In single-celled organisms such as protists, and small multicellular organisms, essential molecules will move to where they're needed by diffusion. Once an organism is beyond a certain size, it cannot get essential molecules into and out of cells solely by diffusion. Diffusion is limited by the surface area to volume ratio of the organism.
It's straightforward to model cells using cubes to investigate surface area to volume ratios in different sized organisms. These cubes can be made of agar jelly. Practical work can then be carried out into how easily dye can diffuse into the cubes in comparison to their surface area to volume ratio.
As each cube represents a cell, the more cubes there are the more cells the organism has. This represents the change in surface area to volume ratio as you move from unicellular to multicellular organisms.
So, as the volume increases, the surface area does not increase at the same rate.
If a graph is drawn:
As the volume increases, the surface area does not increase at the same rate.
When the surface area to volume ratio is large, there is a lot of surface area for diffusion and not much volume to travel within. This is the case for unicellular organisms, which can rely on diffusion alone to get the substances they need.
When the surface area to volume ratio is small, then there is not much surface area for substances to diffuse across but there are lots of cells inside that need the substances (a high volume). The multicellular organism can't rely just on diffusion to get the substances that all of its cells need.
In the table below scientists have estimated the surface area:volume ratios of various organisms.
|Organism||Type of organism||Surface area in square metres||Volume in cube metres||Surface area:volume ratio|
|Bacterium||Unicellular||6 × 10-12||1 × 10-18||6,000,000|
|Blow fly||Multicellular (small)||6 × 10-4||1 × 10-6||600|
|Whale||Multicellular (large)||6 × 104||1 × 106||0.06|
Large, multicellular organisms need ways to ensure that all cells can get the substances that they need to survive. These may include:
Some multicellular organisms living in harsh environmental conditions may reduce their surface area to aid survival, eg cacti only have small spines rather than large leaves to reduce loss of water.