Photosynthesis and Plants



  • In photosynthesis plants make food(sugars and starch) using light energy.
  • The light is trapped by chorophyll in choroplasts in plant leaves.

The word equation for photosynthesis is:

   Carbon dioxide + Water  ---> Glucose + Oxygen 

The balenced chemical equation is:

See the source image

As photosynthesis requires light energy to work,it is an endothermic reaction

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Photosynthesis Experiments

  • The glucose produced during photosynthesis is usually converted into starch for storage. 
  • One way of showing photosythesis has took place in a leave is through a starch test
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Destarching a plant

  • Leave the plant in a dark place for 48hrs.
  • This is to ensure any starch present was produced during the experiment 
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Testing for starch

  • Remove leave from a plant 
  • Boil in water to kill
  • Turn off bunsen burner 
  • Remove leaf put into a boiling tube of ethanol. place boiling tube in hot water for a few minutes 
  • Dip in warm water (this is to soften it and make it less brittle)
  • Spread leave on a white tile and add iodine. 
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Is light needed for photosynthesis?

  • Destarch the plant.
  • Partially cover a leaf on a plant with foil.
  • Put the plant in a bright light for at least 6 hours.
  • Test the leaf for starch.

The results show that starch is only produced in the parts of the leaf that recieved light - showing that light is necessary for photosynthesis.

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Is chlorophyll needed for photosynthesis?

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

  • Destarch a vaariegated plant.
  • Put the plant in bright light for at least 6hrs.
  • Test leave for starch.

The starch test will show that starch is only produced in those parts of the leaf that had chlorophyll (were green).

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Is carbon dioxide needed for photosynthesis?

  • Destarch a plant.
  • Enclose the plant with a plactic with a sodium hydroxide solution - the sosium hydroixide absorbs the carbon dioxe in the air inside the plastic bag.
  • leave plant in bright light for at least 6hrs.
  • test one of the leaves for starch 

A negitive starch test will show that carbon dioxide is need for photosynthesis.

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Is oxygen produced during photosynthesis?

The rate of photosynthesis affects the rate at which the Oxygen bubbles are given off.

By moving the position of the lamp it is possible to investigate the effect of light intensisty of photosynthesis.

    See the source image

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Limiting factors in photosynthesis

  • The environment factors light, carbon dioxide and temperature all effect the rate of photosynthesis.
  • A limiting factor is a factor that limits the rate of photosynthesis due to the factor being present at a sub-optimal level.
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Relationship between photosynthesis and respiratio

  • Hydrogencarbonate (bicarbonate) indicator is red in a normal atmospheric (0.04%) carbon dioxide levels.
  • It turns yellow in increased carbondioxide levels
  • It turns purple in decreased carbon dioxide levels.
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Leaf structure

Leaves are plant organs in which photosynthesis takes place.

See the source image

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Leaf structure

  • Cuticle - waxy, transparent waterproof layer 
  • Epidermis - no chloroplasts 
  • Gaurdcells and stoma - allows gases to diffuse in and out of the leaf
  • The palisade mesopylltightly packed, many chloroplasts, end on to the upper suface.
  • Intercellular Spcae - allows gas trough the leaf. 
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Glossary of terms

Photosynthesis - a process in plants in which light energy is traped by chlorophyll to produce food.

Endothermic reactions - reactions that require energy to be absorbed (taken in) to work.

Starch test - a test to show whether or not starch is present in a plant leaf. 

Limiting factors - and environmental factor that limits the rate of photosynthesis due to thta factor being presnt in too small amount. 

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