Cellular Membranes Chapter 7 Membrane Structure Composed of

Cellular Membranes Chapter 7

Membrane Structure Composed of lipids and proteins ◦ Phospholipid bilayer ◦ each protein type has a specific function ◦ in animal cells cholesterol acts as a buffer for membranes to resist changes in temperature membrane carbohydrates (extracellular surface) aid in recognition of other cells ◦ glycolipids (lipid + carb) ◦ glycoproteins (protein + carb)

Cellular Membranes Selectively Permeable ◦ allows some substances to cross more easily than others ◦ regulates transport across cell boundaries ◦ non-polar molecules cross easily (CO 2, hydrocarbons) hydrophobic ◦ polar molecules can’t cross easily (hydrophilic) ions, sugars, water--pass slowly Fluid Mosaic Model ◦ membrane is a fluid structure that has a variety of proteins scattered throughout ◦ Membranes float in or on phospholipid bilayer

Membrane Proteins determine the membrane’s function ◦ different cell types contain different membrane proteins Transport Enzymes Signal Transduction Cell-Cell Recognition Intercellular joining Attachments to ECM (extra-cellular matrix) Integral Proteins: into the membrane Peripheral Proteins: not embedded in the membrane; loosely bound to the surface

Transport Proteins proteins that allow the passage of specific ions and hydrophilic substances ◦ carrier proteins--hold onto substances and change shape which allows them to pass through membrane ◦ channel proteins--hydrophilic channel that molecules/ions can use to tunnel through the membrane water passes through proteins called aquaporins

Passive Transport Primary role is importing resources and exporting wastes from cell Diffusion: movement of molecules from an area of high to low concentration ◦ any substance will diffuse down a concentration gradient (high to low) ◦ eventually movement of molecules will reach equilibrium (moving at the same rate) no energy is required to move molecules across membrane Non-polar molecules diffuse easiest

Passive Transport Osmosis: diffusion of water across a selectively permeable membrane ◦ osmoregulation--ability for cells/organisms to control solute concentrations and water balance most important in cells without walls Freshwater vs. saltwater fish tonicity: ability of a solution to gain or lose water (depends on concentration of solutes that can’t pass through membrane) ◦ isotonic--no net movement of water ◦ hypertonic--solution with high concentration of solutes unable to cross membrane Ex. increased salinity ◦ hypotonic--solution with low concentration of solutes unable to cross membrane Ex. Distilled water

Passive Transport Facilitated Diffusion: transport proteins help certain polar molecules and ions pass through the cell membrane ◦ molecules move down a concentration gradient, so no energy required! (high to low concentration) ◦ provides efficient passage of a solute through the membrane ◦ channel and carrier proteins ion channels (gated channels)--open and close in response to a stimulus glucose channels

Active Transport membrane proteins move solutes against a concentration gradient (all carrier proteins) ◦ requires energy, provided by ATP allow cells to maintain different internal concentrations of smaller molecules than their environment (create concentration gradients) ◦ sodium/potassium pumps: high internal K+ concentration and low Na+ concentration

Bulk Transport Large molecules (polysaccharides and proteins) move in vesicles ◦ Exocytosis: vesicle membrane and cell membrane fuse, releasing material OUT of the cell ◦ Endocytosis: vesicles form from cell membrane to take IN materials Phagocytosis “eating” Pinocytosis “drinking”

Water Potential http: //www. bozemanscience. com/water-potential/ Used to predict the direction in which water will diffuse through living plant tissues ◦ solute potential (solute concentration) ◦ pressure potential (pressure +/- on a solution). ◦ In an open system, pressure potential will be the same as atmospheric pressure and can be ignored (so water potential = solute potential) Water will move from an area of higher water potential to an area of lower water potential ◦ Water potential of solutions at equilibrium will be zero ◦ Higher M concentration = lower water potential (hypertonic) Increase solute = decrease in water Water will diffuse in the direction of the highest M at atmospheric pressure ◦ Pure water (WP= 0) Turgor pressure (cell wall exerts pressure back to prevent excessive uptake of water and bursting of cell)

Water Potential Practice