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Process of photosynthesis | Photosynthesis experiments | Plant nutrition


Process of photosynthesis

Leaves are the major organs of photosynthesis. They have a large surface area for absorbing sunlight.

They have tiny pores (stomata), through which carbon dioxide rapidly enters the leaf for photosynthesis and the excess oxygen is excreted from the leaf.

The photosynthetic cells contain chloroplasts, the ‘little green food-making slaves’ of the plant.

Chlorophyll: the green pigment found within the chloroplasts that absorbs the light for photosynthesis.

Roots anchor the plant and absorb water for photosynthesis.

Root hairs: increase the surface area for mineral and water uptake.
Xylem: transports water and dissolved minerals up the stem to the leaves.
Phloem: transports food, such as sucrose, around the plant.

Supports the leaves, placing them in good light for a high level of photosynthesis.

Photosynthesis process

6CO2 + 6H2O     >>     C6H12O6 + 6O2

Photosynthesis is the process by which plants make glucose from carbon dioxide and water, using the light energy absorbed by chlorophyll.

Oxygen gas is also produced by photosynthesis. Some of this is used by the plant for photosynthesis and the remainder is excreted from the plant. All the oxygen in the air today was made by photosynthesis.

Photosynthesis experiments

1. To test a leaf for starch

  • Place leaf in boiling water to kill the cell and tear open the cell membranes so the iodine stain can easily enter the cells.
  • Place leaf in boiling alcohol to remove the green chlorophyll so the final results can be clearly seen
  • Dip leaf into warm water to remove the alcohol so the iodine stain can penetrate the leaf.
  • Cover leaf with iodine solution to test for the presence of starch.
  • Iodine turns starch blue-black.
  • Those parts of the leaf that stain blue-black contain starch.
  • Those parts that are yellow-brown (the colour of the iodine stain) do not have starch.

In good conditions for photosynthesis, the formation of glucose is much faster than the plant needs at that time. The excess glucose is stored as starch. Starch is a very large molecule composed of thousands of glucose molecules linked together. If a plant can be shown to produce starch, then this is taken as proof that the plant carried out photosynthesis.

2. To see if light is needed for photosynthesis

  • Place a plant in the dark for at least 24 hours to de-starch its leaves.
  • Cover part of a leaf, top and bottom, with tin foil to exclude light.
  • The uncovered part will be exposed to light.
  • Place the plant with de-starched leaves in good light for 6 hours at room temperature (20˚C).
  • Test the leaf for starch using iodine.
  • Covered part (no light) yellow-brown, no starch so no photosynthesis.
  • Exposed part (light) blue-black, starch so photosynthesis took place.
  • Therefore light is needed for photosynthesis.

3. To see if chlorophyll is needed for photosynthesis

  • Place a plant with variegated leaves in the dark for at least 24 hours to make sure that there is no starch in the leaves i.e. ‘de-starching the leaves’.
  • Variegated Leaves: have green and white areas – the green areas have chlorophyll but chlorophyll is absent in the white areas.
  • Leave the plant in the light with de-starched leaves in good light for 6 hours at room temperature (20˚C).
  • Draw a colour map of one of the leaves.
  • Then test the leaf for the presence of starch using iodine.
  • Compare the stain pattern with the colour map.
  • The green chlorophyll rich parts are blue-black with starch.
  • The white parts without chlorophyll are yellow-brown without starch. 
  • Since starch is only produced if chlorophyll is present, then chlorophyll is needed for photosynthesis.

4. To see if carbon dioxide is needed for photosynthesis

  • Place a plant in the dark for at least 24 hours to de-starch its leaves.
  • Enclose a leaf (A) in a clear plastic bag containing soda lime. This leaf does not have a supply of carbon dioxide from the air because soda lime absorbs CO2.
  • Enclose a second leaf (B) in a clear plastic bag containing some water.
  • Leave the plant with de-starched leaves in good light for 6 hours at room temperature (20˚C).
  • The two leaves are tested for starch using iodine.
  • Leaf A: no carbon dioxide, yellow-brown, no starch so no photosynthesis.
  • Leaf B: carbon dioxide, blue-black, starch so photosynthesis took place.
  • Therefore carbon dioxide is needed for photosynthesis.

5. To show that photosynthesis produces oxygen

  • Set up pondweed in good light in a beaker of pond water.
  • Set up a second beaker of pond water with pondweed in darkness – cover the beaker with a black plastic bag to exclude the light.
  • Photosynthesis occurs in light, no photosynthesis in darkness.
  • Any gas given off by the pondweed can be collected over water in a test tube. Use a funnel to direct the gas into the test tube of water.
  • A colourless gas is collected in both. Test these gases.
  • Pondweed in light: the gas is O2 – relights a glowing splint. Pondweed in darkness: the gas is CO2 – turns limewater milky and is absorbed by soda lime.
  • Oxygen is only produced during photosynthesis so photosynthesis makes O2.

Using products of photosynthesis
Glucose provides energy for plants by respiration, which takes place in all living cells all the time.
Respiration is the controlled release of energy from food by living cells.

Respiration: C6H12O6 + 6O2   >>   6CO2+ 6H2O + Energy

The energy is needed for cell work: growth, repair, reproduction, movement, formation of important biochemicals such as protein and, in many animals, to keep the body temperature constant above the external temperature.

Plant nutrition

For healthy growth, plants also need an adequate supply of a wide range of mineral nutrients as well as carbon dioxide and water.

These substances are dissolved in soil water and are absorbed by the roots.

Nitrogen, phosphorus and potassium are important mineral nutrient elements.

Nitrogen is needed for growth, protein and chlorophyll formation.

Phosphorus is needed for growth, respiration and photosynthesis.

Potassium in needed for growth, photosynthesis and chlorophyll formation.

Deficiency of an essential mineral nutrient causes abnormal plant growth.

Deficiency of N, P and K all result in stunted growth and a lack of chlorophyll.

Excess of certain minerals also can result in abnormal plant growth.


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