Biology Coursework

BIOLOGY COURSEWORK: How the Concentration of Sucrose affects Osmosis in plant cells

AIM: To investigate how changing the concentration of sucrose can affect the osmosis in plant cells

RESEARCH

Definition of Osmosis:

Osmosis is the movement of water molecules from an area of high water concentration (weak solution) to an area of low water concentration (strong solution) through a partially permeable membrane. Water moves in both ways in order to balance the concentrations evenly. It is known as a net movement of water into there are where there is less water.

Water molecules move random with a certain amount of kinetic energy. Distilled water separated by a partially permeable membrane looks like in the diagram below:

Water molecules are moving from one side of the membranes to the other but there is NO NET OSMOSIS.

If a substance is dissolved in water, the kinetic energy of the water molecules is LOWERED. This is because some water molecules collect on the surface of the other molecules…

In osmosis, the potential of the water molecules to move- is known as the OSMOTIC POTENTIAL. Distilled water has the highest potential (zero). When water has another substance dissolved in it, the water molecules have less potential to move. The osmotic potential is NEGATIVE. Water molecules always move from less negative to more negative water potential (meaning from pure water to a more concentrated solution).

Now net osmosis can be understood as:LESS NEGATIVE MORE NEGATIVE (look in diagram below)

Year 11 Biology Coursework 1

On the other hand, the osmotic potential of a cell is known as the water potential.

Cell membranes are completely permeable to water therefore, the environment the cell is exposed to can have a dramatic effect on the cell.

When there is a high concentration of sucrose and a low concentration of water- this is called a hypotonic solution. This is where water passes into the vacuole by osmosis since the water molecules in the solution is attracted to the other water molecules in the cell, this is known as net endosmosis occurring and the cell becomes turgid. The vacuole pushes against the cell wall, and this in turn tops the cell from bursting.

When a solution is hypertonic, it contains a low concentration of solute (in this case sucrose) and a high amount of water. In this situation, water is caused to diffuse out of the cell- this process is known as exosmosis. This makes a few changes to the shape of the cell, such as it shrivels and wilts. It becomes flaccid, in which the cell would feel soft, limp and weak. If a lot of water leaves the cell, the cytoplasm starts to peel away from the cell wall (but the cell wall still keeps intact). This is where the cell has undergone plasmolysis.


The cell is turgid.

The cell is flaccid and causes plasmolysis.

For example:Water potential of cytoplasm=

- 50Osmotic potential of solution= - 20

For example:Water potential of cytoplasm=

- 50Osmotic potential of solution= - 80

This is in the units

of osmotic pressure

If the osmotic potential of the solution is the SAME as the water potential of the cell, which in my case, is where the sucrose and distilled water both have the concentration of 0.5M. The solution is called isotonic.

HYPOTHESIS

From my background research, I predict that the more concentrated the solution (the more sucrose it contains), the lower the osmotic potential is. The plant cell (in this case potato cell) would become flaccid, as water has moved out of the cell causing the mass to have decreased. As concentration of sucrose decreases, and the concentration of dilute water increases, the mass of the potato cell will become turgid and strong. It will gain mass as water diffuses into the cell.

At concentration 0.5M, the solution will be isotonic; therefore, the % mass change should be at zero as no net osmosis should occur, therefore no water should diffuse in or out of the cell, and so should not affect the mass of the cell at all.

I have planned a simple procedure in order to carry out this investigation:

Apparatus/Equipments:

Sucrose Solution and Water (two beakers to hold them) Weighing machine


In an isotonic solution, no net osmosis occurs. The cell is not plasmolysed, but it’s neither full turgid either.

For Example:Water potential of cytoplasm= - 50Osmotic potential of solution= - 50(This is the osmotic pressure)

I predict that graph will look like this because if the concentration of sucrose is low, then the water potential is lower on the outside and higher on the inside, and so water will move in the cell, meaning the cell will gain in mass. This will cause the points on the graph to be positive when it comes to percentage mass change, and after 0.5 M, the concentration of water becomes more less than the sucrose solution meaning the water potential is high on the outside but low in the inside of the cell, therefore water moves out. The cell loses mass, so the percentage mass change becomes negative- so it will be placed in the negative section of the graph-below zero.

0.5 M of sucrose solution and dilute

water

2 Measuring Cylinders (small ones- with the range from 0-1cm cubed)

Borer (to cut the potato cell) Stop Watch and Test tube rack 10 test tubes for each of the ten concentrations

Method

1. Put lab coat on and collect all equipments.2. Firstly, we will experiment with the concentration of 0.1 of sucrose

solution. Therefore pour, 0.1cm cubed of using a measuring cylinder of the sucrose, and pour in 0.9cm cubed of water into a beaker.

