chp 11 cell bio

• Only Gases and small uncharged molecules cross membranes by simple diffusion
• 2 combined forces, collectively called the electrochemical gradient drives the diffusion of ions across a membrane :

  • a chemical force
  • an electrical force
• Uniporters transport a single type of molecule down its concentration gradient
• symporters and antiporters are cotransport proteins that catalyze the movement of one molecule against its concentration gradient, driven by the movement of one or more ions down an electrochemical gradient
• The Na+/K+ ARPase in the plasma membrane uses energy released by ATP hydrolysis to pump Na+ out of the cell and K+ inward
• This create a concentration gradient of Na+ and K+ ions across the plasma membrane (rather high concentration of K inside the Na outside of cells) that are used to power the import of amino acids
• Movement of positively charged K+ ions out of the cell through membrane K+ channel proteins creates an electric potential across the plasma membrane- the cytosolic face is negative with respect to the extracellular face
• a Na+/lysine transporter moves 2 Na+ ions together w 1 lysine from the extracellular medium into the cell
• "uphill" movement of the amino acid is powered by "downhill" movement of Na" ions, powered both by the outside-greater-than-inside Na+ concentration gradient and by the negative potential on the inside of the cell membrane which attracts the positively charged Na+ ions.
• The ultimate source of the energy to power amino acid uptake comes from the ATP hydrolyzed by the Na+/K+ ATPase, since this pump creates both Na+ ion concentration gradient and via the K+ channels the membrane potential which together power influx of Na+ ions
• In secondary active transport, also known as coupled transport or co-transport
• In secondary active transport energy is used to transport molecules across a membrane; hw in contrast to primary active transport, there is no direct coupling of ATP; instead it relies upon the electrochemical potential difference created by pumping ions in/out of the cell
• Cotransport occurs when active transport of a solute indirectly drives transport of the other substances
• Most animal cells utilize glucose as a substrate for ATP production; they usually employ a glucose uniporter to take up glucose from the blood or other extracellular fluid
• Many cells utilize channel-like membrane transport "aquaporins" to increase the rate of water movement across their surface membrane
• The protein-mediated transport of a single type of moL such as glucose or other small hydrophilic molecules, down a concentration gradient across a cellular membrane is known as uniport
• The human genome encodes at least 14 highly homologous GLUT proteins (glucose transporters )
• GLUT proteins: GLUT3 in neuronal cells

  GLUT2 in liver cells

  GLUT4 only in fat and muscle cell GLUT5: only GLUT proteins w high specificity (preference) for fructose
• GLUT2 in liver cells ("excess" glucose stored as the polymer glycogen) and the insulin-secreting islet cells of the pancreas (the rise in glucose trigger secretion of the hormone insulin)
• GLUT4 only in fat and muscle cell, in the absence of insulin, GLUT4 resides in intracellular membranes. Insulin stimulation induces translocation of GLUT4 from intracellular vesicles to the plasma membrane.
• GLUT5: only GLUT proteins w high specificity (preference) for fructose. On the apical membrane of intestinal epithelial cells, where it transport dietary fructose from the intestinal lumen inside the cells,
• This is one principal mechanism by which insulin lowers blood glucose; defect in the movement of GLUT4 to the plasma membrane is one of the causes of adult onset, or type II diabetes, a disease marked by continuously high blood glucose
• Osmosis is the diffusion of water across a selectively permeable membrane
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides
• Water balance of cell without cell walls = animal cells.

  Water balance of cell with cell walls = plant cells
• Tonicity : ability of a surrounding solution to cause a cell to gain or lose water
• Isotonic solution : solute concentration is the same as that inside the cell; no net water movement across the plasma membrane
• Hypertonic solution: solute concentration is greater than that inside the cell; cell loses water
• Hypotonic solution: solute concentration is less than that inside the cell; cell gains water
• Osmoregulation, control of solute concentration & water balance is necessary adaptation for life in such environments
• Aquaporins increases the water permeability of cell membranes
• All ATP powered pumps are transmembrane proteins w one or more binding sites for ATP located subunits or segments of the protein that always face the cytosol
• These proteins commonly are called ATPases and they normally do not hydrolyze ATP into ADP and Pi unless ions or other molecules are simultaneously transported
• Note that the members of the 3 of the classes (P,F and V) only transport ions, as do some members of the fourth class, the ABC superfamily
• Most members of the ABC superfamily transport small moL such as amino acids, sugars, peptides, lipids and other small molecules including many types of drugs
• All P-class ion pumps possess 2 identical catalytic alpha subunits each of which contains an ATP-binding side
• Most P-class ion pump have 2 smaller beta subunits that usually have regulatory functions. During transport, at least one alpha subunits becomes phosphorylated (hence the name P class). Only one alpha & beta subunit are depicted.
• P-class pumps

  • Plasma membrane of plants and fungi (H+ pump)

  • Plasma membrane of higher eukaryotes (Na+/K+ pump)

  • Apical plasma membrane of mammalian stomach (H+/K+ pump)

  • Plasma membrane of all eukaryotic cells (Ca2+ pump)

  • Sarcoplasmic reticulum membrane in muscle cells (Ca2+ pump)
• A proton pumps are electrogenic pumps that store energy by generating voltage (charge separation) across membranes.
• A proton pump translocate positive charge in for of hydrogen ions. The voltage & H+ concentration gradient represent a dual energy source that can drive other processes, such as the uptake of nutrient, most proton pumps are powered by ATP