© 2013 Pearson Education, Inc. Membrane Transport: Active Processes Two types of active processes –Active transport –Vesicular transport Both require ATP to move solutes across a living plasma membrane because –Solute too large for channels –Solute not lipid soluble –Solute not able to move down concentration gradient

© 2013 Pearson Education, Inc. Active Transport Requires carrier proteins (solute pumps) –Bind specifically and reversibly with substance Moves solutes against concentration gradient –Requires energy

© 2013 Pearson Education, Inc. Active Transport: Two Types Primary active transport –Required energy directly from ATP hydrolysis Secondary active transport –Required energy indirectly from ionic gradients created by primary active transport

© 2013 Pearson Education, Inc. Primary Active Transport Energy from hydrolysis of ATP causes shape change in transport protein that "pumps" solutes (ions) across membrane E.g., calcium, hydrogen, Na + -K + pumps

© 2013 Pearson Education, Inc. Primary Active Transport Sodium-potassium pump –Most well-studied –Carrier (pump) called Na + -K + ATPase –Located in all plasma membranes –Involved in primary and secondary active transport of nutrients and ions

© 2013 Pearson Education, Inc. Sodium-Potassium Pump Na + and K + channels allow slow leakage down concentration gradients Na + -K + pump works as antiporter –Pumps against Na + and K + gradients to maintain high intracellular K + concentration and high extracellular Na + concentration Maintains electrochemical gradients essential for functions of muscle and nerve tissues Allows all cells to maintain fluid volume

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 1 Extracellular fluid Na + Na + –K + pump K+K+ ATP-binding site Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. K + released 6 Pump protein binds ATP; releases K + to the inside, and Na + sites are ready to bind Na + again. The cycle repeats. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. K + bound 5 K + binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. K+K+ 4 Two extracellular K + bind to pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P P PiPi

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 2 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 3 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. Na + bound P ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 4 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 5 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. K+K+ 4 Two extracellular K + bind to pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P P ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 6 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. K + bound 5 K + binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. K+K+ 4 Two extracellular K + bind to pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P P PiPi ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. Slide 7 Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. K + released 6 Pump protein binds ATP; releases K + to the inside, and Na + sites are ready to bind Na + again. The cycle repeats. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. K + bound 5 K + binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. K+K+ 4 Two extracellular K + bind to pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P P PiPi ATP-binding site

© 2013 Pearson Education, Inc. Figure 3.10 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. PLAY A&P Flix™: Resting Membrane Potential Extracellular fluid Na + Na + –K + pump K+K+ Cytoplasm 1 Three cytoplasmic Na + bind to pump protein. K + released 6 Pump protein binds ATP; releases K + to the inside, and Na + sites are ready to bind Na + again. The cycle repeats. 2 Na + binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. K + bound 5 K + binding triggers release of the phosphate. The dephosphorylated pump resumes its original conformation. K+K+ 4 Two extracellular K + bind to pump. 3 Phosphorylation causes the pump to change shape, expelling Na + to the outside. Na + bound Na + released P P P PiPi ATP-binding site

© 2013 Pearson Education, Inc. Secondary Active Transport Depends on ion gradient created by primary active transport Energy stored in ionic gradients used indirectly to drive transport of other solutes

© 2013 Pearson Education, Inc. Secondary Active Transport Cotransport—always transports more than one substance at a time –Symport system: Substances transported in same direction –Antiport system: Substances transported in opposite directions

© 2013 Pearson Education, Inc. Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Extracellular fluid Na + -glucose symport transporter loads glucose from extracellular fluid Na + -glucose symport transporter releases glucose into the cytoplasm Glucose Na + -K + pump Cytoplasm Primary active transport The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. Secondary active transport As Na + diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell. 1 2 Slide 1

© 2013 Pearson Education, Inc. Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 2 Extracellular fluid Na + -K + pump Cytoplasm Primary active transport The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. 1

© 2013 Pearson Education, Inc. Figure 3.11 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 3 Extracellular fluid Na + -glucose symport transporter loads glucose from extracellular fluid Na + -glucose symport transporter releases glucose into the cytoplasm Glucose Na + -K + pump Cytoplasm Primary active transport The ATP-driven Na + -K + pump stores energy by creating a steep concentration gradient for Na + entry into the cell. Secondary active transport As Na + diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell. 1 2

© 2013 Pearson Education, Inc. Vesicular Transport Transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles Requires cellular energy (e.g., ATP)

© 2013 Pearson Education, Inc. Vesicular Transport Functions: –Exocytosis—transport out of cell –Endocytosis—transport into cell Phagocytosis, pinocytosis, receptor-mediated endocytosis –Transcytosis—transport into, across, and then out of cell –Vesicular trafficking—transport from one area or organelle in cell to another

