An investigation into changing the concentration of sucrose soltion on the rate of osmosis in potato cells.

An Investigation into the effect of changing the concentration of sucrose on the rate of osmosis in potato cells.

Aim-In this experiment I will be attempting to find out if increasing the concentration of sucrose in a solution will result in a decrease in the rate of osmosis.

Hypothesis- If the concentration of sucrose is lower then the rate of osmosis will be higher thus increasing the mass of the potato. Furthermore I predict that when I increase the concentration of sucrose the amount of water which diffuses will decline.

Taking my hypothesis into account the aim of my experiment is to investigate the extent at which sucrose solution affects the rate of osmosis. To do this I must understand which other variables will affect the rate of osmosis therefore increasing the reliability of my results and making the experiment a fair test.

Introduction-

Osmosis can be defined as the movement of water form an area of higher water potential to an area of lower water potential. The equation which is important when understanding water potential is water potential (Ψ) = solute potential (Ψp) + pressure potential (Ψp).It can also be described as the passage of water from a lower concentration of solute to a higher concentration of solute. Osmosis is a process which occurs through a partially permeable membrane, which is a membrane which will only allow certain molecules through E.g. water.

Diagram 1.a showing two solutions before osmosis

As we can that there is a higher water potential on the left solution. Therefore the net movement of water will be from left to right until a state of equilibrium is reached.

The water potential of a cell will be higher when the solute potential is lower. The net movement of water during osmosis will always move from where there’s a higher water potential to a lower water potential. The highest water potential a cell can have is 0 this occurs in pure water (no ions present etc). Solutes lower the water potential therefore when solutes are present the water potential will drop below 0. The diagram below shows two neighbouring cells and due to the difference in water potential the movement of water will be from cell B to cell A.

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Biology 1

Ψ=-300 Ψ= -200

From this diagram we can see that the net movement of water (direction in which the water moves) is going to be moving from right to left. Osmosis will continue until there is a state of equilibrium between the two cells. This is where the water potential becomes equal to each other.

Osmosis in plant cells-As I previously explained water potential is Ψ = Ψp + Ψs. Plant cells are surrounded by a strong cellulose containing cell wall. This means that they can sustain much more pressure than animal cells so pressure potential is more significant when examining plant cells. When water enters a plant cell through the partially permeable membrane the cell becomes turgid: this means that the cell continues to expand until filled. As the volume of the cell increases the protoplast begins to add pressure to the cell wall. This pressure potential inevitably increases the water potential of the cell until equilibrium, although it is surprising the small amount of time which it takes to achieve this. And because the plant cell wall is so strong and rigid the cell will not burst.

Sometimes in plant cells there is a process called plasmolysis which occurs. This is where the protoplast begins to decrease in size and water moves out of the cell, due to a higher water potential outside the cell. This happens in hypertonic solutions and is represented in the diagram below.