Diffusion in Agar Cells: Bigger Isn’t Necessarily BetterName:
Reading Comprehension Check:
- What is a cell?
- How do cells grow?
- How do cells take in nutrients and get rid of wastes?
- If a large surface area is helpful, why do cells not grow to be very large?
Materials:
Agar CellsSodium hydroxide solution, 0.1 M, NaOH, 200 mL
Metric rulerBeaker, 400-mLPlastic knife
Safety Precautions
Sodium hydroxide solutions are corrosive, skin burns are possible, very dangerous to eyes. Wear chemical splash goggles and a chemical-resistant apron.
Predict
Which cell will be the most efficient at moving materials in and out – the 3 cm, 2 cm or the 1cm. Why?
Procedure
1. Each group will cut three agar cubes. A 3-cm cube, 2-cm cube and a 1-cm cube. Cut as accurately as possible.
2. Pour 100 mL of 0.1 M sodium hydroxide solution into the 400-mL beaker. Note the time and immerse the three blocks in the sodium hydroxide solution.
3. Let them soak for 10 minutes with periodic gentle stirring and turning.
- While waiting answer the questions from the reading comprehension paper at your station
4. After 10 minutes, use a spoon or tongs to remove the blocks and blot dry with a paper towel.
5. Promptly cut each block in half and measure the depth to which the pink color has penetrated. Sketch each block’s cross section.
6. Set up and complete the following data table:
CUBE DIMENSIONS / SURFACE AREA (cm2) / VOLUME (cm3) / VOLUME TO AREA RATIO / DEPTH PENETRATED3 cm/side
2 cm/side
1 cm/side
Surface area = length x width x 6 sidesVolume = length x width x height
Analysis (2 points)
- List the agar cubes in order of size, from largest to smallest.
- List the agar cubes in order from largest to smallest ratio.
- What happens to the surface area to volume ratio of a cell as the cell grows?
- What evidence is there that NaOH diffuses into an agar block?
- Is there any evidence that something was diffusing out of the agar blocks? Explain.
- Compare the diffusion rates between the 2 cells.
- Which cell’s volume has been the most affected by the diffusion? Explain why.
9. How does your prediction compare the actual results?
Application (3 points)
10. If the agar blocks were living cells and the NaOH were a vital substance, which block would have the most efficient ratio of surface area to volume?
11. When one cube shaped cell divides into two equal parts, how does the volume of each small cell compare with the one large cell? Does the surface area change in the same proportion? Explain.
12. Why would the growth rate of a cell slow down as it gets larger?
13. How does division of a cell affect the cell’s ability to absorb material for growth?
14. The agar cubes did not have a membrane around them. How might this activity have been affected if the cubes had a membrane around them like cells do?
Synthesis (5 points)
Homeostasis is the adjustment by an organism to changing conditions in order to survive. It is crucial that single-celled organisms maintain a balance in internal life processes (i.e. excretion) although changes occur in the external environment. Which cell size would be most beneficial in terms of maintain homeostasis for a single-celled organism?
CELL DIFFUSION COMPREHENSION CHECK
Introduction
Diffusion and osmosis are among the most commonly studied topics in biology. Virtually all living cells are, to varying degrees, dependent on these processes. This activity will enable students to explore the relationship between diffusion and cell size by experimenting with model “cells.”
Discussion
Cells are the structural and functional unit of all living organisms. Some organisms, such as bacteria, are unicellular, consisting of a single cell. Other organisms, such as humans, are multicellular, or have many cells- an estimated 100,000,000,000,000 (one trillion) cells! Each cell is an amazing world unto itself: it can take in nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary. Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities.Most cells grow, but upon reaching a certain size, a cell will divide becoming two smaller cells. This is how multicellular organisms, like us, grow.
Diffusion is the spontaneous movement of a substance from high to low concentration. It is how many substances naturally move from where there is more to where there is less: such as the smell of perfume moving across the room. Diffusion is one of the very important processes by which substances such as nutrients, water, oxygen, and cellular wastes are transported between living cells and their environment.
The cell membrane is the selectively permeable barrier whose total surface area is an important factor in regulating the substances that diffuse into or out of the cell. In fact, cells that specialize in absorption or secretion often have relatively large surface areas to increase the efficiency of diffusion (e.g. intestinal cells contain villi and microvilli).
