Light, CO₂ and temperature
In September 2026 the air at Mauna Loa held 426.07 parts per million of carbon dioxide, about 52 percent more than the roughly 280 ppm of the air before the Industrial Revolution.
The slowest step sets the pace
Imagine a sandwich shop where one person slices bread and another fills it. If the bread slicer can do 30 a minute and the filler only 10, the shop makes 10 sandwiches a minute, and hiring a faster slicer changes nothing. In 1905 the Cambridge botanist Frederick Blackman published a short paper, “Optima and limiting factors”, that put the same idea into plant physiology. When a process depends on several factors, its rate is held back by whichever one is in shortest supply.
For photosynthesis the three classic factors are light, carbon dioxide and temperature. Light powers the light-dependent reactions. Carbon dioxide is the raw material that RuBisCO fixes in the Calvin cycle. Temperature sets how fast enzymes work. In the lab below the rate is the product of a temperature factor and the smaller of a light factor and a CO₂ factor. That is a simplified teaching model built on Blackman’s idea, so the numbers are practice numbers, not measurements.
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Light
In dim light, more light means more photosynthesis almost in proportion. As the light gets brighter, the curve bends and then levels off: the chlorophyll is catching more energy than the Calvin cycle can use. In the model, with CO₂ at 420 ppm and 25 °C, the rate is 35.7 at 10 percent of full sunlight, 55.6 at 20 percent and 61.8 at 60 percent. From 60 percent to 100 percent nothing changes, because CO₂ has become the limit.
Carbon dioxide
Carbon dioxide makes up a tiny share of air. NOAA’s Global Monitoring Laboratory reported a monthly mean of 426.07 ppm at Mauna Loa, Hawaii, for September 2026, against about 280 ppm before industrial times, a value taken from ice cores. In the model at 60 percent light, raising CO₂ from 420 to 1,000 ppm lifts the rate from 61.8 to 88.2, and then light becomes the limit again. Many plants in bright light are short of CO₂, not of light.
Temperature
Temperature works differently. It does not supply energy or carbon; it changes how quickly enzymes such as RuBisCO do their jobs. In the cold everything is slow. As it warms, the rate rises to a best temperature (25 °C in the model) and then falls more steeply, because enzymes start to lose their shape. On a hot, dry day there is a second problem: the guard cells close the stomata to save water, and less CO₂ can get in. OpenStax asks this as a question in its photosynthesis chapter, and the next units look at it from the leaf’s side.
| Setting (model) | Rate | Limiting factor |
|---|---|---|
| 60% light, 420 ppm, 25 °C | 61.8 | CO₂ |
| 60% light, 1,000 ppm, 25 °C | 88.2 | light |
| 20% light, 1,000 ppm, 25 °C | 55.6 | light |
| 60% light, 280 ppm, 25 °C | 47.7 | CO₂ |
| 60% light, 420 ppm, 35 °C | 18.0 | temperature |
| 60% light, 420 ppm, 15 °C | 30.8 | temperature |
Real plants are more complicated. Light and CO₂ often limit together rather than taking turns, plants adjust to the light they grow in, and shade plants change the balance of their chlorophylls. Blackman’s idea is still a useful first question: of everything this plant needs right now, what is in shortest supply?
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- Blackman (1905): the factor in shortest supply limits the rate.
- Light curve: steep in dim light, flat in bright light.
- Air today: about 426 ppm CO₂ (NOAA, September 2026); about 280 ppm before industry.
- Temperature changes enzyme speed; too hot is worse than too cold.
- What did Blackman’s 1905 paper describe?
- The slowest factor limits the rate of a process
- The structure of the chloroplast
- The Calvin cycle
- How stomata open
- NOAA: 426.07 ppm in September 2026, about 280 ppm before industry. By what percentage is today’s value higher? (nearest whole number)
- In the model at 60% light and 420 ppm, the rate is 61.8. Raising the light to 100% gives…
- still 61.8, because CO₂ is limiting
- about 100
- about 80
- zero
- In the model, 60% light and 1,000 ppm CO₂ at 25 °C give a rate of 88.2. Before, at 420 ppm, it was 61.8. By how much did the rate rise?
- At 20% light the rate is 55.6 at both 420 and 1,000 ppm. Why?
- Light is the limiting factor
- CO₂ is the limiting factor
- Temperature is too high
- The stomata are closed
- Why does the rate fall quickly above the best temperature?
