Supplemental Lecture (98/03/28 update) by Stephen T. Abedon (abedon.1@osu.edu)
(1)
Chapter title: Osmosis
(a)
A list of vocabulary words is found toward the end
of this document
(2)
Osmosis
(a) In the absence of any opposing force, water tends to flow across semipermeable membranes (membranes which lets water but not solute through).
(b)
Concentration
equilization:
(i) Water will do this until the solute concentration on one side of the membrane is equal to the solute concentration on the other side (i.e., when using a semipermeable membrane that lets the water flow but not the solute).
(ii)
This movement is called osmosis.
(c) If this idea sounds counter intuitive, then consider it in terms of water concentration: The higher the solute concentration, the lower the water concentration, molecular for molecule. Net movement therefore occurs from regions of higher water concentration to regions of lower water concentration, exactly what one would expect.
(a) With pure water on one side of a semipermeable membrane, a solution on the other side is said to have an osmotic pressure equal to the pressure that must be applied to the solution in order to prevent a net influx of water (i.e., dilution).
(b)
Collisions with
membranes:
(i) Osmotic pressure is generated when solute molecules and water molecules are whizzing around due to thermal motion, each able to collide with a semipermeable membrane with approximately equal probability per whizzing molecule (even though solute molecules can't actually pass through the membrane).
(ii) Assuming that semipermeable membrane surface area is limiting (rather than solvent and solute molecules in a solution), then any collision made by a solute molecule prevents the collision of at least one water molecule, thus inhibiting a net movement of water from regions having high solute concentrations to regions having low solute concentrations.
(c)
Net collisions by
water:
(i) Passage of water molecules from one side of a semipermeable membrane is a linear function of collision rates.
(ii) Fewer water molecules colliding on one side of a semipermeable membrane inevitable leads (at least in the absence of any application of pressure) to a net flow of water toward the fewer collisions side.
(iii) Since the fewer collisions side is the side with a higher solute concentration, net flow is from the more dilute to the more concentrated thus leading to an equalization of osmotic pressures.
(iv) Another way of stating this is that water moves from regions of high water concentration (a.k.a. regions of low solute concentration) to regions of low water concentration (a.k.a. regions of high solute concentration).
(4)
Illustration,
osmotic pressure
(a)
Solute
concentration dependent:
(i) Osmotic pressure is a colligative property (like freezing-point depression and boiling-point elevation) which are all direct functions of solute concentration.
(ii) In other words, the more solute particles present, the higher the osmotic pressure, more or less independent of the kind of solute particles present.
(iii) (Osmotic pressure depends only on the number of solute particles present in a solution, not the kinds of solute particles.)
(b)
Example: NaCl vs.
glucose:
(i) Mole for mole, sodium chloride solutions display a higher osmotic pressure than, for example, a glucose solution of the same Molar concentration.
(ii) This is because when glucose dissolves it remains one particle, but when NaCl dissolves, it becomes two particles: Na+ and Cl-.
(c)
Example: glucose
vs. starch:
(i) Furthermore, glucose molecules tend to be stored as starches because a starch molecule containing thousands of glucose monomers is osmotically equivalent to only a single particle.
(ii) Otherwise the thousands of glucose units that make up a molecule of starch would be osmotically equivalent to thousands of glucose molecules, thus leading to dangerously high intracellular osmotic pressures.
(6)
Osmosis and cells [tonicity]
(a)
Tonicity:
(i) "The important thing for a (biologist) to know about osmosis and osmotic pressure is how particles dissolved in fluid environments affect (organisms) in those environments. For this purpose tonicity is a useful concept. Tonicity describes the behavior of cells in a fluid environment. The cells are the reference point, and the fluid environments are compared to the cells." (p. 99, Black, 1996)
(ii) Qualitatively different types of tonicity include:
(1) isotonic
(2) hypertonic (a.k.a., hyperosmotic)
(3) hypotonic (a.k.a., hypoosmotic)
(b)
Highly osmotic:
(i) Because cells tend to concentrate the solutes found in their extracellular environment, they tend to have higher solute concentrations, intracellularly.
(ii) Therefore, water tends to flow into cells with what can be a considerable force.
(c) If the extracellular environment contains solute concentrations in excess of those displayed by the cell cytoplasm (e.g., in brine), then water will tend to flow out of a cell.
(a) An extracellular solution that has effectively the same concentration of solutes as that found within a cell.
(b) This results in no net movement of water into or out of cells.
(c) For example, animal cells are bathed with isotonic solutions, and can be harmed by too dilute or too concentrated solutions.
(d) See movement of water in and out of cells illustration, below.
