Cell Membranes and Signaling

Cell Membranes and Signaling
5 Cell Membranes and Signaling

Chapter 5 Cell Membranes and Signaling
Key Concepts 5.1 Biological Membranes Have a Common Structure and Are Fluid 5.2 Passive Transport across Membranes Requires No Input of Energy 5.3 Active Transport Moves Solutes against Their Concentration Gradients 5.4 Large Molecules Cross Membranes via Vesicles

Chapter 5 Cell Membranes and Signaling
5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals 5.6 Signal Transduction Allows the Cell to Respond to Its Environment

Chapter 5 Opening Question
What role does the cell membrane play in the body’s response to caffeine?

Concept 5.1 Biological Membranes Have a Common Structure and Are Fluid
A membrane’s structure and functions are determined by its constituents: lipids, proteins, and carbohydrates. The general design of membranes is known as the fluid mosaic model. Phospholipids form a continuous bilayer which is like a “lake” in which a variety of proteins “float.”

Figure 5.1 Membrane Structure
Figure 5.1 Membrane Structure The general molecular structure of biological membranes is a continuous phospholipid bilayer in which proteins are embedded. The phospholipid bilayer separates two aqueous regions, the external environment outside the cell and the cell cytoplasm.

The lipid molecules are usually phospholipids with two regions: Hydrophilic regions—electrically charged “heads” associate with water molecules Hydrophobic regions—nonpolar fatty acid “tails” that do not dissolve in water

A bilayer is formed when the fatty acid “tails” associate with each other and the polar “heads” face the aqueous environment.

Membranes may differ in lipid composition; there are many types of phospholipids. Phospholipids may differ in: Fatty acid chain length Degree of saturation Kinds of polar groups present

Cholesterol is an important component of animal cell membranes. Hydroxyl groups interact with the polar heads of phospholipids. Cholesterol is important in modulating membrane fluidity; other steroids function as hormones.

In-Text Art, Chapter 5, p. 84 (2)

The fatty acids make the membrane somewhat fluid. This allows some molecules to move laterally within the membrane. Membrane fluidity is influenced by: Lipid composition—short, unsaturated chains increase fluidity Temperature—fluidity decreases in colder conditions

All biological membranes contain proteins; the ratio of proteins to phospholipids varies. Peripheral membrane proteins lack hydrophobic groups and are not embedded in the bilayer. Integral membrane proteins are at least partly embedded in the phospholipid bilayer.

In-Text Art, Chapter 5, p. 85

Anchored membrane proteins have hydrophobic lipid components that anchor them in the bilayer. Proteins are asymmetrically distributed on the inner and outer membrane surfaces. Transmembrane proteins extend through the bilayer; they may have domains with different functions on the inner and outer sides of the membrane.

Some membrane proteins can move within the phosopholipid bilayer; others are restricted. Cell fusion experiments illustrate this migration. Proteins inside the cell can restrict movement of membrane proteins, as can attachments to the cytoskeleton.

Figure 5.2 Rapid Diffusion of Membrane Proteins (Part 1)
Figure 5.2 Rapid Diffusion of Membrane Proteins A human cell can be fused to a mouse cell in the laboratory, forming a single large cell (heterokaryon). This phenomenon was used to test whether membrane proteins can diffuse independently in the plane of the cell membrane.a [a L. Frye and M. Edidin Journal of Cell Science 7: 319–335.]

Diverse carbohydrates are located on the outer cell membrane and play a role in communication. Glycolipid—carbohydrate covalently bonded to a lipid Glycoprotein—one or more oligosaccharides covalently bonded to a protein Proteoglycan—protein with more and longer carbohydrates bonded to it

Cells can adhere to one another through interactions between cell surface carbohydrates and proteins.

Membranes are constantly forming, transforming into other types, fusing, and breaking down. Though membranes appear similar, there are major chemical differences among the membranes of even a single cell.

