B1 - Investigate the need for light and chlorophyll

• Photosynthesis


  In photosynthesis plants make food using light energy. The light is trapped by chlorophyll in chloroplasts in plant leaves
• Word equation of photosynthesis

  Carbon dioxide + water -> glucose + oxygen
• Balanced Chemical Equation for Photosynthesis

  6CO2 + 6H2O -> C6H12O6 + 6O2
• Photosynthesis is an endothermic reaction 


  This is because photosynthesis requires energy to work. An endothermic reaction require energy to be taken in
• Photosynthesis experiments 


  The glucose produced during photosynthesis is usually converted into starch for storage. One way of showing that photosynthesis has taken place is by showing that starch is present in a leaf. This can be done using a starch test
• Starch test 


  A test to show whether or not starch is present in a plant leaf
• Carrying out the test for starch

  1. Put the leaf in boiling water - This kills the leaf and stops further reactions
  2. Boil the leaf in ethanol. This must be done in water bath with the very hot/boiling water poured from a kettle as alcohol is flammable - This removes chlorophyll from the leaf
  3. Dip the leaf in boiling water again - This makes the leaf soft and less brittle
  4. Spread the leaf on a white tile and add iodine - If starch is present, the iodine will turn from yellow-brown to blue-black
• Destarching a plant 


  Before carry out investigations into photosynthesis, it is usually necessary to destarch the plant. This involves leaving the plant in darkness for 48 hours. This is necessary to make sure that any starch produced is only produced the investigation
• Showing that light is needed for photosynthesis

  • Destarch a plant
  • Partially cover a leaf of a plant with foil
  • Put the plant in bright light for at least 6 hours
  • Test the leaf for starch (see image)
  • Results show that starch is only produced in the parts of the leaf that received light

  
• Showing that chlorophyll is needed for photosynthesis 


  Some plants have leaves that are part green and part white. These leaves are described as variegated leaves.

  • Destarch a variegated plant
  • Put the plant in bright light for at least 6 hours
  • Test the leaf for starch

  The starch test will show that starch is only produced in those parts of the leaf that had chlorophyll
• Limiting Factors in photosynthesis

  A limiting factor is an environmental factor that limits the rate of photosynthesis due to that factor not being present in too small an amount

  • The environmental factors light, carbon dioxide and temperature all affect the rate of photosynthesis.
  • If all three are present in sufficient quantities, the rate will be at its optimum
  • If any of these factors are at sub-optimal levels, the rate will be reduced
• Graph 


  Limiting factors graph See image

  
• Leaf structure 


  Leaves are plant organs in which photosynthesis occurs. The image shows a cross-section of a mesophytic leaf

  
• Leaf adaptations for light absorption

  • Large surface area
  • Thin, transparent cuticle
  • Presence of many tightly packed palisade mesophyll cells, end on to the upper surface, with many chloroplasts rich in chlorophyll
• Leaf adaptations for gas exchange

  • Spongy mesophyll cells have a large surface area for gas exchange
  • Intercellular spaces in the spongy mesophyll allow carbon dioxide to enter and oxygen to leave the photosynthesising cells which are mainly concentrated in the palisade layer
  • Stomata that allow carbon dioxide and oxygen to enter the leaf; the guard cells can open and close the stomatal pore - in many plants, stomata are open during the day and closed at night
• Compensation Point

  • Labelled Q
  • where the line crosses the x axis
  • may be more than 1 compensation point
• Chlorophyll 


  Absorbs sunlight energy
• Chlorophyll is removed 


  To show a distinct colour change when iodine is added
• Investigating need for chlorophyll

  • Destarch the variegated plant
  • leave in bright light for 6 hours
  • then carry out starch tests
• Need for carbon dioxide

  • Destarch a plant
  • place a clear plastic bag over the plant and add sodium hydroxide
  • the sodium hydroxide will absorb the carbon dioxide
  • test the leaf for starch
  • negative starch test shows carbon dioxide is needed
• Measuring rate of photosynthesis

  By measuring bubbles of oxygen by using an inverted funnel
• Relationship between photosynthesis and respiration 


  Red hydrogencarbonate is used
• Photosynthesis and respiration colour changes

  Changes from red to yellow when in increase of carbon dioxide (respiration)

  Changes from red to purple in decreased carbon dioxide levels (photosynthesis)

  • when yellow - only respiration is taking place, causing in increase in CO2 turning yellow
  • when purple - more photosynthesis causing decreases in CO2 causing it to turn purple
  • remains red - rate of photosynthesis = rate of respiration
• Compensation point 


  The point where the rate of photosynthesis equals the rate of respiration