Photosynthesis

Photosynthesis is a special process used by many living things, such as plants, algae, and cyanobacteria. It helps them change light energy—usually from the sun—into chemical energy. This energy is stored in compounds called carbohydrates, like sugars and starches. These help the plants and other organisms stay alive.

One important type of photosynthesis makes oxygen as a byproduct. This oxygen is what keeps the air full of life-sustaining gas and helps most complex life on Earth.

There are also other kinds of photosynthesis, such as anoxygenic photosynthesis. Some bacteria use this type. They don’t make oxygen. Instead, they may release sulfur or make energy in different ways. But all types of photosynthesis start the same way. Light energy is caught by special molecules called pigments, often chlorophyll, found in parts of plant cells called chloroplasts.

Photosynthesis happened a very long time ago and helped change Earth’s atmosphere. The oxygen made by early photosynthetic organisms, especially cyanobacteria, helped make the planet ready for the complex life we see today. Photosynthesis captures a lot of energy and turns large amounts of carbon into living material every year. It was first discovered in 1779 by Jan Ingenhousz, who showed that plants need light to survive.

Overview

Main article: Biological carbon fixation

Most plants, algae, and cyanobacteria are photoautotrophs. They can make their own food using light energy. They use sunlight to change carbon dioxide and water into sugars. This also puts oxygen into the air. This kind of photosynthesis is called oxygenic photosynthesis. It is the most common type.

There are other kinds of photosynthesis too. These are used mostly by bacteria. They can change carbon dioxide into sugars without putting out oxygen. All these processes help take energy from sunlight and store it in sugars. Organisms can use these sugars for growing and getting energy.

Photosynthetic membranes and organelles

Main articles: Chloroplast and Thylakoid

In photosynthetic bacteria, special proteins that capture light are found in their cell membranes. These membranes can fold into shapes like tubes or spheres.

In plants and algae, photosynthesis happens in tiny structures called chloroplasts. These organelles are surrounded by membranes and contain stacks of flat structures called thylakoids. Plants mainly use a green pigment called chlorophyll to absorb light, which is why most plants are green. Leaves contain many chloroplasts that help turn sunlight into energy.

Light-dependent reactions

Main article: Light-dependent reactions

In the light-dependent reactions, plants use sunlight to make energy. Chlorophyll, a green part in leaves, catches energy from the sun. This energy helps split water into oxygen, which we breathe. It also makes two important energy carriers called ATP and NADPH. These carriers are used in the next step to make food for the plant.

The light-dependent reactions happen in tiny parts of plant cells called chloroplasts. They can work in two ways: one makes both ATP and NADPH, and the other makes only ATP. The oxygen we get from plants is made when water is split by the energy from sunlight.

Light-independent reactions

Main articles: Calvin cycle and Carbon fixation

In the light-independent reactions of photosynthesis, a special enzyme called RuBisCO takes carbon dioxide from the air. It uses energy from sunlight to make three-carbon sugars. These sugars can be combined to make bigger molecules like sucrose and starch. Plants use these for energy and growth.

Plants have smart ways to make sugars even when it is hot and dry. Some plants, like maize and sugarcane, use a process called C₄ carbon fixation to capture carbon dioxide better. Other plants, like cacti, use a process called CAM photosynthesis. They store carbon dioxide at night and use it during the day. These methods help plants stay healthy in different places.

Order and kinetics

Photosynthesis happens in four main steps. Plants and other green organisms use sunlight to change light energy into chemical energy. They store this energy in sugars and other materials. This process also puts oxygen into the air, which helps all life on Earth.

Efficiency

Main article: Photosynthetic efficiency

Plants change sunlight into food with a photosynthetic efficiency of about 3–6%. Most of the sunlight they take in turns into heat. A tiny bit shines back as a special kind of light called chlorophyll fluorescence. This helps scientists learn how plants make food.

Scientists want to help plants get better at turning sunlight into food. They compare plants to solar panels, which change sunlight into electricity very well. By learning more about plants, scientists hope to grow plants that give us more food.

Evolution

Main article: Evolution of photosynthesis

Scientists found very old fossils that look like tiny, thread-like organisms. These fossils are about 3.4 billion years old, and they may have used sunlight for energy. Later studies suggest photosynthesis might have started around that time. The first clear proof of photosynthesis comes from tiny structures in very old rocks that are about 1.75 billion years old.

The type of photosynthesis that makes oxygen, called oxygenic photosynthesis, is the main source of oxygen in our atmosphere today. It likely began around two billion years ago. Modern plants and many tiny water organisms use this kind of photosynthesis, using water to make oxygen as a by-product.

Symbiosis and the origin of chloroplasts

Some animals, like corals, sponges, and sea anemones, have special friendships with tiny green organisms called algae. These algae live inside the animals and help them get energy from sunlight. A few sea creatures, such as certain mollusks, keep tiny parts of algae called chloroplasts inside their bodies. This lets them make their own food from sunlight for several months.

Scientists think that chloroplasts came from tiny green bacteria that lived inside early plant cells a very long time ago. These bacteria changed over time to become part of the plant cells, helping them make food from sunlight. Like tiny power houses called mitochondria, chloroplasts have their own DNA, which looks like the DNA in certain water bacteria called cyanobacteria.

Photosynthetic eukaryotic lineages

There are many different groups of tiny organisms that can do photosynthesis, including glaucophytes, red and green algae, cryptophytes, haptophytes, dinoflagellates, ochrophytes, chlorarachniophytes, and euglenids. Most of these got their ability to make food from sunlight by taking in tiny green bacteria long ago. Some of these organisms can also get food in other ways besides using sunlight.

Photosynthetic prokaryotic lineages

Early tiny organisms that used sunlight for energy did not make oxygen. They used other things, like hydrogen or sulfur, instead of water. Today, there are several groups of tiny organisms that can use sunlight, including cyanobacteria, which are the only ones that make oxygen. Other groups include Chlorobi, Heliobacteria, Chloracidobacterium, Proteobacteria, Chloroflexota, Gemmatimonadota, and Eremiobacterota. Each of these groups uses sunlight in slightly different ways.

Cyanobacteria and the evolution of photosynthesis

The ability to use water to make food from sunlight evolved only once, in the ancestors of cyanobacteria. This important change happened very early in Earth's history, at least 2.45 to 2.32 billion years ago. At that time, Earth’s air had almost no oxygen, so the first cyanobacteria likely did not make oxygen. Cyanobacteria continued to be important oxygen makers for a long time. Later, green algae also started making oxygen, and even today, cyanobacteria are still vital for making oxygen in the oceans.

Experimental history

Scientists have studied photosynthesis for a long time. In the 1600s, Jan van Helmont saw that plants grow bigger from water and air, not just soil. Later, Joseph Priestley learned that plants can clean the air that candles or animals make dirty.

In the 1700s, Jan Ingenhousz showed that sunlight helps plants clean the air. Jean Senebier found that plants take in carbon dioxide and give out oxygen when there is light. These discoveries helped scientists learn how plants make their food using sunlight.

Factors

Photosynthesis needs four main things to work: light irradiance and wavelength, water, carbon dioxide concentration, and temperature. Plants use light for energy, water to help make sugars, and carbon dioxide to build those sugars. The right temperature keeps everything working well.

Light and temperature affect how fast plants make sugars. When light is bright, plants work faster, but they can only go so fast. Temperature also helps — warmer temperatures speed things up, but only up to a point. If there isn’t enough carbon dioxide, plants can have trouble. Sometimes, plants use oxygen by mistake instead of carbon dioxide. This is called photorespiration and it makes it harder for plants to grow well.