Biology · Simulation and virtual lab Free
Diffusion and surface area: agar blocks in acid
Drop pink agar cubes of different sizes into dilute acid, time how long each takes to go colourless, and discover why big organisms cannot rely on diffusion alone.
- You will learn to
- Measure how long acid takes to diffuse into agar blocks of different sizes and shapes.
- Calculate surface area, volume and the SA : V ratio.
- Explain why large organisms need transport systems and thin, flat exchange surfaces.
- You will measure
- Time for each block to become colourless (s); then SA, V and SA : V for each block.
- Time needed
- About 30–40 min
- Equipment
- Nothing extra. Real version: agar with phenolphthalein and sodium hydroxide, knife, ruler, dilute hydrochloric acid, beakers, stopwatch, tile.
- Price
- Free: no payment, no account.
Virtual lab
Follow the steps with the simulation above (go back to the simulation ↑). Each time you tap Record, a row is added to the table and a point to the graph.
Aim
To investigate how the size and shape of a block affect the time taken for a substance to diffuse into it, and to relate this to the surface area to volume ratio of organisms.
What to do
- Keep 0.5 mol/dm³ acid, 25 °C, cubes + flat block, clock × 10. Tap Put the blocks in the acid.
- Watch the pink cores shrink. When each block turns completely colourless its time appears above it.
- Tap Record the results as the blocks go colourless (the 3 cm cube takes about 45 minutes: use × 40).
- Calculate the surface area, volume and SA : V ratio of each block yourself, then check the table.
- Compare the flat block with the 2 cm cube: both have a volume of 8 cm³. Which goes colourless first, and why?
- Repeat with 1.0 mol/dm³ acid or at 45 °C. How does the time change?
Results
Questions
Define diffusion.
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until they are evenly spread.
Calculate the surface area : volume ratio of a 2 cm cube.
Surface area = 6 × 2 × 2 = 24 cm²; volume = 2 × 2 × 2 = 8 cm³; SA : V = 24 ÷ 8 = 3 : 1 (3 cm⁻¹).
Why does the large cube take much longer to go colourless?
Its centre is further from the surface, so the acid must diffuse a longer distance; also it has a smaller surface area for each cm³ of volume (smaller SA : V ratio).
Why do large animals need a transport system such as blood?
Their SA : V ratio is small and the distance from the surface to the inner cells is too long: diffusion alone would be far too slow to supply oxygen and food and remove wastes.
Give two features of the flat block that are also found in exchange surfaces.
It is thin (short diffusion distance) and has a large surface area for its volume, like alveoli, villi, gill filaments and leaves.
Why does the acid diffuse in faster at a higher temperature?
The particles have more kinetic energy and move faster, so they diffuse more quickly.
For teachers: expected results and how the model works
Model: the acid front moves in a distance d = k√t from every face, with k chosen so that a 1 cm cube clears in 300 s with 0.5 mol/dm³ acid at 25 °C; k ∝ √(concentration) and rises 2.5 % per °C. Times at 0.5 mol/dm³, 25 °C: 0.5 cm cube 75 s, 1 cm 300 s, 2 cm 1 200 s, 3 cm 2 700 s, flat 4 × 4 × 0.5 cm block 75 s (16 times faster than the 2 cm cube of the same 8 cm³ volume). 1.0 mol/dm³ halves the times; 0.1 mol/dm³ makes them 5 times longer; 45 °C divides them by 2.25. SA : V: 0.5 cm cube 12, 1 cm 6, 2 cm 3, 3 cm 2, flat block 5. The blocks are drawn as if cut in half so that the shrinking pink core can be seen.
Exam practice Exam set
National-exam style questions on this simulation (16 marks). Try each one on paper first, then open the marking scheme.
A cube of agar has sides of 3 cm. (a) Calculate its surface area, volume and SA : V ratio. (b) Compare with a 1 cm cube and explain which would become colourless first in acid. (6 marks)
Marking scheme: (a) SA = 6 × 9 = 54 cm² (1); V = 27 cm³ (1); SA : V = 2 : 1 (1). (b) The 1 cm cube has SA : V = 6 : 1 (1); it has more surface for its volume and a shorter distance to the centre (1), so it becomes colourless first (1).
Explain how the structure of the villi in the small intestine increases the rate of absorption by diffusion. (4 marks)
Marking scheme: Villi and microvilli give a very large surface area (1); the epithelium is one cell thick, a short diffusion distance (1); a dense capillary network carries absorbed food away (1), keeping a steep concentration gradient (1).
Suggest why a flat worm can survive without lungs or blood vessels, but an earthworm needs blood vessels. (3 marks)
Marking scheme: The flatworm is thin and flat, so every cell is close to the surface and its SA : V ratio is large (1); diffusion is fast enough (1); the earthworm is thicker/cylindrical, so the distance to inner cells is too great and it needs blood to transport oxygen (1).
State three factors that increase the rate of diffusion. (3 marks)
Marking scheme: A steeper concentration gradient (1); a larger surface area / shorter diffusion distance (1); a higher temperature (1).
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Diffusion and surface area: agar blocks in acid
Aim: To investigate how the size and shape of a block affect the time taken for a substance to diffuse into it, and to relate this to the surface area to volume ratio of organisms.
Apparatus (real laboratory): agar with phenolphthalein and sodium hydroxide, knife, ruler, dilute hydrochloric acid, beakers, stopwatch, tile.
