Cells: The Living Units

Transcription

1 Chapter 3 Part B Cells: The Living Units Annie Leibovitz/Contact Press Images PowerPoint Lecture Slides prepared by Karen Dunbar Kareiva Ivy Tech Community College

2 3.4 Active Membrane Transport Two major active membrane transport processes Active transport Vesicular transport Both require ATP to move solutes across a plasma membrane for any of these reasons: Solute is too large for channels, or Solute is not lipid soluble, or Solute is not able to move down concentration gradient

3 Active Transport Requires carrier proteins (solute pumps) Bind specifically and reversibly with substance being moved Some carriers transport more than one substance Antiporters transport one substance into cell while transporting a different substance out of cell Symporters transport two different substances in the same direction Moves solutes against their concentration gradient (from low to high) This requires energy (ATP)

4 Active Transport (cont.) Two types of active transport: Primary active transport Required energy comes directly from ATP hydrolysis Secondary active transport Required energy is obtained indirectly from ionic gradients created by primary active transport

5 Active Transport (cont.) Primary active transport Energy from hydrolysis of ATP causes change in shape of transport protein Shape change causes solutes (ions) bound to protein to be pumped across membrane Example of pumps: calcium, hydrogen (proton), - pumps

6 Active Transport (cont.) Sodium-potassium pump Most studied pump Basically is an enzyme, called - ATPase, that pumps out of cell and back into cell Located in all plasma membranes, but especially active in excitable cells (nerves and muscles)

7 Active Transport (cont.) Leakage channels located in membranes result in leaking of into the cell and leaking of out of cell Both travel down their concentration gradients - pump works as an antiporter that pumps out of cell and back into cell against their concentration gradients Maintains electrochemical gradients, which involve both concentration and electrical charge of ions Essential for functions of muscle and nerve tissues

8 Figure 3.1 Cell diversity. Erythrocytes Epithelial cells Fibroblasts Skeletal muscle cell Smooth muscle cells Cells that connect body parts, form linings, or transport gases Cells that move organs and body parts Macrophage Fat cell Nerve cell Cell that stores nutrients Cell that fights disease Cell that gathers information and controls body functions Cell of reproduction Sperm

9 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 2 Extracellular fluid pump ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein.

10 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 3 Extracellular fluid pump ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein. Na+ bound P ADP 2 binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump.

11 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 4 Extracellular fluid pump ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein. Na+ bound P ADP 2 binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. released P 3 Phosphorylation causes the pump to change shape, expelling to the outside.

12 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 5 Extracellular fluid pump ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein. Na+ bound P ADP 2 binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. released P P 3 Phosphorylation causes the pump to change shape, expelling to the outside. 4 Two extracellular bind to pump.

13 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 6 Extracellular fluid pump ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein. Na+ bound P ADP 2 binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. bound released P P i 5 binding triggers release of 3 Phosphorylation causes the the phosphate. The dephosphorylated pump resumes its original pump to change shape, expelling to the outside. conformation. P 4 Two extracellular bind to pump.

14 Focus Figure 3.1 Primary active transport is the process in which solutes are moved across cell membranes against electrochemical gradients using energy supplied directly by ATP. The action of the - pump is an important example of primary active transport. Slide 7 Extracellular fluid pump ATP ATP ATP-binding site Cytoplasm 1 Three cytoplasmic bind to pump protein. Na+ bound P released ADP 6 Pump protein binds ATP; releases to the inside, and sites are ready to bind again. The cycle repeats. 2 binding promotes hydrolysis of ATP. The energy released during this reaction phosphorylates the pump. bound released P P i 5 binding triggers release of 3 Phosphorylation causes the the phosphate. The dephosphorylated pump resumes its original pump to change shape, expelling to the outside. conformation. P 4 Two extracellular bind to pump.

15 A&P Flix : Resting Membrane Potential

16 Active Transport (cont.) Secondary active transport Depends on ion gradient that was created by primary active transport system Energy stored in gradients is used indirectly to drive transport of other solutes Low concentration that is maintained inside cell by - pump strengthens sodium s drive to want to enter cell can drag other molecules with it as it flows into cell through carrier proteins (usually symporters) in membrane Some sugars, amino acids, and ions are usually transported into cells via secondary active transport

17 Figure 3.10 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 2 Extracellular fluid - pump ATP Cytoplasm 1 Primary active transport The ATP-driven - pump stores energy by creating a steep concentration gradient for entry into the cell.

