Photosynthesis
Photosynthesis is the chemical reaction that plants, algae and some bacteria use to turn light energy (usually from the Sun) into food. They take in carbon dioxide from the air and water from the soil, and use sunlight to combine them into glucose (a sugar) and oxygen. Photosynthesis is the most important reaction on Earth. Almost every animal (including you) depends on it for food, and almost every breath you take depends on it for oxygen.
- What it doesLight + CO2 + water -> sugar + oxygenPlants make their own food
- Where it happensChloroplasts in leavesTiny green factories
- Key pigmentChlorophyllCaptures sunlight, makes leaves green
- Sugar producedGlucose (C6H12O6)Stored as starch
- Oxygen releasedO2Around 28% of Earths total
- Energy storedIn sugar bondsPowers nearly all life
The recipe
The overall reaction of photosynthesis is:
6 CO2 + 6 H2O + light energy -> C6H12O6 + 6 O2
In plain words: six carbon dioxide molecules plus six water molecules, with the help of sunlight, become one glucose molecule plus six oxygen molecules. The plant uses the glucose for energy and as a building material; the oxygen drifts out of the leaf as a waste product (which luckily is exactly what most animals need to breathe).
Where it happens
Photosynthesis happens inside tiny structures called chloroplasts, found mainly in the cells of leaves. Each chloroplast contains a green pigment called chlorophyll. Chlorophyll absorbs red and blue light from the Sun and reflects green light, which is why most plants look green.
The energy chlorophyll absorbs is used to split apart water molecules and rearrange carbon dioxide molecules into sugar. The leftover oxygen atoms join in pairs to form O2 gas.
What plants do with the sugar
The glucose a plant makes is its food, exactly as the food you eat is yours. Plants use sugar in three main ways:
- Energy: plants burn sugar (in a slow controlled way) inside their cells, just like you do, releasing the energy needed for growth, repair and chemistry.
- Building materials: plants use sugar as the raw material to build cellulose (their cell walls), starch (food storage) and many other molecules they need.
- Storage: extra sugar is converted into starch and stored in roots, stems, seeds and fruits for use later. This is why potatoes (roots) and rice (seeds) are starchy.
Photosynthesis and food chains
Photosynthesis is the start of almost every food chain on Earth. Plants (called producers) make their own food. Animals that eat plants (herbivores) get their energy and building blocks from those plants. Animals that eat herbivores (carnivores) get their energy from the herbivores. The energy you used to read this sentence came from a meal you ate, which came from a chain that started with a plant doing photosynthesis.
What plants need
For photosynthesis to happen, a plant needs:
- Light: usually sunlight, but plants can grow under artificial light too.
- Carbon dioxide: from the air, drawn in through tiny holes on leaves called stomata.
- Water: sucked up from the soil through the roots and carried up the stem.
- Chlorophyll: the green pigment that absorbs light energy.
- A reasonable temperature: enzymes work best at around 20 to 30 degrees Celsius. Too cold or too hot and photosynthesis slows or stops.
Why leaves change colour
In autumn, deciduous trees stop making chlorophyll because the days are too short and cold for photosynthesis to be worthwhile. As the green chlorophyll fades, the other pigments in the leaves (which were there all along but hidden by the green) become visible. Yellows and oranges come from carotenoids (the same pigments that make carrots orange). Reds and purples come from anthocyanins, which the tree may even make freshly in autumn. After the colourful display, the tree drops its leaves and survives winter on its stored sugars.
Two halves of the reaction
Photosynthesis actually has two stages, both happening inside the chloroplast:
- Light-dependent reactions: chlorophyll absorbs sunlight and uses the energy to split water molecules into hydrogen and oxygen. The oxygen is released; the hydrogen and the energy are stored in special carrier molecules.
- Light-independent reactions (Calvin cycle): the hydrogen and energy from the first stage are used to combine carbon dioxide molecules into glucose. This stage can happen in dim light or even in the dark, as long as the energy carriers are available.
Deeper dive: could we copy photosynthesis to fix the climate?
Photosynthesis takes carbon dioxide out of the atmosphere and turns it into useful chemicals. As humans pump ever more CO2 into the air by burning fossil fuels, scientists are asking: could we build artificial systems that do the same thing, only faster?
This is called artificial photosynthesis. The idea is to use sunlight to split water into hydrogen and oxygen, and then use the hydrogen as a clean fuel, or to combine it with CO2 from the air to make hydrocarbons that can replace petrol and diesel. The end-product would still release CO2 when burned, but only the CO2 originally taken out of the air, so the net effect would be roughly carbon neutral.
Researchers around the world are working on the chemistry. Some use specially made catalysts (often containing platinum, iridium or cobalt) that mimic the role chlorophyll plays in nature. Others have engineered bacteria and yeasts to produce diesel-like fuels from sugar made by photosynthesis. A few have built solar panels that produce hydrogen gas directly rather than electricity.
The challenges are enormous. Nature has had 2.5 billion years to perfect photosynthesis; our copies are nowhere near as efficient yet. But every year the technology improves. If artificial photosynthesis can be made cheap enough, it could help slow climate change by capturing carbon and producing clean fuels at the same time. Some scientists call it one of the most important goals of 21st-century chemistry.
For more, see water and sugar (glucose).