© 2013 Pearson Education, Inc. Endocytosis and Transcytosis Involve formation of protein-coated vesicles Often receptor mediated, therefore very selective Some pathogens also hijack for transport into cell Once vesicle is inside cell it may –Fuse with lysosome –Undergo transcytosis

© 2013 Pearson Education, Inc. Figure 3.12 Events of endocytosis mediated by protein-coated pits. Slide 1 Coated pit ingests substance. Coat proteins are recycled to plasma membrane. 1 Protein coat (typically clathrin) Protein-coated vesicle deta- ches. Transport vesicle Endosome Uncoated endocytic vesicle Transport vesicle containing Uncoated vesicle fuses with a sorting vesicle called an endosome. Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis). Extracellular fluid Plasma membrane Cytoplasm Lysosome 2 membrane compone -nts moves to the plasma membrane for recycling

© 2013 Pearson Education, Inc. Slide 2 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. 1 Protein coat (typically clathrin) Extracellular fluid Plasma membrane Cytoplasm

© 2013 Pearson Education, Inc. Slide 3 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. 1 Protein coat (typically clathrin) Extracellular fluid Plasma membrane Protein-coated vesicle deta- ches. 2 Cytoplasm

© 2013 Pearson Education, Inc. Slide 4 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. 1 Protein coat (typically clathrin) Extracellular fluid Plasma membrane Protein-coated vesicle deta- ches. 2 Cytoplasm Coat proteins are recycled to plasma membrane. 3

© 2013 Pearson Education, Inc. Slide 5 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. 1 Protein coat (typically clathrin) Extracellular fluid Plasma membrane Protein-coated vesicle deta- ches. 2 Cytoplasm Coat proteins are recycled to plasma membrane. 3 Endosome Uncoated endocytic vesicle Uncoated vesicle fuses with a sorting vesicle called an endosome. 4

© 2013 Pearson Education, Inc. Slide 6 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. 1 Protein coat (typically clathrin) Extracellular fluid Plasma membrane Protein-coated vesicle deta- ches. 2 Cytoplasm Coat proteins are recycled to plasma membrane. 3 Endosome Uncoated endocytic vesicle Uncoated vesicle fuses with a sorting vesicle called an endosome. 4 Transport vesicle membrane compone -nts moves to the plasma membrane for recycling. 5 Transport vesicle containing

© 2013 Pearson Education, Inc. Slide 7 Figure 3.12 Events of endocytosis mediated by protein-coated pits. Coated pit ingests substance. Coat proteins are recycled to plasma membrane. 1 Protein coat (typically clathrin) Protein-coated vesicle deta- ches. Transport vesicle Endosome Uncoated endocytic vesicle Uncoated vesicle fuses with a sorting vesicle called an endosome. Fused vesicle may (a) fuse with lysosome for digestion of its contents, or (b) deliver its contents to the plasma membrane on the opposite side of the cell (transcytosis). Extracellular fluid Plasma membrane Cytoplasm Lysosome Transport vesicle containing membrane compone -nts moves to the plasma membrane for recycling. 5

© 2013 Pearson Education, Inc. Endocytosis Phagocytosis –Pseudopods engulf solids and bring them into cell's interior –Form vesicle called phagosome Used by macrophages and some white blood cells –Move by amoeboid motion Cytoplasm flows into temporary extensions Allows creeping

© 2013 Pearson Education, Inc. Figure 3.13a Comparison of three types of endocytosis. Receptors Phagosome Phagocytosis The cell engulfs a large particle by forming projecting pseudopods ("false feet") around it and enclosing it within a membrane sac called a phagosome. The phagosome is combined with a lysosome. Undigested contents remain in the vesicle (now called a residual body) or are ejected by exocytosis. Vesicle may or may not be protein coated but has receptors capable of binding to microorganisms or solid particles.

© 2013 Pearson Education, Inc. Endocytosis Pinocytosis (fluid-phase endocytosis) –Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell Fuses with endosome –Most cells utilize to "sample" environment –Nutrient absorption in the small intestine –Membrane components recycled back to membrane

© 2013 Pearson Education, Inc. Figure 3.13b Comparison of three types of endocytosis. Vesicle Pinocytosis The cell "gulps" a drop of extracellular fluid containing solutes into tiny vesicles. No receptors are used, so the process is nonspecific. Most vesicles are protein-coated.

