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C1.3.1—Light energy → chemical energy

Photosynthesis captures light with pigments and converts it into chemical energy stored temporarily in ATP and ultimately in organic molecules.

Syllabus
First assessment 2025
Objective
C1.3.1
Level
HL

Exam analysis

Chance of appearing2%of analysed past papers
Latest appearanceNovember 2018
Most common paperPaper2
Typical marks1–8

Common command terms

  • Identify
  • State
  • Outline
  • Describe
  • Explain
  • Distinguish

Recent exam appearances

November 2018Paper3 ["HL"] · TZ03(b)[ 1 ]C1.3.1—Light energy → chemical energy
November 2016Paper2 ["HL"] · TZ08(a)[ 8 ]C1.3.1—Light energy → chemical energy
Practice this objective

Coverage 2016–2018 · Updated 15 Jul 2026

Photosynthesis Converts Light into Chemical Energy

Photosynthesis captures light energy and stores it as chemical energy in organic molecules.

Pigments absorb photons and excite electrons; electron transfer creates ATP and reduced NADP, which then drive carbon fixation. The process depends on light, pigments, membranes, water, carbon dioxide and suitable conditions.

Trace the energy conversion:

  • photon absorbed by pigment
  • electron transfer and proton gradient
  • ATP and reduced NADP formed
  • carbon compounds synthesized

A leaf exposed to light can use ATP and reduced NADP from the thylakoid reactions to build carbohydrate in the stroma.

Light provides energy but is not itself converted directly into glucose molecules.

Light energy → chemical energy

Assessment in practice

1–7 marks
How it is assessed

This objective is assessed through structured response, commonly using Identify / State / Outline.

Command terms

Identify / State / Outline / Describe / Explain / Distinguish

What earns marks

Build the answer around this relationship: Chlorophyll and other pigments absorb light energy inside chloroplasts.

Representative question

Question 1

[Maximum number: 8]

Explain the processes by which light energy is converted into chemical energy.

SL Transfer: Explain Core Photosynthesis

Photosynthesis converts light energy into chemical energy in carbon compounds. Carbon dioxide is reduced to carbohydrate using hydrogen from water, glucose is the main stored product, and released oxygen comes from photolysis of water. Pigment evidence is tested with chromatography and Rf, spectra questions separate absorption from action, and rate/evidence questions use limiting factors, controls, and CO2 enrichment context.

  • Light becomes chemical energy in carbon compounds through chlorophyll-containing photoautotrophs.
  • Carbon in carbohydrate comes from CO2; released oxygen comes from photolysis of water.
  • Chromatography separates pigments using Rf; absorption spectra and action spectra measure different things.
  • Limiting-factor and CO2 enrichment questions require variables, controls, and realistic interpretation.

Concept essentials

  • Chlorophyll and other pigments absorb light energy inside chloroplasts.
  • Light energy is converted into chemical energy during photosynthesis.
  • ATP and reducing power connect light capture to carbon compound formation.
  • Photosynthesis produces organic molecules that store usable chemical energy.