ENZYMES
Acid or alkaline conditions alter the chemical properties of enzymes. Most enzymes work best at a particular pH level, some enzymes cannot work at all. The pH at which an enzymes works best is called its optimum pH.
HYPOTHESIS
I think that acid rain will decrease the rate of germination, or even stop germination altogether as the acid gets stronger.
Rate of
germination
Strength of acid
This is because the acid will denature the enzymes which help break down the food reserves for the seed. Without these food reserves, the plant will not be able to grow or even germinate. The acid will denature the enzymes because enzymes work best at a certain pH and the acid will be a higher pH level than that.
METHOD
To see what affect acid rain has on germination, I am going to try and germinate cress seeds with different concentrations of acid.
INDEPENDENT VARIABLE
I will mix acid with water to from the different concentrations. I will do this using a syringe. This will accurately measure the volume of the acid and water and will easily put the acid and water in the Petri dish.
I am going to have 11 petri dishes with cress seeds in and have a different concentration of acid in each dish. The range of values I am going to use are:
Using this wide range of values will give me accurate results because I will be able to take an average and see any anomalies.
DEPENDENT VARIABLE
I will measure the total length of any roots, shoots or leaves that have germinated from the seed. I am going to measure this because if the length of the plant decreased as the concentration of acid rain decreases it will help prove my hypothesis.
I am going to measure the total length of everything because, from doing my preliminary experiment I discovered that some seedlings did not get as far as growing shoots or leaves. If I, for example, decided to measure the shoot then this would be unfair to the seeds that had germinated but were still only growing roots.
I am also going to measure the number of seeds that germinated in each concentration of acid. This will help me prove my hypothesis if the number of seeds germinated decreases as the strength of the acid increases.
I am going to measure the lengths of the plants precisely by using a magnifying glass and a length of card with millimetres marked on as it is easier to read the length from there than a ruler.
In doing my preliminary experiment, I noticed that not all of the plants grew in a straight line. I am going to try and pull them as straight as possible to get the most accurate lengths of the plants, giving me more accurate readings overall.
CONTROL VARIABLES
The factors I am going to try and keep constant are oxygen, temperature and the total amount of acid and water. This is because from doing my background research I found out that they all affect germination. Not keeping them constant would mean an unfair test and inaccurate results.
The amounts of acid and water I will use will always add up to 20cm3. In my preliminary experiment, I decided on 20cm3 because it was enough for all the seeds but not too much.
To make my results as accurate as possible, I am going to use 10 cress seeds in each concentration of acid and then take an average. Using 10 cress seeds will also show a clearer pattern because I will be able to see any anomalies.
I used 10 cress seeds in my preliminary experiment and there was enough space for them all to grow and enough seeds to get reasonable results.
APPARATUS
- Petri dishes (11)
- Syringe
- Beakers (2)
- Cotton wool
- Cress seeds
- Water
- Acid
In doing my preliminary experiment, I decided to put the seeds on cotton wool instead of filter paper. Although the cotton wool made it harder for me to pull out the seeds to measure, it soaked up the water and acid which was vital for my experiment. The filter didn’t soak up enough water or acid.
SAFETY
I am going to wear goggles to protect my eyes from any acid, tuck in my tie, tie back long hair and generally be careful not to spill the acid.
RESULTS
TABLE TO SHOW THE LENGTHS OF THE SEEDLINGS IN DIFFERENT CONCENTRATIONS OF ACID
TABLE TO SHOW HOW MANY SEEDS GERMINATED IN THE DIFFERENT CONCENTRATIONS OF ACID
CONCLUSION
It is clear from my first graph that acid rain does affect germination. The graph has negative correlation and shows that as the percentage of acid increased, the length of the seedling decreased. The graph slopes steeply downwards at the beginning from 103.5 mm to 13.6 mm. This shows that the acid affects the growth a lot because ‘100% acid, 0% water’ and ‘90% acid, 10% water’ had averages of 98.6 mm and 103.5 mm while all the other seedlings averages were under 15 mm. With anything more than 10% acid the seedlings not grow very well.
This is because the enzymes in the seed were being denatured by the acid. Seeds have food reserves (in the forms of starch, proteins, lipids, polysaccharides). The enzymes break down the food reserves the seed needs for growth.
Proteases
Proteins Amino acids
Carbohydrases
Polysaccharides Sugars
Amylase Maltase
Starch Maltose Glucose
Lipases
Lipids Fatty Acids + Glycerol
Without all these food reserves, the seed is unable to grow to its potential.
Acid or alkaline conditions alter the chemical properties of enzymes. Most enzymes work best at a particular pH level. Without this particular pH level, some enzymes cannot work at all.
As I increased the percentage of acid, it started denaturing the enzymes because they were not at the optimum pH and couldn’t function properly. The enzymes being denatured meant that they couldn’t break down the food reserves the plant needed for growth, meaning the plant did not grow.
The rate of growth was not affected by water because water is needed for germination. As the seed becomes hydrated, this activates the enzymes needed for breaking down the food.
My second graph has no real pattern to it. It shows that all 10 seeds germinated in 100% water but only 8 germinated in 100% acid. This could be that because the enzymes are being denatured by the acid and unable to break down the food reserves needed for growth, that the seeds are growing/germinating slowly.
Most of the results I obtained support my original prediction. The length of the seedling did decrease as the percentage of acid increased. However, I did expect less than 8 seeds to germinate in 100% acid. This is because water activates the enzymes for breaking down food reserves enabling the plant to grow. I thought that 100% acid wouldn’t activate these enzymes but it did (before denaturing them).
EVALUATION
The only problem I had whilst doing the practical work was taking the seedlings from the cotton wool to measure them. They kept getting caught in the cotton wool and I had to be very careful not to break them.
Filter paper enables you to easily pick up the seedlings to measure, but it doesn’t absorb much water so I chose cotton wool. If I had to repeat the experiment, I would place cotton wool at the bottom of the petri dish to hold the water and acid, and then place filter paper on top so I could measure the seedlings easily.
I think that my data is reliable because I used 10 seeds and then took the average. I also used 11 different concentrations if acid and water because this would hopefully make a clearer pattern then using fewer concentrations and help me see any anomalies.
I could improve the reliability of my data by using more than 10 seeds (20+) and then taking the average of those. However, I do think that 10 seeds are enough to get sufficient results. I could also use more concentrations of acid, going up every 5% instead of every 10%.
I have no real anomalous results.
To provide additional evidence to support my conclusion I could do more investigations about acid rain affecting germination. For example, I could do exactly the same experiment but use a variety of different seeds apart from cress seeds. This would help me to find out if my conclusion applies to all seeds.
I could extend my investigation into related fields by maybe doing on experiments on already germinated plants to see if acid rain affects the growth of a plant after germination. I would germinate some seeds in 100% water and then add different concentrations of acid to see if they had any affect on the growth of the plant.
