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Surface Area to Volume Ratio (Melissa Savage)


 The ratio between the surface area and volume of cells and organisms has an enormous impact on
their biology. For example, many aquatic microorganisms have increased surface area to increase their drag in the water. This reduces their rate of sink and allows them to remain near the surface with less energy expenditure. Humans and other large animals cannot rely on diffusion for absorption and rejection of respiratory gases for their whole body; however, animals such as flatworms and leeches can, as they have more surface area per unit volume. For similar reasons, surface to volume ratio places a maximum limit on the size of a cell.

    An increased surface area to volume ratio also means increased exposure to the environment. The many tentacles of jellyfish and anemones provide increased surface area for the acquisition of food. Greater surface area allows more of the surrounding water to be sifted for nutrients.

    Individual organs in animals are often shaped by requirements of surface area to volume ratio. The numerous internal branching of the lung increase the surface area through which oxygen is passed

into the blood and carbon dioxide is released from

the blood. The intestine has a finely wrinkled internal surface, increasing the area through which nutrients are absorbed by the body.

    Smaller single celled organisms need to have a high surface area to volume ratio in order to survive. This is because they rely on oxygen diffusing into the cell. The higher the SA:Volume ratio they have, the more efficient this process can be.

    A wide and thin cell, such as a nerve cell, or one with membrane protrusions such as microvilli has a greater surface-area-to-volume ratio than a spheroidal one.

    Increased surface area can also lead to biological problems. More contact with the environment through the surface of a cell or an organ (relative to its volume) increases loss of water and dissolved substances. High surface-area-to-volume ratios also present problems of temperature control in unfavorable environments.





California State Standards - Cell Biology

1. The fundamental life processes of plants and animals depend on a variety of chemical reactions that occur in specialized areas of the organism’s cells. As a basis for understanding this concept:

a.      Students know cells are enclosed within semipermeable membranes that regulate their internal surroundings.


Students will understand how the efficiency of a cell is dependent on the cell’s ability to control cell size. Students will understand how to connection between surface area and volume of a cell.



** For ease of calculations, we will measure circumference of the filled water balloon.

Higher level students should calculate surface area of the filled balloon



4 small regular balloons/class (do not use water balloons)


Measuring tape

Large beaker or container

1000ml graduated cylinder


Optional Extension: M & M candy of different sizes


Start by explaining surface area, volume, and surface-area-to-volume ratio to the students. Use analogies when possible


Demonstration: Have students help with the demonstration.

·         Fill the first water balloon from the faucet.

·         Measure the circumference of the widest part of the balloon after filling

·         Place the balloon into a container and pop it.

·         Pour the water into the 1000mL graduated cylinder and record the volume.


Higher level thinking: calculate the surface area to volume ratio


Follow the same procedure above for the next three balloons. Fill the balloons one at a time. Increase the amount of water each time ( I know…give it your best estimate). Each balloon should be larger when filled then the previous balloon. Have the students guess the volume before popping the balloon.


The students will begin to see that the volume of the balloon increases at a much greater rate than the surface area.


Explain why cells must control their size. Relate the idea to efficiency of the cell.


Extension: (because Melissa loves M & M’s): How are M & M’s related to cell size and efficiency (taste)


Post-demo questions:

1.      What happens to the surface area to volume ratio as the balloon increases in size?

2.      Why is it important for a cell to control size?

3.      How does the cell membrane help to regulate cell size?

4.      Provide an analogy to the efficiency of a cell. Explain your analogy in 3 – 5 complete sentences.
5.   Which will dissolve faster in you mouth
        -10 Small jawbreakers (1ml each)
        -1 Large jawbreaker (10ml)
6.   Why is it more dangerous to leave a small child or animal in a hot car than for you to be in that same car?
7.    How does surface area to volume relate to the alveoli in the lungs and the villi in the small intestines?


 Interactive Surface Area to Volume Website


Melissa Savage,
Sep 26, 2011, 11:15 AM
Melissa Savage,
Sep 26, 2011, 4:43 PM