How Photosynthesis Feeds the World

Plants cannot eat in the way animals do, so they make their own food. In photosynthesis, a plant uses light energy to rearrange carbon dioxide from the air and water from the soil into glucose, a simple sugar. Oxygen is released along the way.

The important idea is not the formula. It is the change in energy. Sunlight arrives as light, then the plant stores part of that energy inside the chemical bonds of glucose. The sugar can be used straight away, moved to another part of the plant, or turned into starch for later.

Plants also use the glucose they make. During respiration, cells release stored energy from glucose so the plant can build roots, repair damage, open flowers and produce seeds. Some glucose becomes cellulose for strong cell walls, while some helps form oils and proteins. Photosynthesis is therefore not just about making a sweet substance. It provides both fuel and building material.

Most photosynthesis happens in leaves because their shape is perfect for the job. A broad, thin leaf catches plenty of light and lets gases move in and out over a large surface. Tiny openings called stomata allow carbon dioxide to enter, while roots supply water through tubes in the stem.

Inside leaf cells are chloroplasts, small structures containing the green pigment chlorophyll. Chlorophyll absorbs light, especially red and blue light, and starts the reactions that make sugar. The main ingredients are simple:

• •Light from the Sun provides energy.
• •Carbon dioxide enters from the air.
• •Water travels up from the roots.
• •Chlorophyll captures the light.
• •Glucose stores energy for the plant.

Leaves are adapted to balance two needs. They must take in carbon dioxide, but open stomata also let water escape. On hot or dry days, a plant may close some stomata to save water, which can slow photosynthesis. This trade-off helps explain why plants have different leaf shapes and habits in deserts, forests and wetlands.

When you eat a carrot, you receive energy that the carrot plant stored from sunlight. When you eat bread, the same is true of the wheat. Even meat begins with photosynthesis, because the animal ate plants or ate another animal that once ate plants.

Photosynthesis also helps keep oxygen in the atmosphere. Plants, algae and some bacteria release oxygen as they make food, while living things use oxygen during respiration. The two processes are linked parts of a much larger cycle. Seeing that connection makes a leaf feel less like decoration and more like a quiet engine for life.

Ask where the wood in a tree came from and most people say the soil. It did not. In an experiment published in 1648, Jan Baptist van Helmont grew a willow in a weighed tub of earth for five years, watering it and nothing else; the tree gained about 74 kilograms and the soil lost a couple of ounces. He concluded the mass came from water, which was half right. The rest of the answer is carbon dioxide: the carbon in every trunk, leaf and root entered through the stomata as a gas and was fixed into sugar, then cellulose, by photosynthesis.

That is worth holding onto because it reverses the intuition. A forest is mostly air, rearranged, and when wood burns the carbon goes back to where it came from. It is also why forests matter in the carbon cycle: growing trees are pulling carbon out of the atmosphere at the rate their leaves can capture light.

Most plants, including wheat, rice and most trees, use the standard route in which carbon dioxide is fixed directly by an enzyme called rubisco. Rubisco has a weakness: when carbon dioxide inside the leaf runs low, for example because stomata have closed on a hot day, it starts grabbing oxygen instead. That wasteful side reaction, called photorespiration, uses energy and releases carbon that the plant had already captured, so hot and dry conditions hit these plants twice.

Some plants have evolved ways round the problem. Maize, sugar cane and many tropical grasses use the C4 pathway, which first traps carbon dioxide in one set of cells and then pumps it at high concentration to rubisco in another, keeping the oxygen mistake rare. Cacti, pineapples and many succulents use CAM photosynthesis, opening their stomata at night when the air is cooler and storing carbon dioxide as an acid. During the day the stomata stay shut to save water, and the stored carbon is released inside the leaf for the light-powered reactions.

In 1771 Joseph Priestley found that a sprig of mint could restore the air in a sealed jar where a candle had burned out, so that a candle could burn there again. A few years later the Dutch physician Jan Ingenhousz showed that this happened only when the green parts of plants were in light. In darkness, plants made the air worse, much as animals did. These experiments linked plants to the air long before anyone understood oxygen or carbon dioxide, and they showed that light was an essential ingredient rather than just a helpful condition.

A natural guess was that the released oxygen came from carbon dioxide, since that molecule contains two oxygen atoms. Experiments in the early 1940s showed otherwise. Researchers gave algae water containing a heavier isotope of oxygen, oxygen-18, which can be detected by its mass. The oxygen gas released carried the heavy label, showing that it came from the water. When the label was placed in the carbon dioxide instead, it ended up in the sugar and other products, not in the oxygen gas. Photosynthesis therefore splits water, and the oxygen you breathe is a by-product of that step.

Photosynthesis is far less efficient than its importance suggests. Only about half of the energy in sunlight falls within the range of wavelengths that chlorophyll can use. Some of the usable light is reflected or passes straight through leaves, and more energy is lost as the light-driven reactions pass it along a chain of steps. The plant then spends part of the sugar it makes on its own respiration. Researchers have calculated that the theoretical best for most crops is only around 5 per cent of incoming solar energy, and in real fields over a whole growing season, typical crops store about 1 per cent or less.

Part of the gap comes from sunlight that leaves cannot use fast enough. On a bright day the upper leaves of a crop receive more light than their reactions can handle, so they switch on protective mechanisms that safely shed the excess as heat. When a cloud passes or a leaf falls into shade, those protections switch off only slowly, wasting light for minutes at a time. Scientists have genetically engineered tobacco and soybean plants that relax this protection more quickly, and in field trials the modified plants produced more growth. Fixing small inefficiencies like this is one way researchers hope to raise food production.

Photosynthesis turns light, water and carbon dioxide into stored food, with oxygen released as a useful extra. Learn the energy journey first, then the names and equations become much easier to remember.