3. Cut out using the borer, 10 (same size) blocks of potato pieces (and make sure you know which one is for which test tube- for this it would be a good idea to put it into chronological order)

4. Weigh all twenty potato blocks and record their mass5. Place them all into their supposed test tube with the right

concentration into at the same time, and allow half an hour for osmosis to carry out.

6. After half an hour, dry the potato blocks/cells 7. Weigh them and record the results

The evidence I will collect is the measurement of the mass loss/gained. To do this I will record the weight of the plant cell at first and then after adding it into the concentration, I will leave it there for a certain amount if time and then record the mass of the plant cell afterwards. I will then subtract the initial mass from that which will give me the difference. This will either be the mass gained or loss (resulting a value either negative or positive). I will also find out the % mass change, in which I will divide mass change by initial mass and then multiply by a hundred.

Variables (Dependant): Percentage change in mass

Controls (Independent Variables):

Shape of sample potato Concentration of surface- range Container- pressure of fluid Temperature Time Method of drying Volume of solution used

I will record my results in a table, and then represent them on a graph for visual evidence; this will allow me to distinguish any patterns of such.

The range of concentration I am going to test is from pure water with the concentration of zero molars, to one molar- complete sucrose solution, testing each one: 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and finally 1 molar. I will use the measuring cylinders to dilute the sucrose solution according to the amount needed to make that particular concentration. (E.g. I will dilute 0.6M with distil water to make a sucrose solution of 0.4).

PRELIMINARY WORK


To get an idea of what the experiment is like at first hand I did some preliminary work. This way I can find what apparatus we would be using, be more efficient in the real experiment and finally to help us make a decision such as how long to leave the cell potatoes in the solution and the size of potato cell. We can be more prepared in the real experiment we would be more prepared, more in control, and to see if we wanted to change anything, such as perhaps use a different range of concentration and maybe better equipment that make the results more accurate.

Even before the preliminary, I prepared a table to record the results and of what I will be measuring:

Concentration (M) RepeatMass Before

(g)Mass After

(g)Mass Change

(g)% Change in

MassAverage

%

0.112

0.212

0.312

0.412

0.512

0.612

0.712

0.812

0.912

112

During the preliminary, my partner collected the ten test tubes (as we were testing ten different concentrations), along with a test tube rack. One of us filled each test tube with the correct amount of distilled water, while the other one did the amount of sucrose. Once that was sorted, grabbing one potato, we first made the mistake of cutting the potato in half and slicing it into cubes. We did not realise later on that we were supposed to use a special tool (NAME) that would cut out cylinder tube-like pieces of potato. So using another potato, we skinned the skin (since the skin is like a barrier and so getting rid of it would allow the process of osmosis to occur more efficiently) and placing them in chronologic order on a tile, we weighed each one.


Recording the measurement, we placed them in the test tubes which were also placed in the same chronological order (so we don’t get mixed up) as fast as we could as we wanted all the potato blocks/cells to have the same amount of time to be in the solution. Once we did that, we left it for half an hour.

Concentration

Weight Before Weight After Mass Change

% Change

0.1 1.30 1.29 -0.01 -0.770.2 1.20 1.18 -0.02 -1.670.3 1.25 1.17 -0.08 -6.400.4 1.28 1.18 -0.1 -7.810.5 1.22 1.10 -0.12 -9.840.6 1.24 1.09 -0.15 -12.100.7 1.11 0.98 -0.13 -11.710.8 1.18 1.03 -0.15 -12.710.9 1.20 0.95 -0.25 -20.831 1.28 1.08 -0.2 -15.63

We did not do any repeats (because we forgot); we found that our results came out to what we didn’t quite expect. I expected it to be having gained mass for the first five concentrations. In fact, according to my research, I had expected that at concentration 0.5, there would not have been any mass gained or lost (but I believe I’m expecting too much from this experiment to receive exactly 0.00 g not gained or lost). After concentration 0.5, I would think it would start losing mass- from our results, I can see that this much has at least happened.

WHAT WE LEARNT FROM THE PRELIMINARY

Leave it for a longer amount of time (for example three hours, instead of half an hour)

Use bigger potato blocks (hence use a bigger of that weird tool) We decided to keep the same range of concentrations Use a pipette to make accurate measurements Measure mass up to two decimals Next time to have 20 test tubes, to repeat the experiment twice so

we can obtain reliable results

IN REAL EXPERIMENT:

Apparatus/Equipment

Sucrose Solution and Water (two beakers to hold them) Weighing machine 2 Measuring Cylinders (from the range of 0 to 1cm cubed) Stop Watch Borer (to cut the potato cell) 20 test tubes for each of the ten concentrations (and repeated

again) Pipette Paper Towels to dry/rinse the potato cell when they come out of the

test tube Test tube rack


Method

1. Put lab coat on and collect all equipments.2. Firstly, we will experiment with the concentration of 0.1 of sucrose

solution. Therefore pour, 1cm cubed of using a measuring cylinder of the sucrose, and pour in 9cm cubed of water into a beaker.

