Explanation of Trends, Biology Experiments

Example of explanation of trends

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Trend Explanations
Experiment One:
As the sucrose concentration increases from 0 moldm-3 to 0.46 moldm-3, the % increase in
mass decreases from 3% to 0%. This is because the solution is hypotonic to the potato; there is a
higher water potential in the solution than there is in the potato. As a result, water molecules move
from the area of high water potential in the solution to an area of low water potential in the potato
through a selectively permeable membrane by osmosis. The mass of the potato increases due to the
potatoes uptake of water by osmosis. There is a bigger increase in mass for lower concentrations of
sucrose in the water because there is a greater difference between the water potential of the
potato and the water potential of the solution therefore the rate of osmosis increases and hence the
potatoes mass increases more as more water moves into the potato cells by osmosis.
As the sucrose concentration increases from 0.46 moldm-3 to 1.00 moldm-3, the % decrease
in mass increases from 0% to -3.5%. This is because the solution is hypertonic to the potato; there is
a lower water potential in the solution than there is in the potato. Thus water molecules move from
the area of high water potential in the potato to the area of low water potential in the solution
through a selectively permeable membrane by osmosis. The mass of the potato decreases due to
the potatoes loss of water by osmosis. There is a bigger decrease in mass for higher concentrations
of sucrose in the water because there is a greater difference between the water potential of the
potato and the water potential of the solution therefore the rate of osmosis increases and hence the
potatoes mass decreases more as more water moves out of the potato cells by osmosis.
Experiment Two:
As the sucrose concentration increases from 0.2 moldm-3 to 0.8 moldm-3, the % of
plasmolysed cells increases from 0% to 100%. This is because water molecules move from areas of
high water potential to areas of low water potential through a selectively permeable membrane by
osmosis. When the solution concentration was 0.2 moldm-3, 0% of the cells were shown to be
plasmolysed; all were turgid. This shows that water was either moving into the onion cells, or there
was an equilibrium between the random movement of water molecules in both directions, thus the
solution must have been either hypotonic or isotonic to all the onion cells. When the solution
concentration was 0.8 moldm-3, 100% of the cells were plasmolysed, therefore the cells must have
been losing water. For this to happen, the solution must have been hypertonic to all the onion cells.
For solution concentrations between 0.2 moldm-3 and 0.8 moldm-3, there was neither 100%
plasmolysed cells, nor 100% turgid cells. This shows that the solution was hypertonic to some cells,
allowing water to move out of the cells making them plasmolysed, and hypotonic to others, allowing
water to pass from the solution into the cells making the cells turgid. Therefore the water potential
of onion cells must vary.

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