RP8 Flashcards
Controls:
Plant species
Volume of DCPIP
Volume of Chloroplast suspension
Volume of isolation medium
Independent variable
Colour of light
- White
- Red
- Green
[Could do intensity of light]
Dependent variable
Rate of dehydrogenase demonstrated by Absorbance
Or Time taken for DCPIP to decolourise
Why not use the midrib of leaf stalk?
No chloroplasts present in midrib. Midrib is tough material so it will not be cut up by blender.
Why use distilled water?
No other ions that affect colour of DCPIP
Suggest why leaves are blended for no longer than 15 seconds?
free chloroplasts without damaging - avoid heating
Why the isolation medium was ice cold and isotonic?
Cold – prevent enzyme activity so chloroplasts aren’t damaged
Isotonic –Same water potential as plant tissue so no net movement of water by osmosis. This means the chloroplasts won’t burst or shrivel
Why does DCPIP change colour?
blue (oxidised) to colourless (reduced) - takes place of NADP - accepts electrons from electron transfer chain and photolysis of water
How would lower light intensity affect the absorbance?
Lower light levels decreases rate of photoionisation. Less e- released and accepted by DCPIP. DCPIP less oxidised and so stays blue [high absorbance].
Lower light intensity = dehydrogenase activity is lower = higher absorbance [blue].
Higher light intensity = dehydrogenase activity is higher = lower absorbance [clear]
Which colour light would cause DCPIP to decolourise the quickest/ have lowest absorbance?
White then red then green. Green would not decolourise as the green light is not
absorbed
What happens if ammonium hydroxide is added?
Ammonium hydroxide accepts the e- and changes pH so that proteins denature. This prevents e- from passing to DCPIP.
DCPIP stays blue = absorbance remains high or time taken to decolourise is very high
Why do you cover one tube in foil?
Control experiment – allows you to see the effect of no light as a comparison. DCPIP remains blue
1 mark
You were told not to put the leaf midrib or leaf stalk in the blender
Suggest why
Few/no chloroplasts present
1 mark
What was the purpose of using a blender?
Break open cells
3 marks
The isolation medium was cold and was also isotonic with the spinach tissue. Explain why it was important that the isolation medium was cold.
1 Slows/stops enzyme activity
2 Enzymes released during blending
3 Chloroplasts not damaged and photosynthesis continues
OR:
4 Blending releases heat
5 Heat changes the tertiary structure of enzymes - denatures enzymes
6 Chloroplasts not damaged and photosynthesis continues
3 marks
There are large pieces of tissue and other organelles in your chloroplast suspension.
Describe how you could isolate the chloroplasts from the other components in the suspension.
1 Filter/centrifuge
2 At low speed then high speed
3 Collect pellet/sediment
2 marks
You were told to wrap tube A in aluminium foil. Explain why
Prevent entry of light- photosynthesis only occurs in the light
2 marks
DCPIP is decolourised more slowly when ammonium hydroxide is added. Use your
knowledge of the light-dependent reaction to suggest a reason why
1 Ammonium hydroxide has stopped/slowed transfer of electrons
2 Along electron transport chain/to DCPIP
OR
3 Ammonium hydroxide accepts electrons
4 Does not pass along electron transport chain / to DCPIP
OR
5 Ammonium hydroxide affects pH
6 pH affects proteins in electron transport chain
2 marks
Some weed killers are substances that accept electrons during photosynthesis.
Suggest how these substances kill weeds.
1 Stop light-dependent reaction
2 So less production of reduced NADP & ATP
2 marks
Suggest one disadvantage of using these weed killers on weeds growing in a crop.
Explain your answer.
1 Substance in weed killer absorbed by crop
2 Reduce yield of crop
3 Toxic effects on other wild life/when crop eaten
4 Slows respiration