Scientific evidence suggests that once a cell grows to a certain size it becomes too large for the complete diffusion of needed substances throughout its cytoplasm. As a cell grows, its volume increases faster than its surface area; as a result, the surface area of the cell membrane gradually becomes less efficient for the diffusion of substances throughout the entire volume of the cell. The relationship between the surface area and volume of a cell can be expressed as a ratio, and this ratio is believed to be a significant factor in triggering a cell to divide, preventing cells from becoming too large, and maintaining efficient diffusion throughout the cell.
Diffusion in Agar Cells: Bigger Isn’t Necessarily BetterName:
Materials:
Agar CellsSodium hydroxide solution, 0.1 M, NaOH, 200 mL
Metric rulerBeaker, 400-mLPlastic knife
Safety Precautions
Sodium hydroxide solutions are corrosive, skin burns are possible, very dangerous to eyes. Wear chemical splash goggles and a chemical-resistant apron.
Predict
Which cell will be the most efficient at moving materials in and out – the 3 cm, 2 cm or the 1cm. Why?
Procedure
1. Each group will cut three agar cubes. A 3-cm cube, 2-cm cube and a 1-cm cube. Cut as accurately as possible.
2. Pour 100 mL of 0.1 M sodium hydroxide solution into the 400-mL beaker. Note the time and immerse the three blocks in the sodium hydroxide solution.
3. Let them soak for 10 minutes with periodic gentle stirring and turning.
- While waiting answer the questions from the reading comprehension paper at your station
4. After 10 minutes, use a spoon or tongs to remove the blocks and blot dry with a paper towel.
5. Promptly cut each block in half and measure the depth to which the pink color has penetrated. Sketch each block’s cross section.
6. Set up and complete the following data table:
Calculating % diffusion in each cube:
Volume = L x W x H
- To calculate volume that did not turn pink: calculate total volume of the small portion of the cube that did not turn pink – use the same formula L x W x H)
Volume diffused = total volume – volume not pinkPercent diffusion = Volume diffused /total volume x 100
Surface area of a cube = L x W x # of sides
Cube Size / Total cube volume (cm3) / Total volume that was not pink(cm3) / Sketch of each
Cube / Volume of the diffused cube
(total volume – volume that was not pink) /
Percent
Diffusion
/ Surface area of cube (cm2) / Surface area to volume ratio1cm
2cm
3cm
1 cm Cell Sketch / 2 cm Cell Sketch / 3 cm Cell Sketch
Reading Comprehension Check:
- What is a cell?
- How do cells grow?
- How do cells take in nutrients and get rid of wastes?
- If a large surface area is helpful, why do cells not grow to be very large?
Analysis (2 points)
- What evidence is there that NaOH diffuses into an agar block?
- Is there any evidence that something was diffusing out of the agar blocks? Explain.
- Compare the diffusion rates between the 3 cells.
- Which cell’s volume has been the most affected by the diffusion? Explain why.
- How does your prediction compare the actual results?
- If a large surface area is helpful to cells, why do cells not grow to be very large?
- You have three cubes, A, B, and C. They have surface to volume ratios of 3:1, 5:2, and 4:1 respectively. Which of these cubes is going to be the most effective at maximizing diffusion, how do you know this?
- How does your body adapt surface area-to-volume ratios to help exchange gases?
- Why can’t certain cells, like bacteria, get to be the size of a small fish?
Application (3 points)
10. If the agar blocks were living cells and the NaOH were a vital substance, which block would have the most efficient ratio of surface area to volume?
11. When one cube shaped cell divides into two equal parts, how does the volume of each small cell compare with the one large cell? Does the surface area change in the same proportion? Explain.
12. Why would the growth rate of a cell slow down as it gets larger?
13. How does division of a cell affect the cell’s ability to absorb material for growth?
14. The agar cubes did not have a membrane around them. How might this activity have been affected if the cubes had a membrane around them like cells do?
Synthesis (5 points)
Homeostasis is the adjustment by an organism to changing conditions in order to survive. It is crucial that single-celled organisms maintain a balance in internal life processes (i.e. excretion) although changes occur in the external environment. Which cell size would be most beneficial in terms of maintain homeostasis for a single-celled organism?