© 2013 Pearson Education, Inc. Endocytosis Receptor-mediated endocytosis –Allows specific endocytosis and transcytosis Cells use to concentrate materials in limited supply –Clathrin-coated pits provide main route for endocytosis and transcytosis Uptake of enzymes, low-density lipoproteins, iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins

© 2013 Pearson Education, Inc. Receptor-Mediated Endocytosis Different coat proteins –Caveolae Capture specific molecules (folic acid, tetanus toxin) and use transcytosis Involved in cell signaling but exact function unknown –Coatomer Function in vesicular trafficking

© 2013 Pearson Education, Inc. Figure 3.13c Comparison of three types of endocytosis. Vesicle Receptor-mediated endocytosis Extracellular substances bind to specific receptor proteins, enabling the cell to ingest and concentrate specific substances (ligands) in protein-coated vesicles. Ligands may simply be released inside the cell, or combined with a lysosome to digest contents. Receptors are recycled to the plasma membrane in vesicles.

© 2013 Pearson Education, Inc. Exocytosis Usually activated by cell-surface signal or change in membrane voltage Substance enclosed in secretory vesicle v-SNAREs ("v" = vesicle) on vesicle find t-SNAREs ("t" = target) on membrane and bind Functions –Hormone secretion, neurotransmitter release, mucus secretion, ejection of wastes

© 2013 Pearson Education, Inc. Figure 3.14 Exocytosis. Slide 1 Extracellular fluid Plasma membrane SNARE (t-SNARE) The process of exocytosis Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm 1 The membrane- bound vesicle migrates to the plasma membrane. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs 3 The vesicle and plasma membrane fuse and a pore opens up. Fusion pore formed 4 Vesicle contents are released to the cell exterior.

© 2013 Pearson Education, Inc. Slide 2 Figure 3.14 Exocytosis. Extracellular fluid Plasma membrane SNARE (t-SNARE) The process of exocytosis Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm 1 The membrane- bound vesicle migrates to the plasma membrane.

© 2013 Pearson Education, Inc. Slide 3 Figure 3.14 Exocytosis. Extracellular fluid Plasma membrane SNARE (t-SNARE) The process of exocytosis Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm 1 The membrane- bound vesicle migrates to the plasma membrane. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs

© 2013 Pearson Education, Inc. Slide 4 Figure 3.14 Exocytosis. Extracellular fluid Plasma membrane SNARE (t-SNARE) The process of exocytosis Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm 1 The membrane- bound vesicle migrates to the plasma membrane. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs 3 The vesicle and plasma membrane fuse and a pore opens up. Fusion pore formed

© 2013 Pearson Education, Inc. Slide 5 Figure 3.14 Exocytosis. Extracellular fluid Plasma membrane SNARE (t-SNARE) The process of exocytosis Secretory vesicle Vesicle SNARE (v-SNARE) Molecule to be secreted Cytoplasm 1 The membrane- bound vesicle migrates to the plasma membrane. 2 There, proteins at the vesicle surface (v-SNAREs) bind with t-SNAREs (plasma membrane proteins). Fused v- and t-SNAREs 3 The vesicle and plasma membrane fuse and a pore opens up. Fusion pore formed 4 Vesicle contents are released to the cell exterior.

© 2013 Pearson Education, Inc. Figure 3.14b Exocytosis. Photomicrograph of a secretory vesicle releasing its contents by exocytosis (100,000x)

© 2013 Pearson Education, Inc. Table 3.2 Active Membrane Transport Processes (1 of 2)

© 2013 Pearson Education, Inc. Table 3.2 Active Membrane Transport Processes (2 of 2)

© 2013 Pearson Education, Inc. Generation of a Resting Membrane Potential Resting membrane potential (RMP) –Produced by separation of oppositely charged particles (voltage) across membrane in all cells Cells described as polarized –Voltage (electrical potential energy) only at membrane Ranges from –50 to –100 mV in different cells –"–" indicates inside negative relative to outside

© 2013 Pearson Education, Inc. Selective Diffusion Establishes RMP Electrochemical gradient established –Electro (charge); chemical (ion concentration) K + diffuses out of cell through K + leakage channels, proteins cannot  inside cell membrane more negative  K + attracted back as inner face more negative K + equalizes across membrane at –90 mV when K + concentration gradient balanced by electrical gradient = RMP

© 2013 Pearson Education, Inc. Figure 3.15 The key role of K + in generating the resting membrane potential. 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. 2 K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 A negative membrane potential (–90 mV) is established when the movement of K + out of the cell equals K + movement into the cell. At this point, the concentration gradient promoting K + exit exactly opposes the electrical gradient for K + entry. Extracellular fluid Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm – – – – – – – – Slide 1

© 2013 Pearson Education, Inc. Figure 3.15 The key role of K + in generating the resting membrane potential. 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. Extracellular fluid Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm – – – – – – – – Slide 2

© 2013 Pearson Education, Inc. Figure 3.15 The key role of K + in generating the resting membrane potential. 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. 2 K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. Extracellular fluid Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm – – – – – – – – Slide 3

