7.1.9Practical Skills
Practical: Surface Area: Volume Ratio — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions
5 min read 6 parts Lesson 10 of 11
Section 1 of 6The idea behind the investigation
Why ratio matters
- Why ratio matters Substances enter a cell or organism across its surface, but they are needed by the whole volume.
- The surface area to volume ratio therefore controls how quickly the inside can be supplied.
- As an object gets bigger, its volume increases faster than its surface area, so the ratio falls.
A model for a cell
- A model for a cell Cubes of agar jelly stand in for cells of different sizes.
- Agar is mostly water, so dissolved particles can diffuse into it in a way that is similar to diffusion into living tissue.
- The agar contains an indicator, so the colour change shows how far the diffusing acid has travelled.
Prediction
- Prediction Small cubes have a large surface area to volume ratio, so the acid should reach their centre quickly.
- Large cubes have a small ratio, so diffusion to the middle should be slower.
Section 2 of 6Apparatus and method
1
Prepare agar containing Universal Indicator and a little dilute sodium hydroxide, so that it is purple (alkaline). Let it set in a tray.
2
Use a ruler and a sharp knife to cut cubes with sides of 0.5 cm, 1 cm and 2 cm. Cut cleanly so that the faces are flat and the dimensions are accurate.
3
Calculate the surface area, volume and surface area to volume ratio of each cube and record them in a table before starting.
4
Place each cube in a separate boiling tube, then add the same volume of dilute hydrochloric acid to each tube so that the cube is fully covered. Start the stopwatch.
5
Either record the time taken for the purple colour to disappear completely from the cube, or remove the cube after a fixed time (for example 5 minutes), cut it open and measure how far the colour change has spread in from each face.
6
Repeat each size at least three times and calculate a mean, to reduce the effect of random error.
Section 3 of 6Variables and safety
| Type | Detail |
|---|---|
| Independent variable | The size of the agar cube, which changes its surface area to volume ratio. |
| Dependent variable | The time for the colour to change throughout the cube, or the distance the acid has diffused in a set time. |
| Control variables | Concentration and volume of acid, temperature, agar recipe, and the way the time is judged. |
| Safety | Wear eye protection. Dilute hydrochloric acid is an irritant, so wipe up spills. Take care with the knife and cut on a hard surface away from your fingers. |
Section 4 of 6Processing the results
Calculating the geometry
- Calculating the geometry For a cube of side L: surface area = 6 × L2 and volume = L3.
- Worked example: a 2 cm cube has a surface area of 6 × 4 = 24 cm2 and a volume of 8 cm3, so the ratio is 24 : 8 = 3 : 1.
- A 1 cm cube has 6 cm2 to 1 cm3, a ratio of 6 : 1.
Converting time to rate
- Converting time to rate If you recorded the time for the colour to vanish, convert it into a rate: rate = 1 ÷ time.
- A shorter time means a faster rate of diffusion.
- Plot a graph of rate of diffusion against surface area to volume ratio.
Conclusion
- Conclusion The graph should show that as the ratio increases, the rate of diffusion increases.
- Cubes with a large ratio are fully coloured sooner,
- which links to why small cells and organisms with large exchange surfaces can obtain materials efficiently.
Section 5 of 6Key terms
| Term | Meaning |
|---|---|
| Diffusion | The net movement of particles from a region of higher concentration to a region of lower concentration. |
| Surface area to volume ratio | The surface area of an object divided by its volume; it falls as the object gets larger. |
| Agar | A jelly used here as a model for cell material because particles can diffuse through it. |
| Universal Indicator | A mixture of dyes that changes colour with pH, used to show where acid has reached. |
| Rate | How quickly something happens; here calculated as 1 divided by the time taken. |
Section 6 of 6Exam technique
1
Saying the larger cube has a larger ratio because it has a larger surface area — its ratio is actually smaller, because volume grows faster than area.
2
Not keeping the acid volume, concentration and temperature the same for every cube, which makes the test unfair.
3
Forgetting to convert times to rates before plotting against the ratio, so that the graph slopes the wrong way.
4
Measuring the distance travelled from only one face of the cube instead of from the outside edge to the colour boundary all around.
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