An even closer form of symbiosis may explain the origin of chloroplasts. Chloroplasts have many similarities with , including a circular , prokaryotic-type , and similar proteins in the photosynthetic reaction center. The suggests that photosynthetic bacteria were acquired (by ) by early cells to form the first cells. Therefore, chloroplasts may be photosynthetic bacteria that adapted to life inside plant cells. Like , chloroplasts still possess their own DNA, separate from the of their plant host cells and the genes in this chloroplast DNA resemble those in . DNA in chloroplasts codes for proteins such as photosynthetic reaction centers. The proposes that this Co-location is required for Redox Regulation.
Several groups of animals have formed relationships with photosynthetic algae. These are most common in , and . It is presumed that this is due to the particularly simple and large surface areas of these animals compared to their volumes. In addition, a few marine and also maintain a symbiotic relationship with chloroplasts they capture from the algae in their diet and then store in their bodies. This allows the mollusks to survive solely by photosynthesis for several months at a time. Some of the genes from the plant have even been transferred to the slugs, so that the chloroplasts can be supplied with proteins that they need to survive.
Although photosynthesis can happen in different ways in different species, some features are always the same. For example, the process always begins when energy from light is absorbed by called that contain . In plants, these proteins are held inside called , while in bacteria they are embedded in the . Some of the light energy gathered by chlorophylls is stored in the form of (ATP). The rest of the energy is used to remove from a substance such as water. These electrons are then used in the reactions that turn carbon dioxide into organic compounds. In plants, algae and cyanobacteria, this is done by a sequence of reactions called the , but different sets of reactions are found in some bacteria, such as the in . Many photosynthetic organisms have adaptations that concentrate or store carbon dioxide. This helps reduce a wasteful process called that can consume part of the sugar produced during photosynthesis.
The first photosynthetic organisms probably evolved about 350 mya, early in the evolutionary history of life, when all forms of life on Earth were and the atmosphere had much more carbon dioxide. They most likely used or as sources of electrons, rather than water. Cyanobacteria appeared later, around , and drastically changed the Earth when they began to , beginning about . This new atmosphere allowed the such as . Eventually, no later than a billion years ago, one of these protists formed a with a cyanobacterium, producing the ancestor of many plants and algae. The chloroplasts in modern plants are the descendants of these ancient symbiotic cyanobacteria.
The proteins that gather light for photosynthesis are embedded within . The simplest way these are arranged is in photosynthetic bacteria, where these proteins are held within the plasma membrane. However, this membrane may be tightly folded into cylindrical sheets called , or bunched up into round called . These structures can fill most of the interior of a cell, giving the membrane a very large surface area and therefore increasing the amount of light that the bacteria can absorb.
In plants and algae, photosynthesis takes place in called . A typical contains about 10 to 100 chloroplasts. The chloroplast is enclosed by a membrane. This membrane is composed of a phospholipid inner membrane, a phospholipid outer membrane, and an intermembrane space between them. Within the membrane is an aqueous fluid called the stroma. The stroma contains stacks (grana) of thylakoids, which are the site of photosynthesis. The thylakoids are flattened disks, bounded by a membrane with a lumen or thylakoid space within it. The site of photosynthesis is the thylakoid membrane, which contains integral and complexes, including the pigments that absorb light energy, which form the photosystems.
Although all cells in the green parts of a plant have chloroplasts, most of the energy is captured in the . The cells in the interior tissues of a leaf, called the , can contain between 450,000 and 800,000 chloroplasts for every square millimeter of leaf. The surface of the leaf is uniformly coated with a water-resistant that protects the leaf from excessive of water and decreases the absorption of or to reduce . The transparent layer allows light to pass through to the mesophyll cells where most of the photosynthesis takes place.
Not all of light can support photosynthesis. The photosynthetic action spectrum depends on the type of present. For example, in green plants, the resembles the for and with peaks for violet-blue and red light. In red algae, the action spectrum overlaps with the absorption spectrum of for red blue-green light, which allows these algae to grow in deeper waters that filter out the longer wavelengths used by green plants. The non-absorbed part of the is what gives photosynthetic organisms their color (e.g., green plants, red algae, purple bacteria) and is the least effective for photosynthesis in the respective organisms.
To be more specific, carbon fixation produces an intermediate product, which is then converted to the final carbohydrate products. The carbon skeletons produced by photosynthesis are then variously used to form other organic compounds, such as the building material , as precursors for and biosynthesis, or as a fuel in . The latter occurs not only in plants but also in when the energy from plants gets passed through a .
The biochemical capacity to use water as the source for electrons in photosynthesis evolved once, in a of extant . The geological record indicates that this transforming event took place early in Earth's history, at least 2450–2320 million years ago (Ma), and, it is speculated, much earlier. Available evidence from geobiological studies of (>2500 Ma) indicates that life existed 3500 Ma, but the question of when oxygenic photosynthesis evolved is still unanswered. A clear paleontological window on cyanobacterial opened about 2000 Ma, revealing an already-diverse biota of blue-greens. remained principal throughout the (2500–543 Ma), in part because the redox structure of the oceans favored photoautotrophs capable of . joined blue-greens as major primary producers on near the end of the , but only with the (251–65 Ma) radiations of dinoflagellates, coccolithophorids, and diatoms did in marine shelf waters take modern form. Cyanobacteria remain critical to as primary producers in oceanic gyres, as agents of biological nitrogen fixation, and, in modified form, as the of marine algae.
Although some of the steps in photosynthesis are still not completely understood, the overall photosynthetic equation has been known since the 19th century.
thought that a complex of reactions consisting of an intermediate to cytochrome b6 (now a plastoquinone), another is from cytochrome f to a step in the carbohydrate-generating mechanisms. These are linked by plastoquinone, which does require energy to reduce cytochrome f for it is a sufficient reductant. Further experiments to prove that the oxygen developed during the photosynthesis of green plants came from water, were performed by Hill in 1937 and 1939. He showed that isolated give off oxygen in the presence of unnatural reducing agents like , or after exposure to light. The Hill reaction is as follows: