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B4.2.3—Photosynthesis as nutrition mode

Photosynthetic nutrition uses light energy to make organic molecules from carbon dioxide and historically released oxygen into the atmosphere over time.

Syllabus
First assessment 2025
Objective
B4.2.3
Level
HL

Photosynthesis Uses Light as an Energy Input

Photosynthetic nutrition uses light energy to build organic molecules from carbon dioxide in plants, algae and several groups of photosynthetic prokaryotes.

Photosynthetic pigments absorb light and drive energy conversion; the resulting chemical energy supports carbon fixation into organic compounds used for growth and respiration.

Plants and algae are major eukaryotic photoautotrophs. Cyanobacteria are photosynthetic prokaryotes whose oxygen-producing photosynthesis contributed to the rise of atmospheric oxygen; detailed prokaryotic pathway types are outside this objective.

A cyanobacterium and a green plant both use light as the energy source and carbon dioxide as a carbon source, even though one is prokaryotic and the other is eukaryotic.

Photosynthesis is not 'feeding on sunlight': light supplies energy, while carbon atoms come from inorganic carbon. Do not add detailed prokaryotic photosynthesis pathways beyond the syllabus.

Photosynthesis as nutrition mode

Assessment in practice

3 marks
How it is assessed

This objective is assessed through structured response.

What earns marks

Build the answer around this relationship: Photoautotrophs use light energy to produce organic compounds from carbon dioxide.

Representative question

Question 1

[Maximum number: 3]

There is evidence that prokaryotes were responsible for changes in the atmospheric gases 3.5 billion years ago. Outline the role of bacteria in producing an oxygen-rich atmosphere.

Niches, Nutrition, And Competition

A niche is the role of a species in a community, including habitat, abiotic tolerances, nutrition, activity, reproduction, and interactions. The evidence should match the question: nutrition mode, oxygen tolerance, feeding adaptation, predator-prey interaction, plant light strategy, fundamental versus realized niche, or competitive exclusion.

  • Definition answers should include role plus habitat/resources/interactions, not only place.
  • Nutrition modes explain how organisms obtain carbon, energy, or organic matter.
  • Adaptation examples should be linked to feeding, defence, light harvesting, or competition.
  • Fundamental versus realized niche and competitive exclusion are tested together because interactions narrow where species actually live.

Concept essentials

  • Photoautotrophs use light energy to produce organic compounds from carbon dioxide.
  • Cyanobacteria were early oxygen-producing photosynthetic organisms.
  • Oxygen released by photosynthesis accumulated in the atmosphere over time.
  • Photosynthetic organisms act as producers in ecosystems.