(a) A hypertonic (or hyperosmotic) solution has higher solute concentrations on the outside.
(b) This results, in the absence of any opposing force, in a net movement of water out of cells.
(c) As a consequence, hyertonic solutions possess antibiological activities and are employed in food preservations (i.e., meats, fruits, and vegetables are pickled, salted, or mixed with concentrated sugar solutions in order to prevent bacterial and fungal growth).
(d) See movement of water in and out of cells illustration, below.
(a) A hypotonic (hypoosmotic) solution has a lower concentration on the outside.
(b) This results, absent any opposing force, in a net movement of water into cells.
(c) Cells which typically exist in hypotonic solutions, such as fresh waters, employ various mechanisms to oppose this inevitable influx of water.
(d) See movement of water in and out of cells illustration, below.
(a) Raven & Johnson, 1995 employ an incorrect explanation for why osmosis occurs. They suggest that there are more water molecules present that may flow through a semipermeable membrane if there is a deficit of water molecules participating in hydration shells on one side (see figure 6.12, page 117, Raven & Johnson, 1995). Hence, water will tend to flow net toward hydration shells. This explanation makes great intuitive sense. Unfortunately, it is also wrong.
(b) Hypertonic
(c) Hypotonic
(d) Isotonic
(e) Isotonic (hypotonic, hypoosmotic, hypertonic, hyperosmotic), illustration
(f) Osmosis
(g) Osmotic pressure
(h) Osmotic pressure, illustration
(i) Tonicity
(a) Red blood cells burst upon being thrown into distilled water. What is this a consequence of . . . (circle correct answer) [PEEK]
(i) not isotonic environment
(ii) osmotic pressure
(iii) semipermeability of membrane
(iv) lack of a cell wall
(v) all of the above
(vi) none of the above
(b) Explain osmosis and the mechanism by which it occurs. [PEEK]
(c) Certain bacteria are halophiles. That is, they are not just tolerant to highly osmotic solutions, they are actually able to grow in such solutions. Describe one aspect of the physiology or morphology of such bacteria which might consequently differ from that of less salt tolerant bacteria. [PEEK]
(d) Spirogyra, a group of fresh water algae, normally live in a(n) _________ environment. (circle one correct answer) [PEEK]
(i) isotonic.
(ii) hypotonic.
(iii) hypertonic.
(iv) very salty.
(v) all of the above.
(vi) none of the above.
(e) What aspect of membranes causes various cell wall-less cells (such as red blood cells) to burst when suspended in hypotonic solutions, and cell-walled cells to undergo plasmolysis when suspended in hypertonic solutions? [PEEK]
(f) In terms of tonicity, describe the following extracellular environments: [PEEK]
(i) That surrounding a bacteria which is unable to grow due to a shrinkage of its cytoplasmic volume: __________
(ii) That surrounding a red blood cell in the body (i.e., in situ): __________
(iii) That surrounding a plant cell in a wilted leaf (assume not identical to osmolarity inside cells): __________
(iv) That surrounding a typical bacterial cell growing in a typical, dilute environment: __________
(g) The term (or phrase) "__________" describes the state of being of a plant cell suspended in a hypertonic environment. [PEEK]
(h) Describe the water concentration inside of a cell relative to the environment outside of a cell if there occurs a net movement of water from outside of the cell to inside the cell. Assume that the cell has an intact plasma membrane that is impermeable to all solutes present inside or outside of the cell, but is permeable to water. [PEEK]
(i) Describe a bacterial cell suspended in a hypertonic environment. [PEEK]
(j) You add sugar to an isotonic solution. In terms of tonicity, now what do you have? [PEEK]
(k) In terms of tonicity, describe the interior of a typical bacterial cell relative to its exterior environment. [PEEK]
(l) Water tends to net-flow from within a cell, through an intact, semipermeable membrane, and out into the solution surrounding the cell. Given that this movement of water continues to occur, describe the tonicity of the environment surrounding the cell. [PEEK]
(14)
Practice question
answers
(d) ii, hypotonic. Fresh water is, in general, a hypotonic environment.
(f) (i) hypertonic, (ii) isotonic, (iii) hypertonic, (iv) hypotonic.
(i) the bacterial cell is undergoing plasmolysis
(k) it is a hypertonic solution
(l) it is a hypertonic solution
(a) Black, J.G. (1996). Microbiology. Principles and Applications. Third Edition. Prentice Hall. Upper Saddle River, New Jersey. pp. 98-99.
(b) Mathews, C.K., van Holde, K.E. (1990). Biochemistry. The Benjamin/Cummings publishing co., inc. Redwood City, CA. pp. 298-332.
(c) Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 107-118.
(d) Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, pp. 82-87.