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Understanding Active Transport Mechanisms in Cellular Membranes

Active transport is essential for moving solutes across cellular membranes, requiring ATP and specific carrier proteins. There are various types of active transport systems, including symport, antiport, primary, and secondary active transport. Additionally, vesicular transport enables large particles and macromolecules to move into and out of cells via processes like exocytosis, endocytosis, and transcytosis. Importantly, receptor-mediated endocytosis uses specialized pits for efficient transport. This understanding is crucial for insights into cellular function and signaling.

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Understanding Active Transport Mechanisms in Cellular Membranes

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  1. Cells Part 2

  2. Active Transport • Uses ATP to move solutes across a membrane • Requires carrier proteins

  3. Types of Active Transport • Symport system – two substances are moved across a membrane in the same direction • Antiport system – two substances are moved across a membrane in opposite directions

  4. Types of Active Transport • Primary active transport – hydrolysis of ATP phosphorylates the transport protein causing conformational change • Secondary active transport – use of an exchange pump (such as the Na+-K+ pump) indirectly to drive the transport of other solutes

  5. Types of Active Transport Figure 3.11

  6. Vesicular Transport • Transport of large particles and macromolecules across plasma membranes • Exocytosis – moves substance from the cell interior to the extracellular space • Endocytosis – enables large particles and macromolecules to enter the cell

  7. Vesicular Transport • Transcytosis – moving substances into, across, and then out of a cell • Vesicular trafficking – moving substances from one area in the cell to another • Phagocytosis – pseudopods engulf solids and bring them into the cell’s interior

  8. Vesicular Transport • Fluid-phase endocytosis – the plasma membrane infolds, bringing extracellular fluid and solutes into the interior of the cell • Receptor-mediated endocytosis – clathrin-coated pits provide the main route for endocytosis and transcytosis • Non-clathrin-coated vesicles – caveolae that are platforms for a variety of signaling molecules

  9. Exocytosis Figure 3.12a

  10. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Recycling of membrane and receptors (if present) to plasma membrane 1 Ingested substance Exocytosis of vesicle contents Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle 3 Transcytosis 2 Uncoating To lysosome for digestion and release of contents Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  11. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Ingested substance Plasma membrane (a) Clathrin-mediated endocytosis Figure 3.13a

  12. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Ingested substance Detachment of clathrin- coated vesicle Clathrin- coated vesicle Plasma membrane (a) Clathrin-mediated endocytosis Figure 3.13a

  13. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Ingested substance Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle Uncoating Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  14. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Recycling of membrane and receptors (if present) to plasma membrane 1 Ingested substance Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle Uncoating Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  15. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Ingested substance Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle 2 Uncoating To lysosome for digestion and release of contents Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  16. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Ingested substance Exocytosis of vesicle contents Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle 3 Transcytosis Uncoating Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  17. Clathrin-Mediated Endocytosis Extracellular fluid Extracellular fluid Cytoplasm Plasma membrane Clathrin- coated pit Recycling of membrane and receptors (if present) to plasma membrane 1 Ingested substance Exocytosis of vesicle contents Clathrin protein Endosome Uncoated vesicle Detachment of clathrin- coated vesicle 3 Transcytosis 2 Uncoating To lysosome for digestion and release of contents Clathrin- coated vesicle Plasma membrane Uncoated vesicle fusing with endosome (a) Clathrin-mediated endocytosis Figure 3.13a

  18. Phagocytosis Figure 3.13b

  19. Receptor Mediated Endocytosis Figure 3.13c

  20. Passive Membrane Transport – Review

  21. Active Membrane Transport – Review

  22. Membrane Potential • Voltage across a membrane • Resting membrane potential – the point where K+ potential is balanced by the membrane potential • Ranges from –20 to –200 mV • Results from Na+ and K+ concentration gradients across the membrane • Differential permeability of the plasma membrane to Na+ and K+ • Steady state – potential maintained by active transport of ions

  23. Generation and Maintenance of Membrane Potential PLAY InterActive Physiology ®: Nervous System I: The Membrane Potential Figure 3.15

  24. Cell Adhesion Molecules (CAMs) • Anchor cells to the extracellular matrix • Assist in movement of cells past one another • Rally protective white blood cells to injured or infected areas

  25. Roles of Membrane Receptors • Contact signaling – important in normal development and immunity • Electrical signaling – voltage-regulated “ion gates” in nerve and muscle tissue • Chemical signaling – neurotransmitters bind to chemically gated channel-linked receptors in nerve and muscle tissue • G protein-linked receptors – ligands bind to a receptor which activates a G protein, causing the release of a second messenger, such as cyclic AMP

  26. Operation of a G Protein • An extracellular ligand (first messenger), binds to a specific plasma membrane protein • The receptor activates a G protein that relays the message to an effector protein

  27. Operation of a G Protein • The effector is an enzyme that produces a second messenger inside the cell • The second messenger activates a kinase • The activated kinase can trigger a variety of cellular responses

