C1.3 Photosynthesis
Photosynthesis converts light energy into chemical energy by using pigments, thylakoid reactions and Calvin-cycle carbon fixation to build organic compounds from carbon dioxide.
- Syllabus
- First assessment 2025
- Topic
- C1.3
- Level
- SL
Learning objectives
C1.3.1Light energy → chemical energy• Photosynthesis converts light energy into chemical energy in carbon compounds• Photoautotrophs use chlorophyll in chloroplasts or cyanobacterial membranesC1.3.2CO₂ → glucose• Carbon dioxide is reduced to carbohydrate using hydrogen from water• Glucose represents the main stored product, though many compounds are synthesizedC1.3.3Oxygen as by-product• Oxygen is released from photolysis of water, not directly from CO₂• Oxygenic photosynthesis occurs in plants, algae, and cyanobacteriaC1.3.4Photosynthetic pigment separation• Paper chromatography separates chlorophylls and accessory pigments• Rf values compare pigment movement relative to solvent frontC1.3.5Absorption of specific wavelengths• Chlorophyll and accessory pigments absorb specific wavelengths• Absorbed light excites electrons for light-dependent reactionsC1.3.6Absorption vs. action spectra• Absorption spectra show wavelengths absorbed by pigments• Action spectra show wavelengths most effective for photosynthesis rateC1.3.7Limiting factors investigation• Light intensity, CO₂ concentration, and temperature can limit photosynthesis• Investigations change one variable and estimate rate from O₂ production or CO₂ uptakeC1.3.8CO₂ enrichment experiments• CO₂ enrichment tests whether increased CO₂ raises photosynthesis or growth• Greenhouse and FACE experiments compare realistic crop and ecosystem responses
Photosynthesis Stores Light Energy in Carbon Compounds
Photosynthesis converts light energy into chemical energy stored in organic carbon compounds. It supplies the carbon compounds and most of the chemical energy that enter ecosystems.
| Photoautotroph | Where chlorophyll captures light |
|---|---|
| plant or alga | thylakoid membranes inside chloroplasts |
| cyanobacterium | internal photosynthetic membranes; no chloroplast |
light absorbed by pigments → excited electrons → ATP and reduced NADP → CO₂ reduced into carbon compounds
Carbon Comes from CO₂; Released Oxygen Comes from Water
Overall matter equation
6CO₂ + 6H₂O + light energy →
C₆H₁₂O₆ + 6O₂
| Atom or product | Source and fate |
|---|---|
| carbon | CO₂ is fixed and reduced into carbohydrate |
| hydrogen | water supplies hydrogen carried by reduced NADP |
| released O₂ | produced by photolysis of water, not removed from CO₂ |
Plants, algae and cyanobacteria carry out oxygenic photosynthesis. Glucose is a useful summary product, but fixed carbon also feeds the synthesis of many other compounds.
Chromatography Separates a Leaf's Pigment Mixture
Pigments separate because each partitions differently between a moving solvent and the stationary phase. A pigment that is more soluble in the solvent and less strongly attracted to the stationary phase moves farther.
- Extract pigments in an organic solvent and place a concentrated spot on the origin.
- Keep the origin above the solvent; let the solvent rise.
- Mark the solvent front immediately, then measure from the origin to each spot's centre.
- Compare colour and Rf only with standards run under the same solvent, temperature and stationary phase.
Pigments Absorb Selected Wavelengths, Not Every Colour
A photon is absorbed only when its energy matches an allowed change in a pigment's electrons. Absorption raises an electron to an excited state, beginning the energy transfers of the light-dependent reactions.
Chlorophylls absorb strongly in blue and red regions but weakly in much of the green region, so green light is reflected or transmitted. Accessory pigments absorb additional wavelengths and transfer excitation energy toward chlorophyll a.
A Plateau Means the Limiting Factor Has Changed
A limiting factor is the condition in shortest effective supply relative to the requirements of photosynthesis. Increasing it raises the rate only until another factor becomes limiting.
| Region of curve | Interpretation |
|---|---|
| steep rise as light increases | light is limiting |
| plateau at low CO₂ | added light cannot overcome carbon limitation |
| higher plateau at high CO₂ | relieving CO₂ limitation reveals a higher maximum under those conditions |
Measure Photosynthesis as a Rate, Not a Bubble Count Alone
Estimate photosynthesis from oxygen volume produced per unit time or carbon dioxide removed per unit time. Bubble number is less valid because bubble size varies.
| Experimental role | Example |
|---|---|
| independent variable | light intensity, CO₂ concentration or temperature |
| dependent variable | initial O₂-production or CO₂-uptake rate |
| controls | plant species and amount, measurement time, wavelength, the other limiting factors |
| reliability | repeat each treatment and compare means with variation |
Allow the apparatus to equilibrate, change one independent variable across a suitable range, measure an initial rate, reset the gas bubble or sensor, and repeat. Use a heat shield or water bath when changing lamp distance so temperature does not change with light.
Temperature Produces an Optimum, Not a Plateau
| Temperature region | Why rate changes |
|---|---|
| below optimum | greater kinetic energy increases successful enzyme–substrate collisions |
| optimum | combined photosynthetic processes reach their highest rate under these conditions |
| above optimum | carbon-fixation performance falls and other heat stresses increase; do not assume immediate wholesale enzyme denaturation |
Light and CO₂ usually give rising curves that level off as limitation changes. Temperature affects enzyme-controlled reactions, so its curve normally rises to an optimum and then declines.
CO₂ Enrichment Tests a Conditional Prediction
Extra CO₂ can raise photosynthesis or growth only while light, temperature, water, mineral nutrients and biochemical capacity remain sufficient. An enrichment result therefore supports a conditional prediction, not unlimited future growth.
| Design | Strength | Main limitation |
|---|---|---|
| greenhouse or chamber | close control of CO₂ and other conditions | enclosure changes light, temperature, airflow and scale |
| FACE field plot | exposes intact crops or ecosystems under realistic open-air conditions | rainfall, light and other variables cannot be controlled and must be monitored |
Compare enriched and ambient-CO₂ replicates, measure photosynthesis and biomass over time, and report variation. Differences among species, years or water treatments show interaction with other limiting factors.