CAMPBELL BIOLOGY IN FOCUS URRY CAIN WASSERMAN MINORSKY REECE 5 Membrane Transport and Cell Signaling Lecture Presentations by Kathleen Fitzpatrick and Nicole Tunbridge, Simon Fraser University 2016 Pearson Education, Inc. SECOND EDITION Overview: Life at the Edge The plasma membrane separates the living cell

from its surroundings The plasma membrane exhibits selective permeability, allowing some substances to cross it more easily than others 2016 Pearson Education, Inc. Concept 5.1: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in most membranes Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions A phospholipid bilayer can exist as a stable boundary between two aqueous compartments 2016 Pearson Education, Inc.

Figure 5.2 Fibers of extracellular matrix (ECM) Glycoprotein Carbohydrate Phospholipid Cholesterol Microfilaments Peripheral of cytoskeleton proteins 2016 Pearson Education, Inc. Glycolipid EXTRACELLULAR

SIDE OF MEMBRANE Integral protein CYTOPLASMIC SIDE OF MEMBRANE Most membrane proteins are also amphipathic and reside in the bilayer with their hydrophilic portions protruding The fluid mosaic model states that the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of phospholipids Groups of certain proteins or certain lipids may associate in long-lasting, specialized patches 2016 Pearson Education, Inc.

Figure 5.3 Two phospholipids Hydrophilic head WATER Hydrophobic tail WATER 2016 Pearson Education, Inc. The Fluidity of Membranes Most of the lipids and some proteins in a membrane

can shift about laterally The lateral movement of phospholipids is rapid; proteins move more slowly Some proteins move in a directed manner; others seem to be anchored in place 2016 Pearson Education, Inc. 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 A membrane 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 The steroid cholesterol has different effects on membrane fluidity at different temperatures

At warm temperatures (such as 37C), cholesterol restrains movement of phospholipids At cool temperatures, it maintains fluidity by preventing tight packing 2016 Pearson Education, Inc. Figure 5.5 (a) Unsaturated versus saturated hydrocarbon tails. Fluid Unsaturated tails prevent packing. Viscous Saturated tails pack

together. (b) Cholesterol reduces membrane fluidity at moderate temperatures, but at low temperatures hinders solidification. Cholesterol 2016 Pearson Education, Inc. Membrane Proteins (IPs) 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 membranes specific functions 2016 Pearson Education, Inc.

Integral proteins penetrate the hydrophobic interior of the lipid bilayer 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 helices Peripheral proteins are loosely bound to the surface of the membrane 2016 Pearson Education, Inc. Figure 5.6 N-terminus EXTRACELLULAR SIDE

helix C-terminus 2016 Pearson Education, Inc. CYTOPLASMIC SIDE Six major functions of membrane proteins Transport Enzymatic activity Signal transduction

Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) 2016 Pearson Education, Inc. Figure 5.7 Enzymes ATP (a) Transport (b) Enzymatic activity Signaling molecule

Receptor Signal transduction (c) Signal transduction Glycoprotein (d) Cell-cell recognition 2016 Pearson Education, Inc. (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 containing carbohydrates, on the extracellular surface of 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 2016 Pearson Education, Inc. Synthesis and Sidedness of Membranes Membranes have distinct inside and outside faces The asymmetrical arrangement of proteins, lipids, and associated carbohydrates in the plasma membrane is determined as the membrane is built by the ER and Golgi apparatus

2016 Pearson Education, Inc. Figure 5.8 Transmembrane glycoproteins Secretory protein Golgi apparatus Vesicle Attached carbohydrate ER Glycolipid

ER lumen Plasma membrane: Cytoplasmic face Extracellular face 2016 Pearson Education, Inc. Transmembrane glycoprotein Secreted protein Membrane glycolipid Concept 5.2: Membrane structure results in selective permeability

A cell must regulate transport of substances across cellular boundaries Plasma membranes are selectively permeable, regulating the cells molecular traffic 2016 Pearson Education, Inc. The Permeability of the Lipid Bilayer Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer of the membrane and cross it easily Polar molecules, such as sugars, do not cross the membrane easily 2016 Pearson Education, Inc. 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 2016 Pearson Education, Inc. 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 2016 Pearson Education, Inc. Concept 5.3: 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 Although each molecule moves randomly, diffusion of a population of molecules may be directional At dynamic equilibrium, as many molecules cross the membrane in one direction as in the other 2016 Pearson Education, Inc. Figure 5.9 Molecules of dye Membrane (cross section) WATER

Net diffusion Net diffusion Equilibrium (a) Diffusion of one solute Net diffusion Net diffusion Net diffusion Net diffusion (b) Diffusion of two solutes

2016 Pearson Education, Inc. Equilibriu m Equilibriu m Substances diffuse down their concentration gradient, from where it is more concentrated to where it is less concentrated 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 no energy is expended by the cell to make it happen 2016 Pearson Education, Inc.

