Experiments

• Starch test

  A test to show whether or not starch is present in a plant leaf
• Starch test procedure

  1. Put the leaf in boiling water

  2. Boil the leaf in ethanol (alcohol)

  3. Dip the leaf in boiling water again

  4. Spread the leaf on a white tile and add iodine
• Starch test procedure

  • Kills the leaf and stops further reactions
  • Removes chlorophyll (green colour) from the leaf
  • Makes the leaf soft and less brittle
• Positive starch test

  If starch is present, the iodine will turn from yellow-brown to blue-black
• Before carrying out investigations into photosynthesis, it is usually necessary to destarch the plant
• If plants were not destarched in these investigations the investigation would not be valid, as it would be impossible to say whether any starch present was produced during the investigation or was there before the investigation started
• Showing that light is needed for photosynthesis

  1. Destarch a plant

  2. Partially cover a leaf on a plant with foil

  3. Put the plant in bright light for at least 6 hours

  4. Test the leaf for starch
• The results show that starch is only produced in the parts of the leaf that received light - showing that light is necessary for photosynthesis
• Showing that chlorophyll is needed for photosynthesis

  1. Destarch a variegated plant

  2. Put the plant in bright light for at least 6 hours

  3. Test the leaf for starch
• The starch test will show that starch is only produced in those parts of the leaf that had chlorophyll (were green)
• Showing that carbon dioxide is needed for photosynthesis

  1. Destarch a plant

  2. Set up with sodium hydroxide to absorb carbon dioxide

  3. Leave the plant in bright light for at least 6 hours

  4. Test one of the leaves for starch
• A negative starch test will show that carbon dioxide is necessary for photosynthesis
• Showing that oxygen is produced

  Using apparatus similar to that in Figure 2.3, it is possible to demonstrate that oxygen is produced in photosynthesis
• The rate of photosynthesis affects the rate at which the bubbles of oxygen are given off, and this can be used to compare photosynthesis rates in different conditions
• To investigate the effect of light intensity on photosynthesis


  1. Place Elodea (Canadian pond weed) in a basin of water and place a funnel and measuring cylinder on top

  2. Record the distance from the lamp to the plant

  3. Bubbles of oxygen will be released from the pondweed as it is photosynthesising

  4. Count the number of bubbles produced in 1 minute OR record the time taken to collect 10cm3 of oxygen in the measuring cylinder

  5. Move the lamp further away from the plant

  6. Repeat step 4

  7. Move the lamp further away again

  8. Compare the results
• To investigate the need for light in photosynthesis

  1. De-starch the plant by placing it in a dark cupboard for two days

  2. Cover part of one leaf with foil

  3. Leave the plant in sunlight or next to a lamp for a few hours

  4. Test the leaf for starch
• To investigate the need for chlorophyll in photosynthesis


  1. De-starch the plant by placing it in a dark cupboard over-night

  2. Test a leaf for starch

  3. Leave the plant in sunlight or next to a lamp for a few hours

  4. Test a leaf for starch
• To prove that carbon dioxide is needed for photosynthesis

  1. De starch the plant by placing it in a dark cupboard over-night

  2. Place a leaf in a clear plastic bag while it is st attached to the plant

  3. Place a amat dish of sodium hydrocide inside the bag with the leaf and w

  4. Leave the plant for a few hours

  5. Test the leaf inside the bag for starch

  6. Test any other leaf from the plant for starch and compare the results
• Photosynthesis


  The process by which plants use light energy to convert carbon dioxide and water into glucose and oxygen
• Starch


  A carbohydrate that is the main way plants store energy
• Chlorophyll

  The green pigment in plants that absorbs light energy for photosynthesis
• Carbon dioxide


  A gas that plants need for photosynthesis
• Limiting factor

  A factor that limits the rate of a process, such as photosynthesis, when it is in short supply
• Light intensity increases


  Rate of photosynthesis increases
• Photosynthesis happens at a maximum rate when all factors are optimal
• When one or more factors are in short supply, photosynthesis is said to be limited by that factor
• As a lamp moves closer to the plant, the rate of photosynthesis increases to a maximum, after which light has no further effect and the rate levels out
• Carbohydrates


  Sugars and starches that are the main source of energy for living things
• Proteins


  Large molecules made up of amino acids that are essential for the structure and function of living organisms
• Fats


  Lipids that store energy and insulate the body
• Most foods contain more than one of the food groups: carbohydrates, proteins and fats
• Showing CO2 is

  needed for photosynthesis

  
• showing light is needed for photosynthesis

  
• Showing Oxygen is produced in Photosynthesis

  
• relationship between photosynthesis and respiration

  
• Limiting Factors