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4.1Unit 4: Energy, Environment, Microbiology and Immunity

Photosynthesis — IAL Edexcel Linear Revision Notes & Topical Questions

5 min read 13 parts Lesson 2 of 28 1 quiz

📚 IAL Edexcel Linear Hub / Topical Revision
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Curriculum Equation Key معادلة البناء الضوئي الموزونة - Balanced Photosynthesis EquationFast Exam Solution
6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂
Word Equation & Conditions: Carbon Dioxide + Water (+ Light & Chlorophyll) -> Glucose + Oxygen | ثاني أكسيد الكربون + ماء (في وجود الضوء والكلوروفيل) ← جلوكوز + أكسجين
Location & Specifications: البلاستيدات الخضراء (Chloroplast): التفاعلات الضوئية في الثايلاكويد، ودورة كالفن في الستروما.
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Lesson summary

4.1 Photosynthesis — IAL EDEXCEL LINEAR notes: Describe the light-dependent and light-independent stages of photosynthesis, chloroplast structure, pigments, and the factors limiting the rate of photosynthesis.Photosynthesis — a series of reactions in autotrophs (producers such as…

  • Two stages
  • Hydrolysis
  • Photolysis
  • Non-cyclic photophosphorylation
  • Location and inputs
  • Glucose yield

Part 1 of 11Overview of photosynthesis

1Photosynthesis
  • Photosynthesis a series of reactions in autotrophs that converts light energy into chemical energy stored in biomass.
    • (producers such as plants and algae)
2Equation
  • 6CO2 + 6H2O → C6H12O6 + 6O2 in the presence of light and chlorophyll.
  • Light energy splits water, releasing oxygen as a waste product.
  • Hydrogen from water combines with carbon dioxide to form glucose, which stores chemical energy as fuel for respiration.
3Two stages
  • Two stages light-dependent reactions in the thylakoid membranes and light-independent reactions in the stroma.

Part 2 of 11ATP as the energy carrier

ATP
ATP adenine + ribose + three phosphate groups, a phosphorylated nucleotide that is the universal energy currency.
  • It supplies energy for anabolic reactions, active transport, muscle contraction and nerve impulses.
  • ATP diffuses to where it is needed within cells.
Synthesis
  • ADP + Pi → ATP, a condensation reaction catalysed by ATP synthase.
  • Energy comes from electrons passing along an electron transport chain in mitochondria (respiration) and chloroplasts (photosynthesis).
  • In photosynthesis the energy originates from light, so it is photophosphorylation.
Hydrolysis
  • ATP → ADP + Pi, catalysed by ATPase, releases about 30.5 kJ mol−1 and removes a phosphate.
    • (dephosphorylation)
  • ADP and Pi are recycled to make more ATP.

Part 3 of 11Light-dependent reactions

Location
  • Location they take place across the thylakoid membrane.
Photolysis
Photolysis light-driven splitting of water: 2H2O → 4H+ + 4e− + O2.
  • The oxygen released comes from water, not carbon dioxide.
  • The protons and electrons are used in the light-dependent reactions.
NADP
NADP a coenzyme that transfers hydrogen, and when it gains hydrogen it is reduced to reduced NADP (NADPH).
  • Reduced NADP reduces other molecules and NADP oxidises other molecules by taking hydrogen.
Products
  • ATP and reduced NADP pass to the light-independent stage, while ADP and NADP return to the light-dependent stage.

Part 4 of 11Photophosphorylation

  • Non-cyclic photophosphorylation
    • it uses photosystems II and I and produces ATP and reduced NADP.
1
Light excites two electrons in photosystem II, which leave and enter the electron transport chain.
2
The electrons are replaced by electrons from the photolysis of water.
3
Energy released along the chain pumps H+ from the stroma into the thylakoid space (chemiosmosis).
4
H+ diffuses back into the stroma through ATP synthase, which catalyses ATP production.
5
The electrons pass to photosystem I, where light excites another pair of electrons.
6
These electrons pass along a second chain and combine with H+ from photolysis and NADP to form reduced NADP.
  • Cyclic photophosphorylation
    • it uses photosystem I only and produces ATP only.
    • Excited electrons pass along the chain and drive chemiosmosis, then return to photosystem I in a complete cycle.

Part 5 of 11Light-independent reactions (Calvin cycle)

  • Location and inputs
    • they occur in the stroma and need ATP and reduced NADP from the light-dependent stage but no light directly.
1
Carbon fixation: CO2 combines with the 5C compound ribulose bisphosphate (RuBP), catalysed by rubisco.
2
The unstable 6C compound splits into two molecules of glycerate 3-phosphate (GP), a 3C compound.
3
GP is reduced to glyceraldehyde 3-phosphate (GALP) using ATP and hydrogen from reduced NADP.
4
Five-sixths of the GALP is used with ATP to regenerate RuBP.
  • Glucose yield
    • one turn produces one-sixth of a glucose, so six turns fix 6 CO2 and give 12 GALP, of which 10 regenerate 6 RuBP and 2 make one glucose.

Part 6 of 11Products of photosynthesis

Carbohydrates
  • Carbohydrates glucose enters respiration, is converted to sucrose for transport in phloem,
    • or joins to form starch (storage) and cellulose (cell walls).
Lipids
  • Lipids glycerol and fatty acids from the cycle build triglycerides and phospholipids.
Amino acids
  • GALP combined with nitrates from the soil gives amino acids for proteins.
Nucleic acids
  • Calvin cycle molecules combined with phosphates from the soil make DNA and RNA.
Acetyl coenzyme A
  • Acetyl coenzyme A an important coenzyme in respiration.
Fate
  • Fate many products form new plant biomass that is passed to consumers.

