← Life processes at the cellular level

Photosynthesis: light-dependent and light-independent reactions

Naming the TWO PHASES of photosynthesis, and which reactants/products belong to each, is what separates a full answer from a surface-level one.

The two phases and what each needs

  • LIGHT-DEPENDENT reactions happen in the THYLAKOID membranes: light energy is captured by chlorophyll, splitting water (releasing O₂ as a by-product) and producing energy carriers used in the next phase.
  • LIGHT-INDEPENDENT reactions (the Calvin cycle) happen in the STROMA: using the energy carriers from the light-dependent phase, CO₂ is fixed and built into glucose — this phase does not directly need light, but depends entirely on the products of the phase that does.
  • Because the two phases are linked, a shortage of a reactant for EITHER phase (e.g. light for the first, or CO₂ for the second) limits the whole overall rate, not just its own phase.
Worked example Explaining why limited CO₂ affects an aquatic plant more than a terrestrial one An aquatic plant's photosynthesis rate is more limited by low CO₂ availability than a terrestrial plant's. Explain why. CO₂ is the reactant specifically needed for the LIGHT-INDEPENDENT (Calvin cycle) reactions, where it is fixed into glucose. CO₂ diffuses much more slowly through water than through air, so an aquatic plant surrounded by water has a much lower effective CO₂ concentration reaching its cells than a terrestrial plant surrounded by air. Because the light-independent reactions cannot proceed without CO₂, regardless of how much light energy is available from the light-dependent phase, the aquatic plant's overall photosynthesis rate is limited by this single scarce reactant. ✓ The explanation must specifically name CO₂'s role in the LIGHT-INDEPENDENT phase and the physical reason (slow diffusion through water) it is scarcer there — not just state that "water plants get less CO₂". ✕

Treating photosynthesis as a single, undivided process

A question naming "light-dependent and light-independent reactions" specifically wants each phase discussed separately, with its own location, reactants, and products — collapsing them into one generic description of "photosynthesis happening in the chloroplast" misses the structural distinction the question is testing. Always name which phase you are discussing before describing what happens in it.

Reading a rate-of-photosynthesis graph against changing light intensity

  • At LOW light intensity, rate rises steeply as light increases — the light-dependent phase is the limiting factor.
  • At a certain point, the graph LEVELS OFF even as light keeps increasing — a different factor (usually CO₂ concentration or temperature) has now become the limiting factor instead.
  • Identify WHICH factor is limiting at each part of the graph, rather than assuming light is always the answer.

A fast check before writing a photosynthesis answer

  • Have I named WHICH phase (light-dependent or light-independent) is responsible for the effect I'm describing?
  • Have I named the specific reactant that becomes limiting, not just "a factor"?

Practise just this