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C1.3.18 (HL)—Synthesis of other compounds

Photosynthetic products can be converted into carbohydrates, lipids, amino acids and other compounds needed for plant growth and storage, connecting the mechanism to observable photosynthesis evidence.

Syllabus
First assessment 2025
Objective
C1.3.18
Level
HL

Calvin cycle products and recycling

HL only

Triose phosphate (TP) is the useful carbon product of the Calvin cycle. Some TP is used to make sugars and starch; carbon skeletons can also contribute to lipids, amino acids and other organic compounds when appropriate mineral nutrients are available. Most TP is retained to regenerate RuBP. The light-dependent reactions supply ATP and reduced NADP (NADPH), while the Calvin cycle returns ADP and NADP⁺ for reuse.

  • TP can leave the cycle for carbohydrate and other organic synthesis.
  • Most TP regenerates RuBP, so carbon fixation can continue.
  • Light reactions provide ATP and NADPH; the Calvin cycle returns ADP and NADP⁺.
  • Photosynthesis depends on a continuing supply of CO₂ and light-reaction products.

Trace Photosynthesis Mechanism

HL only

HL photosynthesis links light-dependent reactions to the Calvin cycle. Photosystems are pigment-protein arrays in thylakoid membranes; antenna pigments broaden absorption and pass energy to reaction-centre chlorophyll. PSII uses light to split water, supplying electrons, protons, and oxygen. Electron transport pumps protons into the thylakoid space, ATP synthase makes ATP by photophosphorylation, PSI re-excites electrons, and NADP is reduced. The Calvin cycle uses ATP and reduced NADP: Rubisco fixes CO2 to RuBP to form GP, GP is reduced to triose phosphate, most triose phosphate regenerates RuBP, and some becomes wider biomolecules.

  • Trace energy: pigments -> reaction centres -> ATP and reduced NADP.
  • Trace electrons/protons: water -> PSII -> carriers -> PSI -> NADP; proton gradient -> ATP synthase.
  • Trace carbon: CO2 -> RuBP/GP -> triose phosphate -> RuBP regeneration and biomolecule synthesis.
  • Trace exchange: light-dependent reactions supply ATP/reduced NADP; Calvin cycle returns ADP/NADP.

Concept essentials

  • Calvin-cycle products can be converted into many organic compounds.
  • Starch and sucrose store or transport photosynthetic carbohydrate.
  • Lipids and amino acids can be synthesized from photosynthetic carbon skeletons.
  • Photosynthesis supplies much of the carbon basis for plant growth.
ConceptIB Biology HL