Part 7 of 11Chloroplast structure and function

StructureFunction
Double membrane envelopeKeeps components together; inner membrane transport proteins control exchange with the cytoplasm
StromaContains enzymes for the Calvin cycle, sugars, ribosomes and DNA
Chloroplast DNA and ribosomesCode for and make some photosynthesis proteins
Thylakoid membranesHold ATP synthase, electron carriers and photosystems; the small thylakoid space allows a proton gradient to build quickly
Grana (stacks of thylakoids)Give a large surface area for photosystems and light absorption
LamellaeMembranous channels connecting grana
  • Photosystems
    • photosystem I absorbs light at 700 nm and photosystem II at 680 nm, and each contains chlorophyll a, chlorophyll b and carotene.

Part 8 of 11Pigments and spectra

Chlorophylls
  • Chlorophylls they absorb blue-violet and red light and reflect green.
Carotenoids
  • Carotenoids they absorb mainly blue-violet light and reflect yellow and orange, remaining in leaves in autumn.
PigmentColour
Chlorophyll aBlue-green
Chlorophyll bYellow-green
CaroteneOrange
XanthophyllYellow
Absorption spectrum
Absorption spectrum the amount of light of each wavelength absorbed by a pigment.
Action spectrum
Action spectrum the rate of photosynthesis at different wavelengths, which closely matches the absorption spectra.
  • Several pigments let plants photosynthesise over a wider range of wavelengths.

Part 9 of 11Chromatography of photosynthetic pigments

  • Chromatography
    • it separates a mixture because components travel at different speeds, and paper or thin-layer (silica gel) chromatography can be used.
1
Draw a pencil line about 1 cm from the bottom of the paper and mark a dot at the centre, since ink would itself separate.
2
Grind a healthy leaf with 20 drops of acetone in a mortar, which dissolves the membranes and releases the pigments.
3
Spot the extract on the dot with a capillary tube.
4
Suspend the paper in a small amount of solvent with the solvent level below the pencil line.
5
Remove the paper when the solvent nears the top and mark the solvent front at once in pencil.
6
Measure from the origin to the centre of each spot.
  • Rf = distance travelled by the pigment ÷ distance travelled by the solvent.
    • Smaller, more soluble pigments travel further and have a larger Rf.
    • Carotenoids have the highest Rf (close to 1), chlorophyll b the lowest and chlorophyll a is between them.

Part 10 of 11Limiting factors and rate of photosynthesis

Limiting factors and rate of photosynthesis
Limiting factors
  • Limiting factors light intensity (and wavelength), carbon dioxide concentration and temperature.
  • If any one is below its optimum the rate is reduced even when the others are optimal.
  • Water shortage usually affects other processes first, so it is not a main limiting factor.
Light intensity
  • Light intensity more light gives more ATP and reduced NADP, so the rate rises until it plateaus when another factor limits.
Carbon dioxide
  • Carbon dioxide more CO2 speeds carbon fixation by rubisco until another factor limits.
Temperature
  • Temperature the rate rises to an optimum, then falls as enzymes such as rubisco denature.
Method with aquatic plants (Elodea or Cabomba)
  • Method with aquatic plants (Elodea or Cabomba) oxygen bubbles are collected and measured.
  • Light intensity is varied by lamp distance, wavelength by colour filters, CO2 by sodium hydrogencarbonate and temperature by water baths.
  • A glass tank between lamp and tube absorbs heat so temperature stays constant.
  • Light intensity is proportional to 1/d², where d is the distance from the lamp.
1
Illuminate the plant first and set the lamp 10 cm from the boiling tube in a darkened room.
2
Cut the stem cleanly and submerge it in sodium hydrogencarbonate solution.
3
Measure the volume of gas collected in a set time such as 1 minute and repeat three times.
4
Move the lamp a further 10 cm and repeat.
5
Plot oxygen volume per minute against distance from the lamp.
  • Rate volume of oxygen produced ÷ time, or 1/t where t is the time for a leaf disc to float.

Part 11 of 11More on this topic

  • Limiting factor
    • A factor in shortest supply that restricts a rate.

Key terms

TermMeaning
PhotolysisLight-driven splitting of water into protons, electrons and oxygen.
PhotophosphorylationProduction of ATP using light energy.
Reduced NADPA coenzyme that carries hydrogen from the light-dependent to the light-independent stage.
RuBPThe 5C compound that combines with carbon dioxide in the Calvin cycle.
RubiscoThe enzyme that fixes carbon dioxide onto RuBP.
GALPGlyceraldehyde 3-phosphate, a 3C sugar phosphate formed from GP.
Action spectrumThe rate of photosynthesis at different wavelengths of light.
Rf valueDistance moved by a pigment divided by distance moved by the solvent.

Avoid these mistakes

1
Saying the oxygen released comes from carbon dioxide — it comes from the photolysis of water.
2
Saying the Calvin cycle needs no ATP — it uses ATP and reduced NADP to reduce GP and regenerate RuBP.
3
Confusing GP and GALP — GP is the first product of fixation and GALP is formed by reducing GP.
4
Saying cyclic photophosphorylation makes reduced NADP — it makes ATP only.

Last updated:

Biology 0610 & 9700 Exam Model Limiting Factors Master

Photosynthesis: Limiting Factors & Graph Plateau

6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)

  • Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
  • Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
  • High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.
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