Photosynthesis is a technique for converting sunlight into energy that has been utilized by certain organisms for around 3.4 billion years. The basic formula involves a conversion of carbon dioxide and water into glucose and oxygen, aided by light-dependent reactions. Photosynthesis emerged quickly after the emergence of life itself, thought to have occurred 3.5 billion years ago, after the Earth’s crust cooled. The first photosynthetic organisms were the ancestors of modern-day cyanobacteria.
Photosynthesis takes place inside chloroplasts, special organelles located in the cells of plants and other photosynthesizing organisms. Choloroplasts are green because they utilize the pigment chlorophyll. The primary sun-absorbing organs of plants are the leaves. Although chloroplasts are located in cells throughout a plant, chloroplast density is by far the highest on the leaves, where between 450,000 and 800,000 chloroplasts can be found in every square millimeter.
Chloroplasts are thought to derive from photosynthetic bacteria, with which they have much in common. Like the power plants of eukaryotic (complex) cells, mitochondria, chloroplasts are thought to derive from extremely close symbiotic relationships between early microbes, so close that they became part of the same inseparable entity.
One of the byproducts of photosynthesis is oxygen, the molecule we humans and other animals require to live. Although today oxygen brings life, during a cataclysmic event two billion years ago, it brought death. At that time, the Earth’s atmosphere contained little oxygen, and large iron rocks could be exposed to the surface without rusting. Then, during a geologically sudden period consisting of a few tens of millions of years, oxygen-producing photosynthetic cyanobacteria evolved and covered the Earth, producing massive amounts of oxygen and causing a mass extinction of evolutionary lineages unaccustomed to such high atmospheric oxygen concentrations. This is known as the oxygen catastrophe.
Today, the atmosphere is about 23% oxygen and the remainder nitrogen. The necessity of oxygen from plants is another reason why we should discourage the destruction of rainforests worldwide, particularly in the Amazon.
Photosynthetic organisms serve as the foundation of every ecosystem. In this role they are referred to as producers. Organisms which consume them are accordingly called consumers.