© 2013 Pearson Education, Inc. Figure 3.15 The key role of K + in generating the resting membrane potential. 1 K + diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of K + results in a negative charge on the inner plasma membrane face. 2 K + also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. 3 A negative membrane potential (–90 mV) is established when the movement of K + out of the cell equals K + movement into the cell. At this point, the concentration gradient promoting K + exit exactly opposes the electrical gradient for K + entry. Extracellular fluid Potassium leakage channels Protein anion (unable to follow K + through the membrane) Cytoplasm – – – – – – – – Slide 4

© 2013 Pearson Education, Inc. Selective Diffusion Establishes RMP In many cells Na + affects RMP –Attracted into cell due to negative charge  RMP to –70 mV –Membrane more permeable to K + than Na +, so K + primary influence on RMP Cl – does not influence RMP— concentration and electrical gradients exactly balanced

© 2013 Pearson Education, Inc. Active Transport Maintains Electrochemical Gradients Na + -K + pump continuously ejects 3Na + from cell and carries 2K + in Steady state maintained because rate of active transport equal to and depends on rate of Na + diffusion into cell Neuron and muscle cells "upset" RMP by opening gated Na + and K + channels

© 2013 Pearson Education, Inc. Cell-Environment Interactions Cells interact directly or indirectly by responding to extracellular chemicals Always involves glycocalyx –Cell adhesion molecules (CAMs) –Plasma membrane receptors –Voltage-gated channel proteins

© 2013 Pearson Education, Inc. Roles of Cell Adhesion Molecules Thousands on approximately every cell in body Anchor to extracellular matrix or each other Assist in movement of cells past one another Attract WBCs to injured or infected areas Stimulate synthesis or degradation of adhesive membrane junctions Transmit intracellular signals to direct cell migration, proliferation, and specialization

© 2013 Pearson Education, Inc. Roles of Plasma Membrane Receptors Contact signaling—touching and recognition of cells; e.g., in normal development and immunity Chemical signaling—interaction between receptors and ligands (neurotransmitters, hormones, and paracrines) to alter activity of cell proteins (e.g., enzymes or chemically gated ion channels) –Same ligand can cause different cell responses –Response determined by what receptor linked to inside cell

© 2013 Pearson Education, Inc. Chemical Signaling Ligand binding  receptor structural change  protein alteration –Catalytic receptor proteins become activated enzymes –Chemically gated channel-linked receptors open and close ion gates  changes in excitability –G protein–linked receptors activate G protein, affecting an ion channel or enzyme, or causing release of internal second messenger, such as cyclic AMP

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme) Extracellular fluid G protein GDP Intracellular fluid Cascade of cellular responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Activated kinase enzymes Active 2nd messenger Inactive 2nd messenger Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. Kinase enzymes activate other enzymes. Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various metabolic and structural changes in the cell. * Ligands include hormones and neurotransmitters. Receptor Ligand Slide 1 The sequence described here is like a molecular relay race. Instead of a baton passed from runner to runner, the message (a shape change) is passed from molecule to molecule as it makes its way across the cell membrane from outside to inside the cell.

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Intracellular fluid * Ligands include hormones and neurotransmitters. Receptor Ligand 1 Slide 2

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Intracellular fluid * Ligands include hormones and neurotransmitters. Receptor Ligand 1 The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). G protein GDP 2 Slide 3

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Intracellular fluid * Ligands include hormones and neurotransmitters. Receptor Ligand 1 The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). G protein GDP Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme) 2 3 Slide 4

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Intracellular fluid * Ligands include hormones and neurotransmitters. Receptor Ligand 1 The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). G protein GDP Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme) 2 3 Active 2nd messenger Inactive 2nd messenger Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) 4 Slide 5

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Intracellular fluid * Ligands include hormones and neurotransmitters. Receptor Ligand 1 The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). G protein GDP Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme) 2 3 Active 2nd messenger Inactive 2nd messenger Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) 4 Activated kinase enzymes Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. 5 Slide 6

© 2013 Pearson Education, Inc. Figure 3.16 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand (1st messenger) Receptor G protein Enzyme 2nd messenger Ligand* (1st messeng- er) binds to the receptor. The receptor changes shape and activates. The activated receptor binds to a G protein and acti- vates it. The G protein changes shape (turns “on”), causing it to release GDP and bind GTP (an energy source). Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme) Extracellular fluid G protein GDP Intracellular fluid Cascade of cellular responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Activated kinase enzymes Active 2nd messenger Inactive 2nd messenger Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. Kinase enzymes activate other enzymes. Kinase enzymes transfer phosphate groups from ATP to specific proteins and activate a series of other enzymes that trigger various metabolic and structural changes in the cell. * Ligands include hormones and neurotransmitters. Receptor Ligand Slide 7