Membrane Transport Permeability - the ability of a substance to pass through a membrane Cell membranes are selectively permeable Permeability is determined by A. the phospholipid bilayer and B. transport proteins in the membrane A. Permeability through the Lipid Bilayer 1. molecular size - smaller molecules are more permeable 2. lipid solubility - lipid-soluble (non-polar) molecules are more permeable - polar molecules and ions do not easily cross the lipid bilayer B. Membrane Transport Proteins (channels, carriers, and pumps) - enable certain ions and polar molecules to cross the membrane protein channels form passageways for certain ions 1
Permeability of the Plasma Membrane carrier proteins are used for certain polar molecules such as glucose Highly permeable O2 & CO2 fatty acids steroids H2O (variable: pores) Less permeable Na +, K +, Cl (via channels) glucose, a.a. s (via carriers) Impermeable proteins* DNA & RNA ATP * except via membrane-bound vesicles Transport Across Membranes 1. Simple Diffusion 2. Osmosis 3. Diffusion through channels 4. Facilitated diffusion 5. Primary active transport 6. Secondary active transport 7. Transport via membrane-bound vesicles 1. Simple Diffusion 2. Osmosis Transport Across Membranes 3. Diffusion through channels 4. Facilitated diffusion 5. Primary active transport 6. Secondary active transport Passive Transport - no energy required Active Transport - requires energy Proteinmediated Transport 2
Diffusion in a Solution Diffusion Across a Membrane Diffusion occurs from high to low concentration (down a concentration gradient) Rate of Diffusion Across a Membrane Fick s Law of Diffusion Osmosis passive movement of water across a membrane in response to a solute concentration gradient Selectively-permeable membrane: permeable to water, impermeable to solute Water follows solutes; solutes suck Osmotic pressure: force that results from the difference in concentration of solutes across the membrane 3
Ø Osmolarity and Osmotic Pressure osmosis depends on osmolarity = total concentration of all solute particles in solution Tonicity effect of an extracellular solution on cell volume due to osmosis across the cell membrane depends on concentration of non-penetrating solutes only Ø 1 Osm (= 1,000 mosm) = 1M total solute concentration Hypotonic Solution - water enters the cell by osmosis (0 m Osm ) - cell gains volume (swells) Ø Ø solute concentration (osmolarity) difference results in a large osmotic pressure difference (π = CRT) osmotic pressure difference is the driving force for osmosis Hypertonic Solution - water leaves the cell by osmosis - cell loses volume (shrinks) Crenated RBCs in a Hypertonic Solution Isotonic Solution - concentration of non-penetrating solutes is equal in the ECF and ICF ICF 300 mosm ECF 300 mosm - no net water movement by osmosis - cell volume remains constant Example: 0.9% NaCl (0.9 g/dl) ( normal saline ) Image: Copyright 2004 Dennis Kunkel Microscopy, Inc. 4
1. Simple Diffusion 2. Osmosis Transport Across Membranes Passive Transport - no energy required Protein-Mediated Transport channels, carriers, and pumps 3. Diffusion through channels 4. Facilitated diffusion 5. Primary active transport 6. Secondary active transport Active Transport - requires energy Proteinmediated Transport Diffusion Through Channels 5
Diffusion of ions is influenced by both chemical and electrical driving forces. Electrical force on ions is due to membrane potential which results from slight imbalance of + and charges between the ICF and ECF (more negative inside). No membrane potential chemical force only (outward for K +, inward for Na + ) Membrane potential present chemical and electrical forces = electrochemical gradient For K +, chemical and electrical gradients are in the opposite direction (chem. outward, elec. inward), so the electrochemical gradient is small. The combined force is the electrochemical gradient. For Na +, chemical and electrical gradients are in the same direction (both inward), so the electrochemical gradient is large Facilitated Diffusion 6
GLUT transporters - mediate transport of glucose into most cells Facilitated Diffusion Carrier-mediated transport exhibits saturation. The transport maximum depends on the number of carrier proteins present in the membrane. - the GLUT4 transporter in skeletal muscle and adipose tissue is activated by insulin - Transports molecules against a gradient - Uses energy from ATP directly Primary Active Transport The sodium-potassium pump (Na + -K + -ATPase) Mechanism of the Na + -K + ATPase 7
Secondary Active Transport - Transports molecules against their gradient - Uses potential energy stored in ionic gradients (energy supplied indirectly by ATP ) - Involves coupled transport with an ion moving downhill The sodium-glucose (SGLT) transporter Mechanism of the SGLT Transporter 8
