Photosynthesis is the process by which plants, algae and some bacteria convert light energy into chemical energy, producing organic compounds from carbon dioxide and water and releasing oxygen.
Where it happens
In plants the process takes place in chloroplasts. Inside them are stacked membrane sacs called thylakoids, which contain chlorophyll. Chlorophyll absorbs mainly blue and red light and reflects green, which is why foliage appears green.
Two stages
Light-dependent reactions occur in the thylakoid membranes. Water molecules are split, releasing oxygen, and light energy is converted into the chemical carriers ATP and NADPH.
The Calvin cycle occurs in the stroma. Using the energy captured in the first stage, carbon dioxide is incorporated into carbohydrate. The name "light-independent" is often misunderstood: the cycle does not require darkness, it simply does not use light directly.
Limiting factors
The rate depends on several conditions, and the slowest one sets the pace:
- light intensity, up to a saturation point;
- carbon dioxide concentration, frequently the limiting factor outdoors;
- temperature, since the reactions are enzyme-driven;
- water availability — under drought, stomata close and gas exchange stops.
Why it matters
Photosynthesis is the entry point of energy into almost every food chain, the origin of atmospheric oxygen, and a major influence on the global carbon cycle. Fossil fuels are, in effect, the stored product of photosynthesis from the distant past.
Artificial photosynthesis
Researchers are working on systems that mimic the process to produce hydrogen from water or fuels from carbon dioxide using sunlight. The aim is a carbon-neutral fuel cycle; the challenge is achieving useful efficiency at reasonable cost.