By Thread Academy · 8 September 2026 · Biology
Almost every meal you will ever eat traces back to photosynthesis. Plants, algae, and some bacteria capture light energy and lock it into glucose, and everything else in the food chain — herbivores, carnivores, decomposers, and us — runs on the energy they captured. This one process feeds the world and fills the atmosphere with oxygen.
The equations
You need to know photosynthesis as both a word equation and a balanced symbol equation.
The word equation:
carbon dioxide + water -> glucose + oxygen (in the presence of light energy, with chlorophyll)
The symbol equation:
6CO2 + 6H2O -> C6H12O6 + 6O2 (light energy, chlorophyll)
Notice two things. First, the conditions are part of the equation: light provides the energy input, and chlorophyll absorbs it. Photosynthesis does not happen in the dark, and it does not happen without chlorophyll. Second, the symbol equation is balanced — six carbon dioxide molecules and six water molecules give one glucose molecule and six oxygen molecules. A common exam task is to balance it from an unbalanced version, so practise counting atoms on each side until it feels automatic.
The glucose made is not the end of the story. Plants use it for respiration (releasing energy), convert it into starch for storage, build it into cellulose for cell walls, or combine it with nitrates to make proteins.
Where it happens
Photosynthesis takes place in chloroplasts, which contain chlorophyll, the green pigment that absorbs light. Chlorophyll absorbs mainly red and blue light and reflects green — that is why leaves look green.
Leaves are adapted for the job. They are broad and thin, giving a large surface area for absorbing light and for gas exchange. They have many stomata — tiny pores, mostly on the lower surface — through which carbon dioxide diffuses in and oxygen diffuses out. Inside, the palisade mesophyll layer near the upper surface is packed with chloroplasts and sits closest to the light, while the spongy mesophyll below has air spaces that let gases circulate. Veins carry water in through the xylem and carry glucose away. Every feature of the leaf is an adaptation to making photosynthesis as efficient as possible.
Limiting factors and graph intuition
The rate of photosynthesis depends on several factors, but only one limits the rate at any given moment: the factor in shortest supply. The three limiting factors you must know are light intensity, carbon dioxide concentration, and temperature.
Light intensity
At low light, increasing light intensity increases the rate of photosynthesis — there is a straight-line relationship, because light is the limiting factor. But eventually the graph levels off into a plateau. More light no longer helps, because something else — carbon dioxide or temperature — has become limiting. This plateau shape is the single most important graph pattern in the topic.
Carbon dioxide concentration
The same pattern appears with carbon dioxide. Raising its concentration speeds up photosynthesis while CO2 is limiting, until the rate plateaus when another factor takes over. Commercial greenhouses exploit this by artificially raising CO2 levels — but they know that beyond a point, adding more CO2 is wasted money, because light or temperature limits the rate instead.
Temperature
Temperature behaves differently. As temperature rises, the rate of photosynthesis increases — the enzymes involved work faster — up to an optimum around 25 to 35 degrees Celsius. Beyond the optimum, the rate falls sharply because the enzymes begin to denature: their active sites lose shape and the reactions they catalyse slow down. So the temperature graph rises to a peak and then drops, unlike the light and CO2 graphs which plateau.
Worked example: reading a rate graph
Try this exam-style question:
In an experiment, the rate of photosynthesis is measured as light intensity increases, at two different CO2 concentrations. At low light, the rate is the same for both CO2 levels. At high light, the rate is higher at the higher CO2 level. Explain these results.
Work through it in two stages.
At low light, the limiting factor is light. Both sets of plants have the same light supply, so extra CO2 cannot increase the rate — CO2 is not what is holding them back. That is why the two curves overlap.
At high light, light is no longer limiting. Now the factor in shortest supply is carbon dioxide, so the plants with the higher CO2 concentration photosynthesise faster. That is why the curves separate.
The general strategy for these questions: ask which factor is limiting at each point on the graph. If changing a factor changes the rate, that factor is limiting. If changing it does nothing, something else is limiting.
Another worked example: the optimum temperature
A scientist finds that a plant's rate of photosynthesis peaks at 30 degrees and falls to almost zero at 55 degrees. Explain.
Below 30 degrees, rising temperature speeds up the enzyme-controlled reactions of photosynthesis — more kinetic energy means more frequent successful collisions between enzymes and substrates. At 30 degrees the enzymes work at their fastest. Above 30 degrees, the enzymes begin to denature: the heat breaks the bonds holding their shape, the active sites distort, and the rate collapses. Full marks come from naming denaturation and linking it to the shape of the active site, not just saying "the enzymes are damaged."
Why photosynthesis matters for life on Earth
Photosynthesis does two things on a planetary scale. First, it is the entry point for energy into almost every ecosystem. Producers capture light energy as chemical energy in glucose; every consumer, from rabbit to lion, is ultimately spending energy that a chloroplast captured. Second, it maintains the oxygen in our atmosphere. The oxygen released as a by-product of photosynthesis built up over billions of years into the air we breathe, and it is still being replenished by every green plant and ocean alga today.
There is a third, quieter consequence: the balance of carbon dioxide. Photosynthesis removes CO2 from the atmosphere while respiration and combustion add it back. Understanding this balance is the scientific foundation for understanding climate change — it is no coincidence that the same equation you learn for an exam is also the starting point for some of the most important science of our time.
Key takeaways
- Photosynthesis: carbon dioxide + water -> glucose + oxygen (light energy, chlorophyll); symbol equation 6CO2 + 6H2O -> C6H12O6 + 6O2.
- It happens in chloroplasts, where chlorophyll absorbs light; leaves are adapted with large surface area, stomata, palisade cells packed with chloroplasts, and veins.
- Limiting factors are light intensity, CO2 concentration, and temperature; only the factor in shortest supply limits the rate at any moment.
- Light and CO2 rate graphs rise then plateau; the temperature graph rises to an optimum (around 25-35 degrees) then falls sharply as enzymes denature.
- In graph questions, check which factor is limiting at each point: if changing a factor changes the rate, it is limiting.
- Photosynthesis feeds nearly all life through food chains and maintains atmospheric oxygen, making it one of the most consequential processes on Earth.