Surface to Volume Ratio

You probably learned at some point in your schooling that the cell is the basic unit of life and how things must be able to pass through the cell membrane in both directions. The amount of surface area is very important and as organisms grow larger, their volume increases at a faster rate than their surface area. A single celled organism can rely on the process of diffusion to exchange materials, but large organisms must use other mechanisms such as respiration and excretion.

In some science fiction films an organism has grown to incredible sizes, even larger than an entire spaceship. Most biologists would not suggest that an organism could ever get to such sizes. Some of the dinosaurs were certainly very large, but even they had to have some extraordinary biological mechanisms to allow such growth. Many very small animals must be constantly eating to generate heat and energy because their body has a large surface area that is losing heat at a high rate compared to their body size.

A one centimeter cube has a volume of one centimeter cubed and it has a surface area of 6 centimeters squared. When a cube of sugar dissolves in your cup of tea, the surface is the part where the sugar actually dissolves. So a sugar cube has six sides that are dissolving. Granular sugar is broken into many much smaller pieces, but each piece has a surface area. The granular sugar dissolves much quicker because of the increased surface area. At one time when people drank a cup of coffee they would place a sugar cube in their mouth and then drink which would then pass over the sugar cube allowing the sweet taste to be tasted, but this method tended to use less sugar.

In science class the concept of how to increase the rate at which something can dissolve is based on three key factors: 1) break the substance into smaller particles to increase the process of dissolving, 2) stir the substance to increase the rate of contact with more surfaces, and 3) increase the temperature which increases the rate of contact with surface areas.

A cube that is made of 8 one centimeter cubes would have a volume of 8 cubic centimeters, but potentially could have had a surface area of 48 centimeters cubed, but if the individual cubes are formed into one single large cube the surface area is just 24 centimeters squared because when the individual cubes are combined many of the surfaces are on the interior of the larger cube.

Materials must pass through the nuclear membrane and the cell membrane. Inside the cell the different structures of a cell also have surface to volume ratios which affect the operation of the cell. The movement of water into and out of a cell has a specific term to describe this movement "osmosis". Surface to volume ratios are important in everyday life. We purchase things like milk in containers with a specific volume listed and the shape of the container affects the heating and cooling of the substance inside the container.