Cell size as a specialization (2025)
In these activities students consider the range of cell size in humans including a few examples of cells, male and female gametes, red and white blood cells, neurones and striated muscle fibres. There is also an experiment activity introducing the importance of cell size for surface area to volume ratios.
Lesson description
Starting questions
What are the effects of cell size on the surface area of a cell?
What does a large size do the the volume of cytoplasm?
Activity 1 - Cell size is a type of adaptation.
The slides below introduce the cells used in this activity.
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Cell size - specialization:
Activity 2 - Surface area to volume ratio experiment.
This experiment uses a simple cube model of a cell to illustrate one advantage of small cell size, a large surface area to volume ratio.
The volume of a cell determines the needs of that cell for oxygen and other resources, as the metabolism increases according to the volume of cytoplasm. The surface area controls the rate of uptake of the oxygen.
This experiment shows the effect of increasing size on the rate of diffusion of materials into and out of the cytoplasm of a cell.
Safety
Working with acids - wear lab coats & eye protection.
The agar blocks contain alkali and phenolphthalein so it is important to wear gloves to handle them,
or use tweezers and wash hands afterwards.
Waste disposal: The acid and the agar should be disposed of according to local rules.
Method
- Cut cubes of agar with the length of sides of 2.0 cm, 1.5 cm, 1.0 cm and 0.5 cm.
Several blocks of each size can be prepared. - Measurements should be as precise as possible.Record the uncertainty of their measurements. This is often +/- the smallest unit on the ruler.
- Pour enough 1M HCl into a beaker to cover the largest block.
- Place the agar blocks in the 1M HCl
- Record the time taken for each of the blocks to lose their pink colour completely.
Several blocks can be placed in the same beaker.
Data processing
- Make a table of the class data showing the times taken for colour change to occur in the blocks of different sizes.
- Calculate average time taken for the acid to diffuse to the centre of the cell in each size.
- This is an opportunity to practice calculating standard deviations, if you have five or more values for each size.
- Present this processed data in a scatter graph with error bars showing standard deviation.
- In a conclusion, compare the time taken for the acid to diffuse to the centre of the gel cube with the size of the cube.
- To be more detailed - calculate the surface area and the volume of each of the cubes used.
From these values calculate the surface area to volume ratio.
[See the Theory box for an example of these calculations]
Theory: Calculation of surface area to volume ratios
Thinking of the gel cubes as unit cubes might help for the calculations.
- The volume of a cube can be calculated using the formula; Volume = l x w x h.
- The surface area can be calculated using the formula; SA = 6*(l x h).
- The SA:Vol ratio is calculated by dividing the surface area by the volume.
This table shows how surface area and volume of a cube are related.
Cell size (as a cube) | Surface area | Volume | SA/Vol ratio |
1 cm side | 6 x (1 x 1) = 6 cm2 | 1 x 1 x 1 = 1 cm3 | 6 : 1 |
2 cm side | 6 x (2 x 2) = 24 cm2 | 2 x 2 x 2 = 8 cm3 | 24 : 8 = 3 : 1 |
3 cm side | 6 x (3 x 3) = 54 cm2 | 3 x 3 x 3 = 27 cm3 | 54 : 27 = 2 : 1 |
Looking at the column for SA/Vol ratio, it is clear that as cells get larger the SA:Vol ratio decreases.
Consequences of cell size on transport of materials
The time taken for the colour change to reach the centre of the gel cubes shows that the larger the cube the more time it takes for materials to get to the centre of the cell. This is partly because the rate of diffusion is relatively constant, so the further it is to the centre of the cell from the cell membrane, then the longer it take.
The other factor is that the larger cell has a smaller surface area for each unit of volume. This limits cell size for several reasons.
- Materials, like glucose and oxygen, enter the cell across the plasma membrane surface.
- A larger cell has trouble getting materials in fast enough to meet its metabolic needs.
- Waste molecules accumulate faster than the cell can lose these by diffusion.
The consequence is for cells to become specially adapted to increase the membrane surface area.
1. cells divide to form smaller cells to increase the SA/Vol ratio
2. calls increase their surface area by changing shape, e.g. folding the membrane forming microvilli.
Making the link to heat loss in another way to imagine these adaptations. Why do fingers, toes and ears get colder faster than other body parts? This illustrates the SA/Volume ratio. The larger the surface area the faster heat is lost.
The ecological adaptation to reduce heat loss by having small ears seen in Arctic foxes can be compared to the large size of ears in tropical desert foxes to increase heat loss. It's the same Surface area to volume ratio and more cute!
Activity 3 - Comparisons of specialised cell sizes.
The worksheet shows some examples of specialised cells. They all have different shapes and sizes.
Consider the effects of each of the cell's adaptations by studying the images and diagrams and answering the questions on the sheet.
Comparisons of cell size - student sheet
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Teacher only boxThe following notes are for teachers using the activities on this page.
Activity 1 - The slides introduce the idea that cells are all different sizes which are an important aspect of cells specialisation. The images are used again in activity 3 and some of the concepts prepare students for the experiment.
Activity 2 - Experiment to model cell size, surface area and rate of diffusion.
Preparation - 1 day in advance
This practical takes less than 2 hours but the largest block (2.0 cm sides) can take over 20 minutes. The time remaining after the data collection can be used by the students to collect the class data for data processing. With the organisation and clean up time this could be done in one hour if everyone is well prepared.
The agar cake should be made no more than one day in advance as the agar is not sterilised and so will begin to support microbe growth. This growth can be reduced by refrigerating the agar and covering the surface tightly with cellowrap ( plastic food wrap).
Materials
A stiff agar cake can be made following this recipe. This needs to be tested with local materials so that the cake is quite stiff. If the agar is not stiff enough, the students will have trouble cutting neat blocks.
- Mix 180 g Agar-agar in 3dm3 cool water and bring to a boil, stirring regularly. This will cause the agar-agar to dissolve.
- Add 1 spatula Cresol red for the acid-base indicator, which turn purplish-red in alkali, pale yellow in acid.
- Add 5 ml Sodium hydroxide 1M NaOH to obtain a purplish-red colour. Mix well for uniformity.
- Pour into a cake pan or form ( not used for cakes anymore!) and allow to cool and solidify. This is best overnight.
Further Notes
This process allows for the students to recognise the difficulty with an end point determination- when is the colour really gone. This is an important lesson for students to learn!
An alternative to this experiment is to make a stiff salt agar cake and to use datalogger conductivity probes to measure the changes of conductivity over time as the ions diffuse out of the agar. Different beakers would be needed for each cube.
Activity 3 - This is a worksheet activity and the questions link to the slides in activity 1 with a focus on cell size.