CONCEPT 5.1: Cellular membranes are fluid mosaics of lipids and proteins

Transcription

1 Ch 5 Membrane Transport and Signaling Overview The plasma separates the living cell from its surroundings The plasma exhibits selective permeability, allowing some substances to cross it more easily than others CONCEPT 5.1: Cellular s are fluid mosaics of lipids and s Phospholipids are the most abundant lipid in most s Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The phospholipid bilayer exists as a boundary between two aqueous compartments Most s are also amphipathic and reside in the bilayer with their hydrophilic portions protruding The fluid mosaic model describes the as a mosaic of molecules bobbing in a fluid bilayer of phospholipids Figure 5.2 Figure 5.3 Fibers of extracellular matrix (ECM) Hydrophilic head WATER Glyco Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE WATER Microfilaments of cytoskeleton Cholesterol Peripheral s Integral IC SIDE OF MEMBRANE Hydrophobic tail The Fluidity of Membranes Most of the lipids and some s in a can shift about laterally The lateral movement of phospholipids is rapid; s move more slowly Some s movement is directed; others seem to be anchored in place Mouse cell Membrane s Human cell Hybrid cell Mixed s after 1 hour As temperatures cool, s switch from a fluid state to a solid state The temperature at which a solidifies depends on the types of lipids A remains fluid to a lower temperature if it is rich in phospholipids with unsaturated hydrocarbon tails Membranes must be fluid to work properly; they are usually about as fluid as salad oil 1

2 Figure 5.5 Fluid Viscous The steroid cholesterol has different effects on fluidity at different temperatures At warm temperatures (such as 37 o C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing Evolution Variations in lipid composition of cell s of many species appear to be adaptations to specific environmental conditions Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary Unsaturated tails prevent packing. Saturated tails pack together. (a) Unsaturated versus saturated hydrocarbon tails (b) Cholesterol reduces fluidity at moderate temperatures, but at low temperatures hinders solidification. Cholesterol Membrane Proteins and Their Functions Figure 5.2 A is a collage of different s embedded in the fluid matrix of the lipid bilayer Proteins determine most of the s specific functions Integral s penetrate the hydrophobic interior of the lipid bilayer Integral s that span the are called trans s The hydrophobic regions of an integral consist of one or more stretches of nonpolar amino acids, often coiled into α helices Peripheral s are loosely bound to the surface of the Fibers of extracellular matrix (ECM) Glyco Microfilaments of cytoskeleton Carbohydrate Cholesterol Peripheral s Glycolipid Integral EXTRACELLULAR SIDE OF MEMBRANE IC SIDE OF MEMBRANE Figure 5.7 Enzymes Six major functions of s Transport Enzymatic activity Attachment to the cytoskeleton and extracellular matrix (ECM) Cell-cell recognition Intercellular joining Signal transduction (a) Transport Glyco ATP (b) Enzymatic activity (c) Attachment to the cytoskeleton and extracellular matrix (ECM) Signaling molecule (d) Cell-cell recognition (e) Intercellular joining (f) Signal transduction 2

3 The Role of Membrane Carbohydrates in Cell-Cell Recognition Cells recognize each other by binding to surface molecules, often containing carbohydrates, on the extracellular surface of the plasma Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or, more commonly, to s (forming glycos) Carbohydrates on the external side of the plasma vary among species, individuals, and even cell types in an individual Membranes faces (outside/inside) determined when made by ER and Golgi Plasma : Cytoplasmic face Extracellular face ER ER lumen Glycolipid Trans glycos Trans glyco Secretory Golgi apparatus Membrane glycolipid Vesicle Secreted CONCEPT 5.2: Membrane structure results in selective permeability A cell must regulate transport of substances across cellular boundaries Plasma s are selectively permeable, regulating the cell s molecular traffic Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer of the and cross it easily Polar molecules, such as sugars, do not cross the easily Transport Proteins Transport s allow passage of hydrophilic substances across the Some transport s, called channel s, have a hydrophilic channel that certain molecules or ions can use as a tunnel Channel s called aquaporins facilitate the passage of water Other transport s, called carrier s, bind to molecules and change shape to shuttle them across the A transport is specific for the substance it moves CONCEPT 5.3: Passive transport is diffusion of a substance across a with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules appears directional At dynamic equilibrium, as many molecules cross the in one direction as in the other Figure 5.9 Molecules of dye WATER Net diffusion (a) Diffusion of one solute Membrane (cross section) Net diffusion Equilibrium Net diffusion Net diffusion Equilibrium Net diffusion Net diffusion (b) Diffusion of two solutes Equilibrium 3

