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 semi permeable, 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 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.
Objectives:
The objective for conduction the experiment is to being able to define solvent, solute, solution, selectively permeable, diffusion, osmosis concentration gradient, equilibrium, turgid plasmolyzed, plasmolysis, turgor pressure, tonicity, hypertonic, isotonic, hypertonic and also to describe the effects of hypertonic, isotonic, and hypertonic solutions on Elodea leaf cells and onion scale leafs.
Materials:
- Forceps
- 2 Microscope slides
- 2 coverslips
- Compound microscope
- Elodea in tap water
-
2 Dropping bottles of dH2O
- 2 dropping bottles of 20% sodium chloride (NaCl)
Methods:
- Two young leaves from the tip of an Elodea plant were removed with a forceps.
- One leaf was mounted in a drop of distilled water on a microscope slide and
the other in 20% NaCl solution on a second microscope slide.
- Coverslips was placed over both leaves.
- The leaf in the distilled water was observed with the compound microscope. medium-power objective was used to focus first and then was switched to the high-dry objective.
- The leaf mounted in 20% NaCl solution was observed. The cell has lost water after several minutes causing it to become plasmolyzed.
- The coverslip was removed from a preparation exhibiting plasmolysis slowly and gently to observe deplasmolysis. The salt solution was drained and two drops of distilled distilled water was added. A coverslip was placed on the preparation after 1 minute. The sample was then examined under high-dry for 5 minutes and what happened during deplasmolysis was observed.
Discussion:
When Elodea leaf observed under the magnification of 400X, the colour of Elodea cells appeared colourless except for the green bodies. The green bodies inside the Elodea cells were chloroplasts. Most of the chloroplasts located at the edges of the cell. The chloroplasts which were oval in shape were green in colour due to the presence of chlorophyll, the light-absorbing pigment necessary for photosynthesis. Besides that, the chloroplasts appeared moving around because the cytoplasm within the cell was constantly moving, thereby moving the various subcellular structures within the cell as well. This was known as cytoplasmic streaming.
The large space that can be seen from the elodea cells was the central vacuole. The central vacuole is an organelle in plant cells that stores nutrients and water for the cell. It can take in and release water depending on the cell's needs. Animal cells do not have a central vacuole as they have many small vacuoles which contain proteins, carbohydrates, water and nutrients. The content of the vacuole in the Elodea leaf was hypertonic compared to the water
When the Elodea leaf was mounted with distilled water and observed under the microscope, it was visible that the Elodea cells swelled. This was because water has a lower concentration of dissolved materials compared to the plant cell which has a lower osmotic pressure. causes a net flow of water into the plant cell, causing swelling and expansion. In other words, the distilled water is said to be hypotonic to the plant cell. However, the plant cell did not burst as plant cells have rigid cell walls which will swell, building up turgor pressure against the rigid cell walls.
The cell of the Elodea leaf was observed under the microscope, the plant cell shrunk from its actual size. When mounted with 20% NaCl, the concentration of NaCl was greater than the concentration within the Elodea cells. Osmosis caused the water to flow out of the Elodea cells therefore the plant cell will have relatively less water than its actual content. The 20% NaCl solution was hypertonic relative to the cytoplasm. Its contents shrunk, and the plasma membrane separated from the cell wall and this process was known as plasmolysis. During the plasmolysis process, the two selectively permeable membrane that were present in the Elodea cells were tonoplast and plasma membrane. At first, the Elodea cells shrunk when it was mounted with NaCl. However, when the NaCl was drained and added with two drops of distilled water, the Elodea cells began to expand. Eventually, the Elodea cells will expand larger than its actual size, hence begin to swell.
On the other hand, when the onion was mounted with distilled water and observed under the microscope, it was visible that the onion cells swelled. This was because water has a lower concentration of dissolved materials compared to the plant cell which has a lower osmotic pressure. causes a net flow of water into the plant cell, causing swelling and expansion. However, when mounted with 20% NaCl, the concentration of NaCl was greater than the concentration within the onion cells. Osmosis caused the water to flow out of the onion cells therefore the onion cells will have relatively less water than its actual content. Its contents shrunk, and the plasma membrane separated from the cell wall and this process was known as plasmolysis. But, when the NaCl was drained and added with two drops of distilled water, the onion cells began to expand. Eventually, the onion cells will expand larger than its actual size, hence begin to swell.
Osmosis is a special kind of diffusion that involved the net movement of water through a selectively permeable membrane from a region of higher concentration to a region of lower concentration. Water molecules pass freely in both directions but net movement is from the region where the water molecules are more concentrated to the region where they are less concentrated. Besides that, osmosis releases energy, and can be made to do work, as when a growing tree-root splits a stone. Osmosis is important in biological systems as many biological membranes are semi-permeable.
Ability of a solution to cause water movement is known as tonicity or effective osmolality. Some of the effects of tonicity are isotonic, hypotonic and hypertonic. When a cell is placed in a fluid with exactly the same osmotic pressure and there are no net movements of water molecules occurs, either into or out of the cell and the cell neither swells nor shrinks, the fluid is said to be isotonic. In other words, the concentration of the solute is equal to the fluid within the cell.
A hypotonic solution has a lower osmotic pressure to the cell. Water enters the cell and causes it to swell. Eventually, the cell will burst and die. However for plant cells, it does not swell and burst. Water moves into the cells by osmosis filling the central vacuole and distending the cell. The cells swell, building up turgor pressure against the cell walls. The cell walls eventually reached a steady state whereby their resistance to stretching prevents any further increment in the cell size and halt the next movement of water molecules into the cells. Inversely, a cell that is in a hypertonic environment has a higher concentration of solute in the environment than in the inside of the cell, making the net flow of water out of the cell.
However, tonicity differs between animal cells and plant cells. Plasmolysis is an effect of exosmosis in plants. When plant cells is placed in a hypertonic medium whereby the medium has a higher osmotic pressure, the plant cell loses water to its surroundings, its contents shrink, and the plasma membrane separates from the cell wall. Based on the experiment conducted, when the plant cell was mounted with NaCl, the plant cell shrunk compared its actual size. Besides that, the plasma membrane also separated from the cell wall when viewed under the microscope. This is because NaCl has a higher concentration of solutes than that in the plant cell and as a result, water flowed out of the plant cell by osmosis.
After conducting the experiment using NaCl, the NaCl was drained ad two drops of distilled water was added. By doing so, the plant cell deplasmolyses which means that plant cell will increase in size due to an that has caused excess water to diffuse into the cell. In spite of that, will not burst and die because plant cells have a strong that contains the osmotic pressure, or . Turgor pressure in the plant cells is an important factor in supporting the body of non-woody plants.
Conclusion:
In conclusion, tonicity is different between animal cells and plant cells. When Elodea and onion cells were placed in a hypertonic solution, the cells shrink as it loses water by osmosis. Meanwhile, water as a hypotonic solution that was used during the experiment caused the Elodea and onion cells to expand. The Elodea and onion cells shrunk when mounted with NaCl which indicted hypertonic effect and then expanded again when mounted with distilled water as the distilled water was a hypotonic solution.
References:
- http://biology.arizona.edu/sciconn/lessons/mccandless/reading.html
- http://www.sciencenetlinks.com/lessons.cfm
- http://en.wikipedia.org/wiki/Hypertonic
- http://en.wikipedia.org/wiki/Hypotonic
- http://en.wikipedia.org/wiki/Isotonic
- http://en.wikipedia.org/wiki/Plasmolysis