What to do (with the simulation on www.donatstem.co.rw)
- Keep 0.5 mol/dm³ acid, 25 °C, cubes + flat block, clock × 10. Tap Put the blocks in the acid.
- Watch the pink cores shrink. When each block turns completely colourless its time appears above it.
- Tap Record the results as the blocks go colourless (the 3 cm cube takes about 45 minutes: use × 40).
- Calculate the surface area, volume and SA : V ratio of each block yourself, then check the table.
- Compare the flat block with the 2 cm cube: both have a volume of 8 cm³. Which goes colourless first, and why?
- Repeat with 1.0 mol/dm³ acid or at 45 °C. How does the time change?
Results
| # | Block | SA (cm²) | V (cm³) | SA : V (cm⁻¹) | Time to go colourless (s) |
|---|
| 1 | | | | | |
| 2 | | | | | |
| 3 | | | | | |
| 4 | | | | | |
| 5 | | | | | |
| 6 | | | | | |
| 7 | | | | | |
Graph
Plot the time to go colourless (s) on the vertical axis against the SA : V ratio (cm⁻¹). Describe the shape of the graph.
Questions
- Define diffusion. [2]
- Calculate the surface area : volume ratio of a 2 cm cube. [2]
- Why does the large cube take much longer to go colourless? [2]
- Why do large animals need a transport system such as blood? [2]
- Give two features of the flat block that are also found in exchange surfaces. [2]
- Why does the acid diffuse in faster at a higher temperature? [2]
- A cube of agar has sides of 3 cm. (a) Calculate its surface area, volume and SA : V ratio. (b) Compare with a 1 cm cube and explain which would become colourless first in acid. [6]
- Explain how the structure of the villi in the small intestine increases the rate of absorption by diffusion. [4]
- Suggest why a flat worm can survive without lungs or blood vessels, but an earthworm needs blood vessels. [3]
- State three factors that increase the rate of diffusion. [3]
Donat Sciences and MathsBiology · Virtual lab worksheet
School: ........................................ Name: ................................ Class: ........ Date: ............
Marking scheme: Diffusion and surface area: agar blocks in acid
Expected results
Model: the acid front moves in a distance d = k√t from every face, with k chosen so that a 1 cm cube clears in 300 s with 0.5 mol/dm³ acid at 25 °C; k ∝ √(concentration) and rises 2.5 % per °C. Times at 0.5 mol/dm³, 25 °C: 0.5 cm cube 75 s, 1 cm 300 s, 2 cm 1 200 s, 3 cm 2 700 s, flat 4 × 4 × 0.5 cm block 75 s (16 times faster than the 2 cm cube of the same 8 cm³ volume). 1.0 mol/dm³ halves the times; 0.1 mol/dm³ makes them 5 times longer; 45 °C divides them by 2.25. SA : V: 0.5 cm cube 12, 1 cm 6, 2 cm 3, 3 cm 2, flat block 5. The blocks are drawn as if cut in half so that the shrinking pink core can be seen.
Answers
- Define diffusion. [2]
The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until they are evenly spread.
- Calculate the surface area : volume ratio of a 2 cm cube. [2]
Surface area = 6 × 2 × 2 = 24 cm²; volume = 2 × 2 × 2 = 8 cm³; SA : V = 24 ÷ 8 = 3 : 1 (3 cm⁻¹).
- Why does the large cube take much longer to go colourless? [2]
Its centre is further from the surface, so the acid must diffuse a longer distance; also it has a smaller surface area for each cm³ of volume (smaller SA : V ratio).
- Why do large animals need a transport system such as blood? [2]
Their SA : V ratio is small and the distance from the surface to the inner cells is too long: diffusion alone would be far too slow to supply oxygen and food and remove wastes.
- Give two features of the flat block that are also found in exchange surfaces. [2]
It is thin (short diffusion distance) and has a large surface area for its volume, like alveoli, villi, gill filaments and leaves.
- Why does the acid diffuse in faster at a higher temperature? [2]
The particles have more kinetic energy and move faster, so they diffuse more quickly.
- A cube of agar has sides of 3 cm. (a) Calculate its surface area, volume and SA : V ratio. (b) Compare with a 1 cm cube and explain which would become colourless first in acid. [6]
(a) SA = 6 × 9 = 54 cm² (1); V = 27 cm³ (1); SA : V = 2 : 1 (1). (b) The 1 cm cube has SA : V = 6 : 1 (1); it has more surface for its volume and a shorter distance to the centre (1), so it becomes colourless first (1).
- Explain how the structure of the villi in the small intestine increases the rate of absorption by diffusion. [4]
Villi and microvilli give a very large surface area (1); the epithelium is one cell thick, a short diffusion distance (1); a dense capillary network carries absorbed food away (1), keeping a steep concentration gradient (1).
- Suggest why a flat worm can survive without lungs or blood vessels, but an earthworm needs blood vessels. [3]
The flatworm is thin and flat, so every cell is close to the surface and its SA : V ratio is large (1); diffusion is fast enough (1); the earthworm is thicker/cylindrical, so the distance to inner cells is too great and it needs blood to transport oxygen (1).
- State three factors that increase the rate of diffusion. [3]
A steeper concentration gradient (1); a larger surface area / shorter diffusion distance (1); a higher temperature (1).