Chapter 12 Flashcards
rates at which various solutes cross the lipid bilayer by simple diffusion
small nonpolar molecules: diffuse rapidly
small uncharged polar molecules: only diffuse if they are small enough
larger uncharged polar molecules: hardly cross
ions: never
two main classes of membrane transport proteins
transporters and channels
channels
discriminate mainly on the basis of size and electric charge
only ions of an appropriate size and charge can pass through
transporter
transfers only those molecules or ions that fit into specific binding sites on the protein
is Na2+ most plentiful inside or outside the cell
outside the cell
is K+ most plentiful inside or outside the cell
inside the cell
membrane potential
voltage difference across a membrane due to a slight excess of positive ions on one side and of negative ions on the other
when is the membrane potential expressed as a negative value
interior of the cell is more negatively charged than the exterior
passive transport
spontaneous movement of a solute down its concentration gradient across a cell membrane via a membrane transport protein
active transport
movement of a solute across a membrane against its electrochemical gradient
requires an input of energy such as ATP hydrolysis
membrane potential tends to pull ____ charged ions and molecules into the cell and drive _____ charged solutes out
positively
negatively
electrochemical gradient
driving force that determines which way an ion will move across a membrane
combined influence of the ion’s concentration gradient and the membrane potential
aquaporin
channel that facilitates the transport of water across membranes
osmosis
passive movement of water across a cell membrane from a region where the concentration of water is high to a region where the concentration of water is low
turgor pressure
force that builds as water flows into plant and yeast cells by osmosis
drives the expansion of cells that underlies plant growth and maintains the rigidity of plant stems and leaves