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13.2.213 · Photosynthesis

Investigating the Rate of Photosynthesis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 8 parts Lesson 12 of 13 1 quiz

📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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Lesson summary

13.2.2 Investigating the Rate of Photosynthesis — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe how redox indicators and aquatic plants are used to investigate the rate of photosynthesis.Indicators — DCPIP and methylene blue.Mechanism — the dye takes up the…

  • Colour change
  • Green colour
  • Higher intensity
  • Constant wavelength
  • Constant intensity
  • Light intensity

Part 1 of 6Redox indicators

Indicators
  • DCPIP and methylene blue.
Mechanism
  • Mechanism the dye takes up the high-energy electrons released from chlorophyll a instead of the electron acceptor.
Colour change
  • Colour change oxidised blue to reduced colourless.
Green colour
  • Green colour the reduced solution may look green because of chlorophyll.
Rate
  • Rate a faster colour change means a faster rate of photoactivation and photosynthesis.
Higher intensity
  • Higher intensity or more suitable wavelengths speeds up the reduction.

Part 2 of 6Method with chloroplast extract

1
Crush leaves in an ice-cold isolation medium to make a chloroplast suspension.
2
The medium has the same water potential as the leaf cells and contains a buffer.
3
Set tubes in different light intensities or wavelengths.
4
Add dye and a small volume of leaf extract to each tube.
5
Record the time for the colour change.
Ice cold
  • Ice cold avoids damaging chloroplasts and maintains membrane structure.
Constant wavelength
  • Constant wavelength use the same colour when changing intensity.
Constant intensity
  • Constant intensity use the same intensity when changing wavelength.

Part 3 of 6Aquatic plants

Plants
  • Elodea or Cabomba.
Why
  • Why oxygen is easy to collect and measure.
Light intensity
  • Light intensity change the lamp distance; intensity is proportional to 1/d2.
CO2
  • CO2 add different quantities of sodium hydrogencarbonate.
Temperature
  • Temperature use water baths of different temperatures.
Controls
  • Controls keep the other two factors constant.

Part 4 of 6Method with pondweed

1
Aerate the water so oxygen forms bubbles.
2
Illuminate the plant well before starting.
3
Cut the stem at an angle just before use.
4
Set up in a darkened room with the plant submerged in 1 % sodium hydrogencarbonate.
5
Collect gas in a gas syringe over a set time such as 5 minutes.
6
Repeat at least twice more.
7
Change the independent variable and repeat.
8
Record results and plot oxygen volume per minute against the independent variable.

Part 5 of 6Controlling variables

Heat
  • Heat place a glass tank between the lamp and the tube.
Darkened room
  • Darkened room so the lamp is the only light source.
Temperature
  • Temperature keep the water temperature constant when changing light.
CO2 supply
  • CO2 supply keep the hydrogencarbonate concentration constant.

Part 6 of 6More from the specification

1Pondweed
  • Aquatic plant such as Elodea used to collect oxygen.
2Enzyme
  • A protein that functions as a biological catalyst.
3Calvin cycle
  • Light-independent reactions that fix CO2 in the stroma.
4Light-dependent reactions
  • Stage in thylakoid membranes producing ATP, reduced NADP and oxygen.
5Light-independent stage
  • Stage in the stroma where CO2 is converted to organic molecules.
6Reading limiting factors
  • Reading limiting factors on a rising part of the graph the x-axis factor limits the rate;
    • on a flat part another factor limits it.

Key terms

TermMeaning
DCPIPRedox dye blue when oxidised and colourless when reduced.
Methylene blueRedox indicator changing blue to colourless.
ElodeaPondweed used to measure photosynthesis.
Isolation mediumCold buffered solution matching leaf water potential.
Gas syringeApparatus collecting gas over time.
Inverse square lawLight intensity is proportional to 1/d squared.

Avoid these mistakes

1
Saying redox dye goes from colourless to blue — it goes from blue to colourless.
2
Using different wavelengths at different intensities together — change one at a time.
3
Not aerating the water — oxygen would dissolve instead of forming bubbles.
4
Forgetting that the lamp heats the water — use a heat shield.

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