What are the ratios between surface area and volume?

All living organisms are made up of cells. Some, like humans, have numerous cells while others only have one. With a few exceptions, individual cells are tiny and can only be seen through a microscope. Why are cells so small? This is where the surface area to volume ratio factor comes in.

The surface area to volume ratio is the relationship between the volume of an object and the surface area of that object.¹

What is the difference between the cell size, surface and volume?

The surface area and volume determine the cell size. Most animal and plant cells are between 0.01 and 0.10 mm in size and cannot be seen by the naked eye (the smallest you would be able to see is about 0.05 mm). Cell size is usually measured in micrometre (μm).

The surface area is the external layer of an object. In the case of a cell, it is the plasma membrane.

The volume of a cell refers to the total amount of space in that cell.

Surface area to volume ratio (SA:Vol)

The ratio refers to the amount of surface area per unit volume of an object. The ratio between the surface and volume is calculated by dividing the surface area by the volume. The lower the ratio, the slower the transport of the molecules within the cell and with the surrounding environment.

To help you understand surface to volume ratio, we will use an example of a cube. As the size of the cube increases, the volume will increase more rapidly than the surface area, and the ratio will decrease.

Figure 1

Calculating the ratio of a cube (Figure 1):

SA = area of one side x 6 sides (example: 1 cm x 1 cm x 6 cm) = 6 cm2)

Vol = length x width x height (example: 1 cm x 1 cm x 1 cm = 1 cm2)

Important to note - the area will always be in squared units, and the volume will always be in cubed units!

As we have covered, as the length of the side of the cube increases, the ratio will decrease.

Cells are more of a sphere shape, but they aren't perfectly spherical. Imagine a cell being a sphere. Here is an example.

Figure 2

For a sphere:

Note: π (pi) ~3.14 (3 s.f.)

As the radius of a sphere increases, the surface area will increase as a squared function, and volume will be cubed. Thus, with the increasing radius, the volume will increase more rapidly. At some point, with the expanding size, the ratio will be too low, and the substances will not be able to enter or leave in a sufficient time for the cell to survive. Substances will not be distributed fast enough via diffusion within the cell.

The cell will stop growing when there is just enough surface area to efficiently distribute the substances within the cell and the surrounding environment.

What is the biological importance of size and surface area to volume ratio?

Organisms transfer materials between the inner and the outer environments to survive. Prokaryotic and eukaryotic cells require a smaller size. This is to facilitate efficient substance exchange. Smaller single-celled organisms can rely on diffusion for gasses and material exchange. A higher surface area to volume ratio allows these organisms to be more efficient. Larger organisms, such as animals, need specialised organs to facilitate substance exchange.

The lungs are organs adapted to gas exchange in humans.

Except for the heat, the exchange will happen in two ways:

The size and metabolic rate of the organism will affect the amount of material exchanged. Organisms with higher metabolic rates will need to exchange a larger amount of substances and, in turn, will require a higher SA:Vol ratio.

Increasing surface area to volume ratio

Cells and tissues that are specialised for gas and material exchange will have different adaptations to facilitate an efficient exchange.

We can use an example of the intestinal tissue. The small intestine has adaptations for absorbing nutrients and minerals from food. The inner wall of the small intestine, mucosa, is lined with simple columnar epithelial tissue. The mucosa is covered in folds that are permanent features of the wall increasing the surface area. The folds project finger-like tissue called villi to increase the surface area further. Villi are filled with blood capillaries to increase the amount of dissolved, digested food that can be absorbed into the bloodstream.

Figure 3

Lungs have alveoli, which are tiny sacs at the end of bronchioles. The blood and lungs exchange oxygen and carbon dioxide at alveoli. The walls of alveoli are very thin, and they also have membranous extensions called microvilli, which increases the total membrane surface.

Dangers of increased surface area

We have established that a cell with a high volume would not survive as it would not facilitate efficient material movement within the cell and with the outside environment. The increased surface can cause problems too. More surface area means more contact with the external environment, leading to more water loss, heat loss and loss of dissolved substances. In addition, especially in extremophiles, temperature control could become impaired in unfavourable conditions.

Extremophiles, organisms that live in extreme environments, have a small surface area to volume ratio. They live in difficult or impossible environments, such as the deep ocean bed, geothermal hot springs and deserts.