3. Cut out using the borer, 20 (same size) blocks of potato pieces (and make sure you know which one is for which test tube- for this it would be a good idea to put it into chronological order)

4. Weigh all twenty potato blocks and record their mass5. Place them all into their supposed test tube with the right

concentration into at the same time, and allow half an hour for osmosis to carry out.

6. After half an hour, dry the potato blocks/cells 7. Weigh them and record the result

OBTAINING THE EVIDENCE

We collected evidence safely and were relevant to the aim by weighing the potato cell before putting it into the sucrose solution and then after, if it loses mass then we know that water has left the cell, and if it gains mass we know that water has entered the cell. This will tell us how osmosis works through different concentrations.

We recorded the results in a tale as shown below, and then drawn a graph to show visual evidence and to see if there is a pattern or trend between the results. From the table, I have repeated the experiment and have taken the average of the two sets of results; this has helped me produce more accurate results than if I were to do the investigation only once. I used pipettes/syringes to make accurate measurements when pouring the sucrose solution and diluting it with distilled water.

OBSERVATIONS

I realised that as I went down to the test tubes that held the higher concentrated sucrose solution, when I came to weigh the potato cell, it felt soft, mushy, weak, and limp. I could easily tear it in half or squash it. On the other hand, when I was weighing the potato cells in the less concentrated sucrose solution they felt tough, strong, hard, and rigid. It was after concentration 0.7 M, where quite a lot of water seemed to have diffused out of the cell because, it was then where I felt the cell were really weak, limp, and flaccid. The cell felt hard and turgid at concentration 0.1M of sucrose solution (high concentration of dilute water -0.9 M).

ANALYSIS

My results show that during the process of osmosis within the time of two hours and thirty-five minutes, (12.30pm - 3.05pm) the mass of the potato cells have changed. The results indicate that as the concentration of sucrose increases, the percentage of mass change becomes more negative. The results also tells me that as the concentration of sucrose


decreases, and the amount of dilute water increases, the percentage mass change becomes a more of a positive value.

It also appears that after the concentration of 0.5M (including itself), the % mass change is always a negative value afterwards.

Result Table to show how different concentrations of sucrose solution can affect the mass of a potato cell during osmosis

Concentration (M) RepeatMass Before

(g)Mass After

(g)Mass Change

(g)% Change in

MassAverage

%

0.11 1.62 1.85 0.23 14

142 1.62 1.84 0.22 14

0.21 1.65 1.79 0.14 8

82 1.64 1.75 0.11 7

0.31 1.61 1.71 0.10 6

62 1.63 1.72 0.09 6

0.41 1.56 1.61 0.05 3

32 1.55 1.6 0.05 3

0.51 1.62 1.37 -0.25 -15

-142 1.60 1.39 -0.21 -13

0.61 1.64 1.27 -0.37 -23

-222 1.63 1.27 -0.36 -22

0.71 1.64 1.19 -0.45 -27

-282 1.65 1.17 -0.48 -29

0.81 1.62 1.11 -0.51 -31

-322 1.61 1.09 -0.52 -32

0.91 1.65 0.99 -0.66 -40

-392 1.66 1.02 -0.64 -39

11 1.70 0.97 -0.73 -43

-432 1.71 0.99 -0.72 -42

I made a graph where I have calculated the average percentage change in mass, as it would be easy to plot on the graph. These were my calculations in order to get the results that I have now:


Take the ‘Mass After’ from the ‘Mass Before’ to get the mass change. Then divide ‘Mass Change’ by ‘Mass Before’ and then multiply it by hundred to get the mass change percentage. You do this with second set of result. You then add both ‘% Change in Mass’ and divide by two to work out the ‘Average %’ in change in mass.

The pattern seen in the graph is that the higher the sucrose concentration, the more mass is lost; and the higher the concentration of dilute water, the more mass is gained. You can see this trend/pattern, as I have drawn a line of best fit, which goes from top left to bottom right, meaning there is a negative correlation.

From the graph, I draw a conclusion that when there is a high concentration of sucrose, the water potential is greater on the outside than in the inside of the potato cell, as there is more water inside the cell compared to the outside. Therefore, it tries to even the water potential on both sides, so water diffuses out of the cell and into the solution.

However as the concentrations become stronger, the values begin to become negative (e.g. 0.7 has the percentage mass of -28).


This diagram illustrates the water molecules moving OUT of the cell, when in a HIGH concentrated sucrose solution.

