The Biology of Food 2: Chips

In this series of ten blogs, various parts of the  GCSE Science, IGCSE Biology specifications will be explored through the context of food.  As well as assisting students revising for their GCSE and IGCSE exams, the blogs also provide an every-day context for science which all readers should find accessible, interesting and useful.

Surface area to volume ratio

The topic ‘surface area to volume ratio’ appears in the following subject specifications: AQA GCSE Biology 1; AQA GCSE Science 1: 11.1, and Edexcel Biology IGCSE 2.15, (but note that this principle is relevant to many areas of science, including rates of reaction in chemistry).

It is important to understand this concept because it helps to explain many phenomena. It gives us the answer to such questions as:

• why is frog’s spawn spherical?
• why do elephants have big ears?
• why is a root hair cell the shape it is?
• why do we curl up in bed to stay warm?

The ratio of surface area to volume mathematically measures exactly how big the surface of an object is compared to its volume.  A large surface area compared to volume includes long, thin, or highly convoluted shapes (ones with a wavy surface).  The shape with the smallest surface area compared to its volume is a sphere.

Biologically, objects with a large surface area to volume ratio allow lots of movement of substances or energy across the surface – because there is a lot of surface for their size.  Spherical shapes with their smaller surface area to volume ratio have limited opportunity for transport of substances or energy across their surface.

So what has this go to do with making chips?

Peeling potatoes and cooking chips provides an illustration that can help us to understand what is meant by ‘surface area to volume ratio’.

Consider peeling enough potatoes to feed your family.  If you had a choice between peeling one enormous spherical potato, or a bag of conker-sized potatoes (both with the same final mass of peeled potato), which would you choose?  The total surface area of all the small potatoes far exceeds the surface area of the enormous potato.  Peeling one large potato would be much quicker than the bag of small ones.  The surface area to volume ratio of the large potato is much smaller.

Now on to cutting and cooking chips: we are all advised to cut down the amount of fat in our diet.  When chips are cooked in hot oil, the surface of the chips absorbs some of the cooking fat.  A single portion of thin, matchstick-like chips has an enormous total surface area which can absorb much more fat that a few chunky chips.  You can imagine cutting a large chip up into many smaller ones:  each slice of the knife exposes fresh surfaces which will absorb fat when cooked.

You can discuss the phenomena in the second paragraph over a plate of hot chips (just make sure they are big chips made from big potatoes).

Georgina Kitching, Science tutor OOL