Hypothesis:
Glucose is a monosaccharide, Sucrose is a disaccharide and Starch is a polysaccharide. All three respiratory substrate sugars have different chemical structure and therefore their reaction with yeast will be at different rate.
Glucose (C6H12O6) contains six carbon atoms and an aldehyde group. It is a simple cyclic molecule that is easy to break.
Glucose is a simple cyclic structure molecule; it is easy to break because it is not complex. It requires less energy and time to break down and therefore, the rate of respiration will be higher when glucose breaks down in yeast fermentation and more amount of bubbling will be visible as more energy is released in a short time.
Sucrose (C12H22O11) is a disaccharide made from glucose and fructose, linked by a glycosidic bond.
This is a disaccharide which consists of two monosaccharide linked by a glycosidic bond. It is not a simple molecular so it takes longer than glucose to breakdown and therefore it will have a lower rate of respiration than glucose.
Starch is a polysaccharide which consists of many units of glucose linked together by a glycosidic bond. It is a long chain of molecules and branched molecules.
This is a polysaccharide which has many units of glucose linked together. It is a very compact structure therefore hard to breakdown. It is difficult to break down all these glycosidic bonds and the structure is very compact. Therefore starch will have the lowest rate of respiration and will produce least amount of bubbles.
Glucose is a simple molecule, a monosaccharide so it is easy to break down and therefore it will have the highest rate of respiration. Alcohol will be produced much quicker and more bubbling will be recorded.
Equipment:
- Respiratory substrates (glucose, sucrose, starch)
- Yeast
- Distilled water
- Measuring cylinder
- 3 Test tubes
- 100 cm³ beakers
- 100 cm³ syringe
- 100 cm³ measuring cylinder
- flask
- cork stopper with tube
- rod to stir
- water bath
- weighing scale
- weighing boat
- cork stopper
- 250 cm³ flask
- gloves
- lab coat
Method:
- Get all the equipment ready.
- Weigh 5 grams of glucose and transfer it into a beaker.
- Measure 100cm³ of distilled water using a measuring cylinder and transfer into the beaker which contains glucose. Stir properly. Now there is 5% glucose suspension.
- Weigh 5 grams of yeast and transfer it into another beaker. Then measure 100cm³ of water and transfer into that beaker and stir. Now there is 5% suspension of yeast.
- Using a different measuring cylinder transfer 5cm³ of glucose solution prepared into 3 different test tubes and transfer same amount of yeast solution into 3 different test tubes and label them A-C.
- Prepare the cork stopper tube and attach a syringe to it.
- Transfer the yeast solution from test tube A into the flask then transfer the glucose solution into the flask. Place the flask in the water bath with set temperature of 37ºC for 10 minutes. Then as the bubbling starts immediately place the cork stopper onto the flask. Measure the volume of gas produced.
- Repeat the same procedure for test tubes B and C.
- Repeat the same procedure for sucrose and starch.
- Take the readings.
Fair test:
To ensure that the experiment is fair there are certain variables that must be kept constant. Temperature and pH must be constant in order to obtain accurate results. To keep the temperature constant I will use the water bath to maintain the temperature of 37ºC as this is the optimum temperature for the enzymes to work at its best. I will use the buffer solution to monitor the pH value. I will ensure that I use the same measuring cylinder and same volume of yeast and sugars to maintain the accuracy. I will repeat the experiment three times for each sugar to increase the degree of accuracy.
Resources:
Essential AS Biology for OCR, Glenn and Susan Toole, nelson thornes pages; 28, 30.