- Enzymes such as RuBisCO start to lose their shape
- There is less light
- Chlorophyll turns red
- Water stops being split
- On a hot, dry day the guard cells close the stomata. What happens to photosynthesis?
- It slows, because less CO₂ enters the leaf
- It speeds up, because water is saved
- Nothing changes
- It stops making oxygen but keeps making sugar
- Which factor supplies the raw material that RuBisCO fixes?
- Carbon dioxide
- Light
- Temperature
- Oxygen
- In dim light, doubling the light roughly…
- raises photosynthesis a lot
- has no effect
- lowers photosynthesis
- stops photosynthesis
- In the model at 60% light, 420 ppm and 15 °C, the temperature factor is 49.9% and the CO₂ factor 61.8%. Rate = 100 × 0.499 × 0.618. Work it out to one decimal place.
Sources for this course (16)
- -fix5: gravity calculation corrected using rho*g*h = 1000*9.8*h Pa
- the cited textbook has a tenfold coefficient error. Sources (checked 2026-10-17): OpenStax Biology 2e 8.1 Overview of Photosynthesis (photoautotrophs, mesophyll, stomata on the underside, guard cells, chloroplast, thylakoid, granum, stroma, two stages; openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis)
- 8.2 The Light-Dependent Reactions (400 to 700 nm, chlorophyll a absorbs blue and red, carotenoids, 300 to 400 chlorophylls per antenna, water split at PSII, about 10 percent of the oxygen used by the leaf's mitochondria; .../8-2-the-light-dependent-reactions-of-photosynthesis)
- 8.3 Using Light Energy to Make Organic Molecules (RuBisCO, 3-PGA, G3P, three turns per G3P and six per glucose, "dark reaction" outdated, cacti fix CO2 at night; .../8-3-using-light-energy-to-make-organic-molecules)
- 30.3 Roots (tap and fibrous roots, root cap, three zones in the first centimetre, root hairs, endodermis and Casparian strip; .../30-3-roots)
- 30.4 Leaves (lamina, petiole, midrib, parallel and reticulate venation, Ginkgo, cuticle, trichomes, palisade in one to three layers, spongy mesophyll, guard cells the only epidermal cells with chloroplasts, xylem and phloem, needles and spines; .../30-4-leaves)
- 30.5 Transport of Water and Solutes (redwoods near 116 m, turgor 0.6 to 0.8 MPa and up to 1.5 MPa = 210 psi vs tyres 30 to 34 psi, gravity -0.1 MPa per metre, -2 MPa at the leaf surface, cohesion-tension, up to 90 percent of the water lost, sources and sinks, phloem sap up to 30 percent sugar; .../30-5-transport-of-water-and-solutes-in-plants)
- 31.3 Nutritional Adaptations (rhizobia, nodules, nitrogenase, biological fixation 65 percent of nitrogen used in agriculture, mycorrhizae in more than 80 percent of land plants, up to 20 percent of the carbon; .../31-3-nutritional-adaptations-of-plants)
- 32.2 Pollination and Fertilization (self and cross pollination, bees cannot see red, nectar guides, bats, birds, wind, water, at least 25,000 orchid species, double fertilization 2n zygote and 3n endosperm, seed parts, scutellum, fruit types, dispersal, dormancy; .../32-2-pollination-and-fertilization)
- Britannica, Photosynthesis: history (Priestley 1771, Ingenhousz 1779, 1782: oxygen formed at the expense of fixed air, identified the year before as carbon dioxide) (britannica.com/science/photosynthesis/History-of-the-study-of-photosynthesis)
- Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables, 1779 (library.si.edu/digital-library/book/experimentsuponv00inge)
- van Helmont's account (200 pounds of earth, 5 pound willow, 169 pounds and about 3 ounces after five years, soil short by about 2 ounces)
- quoted from Howe 1965 by S. G. Saupe, College of St. Benedict and St. John's University (employees.csbsju.edu/ssaupe/biol115/van_helmont.htm)
- F. F. Blackman, Optima and Limiting Factors, Annals of Botany 19:281-296, April 1905 (doi.org/10.1093/oxfordjournals.aob.a089000)
- NOAA Global Monitoring Laboratory, Trends in CO2 (gml.noaa.gov/ccgg/trends/: Mauna Loa September 2026 monthly mean 426.07 ppm)
- NOAA Climate.gov, Climate change: atmospheric carbon dioxide (280 ppm or less before the Industrial Revolution of the mid-1700s, from ice cores; climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide).
Checked: 2026-10-17