Investigating Osmosis using Potato Strips

Image result for potato strips in ribena and water

Osmosis is defined as the net movement of particles of a solvent (a substance that dissolves another to form a solution) along its concentration gradient, across a partially permeable membrane, until an equilibrium (a state of rest or balance due to the equal action of opposing forces) is established.

The aim of this experiment is to prove the effect Osmosis has on the object in question, namely the potato strips.

Ethical issues

There are no ethical issues associated with this experiment

Health and Safety

Potatoes are a foodstuff and therefore low hazard. However, its raw state and the location of the experiment, which was in a laboratory, prohibited its consumption.

 In order to achieve this, the following materials and apparatus were used:

-3 Potato strips

-Water (hypotonic solution)

-Ribena (hypertonic solution)

-ruler

-3 identical test tubes

-paper towels

– test tube holder

– a potato chipper

The procedure is as follows:

 

  1. We observed each strip individually by feeling its consistency
  2. We weighted and measured them, recording the initial mass and length

  Initial length Initial mass
A 4,5cm 3,85g
B 4,5cm 3,38g
C 4,5cm 3,69g
  1. We put Ribena in the first test tube, a mixture solution in the second, and water in the third
  2. We placed a strip in each test tube and let them set for about 15min
  3. We remove the strips after 15 minutes and dab on tissue
  4. We weighted and measured the potato strips anew and observed their state
  Final length Final mass
A (Ribena) 4,3cm 3,25g
B (mix) 4,3cm 2,96g
C (water) 4,7cm 3,85g
  1. Finally, we performed % difference calculations for the mass and length using the formula:

(final – initial) x 100% / initial

  Length change Mass change
A -0,04% -0,15%
B -0,04% -0,12%
C 0,05% 0,04%

Limitations

There were several limitations to this experiment, which may have hindered or altered its accuracy and end results:

  • We did not measure the exact amount of liquid put into each test tube: we did not exclude the factor of liquid amount having an effect over the potato strip
  • There was no exact mixture between water and Ribena: the imprecise mixture deprives us from knowing which fluid was the dominant one
  • Use of normal water: normal water is not as hypotonic as distilled water and urges a smaller visible reaction
  • Uneven temperature
  • No time recording: there was no specific time span in which the potatoes stayed in the liquid and they were also placed one after the other, so they did not spend the exact same time inside it
  • The mass and percentage of change was rounded and thus the result is not precise

In order to avoid potential flaws, next time the experiment should be measured precisely, be timed and use distilled water.

 

Results and Conclusions:

The following results and conclusions were deduced by this experiment:

Potatoes in water:

An increase in mass of the potato strip due to the movement of water molecules into the plant cells via osmosis. The water is hypotonic. This means it possesses high water solution and is low on sugar. In contrast, the potato is hypertonic: it has low water potential and a high sugar solution. When put into contact, water will diffuse into the potato and hence expand its size, making the final size greater that its initial results.

This is also relevant to the increase in length. Since the intake on water molecules occupies extra space, it also changes the volume of the potato by pushing against the cell membrane and the proximate cellulose cell wall. This results into an expansion in all dimensions, including length. However, the expansion is limited by the stiffness of the cellulose cell.

The push/force acting upon the surface area of the rigid cell produces turgor pressure and hence strips are turgid.

Potatoes in Ribena:

There is a decrease in the mass of the potato strips due to the movement of water molecules outside of the plant cells. Ribena is mainly a sugary solution and hence is hypertonic to the potato cells, which are hypotonic. The water molecules will move along their concentration gradient out of the cells and thus make the potato strip shrink.

The decrease in the number of water molecules results in loss of volume and length.

Since the turgor pressure exerted by the water molecules against the cell membrane/wall is less, its condition remains flaccid.

Potatoes in Mixture of both:

There was a decrease in weight and length present but not as great as the one by Ribena. The solution still contained a higher sugar rate and is therefore hypertonic. However, the contrast between hypertonic and hypotonic is smaller and hence the result is less visible.

In order to generalize and prove the statements mentioned above, the class compared its results and established the following realizations:

 

  Average change in mass Average change in length
Water -0,7g -0,2cm
Mix -0,15g 0cm
Ribena 0,2g 0,1cm

 

 

 

 

 

 

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