4 Effects of Osmosis on Water Balance Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated No work is done to move substances down the concentration gradient The diffusion of a substance across a biological is passive transport because no energy is expended by the cell to make it happen Osmosis is the diffusion of free water across a selectively permeable Water diffuses across a from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides Figure 5.10 Lower concentration of solute (sugar) Higher concentration of solute More similar concentrations of solute Water Balance of Cells Without Walls Sugar molecule H 2 O Selectively permeable Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as inside the cell; no net water movement across the plasma 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 Osmosis Figure 5.11 Plant cell Animal cell Hypotonic Isotonic H 2 O H 2 O H 2 O Lysed Normal Cell wall H 2 O H 2 O H 2 O Hypertonic H 2 O Shriveled H 2 O Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments The protist Paramecium caudatum, which is hypertonic to its pondwater environment, has a contractile vacuole that can pump excess water out of the cell Contractile vacuole Turgid (normal) Flaccid Plasmolyzed 4

5 Water Balance of Cells with Walls Figure 5.11 Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (very firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell In a hypertonic environment, plant cells lose water; eventually, the pulls away from the wall, a usually lethal effect called plasmolysis Plant cell Animal cell Hypotonic Isotonic H 2 O H 2 O H 2 O Lysed Normal Cell wall H 2 O H 2 O H 2 O Hypertonic H 2 O Shriveled H 2 O Turgid (normal) Plasmolyzed Facilitated Diffusion: Passive Transport Aided by Proteins No net energy input is required Facilitated diffusion: transport s speed passive movement of molecules across the Channel s provide corridors that allow a specific molecule or ion to cross the Channel s include Aquaporins, for facilitated diffusion of water Ion channels that open or close in response to a stimulus (gated channels) Figure 5.13 EXTRA- CELLULAR Channel Solute (a) A channel Carrier s undergo a subtle change in shape that translocates the solute-binding site across the The shape change may be triggered by binding and release of the transported molecule Carrier (b) A carrier Solute CONCEPT 5.4: Active transport uses energy to move solutes against their gradients Figure 5.14 EXTRACELLULAR [Na + ] high [K + ] low Some transport s, however, can move solutes against their concentration gradients [Na + ] low 1 [K + ] high 2 ADP Active transport moves substances against their concentration gradients Active transport requires cell energy, usually in the form of ATP 6 3 Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active transport system 5 4 5

6 Figure 5.15 Passive transport Active transport How Ion Pumps Maintain Membrane Potential Diffusion Facilitated diffusion Membrane potential is the voltage across a Voltage is created by differences in the distribution of positive and negative ions across a Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a A chemical force (the ion s concentration gradient) An electrical force (the effect of the potential on the ion s movement) An electrogenic pump is a transport that generates voltage across a 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 (H + ) Electrogenic pumps help store energy that can be used for cellular work Cotransport: Coupled Transport by a Membrane Protein Cotransport occurs when active transport of a solute indirectly drives transport of other solutes Plant cells use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell Proton pump Proton pump EXTRACELLULAR Sucrose-H + cotransporter Diffusion of H + Sucrose CONCEPT 5.5: Bulk transport across the plasma occurs by exocytosis and endocytosis Small solutes and water enter or leave the cell through the lipid bilayer or by means of transport s Large molecules, such as polysaccharides and s, cross the in bulk by means of vesicles Bulk transport requires energy Endocytosis In endocytosis, the cell takes in molecules and particulate matter by forming new vesicles from the plasma Endocytosis is a reversal of exocytosis There are three types of endocytosis Phagocytosis ( cellular eating ) Pinocytosis ( cellular drinking ) -mediated endocytosis 6

7 Figure 5.18 Phagocytosis EXTRACELLULAR Solutes Pseudopodium Food or other particle Coated pit Pinocytosis Plasma Coat -Mediated Endocytosis Exocytosis In exocytosis, transport vesicles migrate to the, fuse with it, and release their contents Many secretory cells use exocytosis to export products Food vacuole Coated vesicle CONCEPT 5.6: The plasma plays a key role in most cell signaling In multicellular organisms, cell-to-cell communication allows the cells of the body to coordinate their activities Eukaryotic cells may communicate by direct contact Animal and plant cells have junctions that directly connect the cytoplasm of adjacent cells These are called gap junctions (animal cells) and plasmodesmata (plant cells) Local and Long-Distance Signaling In many cases of local signaling, messenger molecules are secreted by a signaling cell Messenger molecules, called local regulators, travel short distances (paracrine signaling) One class of these, growth factors, stimulates nearby cells to grow and divide Another more specialized type of local signaling occurs in the animal nervous system (synaptic signaling) These diffuse across the space between the nerve cell and its target, triggering a response in the target cell (Neurotrasmitters) Figure 5.19 In long-distance signaling, plants and animals use chemicals called hormones In hormonal signaling in animals (called endocrine signaling), specialized cells release hormone molecules that travel via the circulatory system Local signaling Secreting cell Local regulator diffuses through extracellular fluid. (a) Paracrine signaling Target cell Secretory vesicle Target cell is stimulated. (b) Synaptic signaling Electrical signal along nerve cell triggers release of neurotransmitter. Neurotransmitter diffuses across synapse. Long-distance signaling Endocrine cell Target cell specifically binds hormone. Blood vessel Hormone travels in bloodstream. (c) Endocrine (hormonal) signaling 7

