Surface area to volume ratio

• A larger surface area and a shorter diffusion distance speed up the rate of diffusion.

• As organisms grow larger their surface area increases but the surface area to volume ratio decreases.

• This slows the rate of diffusion to the center of the organism since the diffusion distance increases.

• In the blocks below, the following surface areas (SA), volumes (Vol) and surface area to volume ratios (SA:Vol) apply:

  • 1 x 1 x 1 cm: SA = 6 cm2, Vol = 1 cm3, SA:Vol = 6:1

  • 1 x 1 x 2 cm: SA = 10 cm2, Vol = 2 cm3, SA:Vol = 5:1

  • 2 x 2 x 2 cm: SA = 24 cm2, Vol = 8 cm3, SA:Vol = 3:1

  • 1 x 1 x 8 cm: SA = 34 cm2, Vol = 8 cm3, SA:Vol = 4.25:1

Different size and shape blocks display different surface area to volume ratios

Adaptations of single-celled organisms

• Single-celled organisms are usually small to maintain a high surface area to volume ratio.

• Many have folds in their cell membranes to further increase surface area for diffusion.

Adaptations of multicellular organisms

Challenges:

• Multicellular organisms typically have a small surface area to volume ratio.

• This makes it difficult for substances to diffuse efficiently to all cells.

Solutions:

• Systems like the respiratory system significantly increase the surface area of the organism exposed to the environment, and the circulatory system transports substances efficiently throughout the body.

Surface area to volume ratio and heat exchange

Heat Loss:

• Organisms with a larger surface area to volume ratio lose heat more rapidly.

• This is important for mammals and birds, which maintain a constant body temperature through metabolic reactions.

Metabolic Rate:

• Smaller mammals and birds:

  • Have a larger surface area to volume ratio so lose heat quickly and require a higher metabolic rate to generate enough heat.

• Larger organisms:

  • Have a smaller surface area to volume ratio, which helps conserve heat.

Definitions