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13.113 · Photosynthesis

Photosynthesis as an Energy Transfer Process — Cambridge AS & A Level Biology 9700 Revision Notes & Topical Questions

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📚 Cambridge AS & A Level Biology 9700 Hub / Topical Revision
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13.1 Photosynthesis as an Energy Transfer Process — CAMBRIDGE AS & A LEVEL BIOLOGY 9700 — REVISION NOTES notes: Describe chloroplast structure, the two stages of photosynthesis, pigments, photophosphorylation and the Calvin cycle.Overall equation — 6CO2 + 6H2O → C6H12O6 + 6O2, us…

  • Source of oxygen
  • Hydrogen carrier
  • Food chains
  • Thylakoids
  • Stroma lamellae
  • Reflected light

Section 1 of 13Photosynthesis as energy transfer

Overall equation
  • 6CO2 + 6H2O → C6H12O6 + 6O2, using light energy and chlorophyll.
Energy transfer
  • Energy transfer light energy becomes chemical energy in ATP and reduced NADP, then in carbohydrates.
Source of oxygen
  • Source of oxygen oxygen comes from the photolysis of water, not from carbon dioxide.
Hydrogen carrier
  • Hydrogen carrier reduced NADP, not reduced NAD, is used in photosynthesis.
Food chains
  • Food chains the organic molecules made are the energy source for almost all food chains.

Section 2 of 13Chloroplast structure

Chloroplast structure
Envelope
  • Envelope a double membrane, each membrane a phospholipid bilayer.
Stroma
  • Stroma the fluid filling the chloroplast; site of the light-independent stage.
  • Contains enzymes such as rubisco, 70S ribosomes, a loop of DNA, starch grains and lipid droplets.
  • Chloroplast DNA codes for some chloroplast proteins, made on the 70S ribosomes;
    • nuclear DNA codes for the rest.
Thylakoids
  • Thylakoids flattened fluid-filled sacs that stack to form grana; site of the light-dependent stage.
  • Membranes hold the pigments, enzymes and electron carriers.
  • The small thylakoid space allows a steep proton gradient.
  • Large membrane area holds many pigment molecules and captures more light.
Stroma lamellae
  • Stroma lamellae membrane channels that join grana but keep them apart.

Section 3 of 13The two stages

FeatureLight-dependentLight-independent (Calvin cycle)
LocationThylakoid membranesStroma
Needs light directlyYesNo, but needs ATP and reduced NADP
InputsLight, H2O, ADP + Pi, NADPCO2, RuBP, ATP, reduced NADP
OutputsATP, reduced NADP, O2Triose phosphate, ADP, NADP, RuBP
Key proteinsPhotosystems, electron carriers, ATP synthaseRubisco

Section 4 of 13Chloroplast pigments

PigmentColourAbsorbsRole
Chlorophyll aBlue-greenBlue-violet and redPrimary pigment of reaction centres
Chlorophyll bYellow-greenBlue and red-orangeAccessory, passes energy to chlorophyll a
CaroteneOrangeBlue-violetAccessory
XanthophyllYellowBlue-violetAccessory
Chloroplast pigments
Reflected light
  • Reflected light green light is mostly reflected, so leaves look green.
Photosystems
  • Photosystems pigments are arranged in funnel-like clusters that pass energy to a primary pigment reaction centre.

Section 5 of 13Absorption and action spectra

Absorption spectrum
  • Absorption spectrum a graph of the absorbance of each wavelength by a pigment.
Action spectrum
  • Action spectrum a graph of the rate of photosynthesis at each wavelength.
Match
  • Match the action spectrum has peaks in the blue-violet and red and a trough in green-yellow.
Conclusion
  • Conclusion the match with combined absorption spectra shows these pigments drive photosynthesis.

