEDIATED ENDOCYTOSIS Coat protein Receptor Coated vesicle Coated pit Ligand A coated pit Coat and a coated protein vesicle formed during receptor- mediated endocytosis (TEMs) Plasma membrane 0.25 µm Fig. 7-UN1 Passive transport: Facilitated diffusion Channel Carrier protein protein Fig. 7-UN2 Active transport: ATP Fig. 7-UN3 Environment: “Cell” 0.03 M sucrose 0.05 M sucrose 0.02 M glucose 0.03 M glucose 0.01 M fructose Fig. 7-UN4  Cell communication A signal transduction pathway is a series of steps by which a signal on a cell’s surface is converted into a specific cellular response Signal transduction pathways convert signals on a cell’s surface into cellular responses Fig. 11-2  factor Receptor 1 Exchange a  of mating factors a factor Yeast cell, Yeast cell, mating type a mating type  2 Mating a  3 New a/ a/ cell  Local and Long-Distance Signaling Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells In local signaling, animal cells may communicate by direct contact, or cell-cell recognition Fig. 11-4 Plasma membranes Gap junctions Plasmodesmata between animal cells between plant cells (a) Cell junctions (b) Cell-cell recognition  In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances In long-distance signaling, plants and animals use chemicals called hormones Fig. 11-5 Local signaling Long-distance signaling Target cell Electrical signal Endocrine cell Blood along nerve cell vessel triggers release of neurotransmitter Neurotransmitter Secreting Secretory diffuses across cell vesicle synapse Hormone travels in bloodstream to target cells Local regulator diffuses through Target cell Target extracellular fluid is stimulated cell (a) Paracrine signaling (b) Synaptic signaling (c) Hormonal signaling  Three stages of cell signaling Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals went through three processes: Reception Transduction Response Fig. 11-6-1 EXTRACELLULAR CYTOPLASM FLUID Plasma membrane 1 Reception Receptor Signaling molecule Fig. 11-6-2 EXTRACELLULAR CYTOPLASM FLUID Plasma membrane 1 Reception 2 Transduction Receptor Relay molecules in a signal transduction pathway Signaling molecule Fig. 11-6-3 EXTRACELLULAR CYTOPLASM FLUID Plasma membrane 1 Reception 2 Transduction 3 Response Receptor Activation of cellular response Relay molecules in a signal transduction pathway Signaling molecule Reception: A signal molecule binds to a receptor protein, causing it to change shape The binding between a signal molecule (ligand) and receptor is highly specific A shape change in a receptor is often the initial transduction of the signal Most signal receptors are plasma membrane proteins  Receptors in the Plasma Membrane Most water-soluble signal molecules bind to specific sites on receptor proteins in the plasma membrane There are three main types of membrane receptors: G protein-coupled receptors Receptor tyrosine kinases Ion channel receptors  A G protein-coupled receptor is a plasma membrane receptor that works with the help of a G protein The G protein acts as an on/off switch: If GDP is bound to the G protein, the G protein is inactive Fig. 11-7a Signaling-molecule binding site Segment that interacts with G proteins G protein-coupled receptor Fig. 11-7b G protein-coupled Plasma Inactive membrane Activated Signaling molecule enzyme receptor receptor GDP G protein Enzyme GDP GTP CYTOPLASM (inactive) 1 2 Activated enzyme GTP GDP Pi Cellular response 3 4  Receptor tyrosine kinases are membrane receptors that attach phosphates to tyrosines A receptor tyrosine kinase can trigger multiple signal transduction pathways at once Fig. 11-7c Signaling Ligand-binding site molecule (ligand) Signaling molecule  Helix Tyr Tyr Tyr Tyr Tyr Tyr Tyrosines Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Tyr Receptor tyrosine kinase proteins Dimer CYTOPLASM 1 2 Activated relay proteins Cellular Tyr Tyr P Tyr Tyr P Tyr Tyr P response 1 P Tyr Tyr P Tyr Tyr P Tyr Tyr P P Tyr Tyr P Tyr Tyr P P Tyr Tyr P Cellular 6 ATP 6 ADP response 2 Activated tyrosine Fully activated receptor kinase regions tyrosine kinase Inactive relay proteins 3 4  A ligand-gated ion channel receptor acts as a gate when the receptor changes shape When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor Fig. 11-7d 1 Signaling Gate molecule closed Ions (ligand) Plasma Ligand-gated membrane ion channel receptor 2 Gate open Cellular response 3 Gate closed  Transduction: Cascades of molecular interactions relay signals from receptors to target molecules in the cell Signal transduction usually involves multiple steps Multistep pathways can amplify a signal: A few molecules can produce a large cellular response Multistep pathways provide more opportunities for coordination and regulation of the cellular response  Signal Transduction Pathway The molecules that relay a signal from receptor to response are mostly proteins Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated. At each step, the signal is transduced into a different form, usually a shape change in a protein  Protein Phosphorylation and Dephosphorylation In many pathways, the signal is transmitted by a cascade of protein phosphorylations Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation Fig. 11-9 Signaling molecule Receptor Activated relay molecule Inactive protein kinase 1 Active protein kinase 1 Inactive protein kinase ATP ADP Active P 2 protein PP kinase Pi 2 Inactive protein kinase ATP ADP Active P 3 protein PP kinase Pi 3 Inactive protein ATP ADP P Active Cellular protein response PP Pi  Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off  Response: Cell signaling leads to regulation of transcription or cytoplasmic activities The cell’s response to an extracellular signal is sometimes called the “output response” Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or may involve action in the nucleus  Response: Cell signaling leads to regulation of transcription or cytoplasmic activities Many signaling pathways regulate the synthesis of enzymes or other proteins, usually by turning genes on or off in the nucleus The final activated molecule may function as a transcription factor Fig. 11-14 Growth factor Reception Receptor Phosphorylation cascade Transduction CYTOPLASM Inactive Active transcription transcription factor factor Response P DNA Gene NUCLEUS mRNA Other pathways Reception Binding of epinephrine to G protein-coupled receptor (1 molecule) regulate the Transduction activity of Inactive G protein Active G protein (102 molecules) enzymes Inactive adenylyl cyclase Active adenylyl cyclase (102) ATP Cyclic AMP (104) Inactive protein kinase A Active protein kinase A (104) Inactive phosphorylase kinase Active phosphorylase kinase (105) Inactive glycogen phosphorylase Active glycogen phosphorylase (106) Response Glycogen Glucose-1-phosphate (108 molecules)  Signaling pathways can also affect the physical characteristics of a cell, for example, cell shape  Active/ No help/ Passive membrane proteins/ vesicles DIFFUSION FACILITATED DIFFUSIO-- CARRIER PROTEINS FACILITATED DIFFUSIO– AQUAPORINS ACILITATED DIFFUSION CHANNEL PROTEINS Na+ - K + PUMP  Active/ No help/ Passive membrane proteins/ vesicles PROTON PUMP CO-TRANSPORT RECEPTOR MEDIATED ENDOCYTOSIS PHAGOCYTOSIS PINOCYTOSIS EXOCYOTOSIS