Chapter 4 – The structure and function of the Plasma Membrane
4.7 The movement of substances across cell membranes
Plasma membrane has a dual function. It must retain the dissolved materials of the cell, and it must
allow the necessary exchange of materials into and out of the cell. Two means of movement of
substances, passively by diffusion or actively by an energy coupled transport process. Net flux indicates
movement of substance into cell (influx) and out of cell (efflux) is not balanced.
Simple diffusion through lipid bilayer, simple diffusion through an aqueous protein loined channel,
diffusion that is facilitated by a protein transporter, and active transport which requires a pump.
The energetic of solute movement
Diffusion is a spontaneous process in which a substance moves from a region of high concentration to a
region of low concentration, eventually eliminating the concentration difference.
Diffusion of Substances through Membranes
Two qualifications. The substance must be present at a higher concentration on one side of the
membrane, and the membrane must e permeable to the substance. Permeable to a given solute either
because solute passes through lipid bilayer, or solute can traverse an aqueous pore that spans the
Partition coefficient, the ratioe of its solubility in a nonpolar solvent, greater the lipid solubility, the
faster the penetration. Another factor is its size. If two molecules ahve equivalent partition coefficients,
the smaller molecule tends to penetrate the lipid bilayer of a membrane more rapidly than the larger
one . Membranes are highly permeable to small inorganic molecules, such as oxygen, carbon dioxide, No
and water. Larger polar molecules (amino acids) exhibit poor membrane permeability.
Diffusion of Water through membranes
Water molecules move much more rapidly through a cell membrane than do dissolved ions or small
polar organic solutes. Membranes are said to be semipermeable ecause of difference in permeability of
water versus solutes. Water moves readily through a semipermeable membrane form a region of lower
solute concentration to a region of higher. Is called Osmosis.
During diffusion The compartment of higher solute concentration is hypertonic and the lower
concentration is hypotonic. Once the internal solute concentration equals the external solute
concentration they are isotonic. Plant cells are generally hypertonic compared to their fluid
environment. The tendency for water to enter the cell, causing it to develop an internal (turgor)
Turgor pressure provides support. If a plant cell is placed into a hyptertonic medium, its volume shrinks
as the plasma membrane pulls away form the surrounding cell wall, a process called plasmolysis. Loss of water causes plants too lose their support and wilt. Aquaporins, that allow the passive movem