Water moves from a hypotonic solution across a membrane to a hypertonic

Water moves from a hypotonic solution across a

This preview shows page 10 - 13 out of 20 pages.

-Water moves from a hypotonic solution across a membrane to a hypertonic solution -Regulation of the solute concentration of body fluids is an important process for organisms without cell walls
Image of page 10
Organisms with cell walls can limit their volumes and keep them from bursting Turgor pressure: Cells with sturdy walls take up a limited amount of water, and in so doing they build up internal pressure against the cell wall, which prevents further water from entering o Keeps plants upright -The following about osmosis are TRUE: It obeys the laws of diffusion In animal tissues, water moves into cells if they are hypertonic to their environment Two cells with identical solute concentrations are isotonic to each other Solute concentration is the principal factor in osmosis -Polar and charged substances can cross passively in one of two ways: Channel proteins: Integral membrane proteins that form channels across the membrane through which certain substances can pass Carrier proteins: Speed up their diffusion through the phospholipid bilayer -Best studied channel proteins are the ion channels Integral membrane protein that allows ions to diffuse across the membrane in which it is embedded Structure: Hydrophilic pore that allows a particular ion to move through it -Mos t ion channels have “gates” Gated channel: Opens when a stimulus causes a change in the 3D shape of the channel Ligand: Any molecule that binds to a receptor site of another (usually larger) molecule o Channels controlled this way ligand-gated channels Voltage-gated channel: Stimulated to open or close by a change in the voltage across the membrane -Because there is an imbalance of ion concentrations across the plasma membrane, there is a voltage or membrane potential When an ion channel opens, millions of ions rush through, how fast depends on… o The concentration gradient
Image of page 11
o Magnitude of the voltage -Membrane potential: The difference in electrical charge between the inside and the outside of a cell, caused by a difference in the distribution of ions -Membrane potential is related to the concentration imbalance of K+ by the Nernst equation: R=gas content F=Faraday constant T=temperature Z=charge on the ion Ek= Membrane potential o Results from the ratio of K+ concentrations outside the cell [K]0 and inside the cell [K]i -Equation at room temperature: -Small changes in K+ concentration can have a large effect on the electrical potential across the membrane Example: Nervous system -The specificity of ion channels: The gate that opens/closes the channel appears to be an interaction between positively charged arginine residues on the protein and negative charges on membrane phospholipids o Functional interactions of proteins/lipids -Aquaporins for water: Water can attach onto ions to pass through membranes Even faster way aquaporins o Transport protein in plant and animal cell membranes through which water passes in osmosis o Highly specific o First identified by Peter Agre -Facilitated diffusion: Passive movement through a membrane involving a specific carrier protein; does not proceed against a concentration gradient
Image of page 12
Image of page 13

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture

  • Left Quote Icon

    Student Picture