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13 · Photosynthesis

Cambridge A Level Biology — Full F-drive source: 13 · Photosynthesis — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

5 min read 9 parts Lesson 13 of 13 Unit quiz

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

13. Photosynthesis: overview — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Outline chloroplast structure, the two stages, pigments and limiting factors.Envelope — a double membrane around the chloroplast.Thylakoids and grana — membranes holding pigments, electron …

  • Photolysis
  • Electron transport
  • Chemiosmosis
  • Regeneration
  • Chlorophylls
  • Carotenoids

Section 1 of 9Chloroplast structure and function

Chloroplast structure and function
Envelope
  • Envelope a double membrane around the chloroplast.
Thylakoids and grana
  • Thylakoids and grana membranes holding pigments, electron carriers and ATP synthase for the light-dependent stage.
Stroma
  • Stroma fluid containing Calvin cycle enzymes, 70S ribosomes, DNA and starch grains.

Section 2 of 9Light-dependent stage

1Photolysis
  • Photolysis splits water to give protons, electrons and oxygen.
2Electron transport
  • Electron transport electrons pass from PSII to PSI and then to NADP.
3Chemiosmosis
  • Chemiosmosis protons flow through ATP synthase, making ATP.
4Products
  • ATP, reduced NADP and oxygen.

Section 3 of 9Calvin cycle

1Fixation
  • CO2 combines with RuBP, catalysed by rubisco, to give two GP.
2Reduction
  • GP is reduced to TP using ATP and reduced NADP.
3Regeneration
  • Regeneration most TP regenerates RuBP using ATP.
4Products
  • Products the remaining TP makes carbohydrates, lipids and amino acids.

Section 4 of 9Pigments

Chlorophylls
  • Chlorophylls chlorophyll a and b absorb blue-violet and red light.
Carotenoids
  • Carotenoids carotene and xanthophyll absorb mainly blue-violet light.
Rf values
  • Rf values carotenoids have the highest, chlorophyll b the lowest.
Action spectrum
  • Action spectrum matches the combined absorption spectra.

Section 5 of 9Limiting factors

Main factors
  • Main factors light intensity, CO2 concentration and temperature.
Graphs
  • Graphs where the line rises the x-axis factor limits; where it is flat another factor limits.
Temperature
  • Temperature mostly affects the enzyme-controlled Calvin cycle.

Section 6 of 9Investigating photosynthesis

Redox dye
  • DCPIP decolourises as it accepts electrons from chloroplasts.
Pondweed
  • Pondweed oxygen collected over time gives the rate.
Light intensity
  • Light intensity varied by lamp distance, with intensity proportional to 1/d2.

Section 7 of 9More from the specification

1
Carbon Dioxide Concentration as a Limiting Factor
  • Carbon dioxide is an essential substrate for the light-independent reactions of photosynthesis;
  • increasing CO2 concentration increases the rate of carbon fixation by RuBisCO until another factor becomes limiting.
2
Investigating Photosynthetic Rates with Aquatic Plants
  • In experimental investigations using aquatic plants like Elodea or Cabomba, photosynthetic rate is measured by counting oxygen bubbles evolved per minute or by measuring volume using an oxygen-collecting microburet.
3
Using pH Indicators to Monitor Carbon Dioxide Consumption
  • Hydrogencarbonate indicator solution changes colour from yellow through red (atmospheric CO2) to purple (low CO2 / alkaline) as an aquatic plant removes dissolved carbon dioxide during light-dependent photosynthetic reactions.
    • (high CO2 / acidic)
4
Controlling Experimental Variables
  • To ensure valid and reliable investigations into the effect of light intensity or carbon dioxide, researchers must control temperature using a thermostated water bath and ensure consistent light spectrum from illumination sources.
5
Independent variable
  • The factor deliberately changed in an investigation.
6
Light-dependent reactions
  • Stage in thylakoid membranes producing ATP, reduced NADP and oxygen.
7
Light-independent stage
  • Stage in the stroma where CO2 is converted to organic molecules.
8
Carbon
  • Element with four valence electrons that forms the backbone of organic molecules.
9
Indicator
  • A chemical that changes colour depending on pH.

Section 8 of 9Key terms

TermMeaning
PhotosynthesisConversion of light energy to chemical energy in organic molecules.
PhotolysisLight-driven splitting of water.
RubiscoEnzyme fixing carbon dioxide onto RuBP.
Limiting factorFactor in shortest supply limiting the rate.
Action spectrumGraph of photosynthesis rate against wavelength.
ThylakoidMembrane sac holding pigments.

Section 9 of 9Exam technique

1
Saying oxygen comes from CO2 — it comes from the photolysis of water.
2
Saying the Calvin cycle is light-dependent — it needs ATP and reduced NADP, not light directly.
3
Saying all pigments absorb green light — chlorophylls reflect it.
4
Mixing up the stages and locations.

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