Two processes of transport across membranes:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Selective permeability: biological membranes allow some substances, but not others, to pass Two processes of transport across membranes: 1. Passive transport does not require metabolic energy. A substance moves down its concentration gradient.

2. Active transport does require input of metabolic energy.
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy 2. Active transport does require input of metabolic energy. A substance moves against its concentration gradient.

Passive transport can occur by:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Passive transport can occur by: Simple diffusion through the phospholipid bilayer Facilitated diffusion through channel proteins or aided by carrier proteins

Concept 5.2 Passive Transport across Membranes Requires No Input of Energy
Diffusion is the process of random movement toward equilibrium; a net movement from regions of greater concentration to regions of lesser concentration.

Speed of diffusion depends on three factors:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Speed of diffusion depends on three factors: Diameter of the molecules—smaller molecules diffuse faster. Temperature of the solution—higher temperatures lead to faster diffusion. Concentration gradient—the greater the concentration gradient, the faster a substance will diffuse.

Diffusion of each solute depends only on its own concentration.
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Cell cytoplasm is an aqueous solution, as is the surrounding environment. Diffusion of each solute depends only on its own concentration. A higher concentration inside the cell causes the solute to diffuse out; higher concentration outside causes the solute to diffuse in.

Some molecules cross the phospholipid bilayer by simple diffusion:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Some molecules cross the phospholipid bilayer by simple diffusion: O2, CO2, and small, nonpolar, lipid-soluble molecules. Polar (hydrophilic) molecules do not pass through—they are not soluble in the hydrophobic interior of the membrane. Amino acids, sugars, ions, water

Osmosis is the diffusion of water across membranes through special channels. It depends on the concentration of water molecules on either side of the membrane— water moves down its concentration gradient. The higher the total solute concentration, the lower the concentration of water molecules.

Osmotic pressure: pressure that must be applied to a solution to prevent flow of water across a membrane by osmosis Π = cRT c = total solute concentration R = the gas constant T = absolute temperature

The higher concentration of a substance on one side of a membrane represents stored energy. If a membrane allows water, but not solutes, to pass through, the net movement of water molecules will be toward the solution with the higher solute concentration and the lower concentration of water molecules.

When comparing two solutions separated by a membrane:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy When comparing two solutions separated by a membrane: A hypertonic solution has a higher solute concentration. Isotonic solutions have equal solute concentrations. A hypotonic solution has a lower solute concentration.

Figure 5.3 Osmosis Can Modify the Shapes of Cells
Figure 5.3 Osmosis Can Modify the Shapes of Cells (A) In a solution that is hypertonic to the cytoplasm of a plant or animal cell, water flows out of the cell. (B) In a solution that is isotonic with the cytoplasm, the cell maintains a consistent, characteristic shape because there is no net movement of water into or out of the cell. (C) In a solution that is hypotonic to the cytoplasm, water enters the cell. An animal cell will swell and may burst under these conditions; a plant cell will not swell too much because of its rigid cell wall.

Concentration of solutes in the environment determines the direction of osmosis in all animal cells. In other organisms, cell walls limit the volume of water that can be taken up. Turgor pressure is the internal pressure against the cell wall—as it builds up, it prevents more water from entering.

Facilitated diffusion:
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Facilitated diffusion: Channel proteins are integral membrane proteins that form channels across the membrane through which some substances can pass. Substances can also bind to carrier proteins to speed up diffusion. Both processes operate in either direction.

Ligand-gated—the stimulus is a ligand, a chemical signal.
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Ion channels: channel proteins that allow specific ions to pass through Most are gated channels—they open when a stimulus causes the protein to change shape. Ligand-gated—the stimulus is a ligand, a chemical signal. Voltage-gated—the stimulus is a change in electrical charge difference across the membrane.

Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus
Figure 5.4 A Ligand-Gated Channel Protein Opens in Response to a Stimulus The channel protein is anchored in the lipid bilayer by the nonpolar (hydrophobic) amino acids exposed on the protein’s surface. The protein changes its three-dimensional shape when a stimulus molecule (ligand) binds to it, opening a pore lined with polar amino acids. This allows hydrophilic, polar substances to pass through.

