Biology RP8

Investigation into the effect of a named factor on the rate of dehydrogenase activity in extracts of chloroplasts

Which reaction in photosynthesis does dehydrogenase catalyse?

In the light dependent reaction, when NADP accepts electrons.

What type of chemical is DCPIP?

A redox indicator dye.

True or false? When DCPIP is present, it will accept the electrons that NADP would usually accept.

True.

True or false? The longer the time it takes for DCPIP to decolourise, the greater the rate of photosynthesis.

False. THE LONGER THE TIME IT TAKES FOR DCPIP TO DECOLOURISE, THE LOWER THE RATE OF PHOTOSYNTHESIS.

What equipment and materials do you need for preparing the chloroplast suspension?

• Leaves.

• Blender.

• Buffered, isotonic isolation solution.

• Ice bath.

• Blender.

• Muslin cloth.

• Beaker.

• Centrifuge.

Talk me through and explain how to prepare the chloroplast suspension.

1) Remove the stalk and midribs from the leaves (as they will be difficult to blend and don’t contain many chloroplasts).

2) Blend the leaves in the blender (to break open the cells).

3) Filter the blended mixture with the muslin cloth into the beaker (to remove large cell debris).

4) Immediately place the beaker containing the blended mixture into the ice bath (to stop any enzyme activity from enzymes released when the cells were broken open from damaging the chloroplasts).

5) Mix some isolation solution with the blended mixture (buffered to resist pH changes that could damage the chloroplasts, isotonic so no net movement of water via osmosis that could burst the chloroplasts).

6) Spin in a centrifuge for 10 minutes, mix the pellet formed with distilled water to get the chloroplast suspension

What equipment and materials do you need to carry out the main part of the investigation?

• Chloroplast suspension.

• Boiling tubes.

• Test tube rack.

• Tin foil.

• Whatever your independent variable is.

List seven things that could be the independent variable of this investigation.

• pH

• Light wavelength.

• Light intensity.

• Temperature.

• Presence of weedkiller.

• Type of weedkiller.

• Concentration of weedkiller.

List four things that are always control variables.

• Source of chloroplasts.

• Volume of chloroplast suspension.

• Volume of DCPIP.

• Concentration of DCPIP.

What is the dependent variable of this investigation?

The time taken for DCPIP to decolourise (time taken from colour to change from dark blue to pale green).

What colour is DCPIP?

Dark blue.

What colour is reduced DCPIP?

Colourless.

Give and explain two control tubes you must set up.

• A tube containing chloroplast suspension and DCPIP that is wrapped in tinfoil (so no light can pass through).

• A tube containing only chloroplast suspension (gives colour reference for when DCPIP has been fully reduced).

How do you work out the rate of photosynthesis with your results?

Rate of photosynthesis = 1/time