Photosynthesis

Photosynthesis is a fascinating system of biological processes used by many organisms, such as plants, algae, and cyanobacteria, to turn light energy—usually from the sun—into chemical energy. This energy is stored in special compounds called carbohydrates, like sugars and starches, which the organisms use to power their lives. One of the most important types of photosynthesis is oxygenic photosynthesis, which releases oxygen as a byproduct. This oxygen is what keeps the air we breathe rich in life-sustaining gas and supports most complex life on Earth.

There are also other kinds of photosynthesis, such as anoxygenic photosynthesis, used by some bacteria. These organisms don’t produce oxygen; instead, they may release sulfur or create energy in different ways. No matter the type, photosynthesis always starts when light energy is captured by special molecules called pigments, often chlorophyll, found in structures like chloroplasts in plant cells.

Photosynthesis happened a very long time ago and played a huge role in changing Earth’s atmosphere. The oxygen produced by early photosynthetic organisms, especially cyanobacteria, helped make the planet suitable for the complex life we see today. Today, photosynthesis captures an amazing amount of energy—about 130 terawatts—and turns vast quantities 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, meaning they can make their own food using light energy. They use sunlight to turn carbon dioxide and water into sugars, a process that also releases oxygen into the air. This kind of photosynthesis is called oxygenic photosynthesis and is the most common type.

There are other types of photosynthesis too, used mostly by bacteria. These can turn carbon dioxide into sugars without releasing oxygen. All these processes help capture energy from sunlight and store it in sugars, which organisms can then use for growth and 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 to increase the space for capturing light.

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 where the process occurs. Plants mainly use a green pigment called chlorophyll to absorb light, which is why most plants are green. Leaves, the main photosynthetic organs in most plants, 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 pigment in leaves, catches energy from sunlight. This energy helps split water into oxygen, which we breathe, and makes two important energy carriers called ATP and NADPH. These carriers are used in the next stage of photosynthesis to make food for the plant.

The light-dependent reactions happen in tiny structures inside 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 a by-product of these reactions, 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 and, through a process called the Calvin cycle, uses energy from sunlight to create three-carbon sugars. These sugars can then be combined to make bigger molecules like sucrose and starch, which plants use for energy and growth.

Plants have clever ways to make sure they can still create these sugars even when it’s hot and dry outside. Some plants, like maize and sugarcane, use a process called C₄ carbon fixation to better capture carbon dioxide. Other plants, like cacti, use a process called CAM photosynthesis, where they store carbon dioxide at night and use it during the day. These methods help plants make the most of their resources and stay healthy in different environments.

Order and kinetics

The process of photosynthesis happens in four main stages. Plants and other autotrophic organisms use sunlight to turn light energy into chemical energy, which they store in sugars and other compounds. This process also releases oxygen into the air, which is important for life on Earth.

Efficiency

Main article: Photosynthetic efficiency

Plants usually change sunlight into food with a photosynthetic efficiency of 3–6%. Most of the sunlight they take in but don't use turns into heat, and a tiny bit shines back as a special kind of light called chlorophyll fluorescence. This helps scientists study how plants make food.

Scientists are working to make plants even better at turning sunlight into food. They compare plants to solar panels, which can change sunlight into electricity much better than plants can. By studying how plants work, scientists hope to create plants that give us more food.

Evolution

Main article: Evolution of photosynthesis

Scientists have found very old fossils that look like tiny, thread-like organisms that may have used sunlight for energy. These fossils are about 3.4 billion years old. Later studies also suggest that photosynthesis might have started around this 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 been studying photosynthesis for a long time. In the 1600s, Jan van Helmont noticed that plants gain mass from water and air, not just soil. Later, Joseph Priestley found that plants can clean the air that candles or animals “dirty.”

In the 1700s, Jan Ingenhousz showed that sunlight helps plants clean the air. Jean Senebier discovered that plants take in carbon dioxide and release oxygen when exposed to light. These discoveries helped scientists understand how plants make their food using sunlight.

Factors

Photosynthesis mainly depends on four key factors: light irradiance and wavelength, water absorption, carbon dioxide concentration, and temperature. Plants need light to capture energy, water to help make sugars, carbon dioxide to build those sugars, and the right temperature to keep everything working well.

Light and temperature play big roles in how fast plants can make sugars. When light is bright, plants work faster, but they can only go so fast before they level off. Temperature helps too — warmer temperatures speed things up, but only up to a point. Plants also face challenges when there isn’t enough carbon dioxide around. Sometimes, instead of using carbon dioxide, plants accidentally use oxygen, which wastes energy and can even hurt the plant. This process is called photorespiration and it makes it harder for plants to grow efficiently.