Water crosses membranes at a faster rate than simple diffusion.
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Water crosses membranes at a faster rate than simple diffusion. It may “hitchhike” with ions such as Na+ as they pass through ion channels. Aquaporins are channels that allow large amounts of water to move along its concentration gradient.

Figure 5.5 Aquaporins Increase Membrane Permeability to Water (Part 1)
Figure 5.5 Aquaporins Increase Membrane Permeability to Water A protein was isolated from the membranes of cells in which water diffuses rapidly across the membranes. When mRNA encoding the protein was inserted into and translated in oocytes, which do not normally have the protein, the water permeability of the oocytes was greatly increased.a [a G. M. Preston et al Science 256: 385–387.]

Glucose transporters are carrier proteins in mammalian cells.
Concept 5.2 Passive Transport across Membranes Requires No Input of Energy Carrier proteins in the membrane facilitate diffusion by binding substances. Glucose transporters are carrier proteins in mammalian cells. Glucose molecules bind to the carrier protein and cause the protein to change shape—it releases glucose on the other side of the membrane.

Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 1)
Figure 5.6 A Carrier Protein Facilitates Diffusion The glucose transporter is a carrier protein that allows glucose to enter the cell at a faster rate than would be possible by simple diffusion. (A) The transporter binds to glucose, and as it does so, it changes shape, releasing the glucose into the cell cytoplasm. (B) The graph shows the rate of glucose entry via a carrier versus the concentration of glucose outside the cell. As the glucose concentration increases, the rate of diffusion increases until the point at which all the available transporters are being used (the system is saturated).

Glucose is quickly broken down in the cell, so there is always a strong concentration gradient that favors glucose uptake. But the system can become saturated—when all of the carrier molecules are bound, the rate of diffusion reaches a maximum.

Figure 5.6 A Carrier Protein Facilitates Diffusion (Part 2)
Figure 5.6 A Carrier Protein Facilitates Diffusion The glucose transporter is a carrier protein that allows glucose to enter the cell at a faster rate than would be possible by simple diffusion. (A) The transporter binds to glucose, and as it does so, it changes shape, releasing the glucose into the cell cytoplasm. (B) The graph shows the rate of glucose entry via a carrier versus the concentration of glucose outside the cell. As the glucose concentration increases, the rate of diffusion increases until the point at which all the available transporters are being used (the system is saturated).

Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients
Cells maintain an internal environment with a different composition than the outside environment. This requires work—energy from ATP is needed to move substances against their concentration gradients (active transport). Specific carrier proteins move substances in only one direction, either into or out of the cell.

Table 5.1

Two types of active transport:
Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients Two types of active transport: Primary active transport involves direct hydrolysis of ATP for energy. Secondary active transport uses the energy from an ion concentration gradient or an electrical gradient. The gradients are established by primary active transport.

One molecule of ATP moves two K+ and three Na+ ions.
Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients The sodium–potassium (Na+–K+) pump is an integral membrane protein that pumps Na+ out of a cell and K+ in. One molecule of ATP moves two K+ and three Na+ ions.

Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump
Figure 5.7 Primary Active Transport: The Sodium–Potassium Pump In active transport, energy is used to move a solute against its concentration gradient. Here, energy from ATP is used to move Na+ and K+ against their concentration gradients.

Concept 5.3 Active Transport Moves Solutes against Their Concentration Gradients
Secondary active transport uses energy that is “regained” by letting ions move across the membrane with their concentration gradients. Example: after the Na+–K+ pump establishes a concentration gradient of Na+, then passive diffusion of Na+ back into the cell can provide energy for glucose transport. One protein usually moves both the ion and the transported molecule across the membrane.

Concept 5.4 Large Molecules Cross Membranes via Vesicles
Macromolecules are too large or too charged to pass through biological membranes, so instead they cross within vesicles. To take up or to secrete macromolecules, cells must use endocytosis and exocytosis.

