Agar Blocks Lab Report

Divya Aishwarya Gandi

DIFFUSION IN AGAR BLOCKS LAB REPORT Research Question: how does the surface area to volume ratio of agar blocks, of length 0.2cm, 0.4cm, 0.6cm, 0.8 and 1.0 cm effect the time taken for their decolourisation after being stained with phenolphthalein indicator? Background information: Diffusion is one of the very important processes by which substances are transported between cells and their environment. Understanding the relationship between the size of cells (volume) and their surface area helps us understand how the physical limitation of the size of cells is important for them to function effectively. Surface area to volume ratio is extremely important because so many biological functions happen on the surface of objects. For example: 

Ions diffuse in and out of cells through channels and pores on their surface.



Oxygen diffuses in and out of cells across their plasma membranes.



Heat diffuses out of our bodies through skin that covers our surface.

As an object grows in size, its surface area will grow too. As a cell gets larger, the space inside the cell increases faster than the surface around it. In theory, a cell could grow so large that there would not be enough surface area for a sufficient amount of ions and oxygen to diffuse across. In other words, the SA/V ratio can become too small for a cell to survive. This is one of the reasons why cells are small; if they were larger, not enough oxygen would be able to diffuse into them. Surface area to volume ratio affects more than just cells. It is important to large organisms as well. Humans and other large animals cannot rely on diffusion to move oxygen and carbon dioxide directly in and out of their body. They must have a transport system located close to individual cells to transport the gases in and out. In comparison, animals such as flatworms are thin and have a large enough surface area that they can absorb oxygen directly from their environment. An increased surface area to volume ratio also means increased exposure to the environment. Jellyfish and anemones have many tentacles that increase surface area for getting food. More surface area lets them sift through more of the surrounding water for food. The shape of individual organs in animals is often related to SA/V ratio. Our lungs have numerous branches that lead to small thin grape-like sacs called alveoli. These increase the surface area for gas exchange in our lungs; without the millions of sacs, our lungs would not have enough surface area to absorb all the oxygen we need. By increasing the surface area of the agar blocks, the rate of diffusion can be increased as the hydrochloric acid has more space to act over. The acid is used in order to neutralise the alkaline agar blocks and create a colour change that can be observed when the phenolphthalein indicator diffuses out of the block. Phenolphthalein indicator is colourless in neutral and acidic mediums and a dark pink/fuchsia in alkaline mediums. Therefore the decolourisation of the agar blocks would determine whether all the indicator has reacted with the excess acid. The indicator diffuses out quickly, reacting with the acid to become neutral and hence colourless. In this experiment the time taken for the decolourisation of the phenolphthalein indicator to occur is the dependent variable, while the length of the sides of the cube is the independent (0.2cm, 0.4cm, 0.6cm, 0.8cm, 1.0cm). Various variables are controlled to ensure that the only factor affecting the time of decolourisation is the size of the cube.

Divya Aishwarya Gandi

Hypothesis: I think that the higher the surface area to volume ratio, the greater the rate of diffusion will be and the less time it will take for the agar blocks to decolourise. This is because there is a shorter length for the base to travel before it reacts with the acid and changes colour. Variables: Independent

Dependent

Controlled

Variable Length of the sides of the cube

Value(s) 0.2 cm, 0.4 cm, 0.6 cm, 0.8 cm, 1.0 cm

Time taken for decolourisation to occur Water (for preparation of agar)

In minutes

Phenolphthalein concentration

Phenolphthalein volume Mass of agar used to prepare blocks Volume of HCl

25cm3

Concentration of HCl

1M

No swirling of beaker

Kept constant for every trial

Justification Will be the variable that is changed on purpose to observe the effect of it on the time taken to decolourise agar blocks of different surface area to volume ratios. Will be measured to see the difference in time for each surface area to volume ratio. To ensure that the consistency/composition of all the blocks is the same. To ensure that all the blocks contain the same concentration of indicator therefore determining the shade of the colour of the agar blocks. To ensure that all the blocks contain the same amount of phenolphthalein indicator. Once again will affect the composition of the agar blocks. Will affect the time taken for decolourisation to occur and therefore must be kept constant in each beaker. Once again, must be kept constant in order to ensure that the only factor affecting the reaction time is the surface area to volume ratio of the blocks. This will affect reaction time so all beakers must not be swirled so that there are no other factors speeding up the reaction.

Divya Aishwarya Gandi

Materials and Apparatus:           

Agar Water Phenolphthalein indicator. Hydrochloric acid Beaker (50cm3) x5 Measuring cylinder (25cm3) Metric ruler (15cm) Scalper Tile Forceps Stopwatch

Method: 1. Prepare agar block using set amount of water, agar and phenolphthalein indicator. 2. Cut 5 agar cubes of lengths 0.2 cm, 0.4 cm, 0.6 cm, 0.8 cm and 1.0 cm carefully using a scalpel and a ruler for measurement. 3. Measure 25cm3 of hydrochloric acid using a measuring cylinder and empty it gently into each of the 5 beakers. 4. Using the forceps place the 0.2cm cube into the first beaker and immediately start the stopwatch. 5. Stop it as soon as the entire block decolourises and record the time. 6. Repeat steps 4 and 5 for each of the other blocks. 7. Repeat steps 2-6 five more times to get an accurate average time for decolourisation for each length.

