Evaluation
Although the results of the sodium chloride and sucrose experiments support the hypothesis, there are several anomalous results and a large deviation for each result. These could be improved by altering the experiment, for example by keeping the test-tubes in a water bath at a set temperature, by keeping them at a constant pressure, and by measuring the sizes of potato cylinders before and after with a more accurate method, e.g. accurate weight measurement or volumetric displacement. The test might also be more accurate if the potato cylinders were left in the solutions for a longer period of time to allow the solution to penetrate fully to the core of the sample. The test could also be repeated more times for each concentration of solution, and with a greater number of concentrations, as this would decrease the error - a disadvantage of our experiment was that one anomalous result affected the others significantly (e.g. NaCl 0.2 molar concentration). Another factor is that the potato from which the cylinders are taken could be abnormal - this could be prevented by amalgamating sets of results, for example of a whole class, where each experimenter used a different potato.
Results that were not as I would have expected occurred with NaCl solution at 0.2 molar concentration (see above), where the range of results appeared too low. However, although this is apparently an anomalous result, it could have been caused by either experimental error - more significant when a small number of results are used, or a difference in the potato for those cylinders. Either of these would easily be recognised if a larger number of results were collected and used. Another result that appeared unusual was the ‘step’ in the graph for the sucrose solution between 0.25% and 1% solutions - here different results for each cylinder pulled the average upwards by a noticeable amount, a problem that possibly would not occur if more measurements were taken.
For future experimentation we could repeat this experiment using a range of solution strengths very close to the value discovered here of sap osmolarity, to define more exactly its true value. We could also extend the experiment to use tissue samples from other plants, to discover whether the hypothesis is also correct for other tubers, and even for other plant tissues. We would then also be able to compare osmotic pressures inside different plants.
Explanation
First the definition of osmosis:
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Semi-permeable membranes are very thin layers of material (cell membranes are semi-permeable) which allow some things to pass through them but prevent other things from passing through.
Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through.
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A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: there is a high concentration of water.
Some teachers use the definition which starts "Osmosis is the passage of water from a dilute solution to a......" this means exactly the same as the definition I have given.
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A region of low concentration of water is a concentrated solution of something like sucrose. In this case there is much less water.
So you could use the definition "Osmosis is the passage of water from a dilute solution through a semi-permeable membrane to a more concentrated solution.
Now to explain osmosis:
When you put an animal or plant cell into a liquid containing water one of three things will happen.
- If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis.
Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up.
- If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane.
Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size.
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If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis.
Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink.
Firstly what happens to plant cells:
Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight.
When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed.
When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight.
And now for the animal cells:
When animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. In concentrated solutions, water is sucked out of the cell by osmosis and the cell shrinks. In either case there is a problem. So animal cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why we have kidneys. The exact amount of water and salt removed from our blood by our kidneys is under the control of a part of the brain called the hypothalamus. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. My insulin page will tell you more about other homeostatic mechanisms.
Animals which live on dry land must conserve water; so must animals which live in the sea (the sea is very salty!), but animals which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis
Diffusion, Osmosis and Cell Membranes
All living things have certain requirements they must satisfy in order to remain alive. These include exchanging gases (usually CO2 and O2), taking in water, minerals, and food, and eliminating wastes. These tasks ultimately occur at the cellular level, and require that molecules move through the membrane that surrounds the cell. This membrane is a complex structure that is responsible for separating the contents of the cell from its surroundings, for controlling the movement of materials into and out of the cell, and for interacting with the environment surrounding the cell.
There are two ways that the molecules move through the membrane: passive transport and active transport. Active transport requires that the cell use energy that it has obtained from food to move the molecules (or larger particles) through the cell membrane. Passive transport does not require such an energy expenditure, and occurs spontaneously.
The principle means of passive transport is diffusion. Diffusion is the movement of molecules from a region in which they are highly concentrated to a region in which they are less concentrated. It depends on the motion of the molecules and continues until the system in which the molecules are found reaches a state of equilibrium, which means that the molecules are randomly distributed throughout the system.
An important concept in understanding diffusion is the concept of equilibrium. There are two types of equilibrium. Static equilibrium occurs when there is no action taking place. Dynamic equilibrium occurs when two opposing actions occur at the same rate. For example, consider a bucket full of water. It is in a state of static equilibrium because the water level stays the same. The water is not moving. If you were to poke a hole in the bottom of the bucket, water would leak out. This system would not be at equilibrium because there is action taking place - water is leaking out - and the water level in the bucket would drop.
However, if you were to begin pouring water into the bucket at the same rate that it was leaking out, the water level in the bucket would stay the same because the rate at which the water is entering the bucket is equal to the rate at which it is leaking out. This is an example of dynamic equilibrium, and it applies to nearly everything that happens in the natural world.
Diffusion occurs when a system is not at equilibrium. As an example, suppose you drop one drop of ink into a glass of water. At first, all of the ink molecules are in a small space and they are moving around in a random way. They move in straight lines and change direction only when they collide with each other or the surrounding water molecules. Some of the ink molecules near the edge of the drop move away from the center of the drop. As a matter of fact, most of the molecules move away from the center of the drop.
