Biology AS Planning Exercise Jan 08.

As the temperature increases so does the kinetic energy. At this point the substrate and enzyme molecules vibrate energetically and there is more chance of the enzyme and substrate colliding therefore increasing the rate of reaction and moreover the rate of ripening. Reference 1

When the temperature increases even more the enzyme molecules vibrate so energetically that their tertiary structure (H bonds, Van der Waals, disulphide bridges) breaks. This results in the shape of the active site on the enzymes becoming altered/distorted. The substrate can’t fit into the active site anymore and the enzyme is said to be denatured. This is generally irreversible so the reaction stops and the rate of ripening will decline #

rapidly up to a point where the enzyme is completely denatured and the banana stops ripening. Reference 2

Factors Kept Constant

1. The mass of banana will be kept constant as if the mass varied so would the time in which it takes for it to ripen. Each banana will be 25g.

2. The time the banana is heated in water will be kept constant. Each banana slice will be heated in water for 5 days.

Preliminary

I conducted a preliminary experiment to determine which method to use (regarding the measuring of ripeness).

Method:

1. Cut unripe banana into slices
2. Smear slice of ripe banana onto a slide
3. Place a drop of iodine solution on the slide
4. Cover the slide with a cover slip
5. Place the slide under a microscope at low power. Adjust by increasing the power and focusing until banana cells are visible
6. Count the number of starch grains in 20 cells
7. Repeat this procedure using an unripe banana slice

This method (counting starch grains) proved to be very unreliable. It is extremely easy to lose count of the number of starch grains as they are all lying on top of each other which may mean that the some of the starch grains may be counted more than once or missed. Therefore I will be reacting banana filtrate with excess Benedict’s.

Apparatus

Banana

Scalpel

Balance

Thermostatically controlled water bath

Blender

Filter paper

Filter funnel

10cm3 Excess Benedict’s solution

Beaker

Foil Cover

Sticky labels

Colorimeter

Measuring Cylinder

Method

1. Using a scalpel cut a banana into 9 50g slices. Weigh to ensure that they are the correct mass.
2. Place a banana slice in a beaker. Put the boiling tube into the thermostatically controlled water bath at 25oc. Leave for 5 days.

3. Remove the beaker from the thermostatically controlled water bath.

4. Extract the banana from the boiling tube and place in a blender for 30 seconds. Add water.

5. Filter the banana solution.
6. Place 40cm³ filtrate in a boiling tube with excess 10cm³ Benedict’s solution. Boil for 10 minutes.
7. Filter the extra unreacted Benedict’s.
8. Examine the colour of the contents of the Benedict’s filtrate. Use a colorimeter to find the exact colour of the solution. The percentage of light absorbed will be measured. If pale in colour there will be a lot of sugar and little Benedict’s  and the transmission value will be high.
9. Follow this procedure at temperatures of: 30oc, 35oc, 40oc, 45oc, 50oc, 55oc, 60oc, 65oc.
10.  Once you have found the temperature that produced the most sugar i.e. the lightest blue Benedict's filtrate – conduct the experiment again this time using 3 temperatures lower and 3 temperatures above  going down in 1 oc e.g. if the optimum temperature was found to be 35 oc you would use the following temperatures: 32 oc, 33 oc, 34 oc,  35 oc,36 oc, 37 oc, 38 oc

Potential Hazard

As the thermostatically controlled water bath is an electrical appliance, there is a risk of touching it with wet hands and being electrocuted. This risk could be diminished by ensuring that your hands are always dry before using this appliance.

Precision of Results

I will be using a thermostatically controlled water bath, as this will automatically ensure that the temperature in the water bath is kept constantly at the required temperature.

I will also be using a colorimeter to distinguish between the shades of blue that the Benedict’s filtrate is. A colorimeter works by measuring the amount of light absorbed by the solution. This is a superior method as opposed to using judgement because there may be individual differences in judgement of colour, particularly if the individual is colour blind.

 

After the initial set of experiments have been conducted, a further series of experiments will be carried out. These will be using intermediate temperatures around the result obtained in the initial set of experiments. This will enable me to determine the exact temperature at which the rate of ripening is at its peak.

Reliability of Results

Each individual experiment will be carried out 4 times. This will increase reliability as any anomalies will be apparent and so can be disregarded.

Table of Results

Graph To Show the Effect of Temperature on The Ripening of A Banana.

Validity of Results

A problem with this method is that the Benedict’s measures other reducing sugars such as fructose and galactose which are not involved in the ripening of a banana.

References

①  Biology 1                                                                Cambridge Advanced Sciences

② Understanding Biology For Advanced Level           Glenn and Susan Toole