Water moves OUT. Mass decreases. % Mass Change is

negative The cell becomes

flaccid

This diagram illustrates the water molecules moving INTO the cell, when in a LOW concentrated sucrose solution.

From concentrations 0.1 and 0.3, the percentage mass change is positive.

Plasmolysis:

It is hard to actually tell on the graph when EXACTLY plasmolysis had occurred. Yet, I can definitely say that at the concentration of 1M, plasmolysis did happen since according to my background research, a lot of water would have diffused out of the cell, meaning a great amount of mass is lost. My graph can also prove this since the lowest plot/point that ha the greatest mass lost is for concentration of 1M.

However, if I take my observations into account (from obtaining evidence) then according to that, at concentration 0.7M is where the potato blocks started to feel weak, limp and it was looking thinner or narrower than the previous ones.

My conclusion supports my prediction because I had stated that as concentration of sucrose increases, the mass of the potato cell would decrease, so the percentage mass of the potato cell will be negative. On the other hand, when the concentration of distil water increases, water moves into the potato cell and so mass is gained. The percentage mass change is positive. This is proven by the fact that there is a straight line of best, which shows that there is correlation/relationship between my results, and that my plots are very close to the line.

EVALUATION

I think the experiment went overall okay, I believe we could have still got better results, but despite that, I think the results we have are still worth value and consideration as when they are plotted on the graph, it does support my prediction and produces a conclusion.

I have identified several anomalies as they are circled in pencil on the graph. None of them is seriously out of the odd or far out, as all of them are fairly close to the line of best fit.

Possibilities that can explain the anomalies

I did in fact use two different potatoes, where both of them could have grown in a different environment and contain different nutrients and minerals, including different water amounts inside them

The size of the cell: May not have been very accurate, because firstly the potato was round, and was not cut into a perfect cube – this was partly because it would be too small by the time it was cut down to a cube (and there wouldn’t have been enough space to cut out potato cells)


Water moves IN. Mass increases. % Mass Change is

positive The cell becomes

turgid

The borer sometimes did not cut properly, it left out slight gaps in the corners

Despite using pipettes, the measurements of the two solutions may have been inaccurate due to human error

The weighing machine may have had a few faulty measures Drying the potato cell may not have been done properly

How well my results support my conclusion:

I think my results are reliable to a certain extent; because as you see from my table, I believe that my first and second sets of results are moderately close and therefore producing accurate average % mass change results. This produces better evidence and proof that will support my conclusion. Four of my points go through the line of best fit

% Change in Mass14

14876633

-15-13

-23-22-27

-29-31-32-40-39-43-42

The isotonic solution:

In my hypothesis, I predicted that the line would cross the x-axis at concentration 0.5M, which is when it should be in an isotonic solution- the concentration of distil water and sucrose solution should be equal. In theory, no osmosis should occur, so the percentage mass change should be at zero (crosses x-axis). According to my results, and on the graph, the point where this is supposed to happen did not in fact occur at the concentration 0.5M. My line of best fit crosses the x-axis at 0.34 when no osmosis occurs. This could be considered partially as an anomaly as it does not fit in with the theory; however the line of best fit does go through it.

How my experiment was suitable:

I think the time I left it for was reasonable enough


I have three pairs or results that are the same, which shows very good accuracy.

The rest of the results only have the difference of either one or two % which is quite good, if they are closer, this means better accuracy of the results. (The further apart they are the less accurate results you get)

I am pleased with my concentration range, I reckon it was suitable and with enough points to make a good graph with a smooth curve or a straight line

Finding out the mass before the potato cells were put into the solution and then comparing it with mass after osmosis had taken place was a good way of showing how osmosis works and how it affected the plant cells’ shape and structure

Improvements of the Experiment

Repeat it for a third time Use one potato only- and a larger one so you have enough to cut out

2o equal potatoes Use biurettes or syringes (better equipment) to make more accurate

measurements Despite the fact that I am happy with the amount of time I left the

potato cells for osmosis to occur, it would not do any harm if I increased it to an extra hour, as maybe more time can

FURTHER WORK

If I were to do any further work on this investigation that would support my prediction, then I would test the factor of length and maybe width as well. The change in length and width (when cut into cylindrical shape as previously) of the potato cell can tell us if osmosis did occur, and including its effects on the cell.

The longer and less wide the potato cell is means that the cell is flaccid, and water as diffused out. In a hypertonic solution, an exosmosis reaction occurs, this is where the potato cell would become slightly thicker in width and may increase in lengthwise slightly. This is because water has diffused into the vacuole and it is pushing against the cell wall. This makes the cell turgid, and stretches it very slightly, increasing its length and width in a very small amount.

For the graph, you would plot the concentration on the x-axis against the length of the potato cell that would be on the y-axis. You could draw another line graph to show the width (as you may not be able to hare the same scale with the length of the potato cell)


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