DEMO

PHOTOSYNTHESIS

By the end of this subtopic learners should be able to:
a) State the word equation for photosynthesis.
b) Explain the three main conditions necessary for photosynthesis to take place.
c) Describe how the leaf is adaptable to photosynthesis.
  • Photo means light and synthesis means to make something.
  • Photosynthesis is the process by which green plants and other organisms like algae convert light energy to chemical energy (carbohydrates) in the presence of chlorophyll.

  • Green plants are producers.
  • Green plants are the major source of food for all living organisms.
  • Animals cannot make food on their own, instead they get it from plants. Therefore they are called consumers.
Word equation for photosynthesis:




  • Carbon dioxide and water are the raw materials.
    • Plants obtain carbon dioxide from the air through the leaves.
    • Leaves are important in the process of photosynthesis because they contain chlorophyll.
    • Water needed for photosynthesis comes from the ground through the plant roots.
  • Photosynthesis only takes place in the presence of light energy and chlorophyll.
    • The light energy mainly comes from the sun. It is the source of energy for photosynthesis to take place.
    • Chlorophyll is the green pigment/colour which is found in the plants. It is essential for the absorption of light energy.
    • Chlorophyll is mainly found in the plant leaves.
  • The end products of photosynthesis are carbohydrates and oxygen.
    • The main type of carbohydrates produced is glucose which is a simple sugar.
    • Glucose can be easily used and transported to other parts of the plant because it is soluble in water.
    • The carbohydrates are stored as starch granules which are insoluble in water.

EXPERIMENT 1:

AIM:

To testing for starch in a leaf.

Materials/apparatus:

Tripod stand, beaker, test tube, (alcohol) ethanol or methylated spirit, iodine, dropper, green leaf, asbestos gauze, water, burner, white tile or petri dish.

Procedure:

  • Boil the leaf in the beaker to kill it.
  • Remove the leaf and put it in a test tube with alcohol to extract all the chlorophyll. Place the test tube in the beaker with hot water.
Caution: Avoid placing the beaker with alcohol near a naked flame because it is highly flammable. Turn off the burner after using it.
  • Leave it for ten minutes for the alcohol to extract the chlorophyll.
  • Dip the leaf into the hot water to soften it.
  • Spread the leaf on a white tile or petri dish and add iodine to cover the leaf.

Observation

  • The iodine turns blue black in colour.

Conclusion

  • Starch is present in the leaf.



Experiments to show the conditions necessary for photosynthesis to take place.

  • The plants to be used in the experiments must be first de-starched.
  • This is done by placing them in a dark place for 24 hours to make sure that the plant has no starch.
  • Test the plant leaves for starch to ensure that it is completely de-starched.

Experiment 2

Aim:

To find out if carbon dioxide is necessary for photosynthesis.

Apparatus/Materials:

  • 2 conical flasks
  • 2 stands
  • Potted plant
  • Sodium bicarbonate solution
  • Sodium hydroxide solution
  • Reagents for starch test.

Procedure:

  1. De-starch a plant to remove starch by placing the plant in a dark place for 24 hours.
  2. Test the plant leaves for starch to ensure that it is completely de-starched.
  3. Set up the plant in the light and cover one leaf with a conical flask with sodium bicarbonate solution (provides carbon dioxide) and the other with sodium hydroxide solution (absorbs carbon dioxide).
  4. Leave the plant in light for a few hours.
  5. Test the two leaves from the two conical flasks for starch (see experiment 1).

Observation:

  • Leaf A showed the presence of starch.
  • Leaf B showed absence of starch.

Conclusion:

  • Carbon dioxide is needed for photosynthesis to take place.
Experiment setup.




Experiment 3

Aim:

To find out if sunlight is necessary for photosynthesis.

Apparatus/Materials:

  • Potted plant
  • Aluminium foil
  • Scissors
  • Paper clipper
  • Reagents for starch test.

Procedure:

  1. De-starch the plant to be used by placing it in a dark place for 24 hours.
  2. Test the plant leaves for starch to ensure that it is completely de-starched.
  3. Cut a shape from the aluminium foil using a scissors. Cover the leaf with the aluminium foil as shown in the diagram above.
  4. Place the plant in the sunlight for some hours.
  5. Test the leaf for the presence of starch (see experiment 1).

