Download Passive Transport

Passive Transport
Passive transport – movement of a molecule across a cell membrane
using NO energy
Types of passive transport:
1.
diffusion – net movement of molecules from a high
concentration to an area of low concentration (down a
concentration gradient); independent of other molecules
o Result of random motion of molecules
o When they are uniformly distributed, molecules are said to be
at equilibrium
o Some molecules can diffuse past the cell membrane
o Selectively permeable or semi-permeable nature of the cell
membrane:
Membrane is mostly hydrophobic, so:
 Small, nonpolar molecules will diffuse easily
 Water diffuses slowly, but protein channels called
aquaporins help speed it up.
 Large or polar molecules cannot diffuse through
membrane on their own
 Ions cannot diffuse on their own
Other things affecting rate of diffusion:
o Temperature – heat speeds up diffusion
o Electric gradient – based on charges of molecules
o Pressure gradient – difference in pressure being exerted on
both sides of membrane
2.
facilitated diffusion – the diffusion of solutes or water through
channel or carrier proteins in the membrane
Channel proteins – provide tunnel
Carrier proteins – change shape to transport
substance
3.
osmosis – the diffusion of water across a semi-permeable
membrane
Terms you need to know about osmosis:
1. Solvent – in this case, water
2. Solute – substances dissolved in water
3. Tonicity – solute concentrations of two solutions in relation to
each other
a. Hypertonic solution – has more solutes than other solution
b. Hypotonic solution – has less solutes than other solution
c. Isotonic solution – has same solutes as other solution
4. Hydrostatic pressure – pressure that fluid exerts against a
structure enclosing it
Simple animal cell osmosis – only considers tonicity (solutes)
o Will gain water and swell when placed in a hypotonic
solution
If hydrostatic pressure becomes too great, cell will burst
(cytolysis) – no cell wall
o Will lose water and shrink (crenation) when placed in a
hypertonic solution
Complicated plant cell osmosis – considers both pressure and
solutes
Water flows from area of higher water potential to area of
low water potential
Formula for calculating water potential:
Solute Potential:
o Distilled water = 0
o If there are solutes, solute potential is negative
o More solutes = more negative
Pressure Potential:
o Pressure exerted on the cell wall in either direction
o Pressure of an open system (ex. cup of water) = 0
o Turgor pressure – the pressure exerted on a cell wall from
expanding cell volume
o More water in vacuole = more positive pressure
o Cell at “normal” size = pressure is 0
Problem: A potato core with pressure potential = 0.4 and solute
potential = -0.4 is placed into distilled water. What will happen?
o Osmotic pressure – turgor pressure that is so high that it will
not let any more water in, regardless of solute difference
o Plasmolysis – movement of water out of a plant cell that
results in the collapse of the cell; the membrane pulls away
from the cell wall