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# Important principles and relationships in Life Sciences

## Important principles and relationships in Life Sciences (ESG3D)

### Surface area and volume (ESG3F)

Depending on the system it is advantageous to either have a large surface to volume ratio or a small surface to volume ratio.

The concept of surface area to volume ratio is difficult for some learners to grasp. The following explanation may help them to understand the concept better:

• Think of a piece of clay or Prestik. If we roll it into a ball, that ball has a certain volume (how much clay there is) and a certain surface area (how much of its surface is exposed to the air).
• Surface area is measured in SQUARE CM or cm2, while volume is measured in CUBIC CM or cm3.
• This is important – if they are measured in different units, we cannot say 'surface area is bigger than volume'. It’s like trying to decide whether 5kg is smaller than R20! It makes no sense.
• So our ball of clay has a surface area of say 50 cm2 and a volume of say 150 cm3. This is a relatively large volume compared to a relatively small surface area – it is called a small surface to volume ratio.
• If the same ball of clay is now flattened into a log thin strip like a ruler, the volume is still the same (we have not removed or added any clay), but the surface area is much more. It now has a volume of 150 cm3 and a surface area of say 500 cm2. This is called a large surface to volume ratio.
• Surface to volume ratio is critical in Life Science – an animal with a flat or very small body has a bigger surface to volume ratio than an animal with a rounder or bigger body. The first animal will be able to distribute nutrients and gases by means of DIFFUSION, while the second animal will have to have a BLOOD SYSTEM.
• In plants, surface to volume ratio has a direct effect on how easily a plant dehydrates – leaves, for example, must have a large surface to volume ratio to absorb as much light as possible, so they have to have special structures like cuticles / hairs to prevent dehydration.

A cell’s surface area must be large enough to meet the needs of its volume. This is highlighted in the following examples:

• Flatworms and leeches have more surface area to volume to increase the area for diffusion for nutrients and respiratory gases across their whole bodies.
• In animals the shapes of organs are defined by surface area to volume requirements. For example, in the lungs there are many branches to increase the surface area through which gases can be exchanged.
• Cells with a small volume and large surface area are better suited for diffusion, ingestion and excretion because of the relatively large area of the cell membrane.

### Structure and function (ESG3G)

In living organisms, the structure of a particular biological feature is related to what function it performs. Thus for all the structures you will study in Life Sciences, the important questions to ask are the following:

1. What makes this structure suited to its function?
2. How has the structure adapted to its function?
3. Why is this structure so efficient for its function?