The Effect of Surface Area on the Rate of Osmosis

The relationship between an object’s surface area and the rate that substances diffuse into (or out) of it has implications in many areas. For example, some organisms may rely on diffusion to obtain nutrients and get rid of waste. The effect that the surface area of an object has on the rate of diffusion can be investigated using a potato and a digital scale (accurate to 0.01 g).

  1. Peel the potato, and cut it into 2 cubes, each about 2 cm on a side
  2. Cut one cube into 8 smaller pieces of the same size
  3. Wet the 2 cm cube under tap water, then pat it dry with a paper towel and weigh it
  4. Wet the 8 smaller cubes, pat them dry and weigh them together
  5. Place the cubes into a jug of water and start a timer for 15 minutes
  6. After 15 minutes, remove the cubes, dry them with a paper towel, and weigh the large cube, and the 8 smaller cubes
  7. Try to be consistent in the way you dry the cubes, as this will affect their weight
  8. Repeat step 6 three more times, then graph the results
  9. Repeat the experiment if you want – I cut 2 large cubes and 16 small cubes so I could run a duplicate simultaneously

My results are shown in the graph below:

Effect of surface area on rate of osmosis.png

As the potato cells contain lots of solutes in their water, the water in the beaker will move into the potato cells via osmosis, causing them to become heavier. Thus the increase in mass corresponds to the rate of osmosis.

Both the whole cubes had approximately the same rate of increase in mass – about 5%/hr. The diced cubes had a faster rate of about 7.4 and 8.8%/hr. This was predicted, as diffusion occurs through all the surfaces, so will occur faster through a larger surface area.

It would be interesting to do more replicates with different surface areas, and plot the increase in mass against the surface area to volume ratio (SA:V). One would think there would be a linear pattern, but perhaps not. In the graph above, the SA:V ratio was 3.2:1 for the whole cubes, and 6.4:1 for the 8 cubes, as my cubes were originally 2×2×1.7 cm in size.

As a side note, digital scales can be bought quite cheaply off eBay. The scale I used had a capacity of 200g, and can be bought for $9.99 (free shipping; delivery time ~1 week).

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