Most of the molecules continue to move away from the original center of the drop. They move in all different directions, and some may even move back toward the center. Still, more are moving away from the drop than toward it until they find the wall of the glass. Then they start moving back toward the center again. More and more molecules bounce off of the glass until they start moving toward the center, then they pass the center and move toward the other side. Eventually the number of molecules moving away from the center equals the number moving toward the center, and equilibrium is established. At this point the molecules are evenly spread throughout the water, and diffusion stops. Have the molecules stopped moving? Is this a static or dynamic equilibrium?
Several factors affect how fast a molecule will diffuse. The first of these is the kinetic energy of the molecule, which is most frequently measured as the temperature of the system. Molecules in a system at a higher temperature will have more energy and will move faster, and hence diffuse faster, than molecules of the same type in a low-temperature system. The size of the molecule also affects how rapidly it will diffuse. At the same temperature, smaller molecules will move more rapidly than larger molecules because it takes more energy to get the larger molecule moving. Other factors include any charges on the molecule (positive or negative) and the nature of the material that the molecules are moving through.
Diffusion can occur through a cell membrane. The membrane allows small molecules like water (H2O), oxygen (O2), carbon dioxide (CO2), and others to pass through easily. It is said to be permeable to these molecules. If a cell is floating in a water solution (like the ocean) that has some oxygen dissolved in it, the oxygen molecules will move into the cell. They will also move out of the cell at the same rate, and a dynamic equilibrium will exist. However, if the cell uses some of the oxygen as it comes into the cell, more oxygen will move into the cell than out of the cell.
So the oxygen effectively moves from a region of high concentration (the seawater) to a region of low concentration (the cell), and diffusion occurs. Likewise, as the chemical reactions in the cell use up oxygen they produce carbon dioxide. The concentration of carbon dioxide inside the cell increases so that more CO2 molecules strike the inside of the cell and move out than strike the outside of the cell and move in. So the overall effect is that the CO2 moves out of the cell.
Osmosis is a special case of diffusion. In this case, a large molecule like starch is dissolved in water. The starch molecule is too large to pass through the pores in the cell membrane, so it cannot diffuse from one side of the membrane to the other. The water molecules can, and do, pass through the membrane. Hence the membrane is said to be semipermeable, since it allows some molecules to pass through but not others. However, on the side of the membrane with the starch, the starch molecules interfere with the movement of the water molecules, preventing them from leaving as rapidly as they enter. Thus, more water flows into the side with the starch than flows out, and the starch gets diluted.
If the starch (or some other large molecule like a protein) is in a cell, the water moves into the cell faster than it leaves, and the cell swells. The cell membrane acts somewhat like a balloon, and if too much water enters the cell, the cell can burst, which kills the cell. So cells usually have some kind of mechanism for preventing too much water from entering or pumping the water out or simply making a tough outer coat that will not rupture.
Things are more difficult when the starch or other large molecule is on the outside of the cell. Then the cell loses water faster than it comes in, and the cell shrinks, which might not be too bad except that the cell needs the water for the chemical reactions that take place inside that keep it alive. In fact, this principle is used in food preservation. Foods that are packed in salt or sugar prevent bacterial growth by essentially sucking the water out of the bacterial cells (or, more properly, preventing water from entering the cells) and preventing their growth.
There are other ways that cells use to move materials across the cell membrane, most of which involve active transport, requiring the use of energy. The cell membrane also has other functions besides controlling the movement of materials into and out of the cell, and the membranes of specialized cells have very complex functions. So we see that the cell membrane is a very intricate and important component of the cell.
Dynamic Equilibrium
Introduction: The concept of equilibrium is a very important one to scientists in all fields. Static equilibrium refers to a condition in which the parts of a system have stopped moving, and is rare in nature. Dynamic equilibrium refers to a condition in which the parts of a system are in continuous motion, but they move in opposing directions at equal rates so that the system as a whole does not change
Water molecules move randomly. When water is enclosed by a membrane, living or artificial, some of the moving water molecules will hit the membrane, exerting pressure on it. This pressure is known as water potential.
As the number of water molecules increases, the number of collisions between the molecules and the surrounding membrane increases. This causes the pressure on the surrounding membrane to increase so the water potential will increase.
Water potential is represented by the symbol Ψ (Greek letter, "psi"). It is measured in units of pressure, usually kilopascals (kPa). Pure water has a water potential of zero. A solution will have a lower concentration of water molecules so it will have a negative water potential.
Passive Transport
Passive transport is the diffusion of substances across a membrane. As we stated above, this is a spontaneous process and cellular energy is not expended. Molecules will move from where the substance is more concentrated to where it is less concentrated.
Although the process is spontaneous, the rate of diffusion for different substances is affected by membrane permeability. Since membranes are selectively permeable, different molecules will have different rates of diffusion. For instance, water diffuses freely across membranes, an obvious benefit for cells since water is crucial to many cellular processes
is a special case of passive transport. Water will diffuse from a hypotonic solution to a hypertonic one. Generally speaking, the direction of water flow is determined by the solute concentration and not by the "nature" of the solute molecules themselves.
