Lecture 5


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Lecture 5

  1. 1. Lecture 5: Movement of material across cell membranes Covers 5.2
  2. 2. • Now that we have learned about cell theory and the structure of cells, lets talk about how material moves across the membrane.
  3. 3. • Material moves in response to gradients and concentrations • If there is more of molecule A on the inside of the cell than on the outside, molecule A will want to move to an area of lesser concentration (molecule A will want to move OUTSIDE) • If there is more of molecule A on the outside of the cell, molecule A will want to move INSIDE • Molecules always want to move from an area of high concentration to low concentration
  4. 4. Two types of transport* • Passive transport: movement from area of high concentration to lower concentration – 3 types of passive transport: • Simple diffusion • Facilitated diffusion • Osmosis • Energy-Requiring Transport: sometimes it is necessary for molecules to move from an area of low concentration to an area of high concentration. Because this is against the gradient, energy is required to make this happen – 3 types: • Active transport • Endocytosis • Exocytosis
  5. 5. Passive transport* • Simple Diffusion: substances move DOWN their concentration gradient (high to low concentration) THROUGH THE PHOSPHOLIPID BILAYER (water, O2, CO2, and lipid soluble material like ETOH and Vitamins A, D, E) • Facilitated Diffusion: substances move DOWN their concentration gradient THROUGH PROTEIN CHANNELS (ions, sugars, amino acids) • Osmosis: the movement of WATER ONLY THROUGH PROTEIN CHANNELS
  6. 6. Fig. 5-7 water glucose carrier protein aquaporinchannel protein phospho- lipid bilayer (cytoplasm) (extracellular fluid) Cl– O2 (a) Simple diffusion through the phospholipid bilayer (b) Facilitated diffusion through channel proteins (c) Osmosis through aquaporins or the phospholipid bilayer (d) Facilitated diffusion through carrier proteins Types of Diffusion Through the Plasma Membrane
  7. 7. Osmosis affects solute concentration • Solutes are the solid particles inside a cell • Water moving in and out of cell through osmosis can make solutes more concentrated (if water moves out) or less concentrated (if water moves in) • Movement can also depend on solution the cell is bathed in (isotonic/hypotonic/hypertonic) • Ex: Red Blood Cell
  8. 8. Fig. 5-8 No net flow of water Water flows out; the balloon shrinks Water flows in; the balloon expands (a) A balloon in an isotonic solution (b) A balloon in a hypertonic solution (c) A balloon in a hypotonic solution
  9. 9. Fig. 5-9
  10. 10. Energy-Requiring Transport*: Active Transport • substances move AGAINST concentration gradient (low concentration to high concentration) THROUGH PROTEIN CHANNELS. • ATP IS REQUIRED. (more about ATP later, but recall it is a molecule with 3 phosphates attached to it. There is energy in the phosphate bonds that can be used for work in the body.)
  11. 11. Active Transport Fig. 5-11 The transport protein binds both ATP and Ca2+ Energy from ATP changes the shape of the transport protein and moves the ion across the membrane The protein releases the ion and the remnants of ATP (ADP and P) and closes ATP binding site recognition site ATP P ADP Ca2+ (extracellular fluid) (cytoplasm) ATP 1 2 3
  12. 12. Endocytosis • Cells can engulf and bring particles or fluids into the cell through endocytosis. • Plasma membrane will “dimple” inward and create a vesicle (small space encircled by plasma membrane) containing the material to be brought into cell. • Then vesicle will break apart once inside cell and “drop off” the contents. • Three types: – Pinocytosis: movement of FLUIDS into cell – Receptor-Mediated Endocytosis: receptor proteins on cell surface recognize particles that need to be brought into cell and then that material is brought into cell by creation of a vesicle – Phagocytosis: movement of LARGE PARTICLES into cells (in humans, WBC’s can do this)
  13. 13. Pinocytosis Fig. 5-12 (extracellular fluid) (cytoplasm) vesicle containing extracellular fluid (a) Pinocytosis (b) TEM of pinocytosis A dimple forms in the plasma membrane, which deepens and surrounds the extracellular fluid. The membrane encloses the extracellular fluid, forming a vesicle. extracellular fluid cytoplasm 1 2 3 1 2 3 1 2 3
  14. 14. protein coating coated pit coated vesicle extracellular particles bound to receptors plasma membrane (extracellular fluid) (cytoplasm) 0.1 micrometer (cytoplasm) nutrient molecule (extracellular fluid) receptor coated vesicle coated pit (a) Receptor-mediated endocytosis Receptor proteins for specific molecules or complexes of molecules are localized at coated pit sites. A vesicle ("coated vesicle") containing the bound molecules is released into the cytoplasm. The coated pit region of the membrane encloses the receptor-bound molecules. The receptors bind the molecules and the membrane dimples inward. (b) TEM of receptor-mediated endocytosis 1 2 3 4 1 2 3 4 1 2 3 4 Fig. 5-13
  15. 15. Phagocytosis Fig. 5-14 food particle pseudopods food vacuole (a) Phagocytosis (b) An Amoeba engulfs a Paramecium (c) A white blood cell ingests bacteria (extracellular fluid) (cytoplasm) The plasma membrane extends pseudopods toward an extracellular particle (for example, food). The ends of the pseudopods fuse, encircling the particle. A vesicle called a food vacuole is formed containing the engulfed particle. 1 2 3 1 2 3 1 2 3
  16. 16. Exocytosis • Cells can dispose of waste AND send molecules out of cell via exocytosis • A membrane enclosed vesicle carrying the material to be expelled moves toward the plasma membrane. • Vesicle binds to plasma membrane and releases material to the outside of the cell.
  17. 17. Exocytosis Fig. 5-15