4:27 PM


Investigation 5.2 What Happens during Starch Digestion

Procedure:
1 Test all the solutions for starch and reducing sugar using the iodine test and Benedict’s test respectively. Use about 2 cm3 of each test solution (S1 to S5) for starch test and reducing sugar test. By comparing the intensity of blue-black coloration and the amount of orange-red precipitate, the amount of starch and reducing sugar present can be estimated. To test for starch, add a few drops of iodine solution to 2 cm3 of each test solution. Observe for blue-black precipitate. To test for reducing sugar, add 2 cm3 of Benedict’s solution to 2 cm3 of each test solution. Mix well. Put the mixture in a boiling water bath for about 5 min. Observe for orange-red precipitate. Starch is digested to maltose, a reducing sugar, in the gut. So the lesser the amount of starch and the greater the amount of reducing sugar in a solution, the more digestion has occurred.
2 The volume/amount of all solutions and reagents used, the amount of time the mixture of Benedict’s solution and test solution are put in the boiling water bath, the heating temperature for each solution
3
Results of starch test
Sample
Observations
Conclusions
S1


S2


S3


S4


S5


4 By looking at the colour changes and amount of precipitate produced.
5
Results of reducing sugar test
Sample
Observations
Conclusions
S1


S2


S3


S4


S5


6 Arrange from the most starch and least reducing sugar to the least starch and most reducing sugar.
S4, S5, S1, S3, S2 (order in which digestion occurs)

Questions:
1a) Any of the following:
- Colour changes may be hard to distinguish or judge and varied among the observers.
- The amount of starch and reducing sugar is just an estimation based on the observation of colour or intensity of the chemical reactions.
- The exact amount of starch or reducing sugar cannot be determined by observation on the intensity of the colour or the amount of precipitate.
- The quantity of starch or reducing sugar cannot be determined accurately by the qualitative observation of colour intensity or amount of precipitate.
b) Use a colorimeter; compare with a standard.

Conclusion:
Starch is digested to maltose, a reducing sugar, in the alimentary canal. The lesser the amount of starch and the greater the amount of the reducing sugar in a solution, the more digestion has occurred.

Investigation 5.3 Absorption of Carbohydrates in the Mammalian Alimentary Canal
Results:

Samples taken at
Iodine test
0 min
5 mins
10 mins
20 mins
Observations
iodine remains brown
iodine remains brown
iodine remains brown
iodine remains brown
Deductions
starch absent
starch absent
starch absent
starch absent


Samples taken at
Benedict's test
0 min
5 mins
10 mins
20 mins
Observations
solution remains blue
yellow/green precipitate formed
yellow/green precipitate formed
orange/red precipitate formed
Deductions
glucose absent
little glucose present
little glucose present
more glucose present

Questions:
1 Any of the following:
· Reverse the liquids with distilled water inside Visking tubing and solution of glucose and starch outside (test to show that only glucose enters tubing).
· Use a permeable membrane to show that both starch and glucose are capable of diffusion.
· Use an impermeable membrane (e.g. rubber/plastic membrane) to show that no diffusion can occur.
· Test for starch in Visking tubing at the end of the experiment.
2a) Starch is present in the mammalian diet. Starch molecules are large and cannot diffuse across the gut wall. Enzymes in the mammalian body hydrolyse starch to maltose and then to glucose. Glucose molecules are small and soluble and can pass through the gut wall, hence they can be absorbed in the gut.
b) Name any two enzymes that digest carbohydrates and where you can find them (see
chapter 5 of the textbook). Examples include salivary amylase found in buccal cavity; pancreatic amylase found in duodenum; intestinal maltase found in duodenum.
3 Blood plasma/blood.

Conclusion:
Visking tubing asts as small intestine in mammalian alimentary canal. Starch has to be digested into small and soluble glucose by enzymes so that it can be absorbed.


Investigation 6.1 How Can We Test for Starch in a Leaf?
Procedure:
6 The leaf turns blue-black.

Questions:
1 Enzymes are protein in nature. At high temperatures, the weak bonds in proteins are broken, causing the proteins to be denatured. The active site configuration is lost.
2a) Colourless
b) Green
3 It removed the chlorophyll and decolourised the leaf.
4 Starch is present in the leaf.

Conclusion:
Photosynthesis has taken place since starch is present in leaf.

Investigation 6.6A Effect of Different Light Intensities on the Rate of Photosynthesis?
Procedure:
1 Rate of oxygen produced. Oxygen produced can be measured as gas bubbles from
a water plant.
2 Vary the distance of the light bulb from the plant and measure its effect.
8 Less bubbles are produced as the distance between lamp and plant increases.
10 The rate of bubbling increases as the distance between the lamp and the plant
decreases.

Questions:
1 Photosynthesis
2 Light intensity increases as distance of the lamp from the plant decreases.
3 Rate of photosynthesis increases as the light intensity increases.
4 To provide carbon dioxide to the plant.
5 The size of the bubbles may vary. The rate at which oxygen is evolved cannot be
accurately measured, thus this would not be an accurate measurement of the rate of
photosynthesis.
6 Cut end of plant facing upwards in test tube; average number of bubbles taken from 2
readings each.


Conclusion:
Rate of photosynthesis is affected by light intensity. The higher the light intensity, the higher the rate of photosynthesis and vice versa.