Embedded Files
Title: Diffusion Across the Membrane
Principle(s) Investigated:
1. A selectively permeable membrane allows some types of molecules or ions to pass through, but not others.
2. In biological organisms, each cell is surrounded by a selectively permeable cell membrane which regulates what gets into and out of the cell. Water is a small molecule that readily crosses the selectively permeable cell membrane.
3. Movement of water across a selectively permeable membrane is called osmosis; osmosis results in net movement of water from a solution with a lower concentration of solutes to a solution with a higher concentration of solutes.
4. If the solution surrounding a cell has a greater concentration of solutes than the cytosol inside the cell, there will be net movement of water out of the cell. Conversely, if the solution surrounding a cell has a lower concentration of solutes than the cytosol inside the cell, there will be net movement of water into the cell. This can cause animal cells to burst, but in plant cells the influx of water is limited by pressure from the surrounding cell wall.
Standards : MS-LS1-2. "Develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function."
Materials:
1. 2 raw, unpeeled eggs
2. 2 clear, plastic cups
3. Distilled, 5% white vinegar (acetic acid)
4. Water
5. Corn syrup
6. gloves
Procedure:
1. On the first day, students will prepare two clear plastic cups filled with distilled, 5% white vinegar.
2. Students will use gloved hands to place one raw, unpeeled egg into each cup holding the 5% white vinegar solution.
3. Students will take note as they observe bubbles forming in the solution, indicating the reaction occurring between the vinegar solution and the calcium carbonate egg shell.
4. The eggs will be left in the vinegar solutions overnight for 24 hours.
5. On the following day, students will switch out the solutions in their cups. The egg shells will have been removed by the vinegar solution now.
6. Students will discard the vinegar solution, and replace one cup with water and the other cup with corn syrup.
7. Again, the eggs will be left in the cups overnight for 24 hours.
8. On the final day of observation, students will take note of evidence of osmosis.
Student prior knowledge: Molecules are in motion. Diffusion requires molecules to move along a gradient. In a fluid environment, this means that the molecules move from a higher concentration to a lower concentration.
Explanation: Diffusion is the movement of molecules from a higher concentration to a lower concentration. Osmosis is the movement of water across a membrane. During osmosis, the water molecules move from a higher concentration (a low solute concentration) to a lower concentration (a high solute concentration). When there is an equal amount of water molecules moving in and out of a cell, the cell is said to be in an isotonic state. And the cell retains its normal shape and size, maintaining a balanced environment with its external environment. When there is a higher amount of water entering into the cell, the cell is said to be in a hypotonic state. And the cell will continue to swell, eventually burst, releasing the contents of the cell into its external environment. When there is a higher amount of water molecules exiting the cell, the cell is said to be in a hypertonic state. And the cell will continue to shrivel, losing its water potential to its external environment.
Here, in the demonstration, the evidence of osmosis is revealed through the diffusion of water molecules in and out of an egg. The students place two eggs into cups containing distilled, 5% white vinegar. The students observe the initial reaction of the calcium carbonate shell with the acetic acid (2 CH3COOH + CaCO3 – > Ca(CH3COO)2 + H2CO3 – > Ca(CH3COO)2 + H2O + CO2). There will be bubbles foaming to the top. Over a 24-hour period, this should remove the shell layer of each egg. The next day, the students will see that the hard shells have been removed, and will then replace one cup with water, and the other cup with corn syrup. These eggs will be left overnight in their new environment for another 24-hour period. On the final day of observation, the students will observe that the egg in water is in a normal isotonic state. The egg in corn syrup, however, will be observed to be in a hypertonic state. The students will take note of the differences and apply the concept to our own cells.
Questions & Answers:
1. What is osmosis? - Osmosis is the movement of water across a membrane. During osmosis, the water molecules move from a higher concentration (a low solute concentration) to a lower concentration (a high solute concentration).
2. During osmosis, how do the solutes behave? - Osmosis is the diffusion of water, not solutes, across the membrane. However, during simple diffusion, solutes are able to follow their concentration gradient to cross a membrane.
3. How does osmosis affect the cells in our body? - When there is an equal amount of water molecules moving in and out of a cell, the cell is said to be in an isotonic state. And the cell retains its normal shape and size, maintaining a balanced environment with its external environment. When there is a higher amount of water entering into the cell, the cell is said to be in a hypotonic state. And the cell will continue to swell, eventually burst, releasing the contents of the cell into its external environment. When there is a higher amount of water molecules exiting the cell, the cell is said to be in a hypertonic state. And the cell will continue to shrivel, losing its water potential to its external environment.
Applications to Everyday Life: Plant cell wall (turgid vs. flaccid) - When there is a higher amount of water entering into the cell, the cell is said to be in a turgid state. And the cell will continue to swell, becoming bloated. When there is a higher amount of water molecules exiting the cell, the cell is said to be in a flaccid state. And the cell will continue to shrivel, losing its water potential to its external environment. When there is little water in the soil, plant roots are unable to take up water, so the extracellular fluid becomes hypertonic and cells lose turgor pressure which is necessary to support the plant's structure. This is why a plant wilts if there is no rain or you forget to water the plant
Blood cell membrane (hypo- vs. hypertonic) - When there is an equal amount of water molecules moving in and out of a cell, the cell is said to be in an isotonic state. And the cell retains its normal shape and size, maintaining a balanced environment with its external environment. When there is a higher amount of water entering into the cell, the cell is said to be in a hypotonic state. And the cell will continue to swell, eventually burst, releasing the contents of the cell into its external environment. When there is a higher amount of water molecules exiting the cell, the cell is said to be in a hypertonic state. And the cell will continue to shrivel, losing its water potential to its external environment.
Excretory system - The kidneys function to maintain homeostasis between our body and its waste products. Through osmoregulation and diffusion of water and wastes across the membrane, the kidneys are able to retain necessary nutrients and water, while filtering out the wastes from our bodies.
Photographs:
DAY 1: Both eggs sit in distilled, 5% white DAY 2: Both eggs now are only surrounded by
vinegar overnight. The calcium a thin membrane. The shells have been completely
carbonate shell is shown reacting with removed.
the vinegar solution to produce bubbles.
DAY 2: One shell-less egg has been placed into water. And the other shell-less egg has been placed into corn syrup.
Both shell-less eggs have been left in their respective environments overnight.
DAY 3: The shell-less egg that was put into water has retained its normal size and shape. However, the shell-less egg
that was put into corn syrup has lost some of its water contents to its external environment. This shell-less egg
has decreased in size and seems to have deflated in shape.
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