  28. Operation of a G Protein Extracellular fluid First messenger (ligand) Effector (e.g., enzyme) 1 Active second messenger (e.g., cyclic AMP) 4 3 G protein 2 5 Membrane receptor Inactive second messenger Activated (phosphorylated) kinases 6 Cascade of cellular responses (metabolic and structural changes) Cytoplasm Figure 3.16

  29. Operation of a G Protein Extracellular fluid First messenger (ligand) 1 Membrane receptor Cytoplasm Figure 3.16

  30. Operation of a G Protein Extracellular fluid First messenger (ligand) 1 G protein 2 Membrane receptor Cytoplasm Figure 3.16

  31. Operation of a G Protein Extracellular fluid First messenger (ligand) Effector (e.g., enzyme) 1 3 G protein 2 Membrane receptor Cytoplasm Figure 3.16

  32. Operation of a G Protein Extracellular fluid First messenger (ligand) Effector (e.g., enzyme) 1 Active second messenger (e.g., cyclic AMP) 4 3 G protein 2 Membrane receptor Inactive second messenger Cytoplasm Figure 3.16

  33. Operation of a G Protein Extracellular fluid First messenger (ligand) Effector (e.g., enzyme) 1 Active second messenger (e.g., cyclic AMP) 4 3 G protein 2 5 Membrane receptor Inactive second messenger Activated (phosphorylated) kinases Cytoplasm Figure 3.16

  34. Operation of a G Protein Extracellular fluid First messenger (ligand) Effector (e.g., enzyme) 1 Active second messenger (e.g., cyclic AMP) 4 3 G protein 2 5 Membrane receptor Inactive second messenger Activated (phosphorylated) kinases 6 Cascade of cellular responses (metabolic and structural changes) Cytoplasm Figure 3.16

  35. Cytoplasm • Cytoplasm – material between plasma membrane and the nucleus • Cytosol – largely water with dissolved protein, salts, sugars, and other solutes

  36. Cytoplasm • Cytoplasmic organelles – metabolic machinery of the cell • Inclusions – chemical substances such as glycosomes, glycogen granules, and pigment

  37. Cytoplasmic Organelles • Specialized cellular compartments • Membranous • Mitochondria, peroxisomes, lysosomes, endoplasmic reticulum, and Golgi apparatus • Nonmembranous • Cytoskeleton, centrioles, and ribosomes

  38. Mitochondria • Double membrane structure with shelf-like cristae • Provide most of the cell’s ATP via aerobic cellular respiration • Contain their own DNA and RNA

  39. Mitochondria Figure 3.17a, b

  40. Ribosomes • Granules containing protein and rRNA • Site of protein synthesis • Free ribosomes synthesize soluble proteins • Membrane-bound ribosomes synthesize proteins to be incorporated into membranes

  41. Endoplasmic Reticulum (ER) • Interconnected tubes and parallel membranes enclosing cisternae • Continuous with the nuclear membrane • Two varieties – rough ER and smooth ER

  42. Endoplasmic Reticulum (ER) Figure 3.18a, c

  43. Rough (ER) • External surface studded with ribosomes • Manufactures all secreted proteins • Responsible for the synthesis of integral membrane proteins and phospholipids for cell membranes

  44. Signal Mechanism of Protein Synthesis • mRNA – ribosome complex is directed to rough ER by a signal-recognition particle (SRP) • SRP is released and polypeptide grows into cisternae • The protein is released into the cisternae and sugar groups are added

  45. Signal Mechanism of Protein Synthesis • The protein folds into a three-dimensional conformation • The protein is enclosed in a transport vesicle and moves toward the Golgi apparatus

  46. Cytosol Transport vesicle budding off Coatomer- coated transport vesicle 5 Ribosomes mRNA 3 4 Sugar group 2 1 Released glycoprotein Signal sequence removed Signal sequence Receptor site Growing polypeptide Signal- recognition particle (SRP) ER cisterna ER membrane Signal Mechanism of Protein Synthesis Figure 3.19

  47. Cytosol mRNA 1 Signal sequence Receptor site Signal- recognition particle (SRP) ER cisterna ER membrane Signal Mechanism of Protein Synthesis Figure 3.19

  48. Cytosol mRNA 2 1 Signal sequence Receptor site Growing polypeptide Signal- recognition particle (SRP) ER cisterna ER membrane Signal Mechanism of Protein Synthesis Figure 3.19

  49. Cytosol Ribosomes mRNA 3 2 1 Signal sequence removed Signal sequence Receptor site Growing polypeptide Signal- recognition particle (SRP) ER cisterna ER membrane Signal Mechanism of Protein Synthesis Figure 3.19

  50. Cytosol Ribosomes mRNA 3 4 2 1 Released glycoprotein Signal sequence removed Signal sequence Receptor site Growing polypeptide Signal- recognition particle (SRP) ER cisterna ER membrane Signal Mechanism of Protein Synthesis Figure 3.19

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