Observation:

  • The uncovered areas of the leaf show the presence of starch.
  • The covered area of the leaf shows the absence of starch.

Conclusion:

  • Light is needed for photosynthesis to take place.
Experiment setup.


Experiment 4

Aim:

To find if chlorophyll is necessary for photosynthesis to take place.

Procedure/Materials:

  • Variegated leaf
  • Reagents for starch testing.

Procedure:

  1. Take a fresh variegated leaf from the plant.
  2. Test the leaf for the presence of starch (see experiment 1).

Observation:

  • The green areas of the leaf show the presence of starch.
  • The yellow areas of the leaf show the absence of starch.

Conclusion:

  • Chlorophyll (the green pigment in plants) is necessary for photosynthesis to take place.
Experiment setup.









  • From the above experiments it shows that carbon dioxide, light energy and chlorophyll are necessary for photosynthesis to take place.

End products of photosynthesis

  • The end products of photosynthesis are oxygen and carbohydrates.
  • Oxygen is used for respiration by animals and plants.
  • Carbohydrates are used for growth and development of other organisms.

Carbohydrates (starch and sugars)

  • Sugars are used in the plant in the form of glucose which is soluble in water.
  • Glucose is used for plant growth, development and reproduction.
  • Carbohydrates are stored in the form of starch granules which are insoluble in water.
  • The sugars produced during photosynthesis are transported to other parts of the plant like the trunk and the roots for use and storage. The movement process of sugars in a plant is called translocation.

Experiment 5:

To find out if oxygen is a product of photosynthesis.

Materials/Apparatus:

  • Funnel, lamp, test tube, glass jar, elodea plant, wooden splint.

Procedure

  • Submerge and turn over the funnel so that the funnel is on the bottom of the tank with the test tube inverted on the upper end of the funnel.
  • Place the elodea plant under the funnel (the oxygen from the elodea plant should displace the water in the test tube).
  • Keep this set up for a few days.
  • Light the wooden splint and blow out the flame quickly to leave it glowing. You might need a second pair of hands for this step: someone to light the wooden splint and someone to hold the test tube.
  • Invert the tube so that it is upright and the trapped gas is at the top of the tube. Remove your finger and place the glowing splint into the gas of the test tube.
  • Record what you observe.

Observations

  • The glowing splint relights/rekindles.

Conclusion

  • Plants produce oxygen during photosynthesis.
Experiment setup.


Internal structure of a dicotyledonous leaf

  • A dicotyledonous plant has two cotyledons in the seed.
  • A cotyledon is a seed leaf, which is the first leaf of a plant to develop and an important part of the seed and embryo.
  • The diagram below shows the internal cross section of a dicotyledonous leaf.

 
  • Cuticle – it is a waxy waterproof layer which prevents water loss (through evaporation). It also protects the leaf from parasitic fungi.
  • Upper epidermis – it is a transparent single layer of cells that contains no or few chloroplasts.
  • Lower epidermis – this is a layer that contains the holes called the stomata which enables gases to diffuse.
  • Palisade cells – these are cells which contain chloroplasts and that is where most of the photosynthesis takes place.
  • Spongy mesophyll cells – these are cells of irregular shape which contain chlorophyll for photosynthesis.
  • Chloroplasts – these are organelles found in plants which convert light energy into sugars.

How the leaf is adapted to photosynthesis

Adaption Purpose
Large surface area To absorb more light.
Chlorophyll To convert light energy into chemical energy.
Veins Transport water and soluble sugars to other parts of the tree.
The stomata Allows the diffusion of gases.
The leaf is thin Allows light to come through and the easy diffusion of carbon dioxide.

SUMMARY

  • Green plants are the ultimate source of food for all living organisms.
  • Photosynthesis is the process whereby green plants manufacture food in the form of carbohydrates using carbon dioxide and water in the presence of chlorophyll and sunlight.
  • Word equation for photosynthesis:
Carbon dioxide + water ChlorophylSunlightcarbohydrates + oxygen
  • Carbon dioxide, light and chlorophyll are needed for the process of photosynthesis to take place.
  • The end products of photosynthesis are oxygen and carbohydrates.
  • Carbohydrates are translocated to other parts of the plant, some will be turned into starch for storage or into cellulose for plant support.
  • Parts of the internal structure of a dicotyledonous leaf include epidermis, stomata, vascular tissue and mesophyll.