Biological Membranes: Understanding Transport Mechanisms, Summaries of Cell Biology

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Advanced Cell Biology

Biological Membranes Transport

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Passive Transport Is

Facilitated by

Membrane Proteins

Energy changes accompanying passage of a hydrophilic solute through the lipid bilayer of a biological membrane Figure 11.2 Overview of membrane transport proteins.

Figure 11.3 Multiple membrane transport proteins function together in the plasma membrane of metazoan cells.

13 Regulation by insulin of glucose transport by GLUT4 into a myocyte

Effects of Osmosis on Water

Balance

• Osmosis is the diffusion of water across a selectively permeable membrane
• The direction of osmosis is determined only by a difference in total solute concentration
• Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration

Water Balance of Cells Without

Walls

• Tonicity is the ability of a 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
• Animals and other organisms without rigid cell walls have osmotic problems in either a hypertonic or hypotonic environment
• To maintain their internal environment, such organisms must have adaptations for osmoregulation , the control of water balance
• The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

19 ATP-Powered Pumps The four classes of ATP-powered transport proteins

Maintenance of Membrane

Potential by Ion Pumps

• Membrane potential is the voltage difference across a membrane
• Two combined forces, collectively called the electrochemical gradient , drive the diffusion of ions across a membrane: - A chemical force (the ion’s concentration gradient) - An electrical force (the effect of the membrane potential on the ion’s movement)

Figure 11.10 Operational model of the Ca2+^ ATPase in the SR membrane of skeletal muscle cells. P-Type Ca2+^ Pumps Maintain a Low Concentration of Calcium in the Cytosol- Effect of V-class H+^ pumps on H+^ concentration gradients and electric potential gradients across cellular

27 F-Type ATPase Are Reversible, ATP- driven Proton Pumps Structure of the FoF 1 ATPase/ATP synthase Figure 11.15 The multidrug transporter ABCB (MDR1): structure and model of ligand export.

31 The Chloride-Bicarbonate Exchanger Catalyzes Electrochemical Cotransport of Anions across the Plasma Membrane 32 Active Transport Results in Solute Movement against a Concentration or Electrochemical Gradient

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Ion Gradients Provide the Energy for

Secondary Active Transport

34 Lactose uptake in E. coli

Figure 11.31 Acidification of the stomach lumen by parietal cells in the gastric lining. Figure 11.32 Dissolution of bone by polarized osteoclast cells requires a V-class proton pump and the ClC-7 chloride channel protein.

Ion channel

• Movement of ions

through Ion

channel generate

transmembrane

electric potential

39 40 Structure and function of the K+ channel of Streptomyces lividans - Diagram of the K+ channel in cross section