18 Figure 3.10 Secondary active transport is driven by the concentration gradient created by primary active transport. Slide 3 Extracellular fluid - pump -glucose symport transporter loads glucose from extracellular fluid Glucose -glucose symport transporter releases glucose into the cytoplasm ATP Cytoplasm 1 Primary active transport 2 Secondary active transport The ATP-driven - pump stores energy by creating a steep concentration gradient for entry into the cell. As diffuses back across the membrane through a membrane cotransporter protein, it drives glucose against its concentration gradient into the cell.

19 Vesicular Transport Involves transport of large particles, macromolecules, and fluids across membrane in membranous sacs called vesicles Requires cellular energy (usually ATP)

20 Vesicular Transport (cont.) Vesicular transport processes include: Endocytosis: transport into cell 3 different types of endocytosis: phagocytosis, pinocytosis, receptor-mediated endocytosis Exocytosis: transport out of cell Transcytosis: transport into, across, and then out of cell Vesicular trafficking: transport from one area or organelle in cell to another

21 Vesicular Transport (cont.) Endocytosis Involves formation of protein-coated vesicles Usually involve receptors; therefore can be a very selective process Substance being pulled in must be able to bind to its unique receptor Some pathogens are capable of hijacking receptor for transport into cell Once vesicle is pulled inside cell, it may: Fuse with lysosome or Undergo transcytosis

22 Figure 3.11 Events of endocytosis mediated by protein-coated pits. 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Lysosome Endosome 5 Transport vesicle containing membrane components moves to the plasma membrane for recycling. (a) 6 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). (b)

23 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 2 1 Coated pit ingests substance. Protein coat (typically clathrin) Extracellular fluid Cytoplasm Plasma membrane (a) (b)

24 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 3 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane (a) (b)

25 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 4 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane 3 Coat proteins are recycled to plasma membrane. (a) (b)

26 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 5 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane 3 Coat proteins are recycled to plasma membrane. Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Endosome (a) (b)

27 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 6 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Endosome 5 Transport vesicle containing membrane components moves to the plasma membrane for recycling. (a) (b)

28 Figure 3.11 Events of endocytosis mediated by protein-coated pits. Slide 7 1 Coated pit ingests substance. Protein coat (typically clathrin) 2 Protein-coated vesicle detaches. Extracellular fluid Cytoplasm Plasma membrane 3 Coat proteins are recycled to plasma membrane. Transport vesicle Uncoated endocytic vesicle 4 Uncoated vesicle fuses with a sorting vesicle called an endosome. Lysosome Endosome 5 Transport vesicle containing membrane components moves to the plasma membrane for recycling. (a) 6 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). (b)

29 Vesicular Transport (cont.) Phagocytosis: type of endocytosis that is referred to as cell eating Membrane projections called pseudopods form and flow around solid particles that are being engulfed, forming a vesicle which is pulled into cell Formed vesicle is called a phagosome Phagocytosis is used by macrophages and certain other white blood cells Phagocytic cells move by amoeboid motion where cytoplasm flows into temporary extensions that allow cell to creep

30 Figure 3.12a Comparison of three types of endocytosis. Receptors 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. Phagosome

31 Vesicular Transport (cont.) Pinocytosis: type of endocytosis that is referred to as cell drinking or fluid-phase endocytosis Plasma membrane infolds, bringing extracellular fluid and dissolved solutes inside cell Fuses with endosome Used by some cells to sample environment Main way in which nutrient absorption occurs in the small intestine Membrane components are recycled back to membrane

32 Figure 3.12b Comparison of three types of endocytosis. 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. Vesicle

33 Vesicular Transport (cont.) Receptor-mediated endocytosis involves endocytosis and transcytosis of specific molecules Many cells have receptors embedded in clathrin-coated pits, which will be internalized along with the specific molecule bound Examples: enzymes, low-density lipoproteins (LDL), iron, insulin, and, unfortunately, viruses, diphtheria, and cholera toxins may also be taken into a cell this way Caveolae have smaller pits and different protein coat from clathrin, but still capture specific molecules (folic acid, tetanus toxin) and use transcytosis

34 Figure 3.12c 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.

35 Exocytosis Process where material is ejected from cell Usually activated by cell-surface signals or changes in membrane voltage Substance being ejected is enclosed in secretory vesicle Protein on vesicle called v-snare finds and hooks up to target t-snare proteins on membrane Docking process triggers exocytosis Some substances exocytosed: hormones, neurotransmitters, mucus, cellular wastes

36 Figure 3.13a 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 membranebound vesicle migrates to the plasma membrane. Fused v- and t-snares 2 There, proteins at the vesicle surface (v-snares) bind with t-snares (plasma membrane proteins). Fusion pore formed 3 The vesicle and plasma membrane fuse and a pore opens up. 4 Vesicle contents are released to the cell exterior.

