Notes for Diffusion, Osmosis and Transport across the Cell’s Plasma Membrane
Recall That the Membrane Maintains Homeostasis
The cell membrane is the most important part of the cell in regard to homeostasis of substances. The
membrane is semi-permeable, also called selectively permeable, as it will not allow just anything into the
cell. The regulation and maintenance of a steady balance of substances in the body is referred to as
homeostasis. Cells must constantly maintain their pH in order to function properly. Your blood maintains a
pH of about 7.4 which is slightly basic (alkaline), but maintaining this pH balance is crucial to life.
Lysosomes, however, work properly in an acidic environment; a pH of about 5.
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Solute  particles dissolved in solvent (like sugar, salt, etc)
Solvent  liquid in which solute dissolves; we’ll be looking at water as a solvent in this unit.
Solution  Mixture in which solute is evenly distributed (dissolved) in a solvent. An example is saline
solution for cleaning contact lenses. NaCl (sodium chloride) is totally dissolved in water.
Passive Transport of Solute and Solvent; Osmosis and Simple Diffusion
During passive transport, particles move across the membrane; either into or out of the cell without using
any energy (no ATP required). Diffusion and osmosis are both forms of passive transport.
Simple Diffusion  Particles move across membrane (either in or out of the cell) from an area of high
concentration to an area of low concentration. Small particles and molecules such as O2, CO2 and H2O
diffuse directly across the phospholipid bilayer.
Concentration gradient  Difference in solute concentrations on each side of the membrane. This
illustrates particles moving with, or down, the concentration gradient; requires no energy
The illustration above shows particles moving with the concentration gradient (high to low)
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Water Flows via Osmosis in Response to Solute Concentration
Osmosis  The diffusion of water across the membrane from an area where there is more water (thus less
solute) to an area where there is less water (thus more solute).
Water flows with the concentration gradient in response to how much solute is dissolved in it; from a less
concentrated solution, called hypotonic, into a more concentrated area, called hypertonic. Water will
continue to move until equilibrium of concentration of solution on both sides of the cell is reached. Dynamic
equilibrium is the term used as water will move in and out of the cell at a constant rate.
Cells in a hypertonic solution  higher concentration of solute outside of the cell (thus less water). Water
will flow (osmosis) outside of the cell causing it to shrivel. Water flows from hypo- to hyper !!
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Cells in a hypotonic solution  lower concentration of solute outside of the cell (thus more water). Water
will flow (osmosis) into the cell and it will swell (maybe burst if too much pressure). Water flows from
hypo- to hyper !!
Cells in isotonic solution  (Normal state); Solute concentration is equal on both sides of the cell. Water
will flow in and out at a constant rate (dynamic equilibrium). Remember molecules are constantly moving!
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Osmotic Pressure  The osmotic pressure of a solution is the pressure needed to prevent osmosis from
occurring.
Although U-tube B looks uneven, the concentration of solution is the same. Think of it this way. You
can have a gallon of solution that contains 5% salt and a cup of solution that contains 5%.......Same
concentrations, just different volumes of solution!!!
See the illustration above. Water flows from low osmotic pressure (dilute solution) to high osmotic pressure
(concentrated solution). As solute concentrations on both sides equalize, the pressure will stop osmosis from
continuing,
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Transport across the Cell Membrane Continued
Another form of passive transport is called facilitated diffusion. In this case, particles cannot simply diffuse across the
membrane. They need the help of proteins in the membrane but NO energy is required! Molecules still move from an
area of high concentration to an area of low. Down (or with) the concentration gradient!
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Allows diffusion of large, membrane insoluble compounds such as glucose and amino acids
Highly Selective. For example, the glucose transport protein in the liver will carry glucose into the
cell but will not transport fructose
Particles such as ions may pass through channel proteins
Substance binds to carrier protein
Particles move from areas of high concentration to areas of low concentration.
Simple diffusion compared to facilitated diffusion. Channel
proteins as well as carrier proteins may be involved!
Active Transport Requires Energy
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May involve carrier proteins (see below)
Movement across membrane against concentration or electrochemical gradient
o Particles move from area of low concentration to an area of high concentration!
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Endocytosis
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Active Transport
Cells engulf molecules or fluids
Particle comes in contact with membrane, folds inward forming a
Characteristic of white blood cells
Exocytosis
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Active transport
The transport of material out of a cell by means of a sac or vesicle
Examples: transport of proteins like enzymes, hormones and antibodies out of cell
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