For example, the polar bears at the North Pole have a small surface area to volume ratio to minimize heat loss from the tissue and a thick layer of fat to keep warm.

Surface Area to Volume Ratio - Key takeaways

• Cell size, surface area and volume are essential factors of substance exchange. The surface area and volume determine the cell size.
• The ratio between the surface area and volume will determine the speed of material exchange, calculated by dividing the surface area by the volume.
• The surface area and volume will not increase proportionally as the object increases in size.
• Living organisms have a number of adaptations to increase the surface area. For example, alveoli in the lungs have microvilli - membranous extensions to increase the gas exchange area.
• More surface area leads to more contact with the environment. Increased contact of a cell or an organ with the environment will increase water loss, heat loss and loss of dissolved substances.

(1) KeyStageWiki (2021). Surface Area to Volume Ratio. Available at: https://keystagewiki.com/index.php/Surface_Area_to_Volume_Ratio [Accessed: 03/11/2021].

First determine the surface area and the volume of the shape. You will then divide the surface area by the volume to find the ratio.

The amount of surface area per unit volume of an object.

Organisms transfer materials between the environments in order to survive. High ratio between the surface area and volume will allow efficient substance exchange. However, if this ratio is too low, the cell will die as it will be unable to exchange enough substances to survive.

More surface area leads to more contact with the environment. Increased contact of a cell or an organ with the environment will increase heat loss.

We can rearrange the equation for the surface area of a cube. SA = side of a cube x side of a cube x 6 sides. Since we know the length of the side of the cube, we can use that to calculate volume: Volume = length x width x height (of a side of a cube).

Question

The ratio refers to the amount of volume per unit surface area of an object with the formula: Vol:SA. True or False?

Answer

False - The ratio is the amount of surface area per unit volume of an object with the formula: SA:Vol

Question

A side of a cube is 5 cm in length. Calculate the surface area to volume ratio.

Answer

Question

Calculate the surface to volume ratio of a sphere. The surface area can be calculated using the formula: 4πr^2 , and the volume can be calculated using: 4/3πr³. The radius of this sphere is 3 cm.

Question

Alveoli are an important feature of the lungs, and they are a place where gas exchange takes place. What are the adaptations for the alveoli in the lungs to increase the surface area?

Answer

Alveoli have membranous extensions called microvilli, which increase the total membrane surface. This allows an increased rate of material exchange via capillaries.

Question

Extremophiles living in cold environments have a high surface to volume ratio to prevent heat loss to the surrounding environment. True or False?

Answer

Question

The surface area and volume will determine the size of an organism or a cell. What would happen to the cell if its surface area increased too much? 

Answer

More surface area leads to more contact with the environment. Increased contact of a cell or an organ with the environment will increase water loss, heat loss and loss of dissolved substances. The cell will die.

Question

What are the units used for the volume?

Answer

Question

What are the units used for the surface area?

Answer

Question

What is the difference between active and passive transports?

Answer

Active transport requires energy, while passive does not.

Question

Diffusion is an active movement of molecules against the concentration gradient. True or False?

Answer

Question

​Fill in the blanks about SA:Vol ratio. The size and _________ rate will affect the amount of material _________. The __________ the ___________ rate, the ________ the need to exchange larger amounts of substance. Therefore, it will require a _______ SA:Vol ratio. 

Answer

The size and the metabolic rate will affect the amount of material exchanged. The higher the metabolic rate, the higher the need to exchange larger amounts of substance. Therefore, it will require a higher SA:Vol ratio.

Question

Active transport requires energy. Why would a cell in an animal use active transport instead of diffusion?

Answer

Active transport is used by the cell to accumulate high concentrations of minerals the cell requires, such as ions and glucose. There is usually a high concentration of these in the cell, so they would not be able to move down the gradient from the external environment. They are, therefore, moved against the gradient.

Question

What are the two adaptations of the small intestine to increase the SA:Vol ratio?

Answer

Villi and folds in the mucosa.

Question

Why are villi in the small intestine filled with blood capillaries?

Answer

Close contact between blood and small intestine allows a faster exchange of substances.

Question

Give an example of a gas exchange organ in an animal.

Answer