37 Figure 3.13a Exocytosis. Extracellular fluid Plasma membrane SNARE (t-snare) The process of exocytosis Slide 2 Secretory vesicle Vesicle SNARE (v-snare) Molecule to be secreted Cytoplasm 1 The membranebound vesicle migrates to the plasma membrane.

38 Figure 3.13a Exocytosis. Extracellular fluid Plasma membrane SNARE (t-snare) The process of exocytosis Slide 3 Secretory vesicle Vesicle SNARE (v-snare) Molecule to be secreted Cytoplasm 1 The membranebound vesicle migrates to the plasma membrane. Fused v- and t-snares 2 There, proteins at the vesicle surface (v-snares) bind with t-snares (plasma membrane proteins).

39 Figure 3.13a Exocytosis. Extracellular fluid Plasma membrane SNARE (t-snare) The process of exocytosis Slide 4 Secretory vesicle Vesicle SNARE (v-snare) Molecule to be secreted Cytoplasm 1 The membranebound vesicle migrates to the plasma membrane. Fused v- and t-snares 2 There, proteins at the vesicle surface (v-snares) bind with t-snares (plasma membrane proteins). Fusion pore formed 3 The vesicle and plasma membrane fuse and a pore opens up.

40 Figure 3.13a Exocytosis. Extracellular fluid Plasma membrane SNARE (t-snare) The process of exocytosis Slide 5 Secretory vesicle Vesicle SNARE (v-snare) Molecule to be secreted Cytoplasm 1 The membranebound vesicle migrates to the plasma membrane. Fused v- and t-snares 2 There, proteins at the vesicle surface (v-snares) bind with t-snares (plasma membrane proteins). Fusion pore formed 3 The vesicle and plasma membrane fuse and a pore opens up. 4 Vesicle contents are released to the cell exterior.

41 Figure 3.13b Exocytosis. Photomicrograph of a secretory vesicle releasing its contents by exocytosis (100,000 )

42 3.5 Membrane Potential Resting membrane potential (RMP) Electrical potential energy produced by separation of oppositely charged particles across plasma membrane in all cells Difference in electrical charge between two points is referred to as voltage Cells that have a charge are said to be polarized Voltage occurs only at membrane surface Rest of cell and extracellular fluid are neutral Membrane voltages range from 50 to 100 mv in different cells (negative sign ( ) indicates inside of cell is more negative relative to outside of cell)

43 is Key Player in RMP diffuses out of cell through leakage channels down its concentration gradient Negatively charged proteins cannot leave As a result cytoplasmic side of cell membrane becomes more negative is then pulled back by the more negative interior because of its electrical gradient When drive for to leave cell is balanced by its drive to stay, RMP is established Most cells have an RMP around 90 mv Electrochemical gradient of sets RMP

44 is Key Player in RMP (cont.) In many cells, also affects RMP is also attracted to inside of cell because of negative charge If enters cell, it can bring RMP up to 70 mv Membrane is more permeable to than, so primary influence on RMP Cl does not influence RMP because its concentration and electrical gradients are exactly balanced

45 Figure 3.14 The key role of in generating the resting membrane potential. Extracellular fluid CI CI diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of results in a negative charge on the inner plasma membrane face. 2 also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. Potassium leakage channels A A Cytoplasm Protein anion (unable to follow through the membrane) 3 A negative membrane potential ( 90 mv) is established when the movement of out of the cell equals movement into the cell. At this point, the concentration gradient promoting exit exactly opposes the electrical gradient for entry.

46 Figure 3.14 The key role of in generating the resting membrane potential. Slide 2 Extracellular fluid CI CI 1 diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of results in a negative charge on the inner plasma membrane face Potassium leakage channels A A Cytoplasm Protein anion (unable to follow through the membrane)

47 Figure 3.14 The key role of in generating the resting membrane potential. Slide 3 Extracellular fluid CI CI diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of results in a negative charge on the inner plasma membrane face. 2 also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. Potassium leakage channels A A Cytoplasm Protein anion (unable to follow through the membrane)

48 Figure 3.14 The key role of in generating the resting membrane potential. Slide 4 Extracellular fluid CI CI diffuse down their steep concentration gradient (out of the cell) via leakage channels. Loss of results in a negative charge on the inner plasma membrane face. 2 also move into the cell because they are attracted to the negative charge established on the inner plasma membrane face. Potassium leakage channels A A Cytoplasm Protein anion (unable to follow through the membrane) 3 A negative membrane potential ( 90 mv) is established when the movement of out of the cell equals movement into the cell. At this point, the concentration gradient promoting exit exactly opposes the electrical gradient for entry.