Exocytosis moves materials out of the cell in vesicles. The vesicle membrane fuses with the cell membrane and the contents are released into the environment. Exocytosis is important in the secretion of substances made by cells such as digestive enzymes and neurotransmitters.

Endocytosis brings macromolecules and particles into eukaryotic cells. The cell membrane invaginates, or folds around the particle and forms a vesicle. The vesicle then separates from the membrane.

Figure 5.8 Endocytosis and Exocytosis
Figure 5.8 Endocytosis and Exocytosis Eukaryotic cells use endocytosis (A) and exocytosis (B) to take up and release large molecules and particles. Even small cells can be engulfed via endocytosis.

Endocytosis depends on receptors—proteins that bind to specific molecules (ligands). The receptors are integral membrane proteins on the cell membrane. The resulting vesicle includes both the receptor and its ligand, plus other substances present near the site of invagination.

Phagocytosis (“cellular eating”): a specialized cell engulfs a large particle or another cell A food vesicle (phagosome) forms and usually fuses with a lysosome, where the contents are digested. Pinocytosis (“cellular drinking”): vesicles are smaller and bring in fluids and dissolved substances

Receptor endocytosis brings specific large molecules into a cell via specific receptors. This allows cells to control internal processes by controlling location and abundance of each type of receptor on the cell membrane. It also plays a role in cell signaling.

The receptors are located in membrane regions called coated pits. The cytoplasmic surface of a pit is coated by another protein (often clathrin). When receptors bind to their ligands, the coated pit invaginates and forms a coated vesicle. Clathrin stabilizes the vesicle.

Figure 5.9 Receptor Endocytosis
Figure 5.9 Receptor Endocytosis The receptor proteins in a coated pit bind specific macromolecules, which are then carried into the cell by a coated vesicle.

Once inside, the vesicle loses its clathrin coat and fuses with a membrane-enclosed compartment called an endosome. Receptors may be recycled to the cell membrane or degraded in a lysosome. This is an important mechanism for controlling the abundance of each kind of receptor on the cell surface.

Cell signaling: cells can process information from their environment
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Cell signaling: cells can process information from their environment Signals include physical stimuli, such as heat or light, and chemicals (ligands). The cell must have a receptor for the signal in order to respond. Following receptor activation by a signal, a signal transduction pathway is initiated—a sequence of events that lead to a cellular response.

In a multicellular animal, cells are exposed to many chemical signals:
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals In a multicellular animal, cells are exposed to many chemical signals: Autocrine signals affect the same cells that release them. Paracrine signals diffuse to and affect nearby cells. Juxtacrine signaling requires direct contact between the signaling and responding cell. Hormones travel to distant cells.

Figure 5.10 Chemical Signaling Concepts
Figure Chemical Signaling Concepts A signal molecule can act on the cell that produces it, on a nearby cell, or be transported by the organism’s circulatory system to a distant target cell.

Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals
Only cells with the necessary receptors can respond to a signal—the target cell must be able to sense it and respond to it. A signal transduction pathway involves a signal, a receptor, and a response.

Figure 5.11 Signal Transduction Concepts
Figure Signal Transduction Concepts This general pathway is common to many cells and situations. The ultimate cellular responses are either short-term or long-term.

Signal transduction pathways often include allosteric regulation:
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Signal transduction pathways often include allosteric regulation: Protein shape changes as a result of a molecule binding at a site other than the active site (e.g., a ligand-gated channel). A signal transduction pathway may produce short or long term responses.

Receptors can be classified by their location:
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Receptors can be classified by their location: Intracellular receptors are located inside a cell. Their ligands are small or nonpolar and can diffuse across the membrane. Membrane receptors located on the cell surface have large or polar ligands that cannot diffuse through the membrane.

