Photosynthesis

Photosynthesis

Photosynthesis is the manufacture of carbohydrates by a green plant using carbon dioxide, water and light energy in the presences of chlorophyll.

According to the definition above:

  • the raw materials of photosynthesis are carbon dioxide and water.
  • light is the source of the energy required for photosynthesis to take place.
  • chlorophyll is required as a catalyst for the reaction.
  • carbohydrates are the main products of photosynthesis.

Photosynthesis equation

Oxygen is also produced by photosynthesis though it isn’t mentioned in the definition. Oxygen is a by-product of photosynthesis, which means it is not require by the plant. It is therefore released into the atmosphere.

Green plants are called producers because they manufacture food (carbohydrates) from inorganic raw materials (carbon dioxide and water). Only green plants can make food because they contain the green pigment, chlorophyll, which absorbs light and facilitates the reaction of photosynthesis.

Importance of photosynthesis

  • Plants manufacture food for the whole ecosystem through the process of photosynthesis.
  • Photosynthesis removes carbon dioxide from the atmosphere and replaces it with oxygen which is required for respiration by humans and animals.
  • Through photosynthesis, plants convert solar into chemical energy which is stored in plant tissue. That chemical energy is released as heat as we burn wood for fuel.

Experiment – Testing a leaf for starch

Most green plants manufacture glucose during photosynthesis and convert the glucose to an insoluble complex polymer called starch. To test whether a plant has been photosynthesising or not, we test it for starch. Here is an experiment to test a leaf for starch:

Materials:

  • iodine solution
  • dropper
  • alcohol
  • test tube
  • beaker
  • burner
  • tripod
  • gauze
  • white tile
  • green leaf which has been photosynthesising

Method:

  1. Boil the leaf in water to kill it and to soften it.
  2. Boil the leaf in alcohol to remove chlorophyll. For this stage the alcohol is boiled using a water bath because alcohol is highly inflammable. Heating it with naked flames would cause it to catch fire.

    Water bath

  3. Return the leaf to hot water to soften it again and also to remove traces of the alcohol.
  4. Place the leaf on a white tile and cover it with iodine solution.

Observation:

If starch is present is present the iodine solution turns blue-black. If it is absent, the iodine remains brown.

Conditions necessary for photosynthesis

As we stated earlier on, plants require carbon dioxide, water, chlorophyll and light for photosynthesis to occur. We are going to discuss a series of experiments to prove that.

Experiment – Is carbon dioxide necessary for photosynthesis?

Materials:

  • Two potted green plants
  • Soda lime or potassium hydroxide solution
  • Two polythene bags
  • Apparatus and reagents for starch test

Method:

  1. De-starch the two plants by placing them in the dark for 24 to 48 hours. It is necessary for the accuracy of the experiment to remove any starch in the leaves before we start the experiment.
  2. Test one leaf from each plant to make sure there is no starch at the beginning of the experiment.
  3. Cover each plant with a polythene bag after placing a container of soda lime (or potassium hydroxide) in one of the plant’s pots. Both soda lime and potassium hydroxide absorb carbon dioxide. This means that one of the plants will have a lack of carbon dioxide. We are going to call this plant A.
  4. The plant that doesn’t have soda lime is the control as it will have all the conditions necessary for photosynthesis. Lets call this plant B.
    Starch conditions test
  5. Leave the plants in sunlight for up to six hours.
  6. Remove a leaf from each plant and test the two leaves for starch.
  7. Record your observations and explain the results.

Observation

Leaf from plant ALeaf from plant B
Iodine remains brownIodine turns blue black

Conclusion

The results of the starch test indicate that photosynthesis occur in plant B but not in plant A. Since plant A lacked carbon dioxide we can therefore conclude that carbon dioxide is necessary for photosynthesis.

Experiment – Is light necessary for photosynthesis?

Materials:

  • A potted green plants
  • Aluminium foil
  • Scissors
  • Paper clips or tape
  • Apparatus and reagents for starch test

Method:

  1. De-starch the plant.
  2. Cut out a shape in the foil to partly cover one leaf and make sure that the cover is firm enough not to allow light to pass through.
  3. Place the plant in light for up to six hours.
  4. Remove the partly covered leaf and test the leaf for photosynthesis.
  5. Record your observations and explain the results.

Observation

Covered partExposed part
Iodine remains brownIodine turns blue black

Conclusion

The results of the starch test indicate that photosynthesis occur on the exposed part but not on the covered part. The covered part lacked light we can therefore conclude that light is necessary for photosynthesis.

Experiment – Is chlorophyll necessary for photosynthesis?

Materials:

  • A potted plant with variegated leaves (leaves that are partly green)
    Varigated leaf
  • Apparatus and reagents for starch test

Method:

  1. De-starch the plant.
  2. Place it in sunlight for up to 6 hours.
  3. Remove one leaf and test it for starch.
  4. Record your observations and explain the results.

Observation

The white partThe green part
Iodine remains brownIodine turns blue black

Conclusion

The results of the starch test indicate that photosynthesis occur in the green part but not in the white part. Since the white part lacked chlorophyll we can therefore conclude that chlorophyll is necessary for photosynthesis.

Fate of the products of photosynthesis

  • Oxygen is a byproduct of photosynthesis. It diffuses into the atmosphere where it is used for respiration and combustion.
  • Carbohydrates are the main products of photosynthesis. Carbohydrates are transported as sugars to different parts of the plant because sugars are soluble. This process is called translocation.
  • Some of the carbohydrates produced are used as a source of energy by the plant especially at night when there is no sunlight.
  • Not all the carbohydrates are used immediately by the plant. Some are stored in specialised storage organs such fruits, roots and tubers.
  • Some of the carbohydrates are converted into cellulose which gives the plant structural integrity.