Practical: Factors Affecting Starch Production (SQA National 5 Biology): Revision Note

Investigating the factors affecting starch production

• Plants make glucose during photosynthesis, but it is quickly converted into starch for storage

• Testing a leaf for starch therefore shows where photosynthesis has taken place; this process can be used to demonstrate the effect of different factors on photosynthesis, including:

  • chlorophyll

  • light

  • carbon dioxide

Investigating the need for chlorophyll

• Chlorophyll in chloroplasts absorbs light energy for photosynthesis

• Testing for starch in a variegated leaf can demonstrate that chlorophyll is needed for photosynthesis

Equipment

• A plant with variegated leaves

• Beaker

• Tripod and gauze

• Bunsen burner

• Forceps

• Boiling tube

• Ethanol

• White tile

• Iodine solution

Method

1. Boil leaf in a beaker of water, e.g. over a Bunsen burner

2. Remove leaf from hot water using forceps and transfer to a boiling tube containing ethanol; heat leaf in the ethanol until the green colour is lost

   • This can be achieved by placing the boiling tube of ethanol inside the beaker of previously boiled water

   • Ensure that any nearby Bunsen burners have been extinguished before this step, as ethanol is highly flammable

3. Use the forceps to remove the leaf from the ethanol and dip it back into the hot water

   • This removes excess ethanol and further softens the leaf

4. Place leaf on a white tile and test with iodine solution

5. A colour change to blue-black indicates that starch is present

Expected results

• Only the green areas of the variegated leaf should turn blue-black, showing that photosynthesis (and starch formation) occurs only where chlorophyll is present

• The white areas, which contain no chlorophyll, should stay orange-brown, indicating a lack of photosynthesis in these regions

Limitations

Investigating the need for light

• Light provides the energy needed for photosynthesis

• Testing for starch in leaves that have been kept in the light and in the dark can demonstrate that light is needed for photosynthesis

Equipment

• Potted plant with healthy green leaves

• Access to a dark room or cupboard

• Aluminium foil / black paper

• Paper clips / tape

• Beaker

• Tripod and gauze

• Bunsen burner

• Forceps

• Boiling tube

• Ethanol

• White tile

• Iodine

Method

1. Destarch the plant by placing it in darkness for 24 hours

   • This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment

2. Remove the plant from the dark location and cover one leaf with aluminium foil, keeping other leaves uncovered

3. Place the plant in bright light for several hours

4. Remove the covered leaf from the plant and test it for starch as follows:

   • boil leaf in a beaker of water

   • transfer leaf to a boiling tube containing ethanol

   • heat leaf in the ethanol until the green colour is lost

     • Ensure that any nearby Bunsen burners have been extinguished

   • remove the leaf from the ethanol and rinse the ethanol from the leaf

   • place leaf on a white tile and test with iodine solution

   • a colour change to blue-black indicates that starch is present

5. Repeat step 4 for an uncovered leaf

Expected results

• Only the leaves exposed to light should turn blue-black, showing that light is needed for photosynthesis

• The covered leaves will stay orange-brown, showing the absence of starch, and therefore no photosynthesis

Limitations

Investigating the need for carbon dioxide

• Carbon dioxide is fixed during photosynthesis

• Testing for starch in leaves that have been kept in the presence and absence of carbon dioxide can demonstrate that this it is needed for photosynthesis

Equipment

• Two healthy, potted plants

• Access to a dark room or cupboard

• Two bell jars

• Beaker of sodium hydroxide solution (NaOH)

• Beaker of water

• Petroleum jelly

• Beaker

• Tripod and gauze

• Bunsen burner

• Forceps

• Boiling tube

• Ethanol

• White tile

• Iodine

Method

1. Destarch both plants by keeping them in darkness for 24 hours

   • This ensures that any starch already present in the leaves will be used up and will not affect the results of the experiment

2. Remove the plants from the dark location and place one plant under a bell jar with a beaker of NaOH solution

   • NaOH absorbs carbon dioxide from the air under the bell jar

3. Place the other plant under a bell jar with a beaker of water

   • The beaker of water acts as a negative control, demonstrating that it is the lack of carbon dioxide that causes the change in photosynthesis and not any other factor

4. Seal each jar with petroleum jelly to prevent air entry

5. Leave both plants in bright light for several hours

6. Test one leaf from each plant for starch as follows:

   • boil leaf in a beaker of water

   • transfer leaf to a boiling tube containing ethanol

   • heat leaf in the ethanol until the green colour is lost

     • Ensure that any nearby Bunsen burners have been extinguished

   • remove the leaf from the ethanol and rinse the ethanol from the leaf

   • place leaf on a white tile and test with iodine solution

   • a colour change to blue-black indicates that starch is present

Expected results

• We would expect the leaf from the plant in the jar with NaOH to remain orange-brown, showing that no starch forms in the absence of CO₂

• The control plant leaf will turn blue-black, showing that photosynthesis will occur in the presence of CO₂

Limitations