Vesicular Transport Phagocytosis Vesicles created by the cytoskeleton Cell engulfs bacterium or other particle into phagosome Figure 5.18 Phagocytosis Slide 1 Figure 5.18 Phagocytosis Slide 2 Ba c te rium Ba c te rium Phagocyte Lysosome The phagosome containing the bacterium separates from the cell membrane and moves into the cytoplasm. Phagocyte Lysosome The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagosome fuses with lysosomes containing digestive enzymes. The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. The bacterium is killed and digested within the vesicle. 9
Figure 5.18 Phagocytosis Slide 3 Figure 5.18 Phagocytosis Slide 4 Ba c te rium Ba c te rium Phagocyte Lysosome Phagocyte Lysosome The phagosome containing the bacterium separates from the cell membrane and moves into the cytoplasm. The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. Figure 5.18 Phagocytosis Slide 5 Figure 5.18 Phagocytosis Slide 6 Ba c te rium Ba c te rium Phagocyte The phagosome containing the bacterium separates Phagocyte The phagosome containing the bacterium separates Lysosome from the cell membrane and moves into the cytoplasm. Lysosome from the cell membrane and moves into the cytoplasm. The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagosome fuses with lysosomes containing digestive enzymes. The phagocytic white blood cell encounters a bacterium that binds to the cell membrane. The phagosome fuses with lysosomes containing digestive enzymes. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. The phagocyte uses its cytoskeleton to push its cell membrane around the bacterium, creating a large vesicle, the phagosome. The bacterium is killed and digested within the vesicle. 10
Vesicular Transport Exocytosis -ligand migrates to clathrin-coated. Transport vesicle Endocytosis and cell membrane fuse (membrane re c y c ling). Endocytosis Membrane surface indents and forms vesicles Active process that can be nonselective (pinocytosis) or highly selective Receptor-mediated endocytosis uses coated s Transport vesicle with receptors moves to the cell membrane. Vesicle loses clathrin coat. most common protein in coated s Membrane recycling To lysosome or Golgi complex Ligands go to lysosomes or Golgi for processing. Endosome s and ligands separate. 2016 Pears on Education, Inc. -ligand migrates to clathrin-coated. 11
-ligand migrates to clathrin-coated. -ligand migrates to clathrin-coated. Endocytosis Endocytosis Vesicle loses clathrin coat. -ligand migrates to clathrin-coated. -ligand migrates to clathrin-coated. Endocytosis Endocytosis Vesicle loses clathrin coat. Vesicle loses clathrin coat. s and ligands separate. To lysosome or Golgi complex s and ligands separate. Endosome Endosome Ligands go to lysosomes or Golgi for processing. 12
-ligand migrates to clathrin-coated. -ligand migrates to clathrin-coated. Transport vesicle Endocytosis and cell membrane fuse (membrane re c y c ling). Endocytosis Transport vesicle with receptors moves to the cell membrane. Vesicle loses clathrin coat. Transport vesicle with receptors moves to the cell membrane. Vesicle loses clathrin coat. To lysosome or Golgi complex s and ligands separate. To lysosome or Golgi complex s and ligands separate. Endosome Ligands go to lysosomes or Golgi for processing. Endosome Ligands go to lysosomes or Golgi for processing. Ehelial Transport Apical (mucosal) membrane Basal membrane Paracellular transport Through junctions between adjacent cells Transcellular transport Through cells themselves Transcytosis with vesicular transport 13
Ehelial Transport Absorption from lumen to extracellular fluid (ECF) Secretion from ECF to lumen Transcellular transport of glucose uses membrane proteins Apical membrane with microvilli faces the lumen. Tight junction limits movement of substances between the cells. Tra nsport proteins Basolateral membrane faces the ECF. Figure 5.20 Transporting ehelia are polarized Lumen of intestineor kidney Tra nsporting ehelial cell Secretion Absorption (transcellular) Absorption (paracellular) Lumen of kidney or intestine Figure 5.21 Transehelial [Glucose]low Glu Na + absorption of glucose [Na + ]high Na + -glucose symporter brings glucose into cell against its gradient using energy stored in the Na + concentration gradient. Transehelial Transport of Water Apica l membrane GLUT transporter transfers glucose to ECF by facilitated diffusion. Ehelial cell [Glucose]high Glu Na + [Na + ]low Na + -K + -ATPase pumps Na + out of the cell, keeping ICF Na + concentration low. Active transport of solutes into lateral intercellular space Osmosis: H 2O follow solute movement Ba s olate ra l membrane Extracellular fluid [Glucose]low Glu Na + K+ Glu [Na + ] high Na + ATP K+ FIGURE QUESTIONS 1.Ma tc h each transporter to its location. 1. GLUT 2. Na + -glucose symporter 3. Na + -K + -ATPase 2. Is glucose mo v em en t ac ro ss the basolateral me mb ra ne ac ti ve or passive? Explain. Choose either 3. Why doesn't Na + movement at the ap i ca l membrane re quireatp? (a)apical membrane (b)basolateral membrane 14