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De-starching the Leaves of a Plant

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De-starching the Leaves of a Plant

'De-starch (or Destarch) the leaves of a potted plant' is one of the preliminary steps in a photosynthesis experiment.

Biology students often ask the following questions about de-starching.
  • What is  the definition of de-starching?
  • Why it is necessary to de-starch the leaves of a plant before conducting photosynthesis experiments?
  • What happens to the starch stores when the potted plant is deprived of light or carbon dioxide?
  • How to de-starch the leaves of a plant?

What is the definition of de-starching?

De-starching is the process by which the starch reserves in a plant are depleted by depriving the plant of either light or carbon dioxide. 


Why is it necessary to de-starch the leaves of a plant before conducting photosynthesis experiments?

We test for the presence  of starch  in leaves in order to determine that photosynthesis has occurred. Glucose -  the product of photosynthesis - is rapidly converted into starch for storage in the chloroplast and cytoplasm.
Read:  Testing a Leaf for the Presence of Starch
When investigating whether a condition (or a factor) is necessary for photosynthesis is to occur we need to remove all traces of starch in leaves so that we can provide evidence that photosynthesis takes place during the experiment i.e. in  the presence of this factor and not in its absence.

If starch is already present at the beginning of the experiment the results will not be accurate. Hence, it is important to start off with a plant without starch. This is accomplished by de-starching the leaves of a plant.


What happens to the starch stores in leaves when the potted plant is deprived of light or carbon dioxide?

Starch stores are hydrolyzed into glucose.(Glucose may be used during respiration, or as a precursor for other molecules - amino acids, nucleic acids, glycerol,etc. Glucose may also be converted to sucrose which is translocated to other parts of the plant.)

Although levels of starch fall significantly at night  i.e. low light intensity - when photosynthesis ceases -  all the starch in the leaf is typically not all used up by dawn. Even after 24 hours of being placed in a dark cupboard, most plants still have a significant starch reserve. During the day i.e. high light intensity, starch levels rise.

Starch reserves are depleted when low light intensity is sustained - such as in a dark cupboard. Starch reserves may also be depleted when the plant is deprived of carbon dioxide. 

How to de-starch the leaves of a plant?

There are two main ways to de-starch leaves of a plant - the 'Light Exclusion' Method and the 'Carbon Dioxide Deprivation' Method. The 'Light Exclusion' method is a simpler procedure and is used often.
LIGHT EXCLUSION METHOD
Leaves can be destarched by depriving them of light for an extended period of time, usually 24-48 hours.

Potted plants are convenient to work with because they can easily be placed in a dark cupboard, or under a bell jar draped with a black cloth.

Individual leaves on large plants can be de-starched by covering with a black bag to exclude all light.
Picture

CARBON DIOXIDE EXCLUSION METHOD
Leaves can also be de-starched by depriving them of carbon dioxide for about 24-48 hours - N.B. the time varies depending on the type of plant and its size.

Concentrated sodium hydroxide solution or granular soda lime is usually  placed alongside a potted plant in a sealed bell jar; sodium hydroxide reacts readily with atmospheric carbon dioxide, depleting it.

This should not be confused with investigating whether carbon dioxide is necessary for photosynthesis.

How do I know when the leaves are de-starched?

Test a leaf or two for the presence of starch in order to determine whether the plant is completely destarched. If a positive result is obtained - leaves turn blue-black, the plant should be left under in the cupboard or under the coverd bell jar for longer, until a negative result- no colour change - is obtained.

Then carry on with the rest of the experimental procedure.
Sumo

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