Safety Precautions:      

Use latex gloves while handling the acid and the agar blocks as they are both corrosive and may harm the skin if in contact. Be careful while using scalpel in order to avoid injuries. Keep hair tied back. Avoid parallax error by taking readings parallel to the ruler. Make sure the stopwatch and ruler have no zero errors. Use a funnel while measuring acid using measuring cylinder to avoid any spilling.

Raw Data Table: Size of cube/ cm ±0.05 0.2

Surface area/cm2 ±0.05 0.24

Volume/cm3 ±0.05 0.008

Surface area to volume ratio 30:1

Divya Aishwarya Gandi

0.4 0.6 0.8 1.0

0.96 2.16 3.84 6.00

0.064 0.216 0.512 1.000

15:1 10:1 7.5:1 6:1

Time taken for decolourisation of phenolphthalein indicator when placed in 25cm 3 of HCl: Size of cube/ cm ±0.05

Trial 1 Time taken for decolourisation / minutes ±0.005

Trial 2 Time taken for decolourisation / minutes ±0.005

Trial 3 Time taken for decolourisation / minutes ±0.005

Trial 4 Time taken for decolourisation / minutes ±0.005

0.2

0.149

0.147

0.229

0.102

Trial 5 Time taken for decolourisatio n/ minutes ±0.005 0.146

0.4

0.835

0.679

0.738

0.688

0.718

0.6

0.511

1.133

1.516

1.400

1.200

0.8

1.167

1.233

1.983

1.717

1.050

1.0

2.033

1.967

1.917

1.890

1.789

Average time taken for decolourisation of phenolphthalein indicator for each size of cube: Size of cube/ cm ±0.05

Average time taken for decolourisation/minutes ±0.005

Average calculated by adding all trial values and dividing it by 5.

0.2 0.4 0.6 0.8 1.0

0.155 0.732 1.152 1.430 1.919

Ex. 1: (0.149+0.147+0.229+0.102 +0.146)/5=0.155

Observations: Colour change is observed immediately after the agar blocks are dropped into the HCl, the pink starting to turn colourless starting from the outer corners of the cub, towards the centre.

Divya Aishwarya Gandi

Standard deviation for 0.2cm: 0.046 Standard deviation for 0.4cm: 0.062 Standard deviation for 0.6cm: 0.389 Standard deviation for 0.8cm: 0.400 Standard deviation for 1.0cm: 0.091

Analysis of graph: As can be seen from the results and the graph the general trend shows that as the surface area to volume ratio decreases and the length of the sides of the cube increases, the time taken for the decolourisation of the phenolphthalein indicator also increases. In each case, for every length, the time taken for the decolourisation does not vary much as can be seen by the values for standard deviation. However there is one value which is a clear error in both the 0.6 and

Divya Aishwarya Gandi

0.8 trials which increases the value of standard deviation and causes the line graph to not be a completely straight line. The 0.511 minutes reading for the 0.6cm cube was clearly an error as the value was much lower than the consecutive trials. Similarly the 1.050 minutes reading for the 0.8cm cube was also an error; this can be put down to human error or due to the uneven thickness of each cube, which caused it to take less time to decolourise as compared to the other trials. Conclusion: From the experiment and data collected it can be concluded that there is a definite relationship between surface area to volume ratio and diffusion. Supported by the data it can be said that the lower the surface area to volume ratio the slower the rate of diffusion and hence the more time it takes for the decolourisation of phenolphthalein indicator in the agar cubes. From the data we see that the smaller cubes have a larger surface area to volume ratio and hence increase the rate of diffusion is greater. This can be explained through the reasoning that a larger surface area to volume ratio means that there is more surface for the substance, the basic phenolphthalein indicator in this case to pass through, hence more molecules can do it at once as there is less volume for them to diffuse through to reach the acid. This in turn speeds up the rate of diffusion. This proves my hypothesis to be right as the larger the surface area to volume ratio the greater the rate of diffusion and therefore less time is taken for the decolourisation. Evaluation: Strengths The experiment was conducted in fair conditions and the controlled variables were kept constant throughout so I feel as though the experiment yielded quite accurate results. The number of trials done was also a good amount as it allowed me to spot the errors easily and allowed for more accurate results. The 3 decimal point reading for the minutes also allowed for more precise readings and therefore a more precise average. Limitations The main limitation of this experiment however was the accuracy of the size of the cubes. As they were extremely small, it was difficult for each cube to have the exact thickness and this interfered with the accuracy of the results as can be seen by the slightly higher standard deviations. Although there were a good number of trials, more could be conducted to observe the relationship better and have a larger range of data. Improvements The next time this experiment is conducted, moulds or cutters can be used to make sure the agar cubes are uniform in size and thickness hence ensuring that there is no issue with the rate of diffusion. More trials could be conducted using more lengths of the cube to investigate the effect of surface area to volume ratio further. This would also increase the range of the data collected allowing for a deeper understanding.