Section 6 of 13Chromatography of pigments

Extract the pigments from leaves by grinding with a solvent such as propanone.
Spot the extract on paper or a thin-layer plate.
Place in a solvent so the mobile phase moves through the stationary phase.
Mark the pigment and solvent distances.
Calculate Rf
Calculate Rf distance moved by pigment ÷ distance moved by solvent front.
Principle
  • Principle pigments differ in solubility and affinity for the stationary phase, so travel different distances.
Rf order
  • Rf order carotene highest (near 1), then xanthophyll, then chlorophyll a, then chlorophyll b.
Rf
  • Rf has no units and is always less than 1.
Thin-layer chromatography
  • Thin-layer chromatography usually separates faster and more sharply than paper.

Section 7 of 13Non-cyclic photophosphorylation

1
Light is absorbed by PSII (P680) and passed to the reaction centre.
2
Electrons are photoactivated and leave the primary pigment.
3
Photolysis 2H2O → 4H+ + 4e- + O2 replaces the electrons.
4
Electrons pass along the electron transport chain to PSI, releasing energy that pumps protons into the thylakoid space.
5
Protons flow back through ATP synthase, making ATP by chemiosmosis.
6
At PSI (P700) electrons are re-excited and, with H+, reduce NADP to reduced NADP.

Section 8 of 13Cyclic photophosphorylation

1Photosystem
  • Photosystem only PSI is involved.
2Electron flow
  • Electron flow excited electrons return to PSI through the electron transport chain.
3Products
  • ATP only, with no reduced NADP, no photolysis and no O2.
4Purpose
  • Purpose the Calvin cycle needs more ATP than reduced NADP, and cyclic flow supplies extra ATP.
5Proton movement
  • Proton movement protons are pumped from the stroma into the thylakoid space, then return through ATP synthase.

Section 9 of 13The Calvin cycle

1
CO2 combines with RuBP (5C), catalysed by rubisco, to form an unstable 6C compound.
2
The 6C compound splits into two molecules of GP (3C).
3
GP is reduced to triose phosphate (TP) using reduced NADP and ATP.
4
Five-sixths of the TP regenerates RuBP using ATP.
5
One-sixth of the TP leaves the cycle to make useful organic molecules.
1Per glucose
  • 6 turns fix 6 CO2, use 18 ATP and 12 reduced NADP, and form 12 TP: 10 regenerate RuBP and 2 make glucose.
2Without light
  • GP accumulates and RuBP falls because no ATP or reduced NADP is made.
3Low CO2
  • RuBP accumulates and GP falls.

Section 10 of 13Use of triose phosphate

Carbohydrates
  • Carbohydrates hexose phosphates give glucose, sucrose for transport, starch for storage and cellulose for cell walls.
Lipids
  • Lipids glycerol and fatty acids for membranes.
Proteins
  • Proteins amino acids, from GP and TP with nitrate.

Section 11 of 13More from the specification

  • Non-cyclic photophosphorylation
    • ATP formation using PSII and PSI with electrons ending in NADP.

Section 12 of 13Key terms

TermMeaning
PhotolysisSplitting of water by light at PSII.
PhotophosphorylationMaking ATP using light energy, an electron transport chain and chemiosmosis.
RubiscoEnzyme that fixes CO2 onto RuBP.
RuBPFive-carbon CO2 acceptor in the Calvin cycle.
Triose phosphateThree-carbon sugar phosphate made from GP.
Absorption spectrumGraph of absorbance against wavelength for a pigment.
Action spectrumGraph of photosynthesis rate against wavelength.
RfDistance moved by pigment divided by distance moved by solvent.

Section 13 of 13Exam technique

1
Saying oxygen comes from carbon dioxide — it comes from water.
2
Mixing up the reduced hydrogen carriers — photosynthesis uses reduced NADP, not reduced NAD.
3
Saying the Calvin cycle works forever in the dark — it stops when ATP and reduced NADP run out.
4
Giving Rf a unit or a value above 1 — it is a ratio less than 1.

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