Diffusion and Cell Membranes - I
Objectives
1. Define the following terms: solute, solvent, concentration gradient, osmotic pressure, and
selectively permeable.
2. Define the following processes and identify the characteristics that distinguish them from one
another: diffusion, osmosis, facilitated diffusion, and active transport.
3. Give three factors that affect the rate of diffusion and state whether they increase or decrease the
rate.
Introduction
Cells maintain a constant internal environment, a process called homeostasis. In a constant environment
enzymes and other cellular components can operate at optimum efficiency. The ability to selectively
exchange materials with the environment is one component of the cell’s homeostatic mechanism. Ions and
molecules, such as sugars, amino acids and nucleotides, must enter the cell, and the waste products of
cellular processes must leave the cell. Regardless of the direction of movement, the common mediator of
these processes is the cell membrane, or plasma membrane.
The plasma membrane is a fluid, or mobile, mosaic of lipids and proteins. Ions and molecules cross the
plasma membrane by a number of processes. Large particles are engulfed by the membrane, forming a
vesicle or vacuole that can pass into (endocytosis) or out (exocytosis) of the cell. Some small, electrically
neutral molecules diffuse through the spaces between the lipid molecules of the plasma membrane. Others
bind to transport proteins embedded in the plasma membrane and transported into or out of the cell.
Atoms, ions and molecules in solution are in constant motion and continuously collide with each other
because of their kinetic energy. As the temperature is raised, the speed of movement of the molecules
increases and they collide more frequently and with greater force. An observable consequence of this
motion is Brownian motion, an erratic, vibratory motion of small particles suspended in water, which is
caused by the collisions of water molecules with the particles.
Diffusion also results from the kinetic energy of molecules. If a small crystal of a soluble substance is
added to water, molecules of the substance break away from the crystal surface and enter solution. As a
consequence of the collisions with water molecules, molecules of the substance move in a random pattern
in the solution, but always away from the crystal, with some moving to the farthest reaches of the solution.
This process continues until the molecules of the substance are evenly distributed throughout the water (the
solvent). In a general sense, in any localized region of high concentration, the movement of molecules is,
on average, away from the region of highest concentration and towards the region of lowest concentration.
The gradual difference in concentration over the distance between the regions of high and low
concentration is called the concentration gradient. The steeper the concentration gradient, the greater the
rate of diffusion. The rate of diffusion is also directly proportional to the temperature and inversely
proportional to the molecular weight of the diffusing molecules. In other words, the higher the temperature
the faster the rate of diffusion, but larger molecules move slower than smaller molecules at the same
temperature.
Small, electrically neutral substances diffuse into and out of cells by passing through the spaces between
the lipids of the plasma membrane or by dissolving in the lipids or proteins of the membrane. Substances
that are large or electrically charged cannot pass through membranes. Membranes that block or inhibit the
movement of molecules are called differentially permeable, or selectively permeable. Selective
permeability explains the phenomenon of osmosis, the diffusion of water across a membrane under certain
conditions.
If two solutions containing different concentrations of a solute are separated by a selectively permeable
membrane (permeable to water but not to the solute), water will move from the solution with low solute
concentration to the solution of high solute concentration. The water flows in this direction because the
solution with low solute concentration has a high water concentration and the solution with high solute
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