Effects of Osmosis on Water Balance Osmosis is the diffusion of free 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 until the solute concentration is equal on both sides 2016 Pearson Education, Inc. Figure 5.10-1 Lower concentration of solute (sugar) Higher concentration

of solute Sugar molecule H2O 2016 Pearson Education, Inc. More similar concentrations of solute Figure 5.10-2 Selectively permeable membrane Water molecules can pass through

pores, but sugar molecules cannot. Water molecules cluster around sugar molecules. This side has This side has more fewer solute solute molecules molecules and fewer free and water molecules. more free

Osmosis water molecules Water moves from an area of . higher to lower free water concentration (lower to higher solute concentration). 2016 Pearson Education, Inc. Water Balance of Cells Without Walls 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 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 2016 Pearson Education, Inc. Figure 5.11a (a) Animal cell Hypotonic solution 2016 Pearson Education, Inc. H2O Lysed Isotonic solution

H2O Hypertonic solution H2O Normal H2O Shriveled 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 2016 Pearson Education, Inc. Figure 5.12 Contractile vacuole 2016 Pearson Education, Inc. 50 m 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 (very

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 In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis 2016 Pearson Education, Inc. Figure 5.11b (b) Plant cell Hypotonic solution Plasma Cell wall

membrane H2O Turgid (normal) 2016 Pearson Education, Inc. Isotonic solution H2O H2O Flaccid Hypertonic solution Plasma membrane

H2O Plasmolyzed 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) 2016 Pearson Education, Inc.

Figure 5.13 EXTRACELLULAR FLUID (a) A channel protein Channel protein CYTOPLASM 2016 Pearson Education, Inc. Solute Concept 5.4: Active transport uses energy to move solutes against their gradients Facilitated diffusion speeds transport of a solute by providing efficient passage through the membrane

but does not alter the direction of transport Some transport proteins, however, can move solutes against their concentration gradients 2016 Pearson Education, Inc. The Need for Energy in Active Transport Active transport moves substances against their concentration gradients Active transport requires energy, usually in the form of ATP 2016 Pearson Education, Inc. Active transport allows cells to maintain concentration gradients that differ from their surroundings The sodium-potassium pump is one type of active

transport system 2016 Pearson Education, Inc. Figure 5.14 The sodium-potassium pump: a specific case of active transport via carrier IPs EXTRACELLULAR FLUID Na+ [Na+] high [K+] low Na+ Na+

Na+ Na+ CYTOPLASM ATP P ADP + ] low Na+ [Na + [K ] high +

K + K Na+ Na+ + K + K + K

P + K P 2016 Pearson Education, Inc. Pi Na+ Figure 5.14-1 ZOOMED IN EXTRACELLULAR FLUID

[Na+] high [K+] low Na+ Na + Na+ Na+ Na+ Na +

CYTOPLASM [Na+] low + [K ] high Cytoplasmic Na+ binds to the sodium-potassium + pump. The affinity for Na is high when the protein has this shape. 2016 Pearson Education, Inc. P ATP

ADP Na+ binding stimulates phosphorylation by ATP. Figure 5.14-2 ZOOMED IN Na+ Na + + K Na+

+ K P P Pi Phosphorylation leads to The new shape has a high + a change in protein shape, affinity for K , which binds on

+ reducing its affinity for Na , the which is released outside. extracellular side and triggers release of the phosphate group. 2016 Pearson Education, Inc. Figure 5.14-3 ZOOMED IN + K + K

+ K + K Loss of the phosphate group restores the proteins original shape, which has a lower affinity for K+. 2016 Pearson Education, Inc. K+ is released; affinity for Na+ is high again, and the cycle repeats. Figure 5.15

Passive transport Diffusion 2016 Pearson Education, Inc. Facilitated diffusion Active transport ATP How Ion Pumps Maintain Membrane Potential Membrane potential is the voltage across a membrane Voltage is created by differences in the distribution of positive and negative ions across a membrane

2016 Pearson Education, Inc. Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membrane A chemical force (the ions concentration gradient) An electrical force (the effect of the membrane potential on the ions movement) 2016 Pearson Education, Inc. 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

Electrogenic pumps help store energy that can be used for cellular work 2016 Pearson Education, Inc. Figure 5.16 ATP H+ EXTRACELLULAR FLUID H+ H+ Proton pump H+

H+ CYTOPLASM 2016 Pearson Education, Inc. H+ 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 2016 Pearson Education, Inc.

