How Photosynthesis Works
1. Quick Summary
Photosynthesis takes three inputs, light, water and carbon dioxide, and produces two outputs: sugar, which the plant uses as building material and fuel, and oxygen, which it releases. The carbon in the sugar comes from carbon dioxide in the air, not from the soil.
That last point is the one most people find surprising. A tree is mostly made of air and water. Soil contributes minerals in small amounts, but the bulk of the dry mass is carbon that was floating in the atmosphere.
2. What It Means
The process happens inside chloroplasts, structures in plant cells packed with a green pigment called chlorophyll. Chlorophyll absorbs red and blue light and reflects green, which is why leaves look green, and the absorbed energy is what drives the chemistry.
It runs in two stages. The light-dependent reactions capture energy from photons and use it to split water molecules, releasing oxygen and producing two energy-carrying molecules that act as short-term batteries. The second stage uses those molecules to power the assembly of sugar from carbon dioxide.
The second stage is a cycle. Carbon dioxide is attached to an existing molecule, then rearranged and reduced using the energy captured earlier, and part of the result goes on to make sugar while the rest is recycled to keep the cycle turning. An enzyme that catalyses the carbon-capturing step is thought to be the most abundant protein on Earth.
3. Why It Happens
Splitting water is chemically difficult, which is why light energy is needed at all. Water is very stable, and pulling hydrogen out of it requires a strong oxidising agent. The light reactions create exactly that by exciting an electron to a high enough energy state.
The oxygen you breathe is the waste product of that step. When water is split, the oxygen atoms pair up and leave as oxygen gas. Every molecule of oxygen in the atmosphere was produced by this reaction at some point.
The carbon fixation step is where inefficiency enters. The same enzyme that captures carbon dioxide can also react with oxygen instead, which wastes energy and forces the plant into a recovery process. Plants in hot, dry conditions have evolved workarounds that concentrate carbon dioxide around the enzyme to reduce this error.
4. Real Examples
A classic experiment demonstrates where the mass comes from. A tree planted in a weighed pot of soil and watered for years gains a large amount of mass while the soil loses almost none, showing the added material came mostly from air and water.
Leaves are structured for the job. They are thin so light penetrates, broad so they intercept it, and full of pores that open to let carbon dioxide in while unavoidably letting water vapour out, which is why plants in dry climates face a constant trade-off.
Not all photosynthesis is identical. Some plants separate the carbon capture step in time or space to reduce water loss, which is why grasses such as maize and sugarcane outperform many others in high heat, while cacti open their pores only at night.
5. How It Affects Us
Photosynthesis is the entry point for essentially all food. Nearly every organism either performs it or eats something that does, and the energy in fossil fuels is stored photosynthesis from long ago, which is why burning them releases carbon dioxide back.
It also sets the ceiling on agriculture. Crop yield is ultimately limited by how much light a field captures and how efficiently that light is converted, and improvements in yield have mostly come from increasing the fraction of the plant people eat rather than from changing the chemistry.
And it governs the carbon cycle. Forests and oceans absorb a large share of the carbon dioxide released by human activity each year, which currently slows atmospheric accumulation, though the size of that effect depends on conditions the plants are facing.
6. Key Takeaways
- Sugar is built from carbon dioxide in the air and hydrogen from water, powered by light energy.
- The oxygen released comes from splitting water, not from the carbon dioxide.
- A plant’s dry mass comes mostly from air, not from soil; soil supplies minerals in small amounts.
- The carbon-fixing enzyme also reacts with oxygen by mistake, which is a real limit on efficiency.