8 The effect of a named factor on the rate of dehydrogenase activity
in extract of chloroplasts
The effect of ammonium hydroxide on the time taken for chloroplasts to decolourise
DCPIP.
Dehydrogenase is an enzyme that is found in chloroplasts in plant cells and is used in the
light-dependent stage during photosynthesis and therefore the activity of this enzyme can be
used to determine the effect of DCPIP of which changes colour when reduced.
Suggest why an isolation medium is used?
An isolation medium is used in order to preserve biochemical activity while separating the
chloroplasts and to prevent lysis of chloroplasts as it is isotonic.
Suggest why the solution must be kept cold.
The solution must be kept in ice in order to reduce enzyme activity and prevent the
chloroplasts from being damaged
Suggest why the midrib and the leaf stalks are discarded.
There are not many chloroplasts in the midrib of the leaf as this is where the xylem and
phloem are.
Suggest why leaves are blended for no longer than 15 seconds.
The leaves are only blended for less than 15 seconds to prevent the damage of chloroplasts
when separating them and because if they are blended for too long they’ll heat up.
Suggest why muslin cloth is used over filter paper.
This ensures only the chloroplasts are filtered out due it allowing small substances to be
filtered through.
Suggest why DCPIP changes colour.
The DCPIP solution is originally blue and changes to colourless as it is reduced as it acts as
an electron acceptor from the photolysis of water and the electron transport chain. In this
way it replaces NADP.
Explain the effect of ammonium hydroxide on rate of decolourisation of DCPIP hence light
dependent reaction.
The ammonium hydroxide acts as an electron acceptor in order to prevent the DCPIP
accepting electrons and by proxy being reduced. Therefore it inhibits the enzyme
dehydrogenase. Furthermore it affects the pH.
Suggest why a tube containing DCPIP and the leaf extract kept in the dark is prepared.
This experiment results in no colour change which provides evidence that the reaction is
light dependent.
in extract of chloroplasts
The effect of ammonium hydroxide on the time taken for chloroplasts to decolourise
DCPIP.
Dehydrogenase is an enzyme that is found in chloroplasts in plant cells and is used in the
light-dependent stage during photosynthesis and therefore the activity of this enzyme can be
used to determine the effect of DCPIP of which changes colour when reduced.
Suggest why an isolation medium is used?
An isolation medium is used in order to preserve biochemical activity while separating the
chloroplasts and to prevent lysis of chloroplasts as it is isotonic.
Suggest why the solution must be kept cold.
The solution must be kept in ice in order to reduce enzyme activity and prevent the
chloroplasts from being damaged
Suggest why the midrib and the leaf stalks are discarded.
There are not many chloroplasts in the midrib of the leaf as this is where the xylem and
phloem are.
Suggest why leaves are blended for no longer than 15 seconds.
The leaves are only blended for less than 15 seconds to prevent the damage of chloroplasts
when separating them and because if they are blended for too long they’ll heat up.
Suggest why muslin cloth is used over filter paper.
This ensures only the chloroplasts are filtered out due it allowing small substances to be
filtered through.
Suggest why DCPIP changes colour.
The DCPIP solution is originally blue and changes to colourless as it is reduced as it acts as
an electron acceptor from the photolysis of water and the electron transport chain. In this
way it replaces NADP.
Explain the effect of ammonium hydroxide on rate of decolourisation of DCPIP hence light
dependent reaction.
The ammonium hydroxide acts as an electron acceptor in order to prevent the DCPIP
accepting electrons and by proxy being reduced. Therefore it inhibits the enzyme
dehydrogenase. Furthermore it affects the pH.
Suggest why a tube containing DCPIP and the leaf extract kept in the dark is prepared.
This experiment results in no colour change which provides evidence that the reaction is
light dependent.