8 The Three Stages of Cell Signaling: A Preview Earl W. Sutherland discovered how the hormone epinephrine acts on cells Sutherland suggested that cells receiving signals undergo three processes Reception, Transduction, Response EXTRACELLULAR Signaling molecule Reception Plasma Transduction Relay molecules Response Activation Reception, the Binding of a Signaling Molecule to a Protein The binding between a signal molecule (ligand) and receptor is highly specific Ligand binding generally causes a shape change in the receptor, many receptors are directly activated by this shape change Most signal receptors are plasma s Most water-soluble signal molecules bind to specific sites on receptor s that span the plasma There are two main types of receptors G -coupled receptors Ligand-gated ion channels G -coupled receptors (GPCRs) are plasma receptors that work with the help of a G Figure Activated GPCR Signaling molecule Plasma Activated G Inactive enzyme G s bind to the energy-rich molecule GTP The G acts as an on-off switch: If GTP is bound to the G, the G is inactive 2 Activated enzyme Many G s are very similar in structure Cellular response Figure A ligand-gated ion channel receptor acts as a gate for ions 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 Ca 2+, through a channel in the receptor Ligand-gated ion channels are very important in the nervous system The diffusion of ions through open channels may trigger an electric signal 1 Signaling molecule (ligand) Gate closed Ligand-gated ion channel receptor 3 Ions Plasma 2 Gate closed Gate open Cellular response 8

9 Intracellular s Figure 5.23 Hormone (testosterone) EXTRA- CELLULAR Intracellular receptor s are found in the cytosol or nucleus of target cells Small or hydrophobic chemical messengers can cross the and activate receptors Plasma Hormonereceptor complex Examples of hydrophobic messengers are the steroid and thyroid hormones of animals and nitric oxide (NO) in both plants and animals. DNA Cells that contain internal testosterone bind and respond activating the receptor The active form of the receptor enters the nucleus, acts as a transcription factor, and activates genes needed for male sex characteristics mrna NUCLEUS New Transduction by Cascades of Molecular Interactions 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 than simpler systems do The molecules that relay a signal from receptor to response are mostly s At each step, the signal is transduced into a different form, usually a shape change in a Protein Phosphorylation and Dephosphorylation Phosphorylation and dephosphorylation are a widespread cellular mechanism for regulating activity Protein kinases transfer phosphates from ATP to, a process called phosphorylation The addition of phosphate groups often changes the form of a from inactive to active Figure 5.24 Signaling molecule Inactive kinase 1 Inactive kinase 2 Activated relay molecule Active kinase 1 ADP Active kinase 2 Phosphorylation cascade Protein phosphatases remove the phosphates from s, a process called dephosphorylation Phosphatases provide a mechanism for turning off the signal transduction pathway They also make kinases available for reuse, enabling the cell to respond to the signal again Inactive ADP Active Cellular response 9

10 Small Molecules and Ions as Second Messengers The extracellular signal molecule (ligand) that binds to the receptor is a pathway s first messenger Second messengers are small, non, watersoluble molecules or ions that spread throughout a cell by diffusion Cyclic AMP (camp) and calcium ions are common second messengers Adenylyl cyclase, an enzyme in the plasma, rapidly converts ATP to camp in response to a number of extracellular signals Figure 5.25 G -coupled receptor First messenger (signaling molecule such as epinephrine) G Adenylyl cyclase Cellular responses Second messenger Protein kinase A Response: Regulation of Transcription or Cytoplasmic Activities Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may be in the cytoplasm or nucleus Many signaling pathways regulate the synthesis of enzymes or other s, usually by turning genes on or off in the nucleus The final activated molecule in the signaling pathway may function as a transcription factor Other pathways regulate the activity of enzymes rather than their synthesis, such as the opening of an ion channel or a change in cell metabolism Figure 5.26 Inactive transcription factor DNA NUCLEUS Growth factor Phosphorylation cascade Active transcription factor mrna Reception Transduction Gene Response The Evolution of Cell Signaling Universal mechanisms of cellular regulation is evidence of the evolutionary relatedness of all life Scientists think that signaling mechanisms first evolved in ancient prokaryotes and single-celled eukaryotes These mechanisms were adopted for new uses in their multicellular descendants 10

11 Figure 5.UN03 Passive transport: Facilitated diffusion Figure 5.UN04 Active transport Channel Carrier Figure 5.UN05 1 Reception 2 Transduction 3 Response Relay molecules Activation of cellular response Signaling molecule 11