Figure 5.17 + Sucrose Sucrose Sucrose-H+ cotransport er Diffusion of H+ + H

H+ + H+ + H+ H+ H+ ATP

2016 Pearson Education, Inc. H+ Proton pump + H+ H+ Concept 5.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Water and small solutes enter or leave the cell through the lipid bilayer or by means of transport proteins

Large molecules, such as polysaccharides and proteins, cross the membrane in bulk by means of vesicles Bulk transport requires energy 2016 Pearson Education, Inc. Exocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents Many secretory cells use exocytosis to export products Endocytosis In endocytosis, the cell takes in molecules and particulate matter by forming new 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 2016 Pearson Education, Inc. Figure 5.18 Pinocytosis Phagocytosis Receptor-Mediated Endocytosis EXTRACELLULAR FLUID Solutes

Pseudopodium Plasma membrane Coat protein Food or other particle Food vacuole CYTOPLASM 2016 Pearson Education, Inc. Coated pit

Coated vesicle Receptor Concept 5.6: The plasma membrane 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 Communication between cells is also essential for many unicellular organisms 2016 Pearson Education, Inc. Local and Long-Distance Signaling 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) The free passage of substances in the cytosol from one cell to another is a type of local signaling 2016 Pearson Education, Inc. In many other cases of local signaling, messenger molecules are secreted by a signaling cell These messenger molecules, called local regulators, travel only short distances One class of these, growth factors, stimulates nearby cells to grow and divide This type of local signaling in animal cells is called paracrine signaling 2016 Pearson Education, Inc.

Figure 5.19-1 Local signaling Target cells Secreting cell Secretory vesicles Local regulator (a) Paracrine signaling 2016 Pearson Education, Inc. Another more specialized type of local signaling occurs in the animal nervous system

This synaptic signaling consists of an electrical signal moving along a nerve cell that triggers secretion of neurotransmitter molecules These diffuse across the space between the nerve cell and its target, triggering a response in the target cell 2016 Pearson Education, Inc. Figure 5.19-2 Local signaling Electrical signal triggers release of neurotransmitter. Neurotransmitter diffuses across synapse.

Target cell (b) Synaptic signaling 2016 Pearson Education, Inc. 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 Hormones vary widely in size and shape 2016 Pearson Education, Inc. Figure 5.19-3 Long-distance signaling Endocrine cell

Target cell specifically binds hormone. Hormone travels in bloodstream. Blood vessel (c) Endocrine (hormonal) signaling 2016 Pearson Education, Inc. 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 2016 Pearson Education, Inc. Figure 5.20-s1 EXTRACELLULAR FLUID Reception Receptor Signaling molecule 2016 Pearson Education, Inc.

CYTOPLASM Plasma membrane Figure 5.20-s2 EXTRACELLULAR FLUID Reception CYTOPLASM Plasma membrane Transduction Receptor 1 2 Relay molecules

Signaling molecule 2016 Pearson Education, Inc. 3 Figure 5.20-s3 EXTRACELLULAR FLUID Reception CYTOPLASM Plasma membrane Transduction

Response Receptor 1 2 Relay molecules Signaling molecule 2016 Pearson Education, Inc. 3 Activation Reception, the Binding of a Signaling Molecule to a

Receptor 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 membrane proteins 2016 Pearson Education, Inc. Receptors in the Plasma Membrane Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane There are two main types of membrane receptors G protein-coupled receptors

Ligand-gated ion channels 2016 Pearson Education, Inc. G protein-coupled receptors (GPCRs) are plasma membrane receptors that work with the help of a G protein G proteins bind to the energy-rich molecule GTP Many G proteins are very similar in structure GPCR pathways are extremely diverse in function 2016 Pearson Education, Inc. Figure 5.21-s1 Activated GPCR

Signaling molecule GTP CYTOPLASM 2016 Pearson Education, Inc. Activated G protein Inactive enzyme Plasma membrane

Figure 5.21-s2 Activated GPCR Signaling molecule GTP CYTOPLASM Inactive enzyme Plasma membrane

Activated G protein Activated enzyme GTP Cellular response 2016 Pearson Education, Inc. 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 Ca2+, 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 2016 Pearson Education, Inc. Figure 5.22-s1 Signaling molecule (ligand) Gate closed Ligand-gated ion channel receptor 2016 Pearson Education, Inc.