49 Active Transport Maintains Electrochemical Gradients RMP is maintained through action of the - pump, which continuously ejects 3 out of cell and brings 2 back inside Steady state is maintained because rate of active pumping of out of cell equals the rate of diffusion into cell Neuron and muscle cells upset this steady state RMP by intentionally opening gated and channels

50 3.6 Cell-Environment Interactions Cells interact with their environment by responding directly to other cells, or indirectly to extracellular chemicals Interactions always involves glycocalyx Cell adhesion molecules (CAMs) Plasma membrane receptors

51 Role of Cell Adhesion Molecules (CAMs) Every cell has thousands of sticky glycoprotein CAMs projecting from membrane Functions: Anchor cell to extracellular matrix or to 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 (example: tight junctions) Transmit intracellular signals to direct cell migration, proliferation, and specialization

52 Roles of Plasma Membrane Receptors Membrane receptor proteins serve as binding sites for several chemical signals Contact signaling: cells that touch recognize each other by each cell s unique surface membrane receptors Used in normal development and immunity Chemical signaling: interaction between receptors and ligands (chemical messengers) that cause changes in cellular activities In some cells, binding triggers enzyme activation; in others, it opens chemically gated ion channels Examples of ligands: neurotransmitters, hormones, and paracrines

53 Roles of Plasma Membrane Receptors (cont.) Chemical signaling (cont.): Same ligand can cause different responses in different cells depending on chemical pathway that the receptor is part of When ligand binds, receptor protein changes shape and thereby becomes activated Some activated receptors become enzymes; others act to directly open or close ion gates, causing changes in excitability

54 Roles of Plasma Membrane Receptors (cont.) Chemical signaling (cont.): Activated G protein linked receptors indirectly cause cellular changes by activating G proteins, which in turn can affect ion channels, activate other enzymes, or cause release of internal second messenger chemicals such as cyclic AMP or calcium

55 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st 2 The activated receptor binds messenger) binds to the receptor. The receptor changes shape and activates. to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Effector protein (e.g., an enzyme Extracellular fluid Ligand Receptor G protein GDP * Ligands include hormones and neurotransmitters. GTP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) 5 Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. 6 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. Intracellular fluid

56 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 2 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor GTP G protein GDP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes * Ligands include hormones and neurotransmitters. Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Intracellular fluid

57 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 3 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor GTP G protein GDP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes * Ligands include hormones and neurotransmitters. Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Intracellular fluid

58 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 4 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor GTP G protein GDP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes * Ligands include hormones and neurotransmitters. Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Intracellular fluid

59 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 5 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor G protein GDP GTP GTP GTP Inactive 2nd messenger Active 2nd messenger 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) Activated Kinase enzymes * Ligands include hormones and neurotransmitters. Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Intracellular fluid

60 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 6 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor G protein GDP GTP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) 5 Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. * Ligands include hormones and neurotransmitters. Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) Intracellular fluid

61 Focus Figure 3.2 G proteins act as middlemen or relays between extracellular first messengers and intracellular second messengers that cause responses within the cell. Slide 7 Ligand Receptor G protein Enzyme 2nd (1st messenger) messenger 1 Ligand*(1st messenger) binds to the receptor. The receptor changes shape and activates. 2 The activated receptor binds to a G protein and activates it. The G protein changes shape (turns on ), causing it to release GDP and bind GTP (an energy source). 3 Activated G protein activates (or inactivates) an effector protein by causing its shape to change. Extracellular fluid Effector protein (e.g., an enzyme Ligand Receptor G protein GDP * Ligands include hormones and neurotransmitters. GTP GTP GTP Inactive 2nd messenger Active 2nd messenger Activated Kinase enzymes Cascade of cellular Responses (The amplification effect is tremendous. Each enzyme catalyzes hundreds of reactions.) 4 Activated effector enzymes catalyze reactions that produce 2nd messengers in the cell. (Common 2nd messengers include cyclic AMP and Ca 2+.) 5 Second messengers activate other enzymes or ion channels. Cyclic AMP typically activates protein kinase enzymes. 6 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. Intracellular fluid