In order to understand how biological membranes function you need to know some basic and important concepts. A solution is a liquid that contains dissolved molecules. The liquid that dissolved the molecules (the “dissolver”) is known as the solvent and the substance that was dissolved (the “dissolvee”) is the solute. The most basic example of this concept is salt water. The solution is salt water, the solvent is water and the solute is salt. Another solution is blood. Can you think of other examples?
The relative amount of solute in a solution is expressed in terms of a solution’s tonicity. Notice that a solution’s tonicity is a relative term and is only used when comparing similar solutions (in other words these terms are not used to describe a single solution, only one solution relative to other solutions). If a solution contains less dissolved solute than another solution (salt water from the Pacific Ocean versus salt water from the Dead Sea, respectively) the first solution is said to be hypotonic to the second solution (think hypoglycemic, or low blood sugar). If we reverse this scenario we can say that Dead Sea water is hypertonic to Pacific Ocean water or contains more solute (think hyperactive child or a kid that has a lot of energy). If we compared two samples of Dead Sea water we can say that they are isotonic or equal in solute content to one another.
Finally, entropy refers to the increasing randomness of molecules in a solution. Remember, molecules of a solute move randomly in solution. If you pour honey into hot water it is initially very concentrated in one area of the mug but given time the honey will dissolve and the sugar molecules will spread out randomly and evenly in the solution. Entropy is positively correlated with temperature. Recall your first lessons on matter from grade school. Solid elements are a barely-moving matrix of molecules. As temperature increases this matrix breaks down and the molecules move faster and faster; so a liquid contains little order and moving molecules, and a gas contains almost no order and very quickly moving molecules. Entropy is the driving force behind this progression.
All of the above concepts will be essential to understanding this week’s lab and the function of biological membranes.
Biological Membranes, in the most basic sense, are those membranes that separate living cells from their non-living surroundings. You learned about these membranes and their structure in previous labs. In this lab you will learn how these membranes control molecular traffic and water balance for the cell. These membranes have the unique and essential property of selective permeability. This simply means that these membranes allow for certain substances to cross into and out of the cell more easily than others. A cell membrane allows for molecules to move across it two ways: passive diffusion, which requires no energy expenditure by the cell, and active transport, which requires energy expenditure by the cell. (There is a third type of movement known as active diffusion which requires energy to move molecules through special gates, but it will not be covered in this lab.) In order to understand these concepts we must understand concentration gradients. I will use the below illustration to explain:
The inside of this cell has far fewer potassium ions (K+) than the outside of the cell. We would say that the inside of the cell is hypotonic to the outside of the cell and that the outside of the cell is hypertonic to the inside of the cell with respect to the ion of interest, potassium. This means that the inside of the cell has a low concentration of potassium ions and the outside of the cell has a high concentration. If the potassium ions move from an area of high concentration to low concentration (i.e. into the cell) then they are said to be moving along the concentration gradient. If they move out of the cell, however, from an area of low concentration to an area of high concentration then they are moving against the concentration gradient. In passive diffusion molecules move along the concentration gradient which requires no energy input. In active transport the molecules move against the concentration gradient, requiring energy. In this lab we will not look at active transport. You can read more about it in your lecture textbook.
In this lab we will look at several examples of passive diffusion- the movement of molecules from areas of high concentration to areas of low concentration within solutions and across membranes.
Diffusion of a Liquid in a Liquid
This experiment does not involve a membrane but will demonstrate the concepts of entropy and passive diffusion. Remember that entropy is positively correlated with temperature. Let us imagine what happens to the dye of the experiment in room temperature water. Initially, when we drop the dye into the water it is very concentrated. With time, entropy causes the molecules to randomize in the water, moving from an area of high concentration to low concentration (passive diffusion!!!). Now, if entropy is positively correlated with temperature would you expect the dye to passively diffuse more quickly in the hot or cold water of your experiment?
Diffusion of a Liquid in a Solid
In this experiment we have a pseudo-membrane in the form of agar. This 1.5% agar is selective permeable by size. Think of the agar as a matrix where small molecules move very easily and bulkier molecules move slowly and awkwardly. In this experiment you are placing 4 highly concentrated solutions in wells. These molecules should move along the concentration gradient and spread out in the agar. If molecular weight is positively correlated with molecular size, which chemicals would you expect to move quickly through the agar matrix; which would move more slowly?
Osmosis in a Model Cell
Recall the potassium example from above. In the dialysis experiment we will create a cell from dialysis tubing. Inside the cell is Solution “A” which contains protein molecules (albumen), glucose molecules, and starch molecules. Remember that starch and protein are large bulky molecules. The dialysis tubing is selectively permeable by size. Outside of the dialysis cell is distilled water, which, by definition contains no solutes. Therefore, the inside of the cell is extremely hypertonic relative to the outside of the cell. Because of this we expect some of the molecules inside the cell to move outside the cell via passive diffusion. Which molecules do you think will move? Remember, the dialysis tubing is selectively permeable by size. Now, if molecules are moving out of the dialysis bag you expect the dialysis cell to get lighter with time but this is not what happens; the cell actually becomes heavier! This is because osmosis occurred. Osmosis is the movement of water (molecules) across a membrane. Just like other molecules water is subject to the same properties of the concentration gradient. So, while some solute molecules moved across the membrane into the distilled water, many water molecules moved across the same membrane into the cell to dilute Solution “A”. Remember, solutions want to be in equilibrium (isotonic) with one another.
Osmosis in a Living Cell
Unlike the last experiment we will look at osmosis and passive diffusion in a live plant cell. Plant cells are unique from animal cells because they have a cell wall. This rigid structure prevents lysis, or bursting of the cell from excess water intake. If a plant cell is placed in a hypertonic solution all the water in the cell will rush out and the cell will become flaccid (anyone who has ever seen a droopy plant knows what this looks like). This process is known as plasmolysis in plant cells and crenation in animal cells. In the reverse, if a plant cell is placed in a hypotonic solution water will rush into the cell causing it to swell. If this were an animal cell it would almost certainly lyse, since animal cells are only protected by a fragile membrane. In a plant cell, however, the water will simply swell against the cell wall and the cell will be turgid. The first diagram demonstrates these principles. In this experiment you will watch a plant cell become flaccid and turgid when placed in salt water and distilled water respectively.