Ions Plasma membrane Figure 5.22-s2 Signaling molecule (ligand) Gate closed Ligand-gated ion channel receptor 2016 Pearson Education, Inc.

Gate open Ions Plasma membrane Cellular response Figure 5.22-s3 Signaling molecule (ligand) Gate closed

Ligand-gated ion channel receptor Gate open Ions Plasma membrane Gate closed 2016 Pearson Education, Inc. Cellular response Intracellular Receptors Intracellular receptor proteins are found in the

cytosol or nucleus of target cells Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors Examples of hydrophobic messengers are the steroid and thyroid hormones of animals and nitric oxide (NO) in both plants and animals 2016 Pearson Education, Inc. Aldosterone behaves similarly to other steroid hormones It is secreted by cells of the adrenal gland and enters cells all over the body, but only kidney cells contain receptor cells for aldosterone The hormone binds the receptor protein and activates it The active form of the receptor enters the nucleus, acts as a transcription factor, and activates genes

that control water and sodium flow 2016 Pearson Education, Inc. Figure 5.23 Hormone (aldosterone) EXTRACELLULAR FLUID Plasma membrane Receptor protein

Hormonereceptor complex DNA mRNA NUCLEUS CYTOPLASM 2016 Pearson Education, Inc. New protein 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 2016 Pearson Education, Inc. The molecules that relay a signal from receptor to response are often proteins Like falling dominoes, the activated 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, commonly a shape change in a protein 2016 Pearson Education, Inc. Protein Phosphorylation and Dephosphorylation Phosphorylation and dephosphorylation are a

widespread cellular mechanism for regulating protein activity Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation A signaling pathway involving phosphorylation and dephosphorylation can be referred to as a phosphorylation cascade The addition of phosphate groups often changes the form of a protein from inactive to active 2016 Pearson Education, Inc. Figure 5.24 Signaling molecule Activated relay molecule Receptor

o Ph Inactive protein kinase 1 n ATP Active protein kinase 2 Pi

Inactive protein e ad sc ca ADP PP P ATP ADP Pi

2016 Pearson Education, Inc. tio yla or Inactive protein kinase 2 h sp Active protein kinase 1

PP P Active protein Cellular response Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation Phosphatases provide a mechanism for turning off the signal transduction pathway They also make protein kinases available for reuse, enabling the cell to respond to the signal again 2016 Pearson Education, Inc.

Small Molecules and Ions as Second Messengers The extracellular signal molecule (ligand) that binds to the receptor is a pathways first messenger Second messengers are small, nonprotein, watersoluble molecules or ions that spread throughout a cell by diffusion Cyclic AMP and calcium ions are common second messengers 2016 Pearson Education, Inc. Cyclic AMP (cAMP) is one of the most widely used second messengers Adenylyl cyclase, an enzyme in the plasma membrane, rapidly converts ATP to cAMP in response to a number of extracellular signals The immediate effect of cAMP is usually the activation of protein kinase A, which then

phosphorylates a variety of other proteins 2016 Pearson Education, Inc. Figure 5.25 First messenger (signaling molecule such as epinephrine) G protein Adenylyl cyclase GTP G protein-coupled receptor

ATP cAMP Second messenger Protein kinase A Cellular responses 2016 Pearson Education, Inc. Response: Regulation of Transcription or Cytoplasmic Activities Ultimately, a signal transduction pathway leads to regulation of one or more cellular activities The response may occur in the cytoplasm or in the

nucleus 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 in the signaling pathway may function as a transcription factor 2016 Pearson Education, Inc. Figure 5.26 Growth factor Receptor Phosphorylation cascade Reception

Transduction CYTOPLASM Inactive transcription factor Active transcription factor P Response DNA Gene

NUCLEUS 2016 Pearson Education, Inc. mRNA 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 2016 Pearson Education, Inc. Figure 5.UN04 Passive transport: Facilitated diffusion Channel protein

2016 Pearson Education, Inc. Carrier protein Figure 5.UN05 Active transport ATP 2016 Pearson Education, Inc. Figure 5.UN06 Transduction

Reception Response Receptor 1 2 Relay molecules Signaling molecule 2016 Pearson Education, Inc. 3 Activation

of cellular response

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