A chemical ligand fits into a 3-D site on the receptor protein.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Membrane receptors: A chemical ligand fits into a 3-D site on the receptor protein. The receptor may have a catalytic domain on the cytoplasmic side. The ligand is an allosteric regulator—it exposes the active site on the catalytic domain.

Figure 5.12 A Signal Binds to Its Receptor
Figure A Signal Binds to Its Receptor Human growth factor fits into its membrane-bound receptor (a protein with two subunits) and binds to it noncovalently.

Ligand-receptor binding is noncovalent and reversible.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Ligand-receptor binding is noncovalent and reversible.

Inhibitors, or antagonists, can bind in place of the normal ligand.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Reversible binding is important because cells need to stop responding to a signal after the appropriate response has occurred. Inhibitors, or antagonists, can bind in place of the normal ligand. Caffeine binds to receptors in the brain, preventing binding by the normal ligands.

Ion channel receptors are ligand-gated ion channels; they change shape when a ligand binds. Acetylcholine receptors on skeletal muscle cells bind acetylcholine to open the channel and allow Na+ to diffuse into the cell.

Protein kinase receptors also change shape when a ligand binds.
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Protein kinase receptors also change shape when a ligand binds. The new shape exposes or activates a cytoplasmic domain that has protein kinase activity—it modifies proteins by adding phosphate groups. (Not all protein kinases are receptors.)

Figure 5.13 A Protein Kinase Receptor
Figure A Protein Kinase Receptor The mammalian hormone insulin binds to a protein kinase receptor on the outside surface of the cell and initiates a response.

G protein–linked receptors: ligand binding on the surface exposes a site on the cytoplasmic side that binds to a mobile membrane protein, a G protein The G protein is partially inserted in the lipid bilayer and partially exposed on the cytoplasmic surface.

GDP and GTP, used for energy transfer
Concept 5.5 The Membrane Plays a Key Role in a Cell’s Response to Environmental Signals Many G proteins have three subunits and can bind three different molecules: The receptor GDP and GTP, used for energy transfer An effector protein that causes an effect in the cell

The activated G protein–linked receptor exchanges a GDP nucleotide bound to the G protein for a higher energy GTP. The activated G protein activates the effector protein, leading to signal amplification.

Figure 5.14 A G Protein–Linked Receptor
Figure A G Protein–Linked Receptor The G protein is an intermediary between the receptor and an effector protein.

Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment
Signal activation of a specific receptor leads to a cellular response, mediated by a signal transduction pathway. Signaling can initiate a cascade of protein interactions—the initial signal is amplified and distributed to cause different responses, ultimately leading to changes in cell function.

There are many ways in which cells respond to environmental signals:
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment There are many ways in which cells respond to environmental signals: Opening of ion channels—changes the balance of ion concentrations between the outside and inside of the cell and results in change in the electrical potential across the membrane.

Alterations in gene expression—genes may be switched on (upregulated) or switched off (downregulated). This affects the abundance of proteins (often enzymes), thus changing cell function. Alteration of enzyme activities—more rapid response than those involving change in gene expression.

The same signal can lead to different responses in different types of cells. Example: Heart and digestive tract muscle cells respond differently to epinephrine because the signal transduction pathways stimulated are different in the two cell types.

Often there is a small molecule intermediary, a “second messenger,” between the activated receptor and the cascade of responses that ensues. In the fight-or-flight response, epinephrine (adrenaline) activates the liver enzyme glycogen phosphorylase, which catalyzes breakdown of glycogen for quick energy.

Researchers found that glycogen phosphorylase could be activated by membrane-bound epinephrine in broken cells, as long as all parts were present. They discovered that another molecule delivered the message from the “first messenger,” epinephrine, to the enzyme.

Figure 5.15 The Discovery of a Second Messenger (Part 1)
Figure The Discovery of a Second Messenger Glycogen phosphorylase is activated in liver cells after epinephrine binds to a membrane receptor. Sutherland and his colleagues observed that this activation could occur in a test tube only if fragments of the cell membrane were present. They designed experiments to show that a second messenger caused the activation of glycogen phosphorylase.a [a T. W. Rall et al Journal of Biological Chemistry 224: 463–470.]

