Cells run on the chemical energy found mainly in carbohydrate molecules, and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy stored in the bonds to hold these molecules together is released when an organism breaks down food. Cells then use this energy to perform work, such as movement. The energy harnessed from photosynthesis enters our planet’s ecosystems continuously and is transferred from one organism to another. Therefore, directly or indirectly, photosynthesis provides most of the energy required by living things on Earth. Photosynthesis also results in the release of oxygen into the atmosphere. In short, humans depend almost entirely on the organisms that carry out photosynthesis to eat and breathe.

Solar Dependence and Food Production

Some organisms can carry out photosynthesis, whereas others cannot. An autotroph is an organism that can produce its own food. The Greek roots of the word autotroph mean “self” (auto) or “feeder” (troph). Plants are the best-known autotrophs, but others exist, including certain types of bacteria and algae (Figure 1). Marine algae contribute enormous quantities of food and oxygen to global food chains. More specifically, plants are photoautotrophs, a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates. All organisms carrying out photosynthesis require sunlight.

Heterotrophs are organisms incapable of photosynthesis that must obtain energy and carbon from food by consuming other organisms. The Greek roots of the word heterotroph mean “other” (hetero) “feeder” (troph), meaning that their food comes from other organisms. Even if the organism being consumed is another animal, it traces its stored energy back to autotrophs and photosynthesis. Humans are heterotrophs, as are all animals and fungi. Heterotrophs depend on autotrophs, either directly or indirectly. For example, a deer obtains energy by eating plants. A wolf eating a deer obtains energy that originally came from the plants eaten by that deer (Figure 2). Using this reasoning, all food eaten by humans can be traced back to autotrophs that carry out photosynthesis.

Photosynthesis requires sunlight, carbon dioxide, and water as starting reactants (Figure 3). After the process is complete, photosynthesis releases oxygen and produces carbohydrate molecules, most commonly glucose. These sugar molecules contain the energy that living things need to survive. The chemical equation shown in Figure 4 below summarizes the complex reactions of photosynthesis.

Although the equation looks simple, the many steps during photosynthesis are quite complex. In plants, photosynthesis takes place primarily in the chloroplasts of leaves. Chloroplasts have a double (inner and outer) membrane. Within the chloroplast is a third membrane that forms stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane are molecules of chlorophyll, a pigment (a molecule that absorbs light) through which the entire process of photosynthesis begins.

The Two Parts of Photosynthesis

Photosynthesis occurs in two stages: the light-dependent reactions and the Calvin cycle. In light-dependent reactions, chlorophyll absorbs energy from sunlight and then converts it into chemical energy with the aid of water. The light-dependent reactions release oxygen as a byproduct of the splitting of water. In the Calvin cycle, the chemical energy derived from the light-dependent reactions drives carbon capture in carbon dioxide molecules and the subsequent assembly of sugar molecules.

The Global Significance of Photosynthesis

The process of photosynthesis is crucially important to the biosphere for the following reasons:

1. It creates O₂, which is important for two reasons. The molecular oxygen in Earth’s atmosphere was created by photosynthetic organisms; without photosynthesis, there would be no O₂ to support cellular respiration (see chapter 3.2) needed by complex, multicellular life. Photosynthetic bacteria were likely the first organisms to perform photosynthesis, dating back 2-3 billion years ago. Thanks to their activity and diversity of present-day photosynthesizing organisms, Earth’s atmosphere is currently about 21% O₂. Also, this O₂ is vital for creating the ozone layer (see chapter 10.2), which protects life from harmful ultraviolet radiation emitted by the sun. Ozone (O₃) is created from the breakdown and reassembly of O₂.
2. It provides energy for nearly all ecosystems. By transforming light energy into chemical energy, photosynthesis provides the energy used by organisms, whether those organisms are plants, grasshoppers, wolves, or fungi. The only exceptions are found in very rare and isolated ecosystems, such as near deep-sea hydrothermal vents where organisms get energy that originally came from minerals, not the sun.
3. It provides the carbon needed for organic molecules. Organisms are primarily made of two things: water and organic molecules, the latter being carbon-based. Through the process of carbon fixation, photosynthesis takes carbon from CO₂ and converts it into sugars (which are organic). Carbon in these sugars can be re-purposed to create the other types of organic molecules that organisms need, such as lipids, proteins, and nucleic acids. For example, the carbon used to make your DNA was once CO₂ used by photosynthetic organisms (see section 3.1 for more information on food webs).