7.1: Background

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Why must a sparrow, relative to size, eat more than a lion? Why the polar bear is bigger then any tropical bear? Why don’t we see insects bigger then a rat? Why then, in Carboniferous period, were insects bigger than a rat? Why are dinosaurs suspected to be warm-blooded? Why were dinosaur eggs so small? Why are leaves of plants flat and often dissected?

To answer these, and many more, questions, you need to understand one of the deepest laws of biology, the law of volume and surface.

We want you to start before the lab time, with your own simple experiment:

Experiment 1. Take two cans, one big and one small. Boil water in both. Simultaneously take them from the fire to a cooler place. Wait 30 minutes. Which can is now warmer? Why?

The explanation lays in geometry. When the linear size grows, the volume of an object grows as the cube of it, whereas the surface area grows only as square. As a result, there will be more and more volume on the surface of object. The relative surface area will shrink when size grows!

This is the true limit to the life on Earth. In general, evolution always tries to make organisms bigger. It is beneficial for everybody: bigger predators catch more prey; bigger prey escape from predators better; bigger plants take in more sun and shade more competitors; and so on.

However, a bigger organism will have more volume per surface area (i.e., smaller relative surface). Therefore, the most important feature of life, the gas exchange between an organism and its environment, will be hindered, assuming respiration is cutaneous. The pressure of weight (which corresponds tightly with volume) will also increase. How might an organism overcome these restrictions?

Today we will practice the laws of surface area to volume using two “organisms” made from the modeling clay. We will call them Plasticizoans since they are made from plasticine.