The second messenger was later discovered to be cyclic AMP (cAMP).
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment The second messenger was later discovered to be cyclic AMP (cAMP). Second messengers regulate target enzymes by binding to them noncovalently. They allow the cell to respond to a single membrane event with many events inside the cell—they distribute the signal. They amplify the signal by activating more than one enzyme target.

Figure 5.16 The Formation of Cyclic AMP
Figure The Formation of Cyclic AMP The formation of cAMP from ATP is catalyzed by adenylyl cyclase, an enzyme that is activated by G proteins.

Signal transduction pathways involve multiple steps in which enzymes are either activated or inhibited by other enzymes. In liver cells, a signal cascade begins when epinephrine stimulates a G protein–mediated protein kinase pathway.

cAMP is produced and activates protein kinase A, which phosphorylates two other enzymes, with opposite effects: Inhibition—glycogen synthase is inactivated by phosphorylation, which prevents glucose storage. Activation—phosphorylase kinase is phosphorylated and starts a cascade that results in the liberation of glucose molecules from glycogen.

Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 1)
Figure A Cascade of Reactions Leads to Altered Enzyme Activity Liver cells respond to epinephrine by activating G proteins, which in turn activate the synthesis of the second messenger cAMP. Cyclic AMP initiates a protein kinase cascade, greatly amplifying the epinephrine signal, as indicated by the blue numbers. The cascade both inhibits the conversion of glucose to glycogen and stimulates the release of previously stored glucose.

Figure 5.17 A Cascade of Reactions Leads to Altered Enzyme Activity (Part 2)
Figure A Cascade of Reactions Leads to Altered Enzyme Activity Liver cells respond to epinephrine by activating G proteins, which in turn activate the synthesis of the second messenger cAMP. Cyclic AMP initiates a protein kinase cascade, greatly amplifying the epinephrine signal, as indicated by the blue numbers. The cascade both inhibits the conversion of glucose to glycogen and stimulates the release of previously stored glucose.

The original signal is amplified at every step in the cascade.
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment The original signal is amplified at every step in the cascade. Each molecule of epinephrine that arrives at the cell membrane ultimately results in 10,000 molecules of blood glucose.

Signal transduction ends after the cell responds—enzymes convert each transducer back to its inactive precursor. The balance between the regulating enzymes and the signal enzymes determines the cell’s ultimate response.

Figure 5.18 Signal Transduction Regulatory Mechanisms
Figure Signal Transduction Regulatory Mechanisms Some signals lead to the production of active signal transduction molecules such as (A) protein kinases, (B) G proteins, and (C) cAMP. Other enzymes (red type) inactivate or remove these active molecules.

Cells can alter the balance of enzymes in two ways:
Concept 5.6 Signal Transduction Allows the Cell to Respond to Its Environment Cells can alter the balance of enzymes in two ways: Synthesis or breakdown of the enzyme Activation or inhibition of the enzymes by other molecules

Answer to Opening Question
Caffeine is a large, polar molecule that binds to receptors on nerve cells in the brain. Its structure is similar to adenosine, which binds to receptors after activity or stress and results in drowsiness. Caffeine binds to the same receptor, but does not activate it—the result is that the person remains alert.

Figure 5.19 Caffeine and the Cell Membrane
Figure Caffeine and the Cell Membrane (A) The adenosine 2A receptor is present in the human brain, where it is involved in inhibiting arousal. (B) Adenosine is the normal ligand for the receptor. Caffeine has a structure similar to that of adenosine and can act as an antagonist that binds the receptor and prevents its normal functioning.

Cell Membranes and Signaling

Similar presentations

Presentation on theme: "Cell Membranes and Signaling"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Cell Membranes and Signaling

Similar presentations

Presentation on theme: "Cell Membranes and Signaling"— Presentation transcript:

Similar presentations

About project

Feedback