Photosynthesis

Plants, unlike animals, do not get food by eating other organisms (as always in nature, there are exceptions: carnivorous plants such the Venus fly trap). They make their own food, usually in the form of glucose, from the inorganic compounds[?] carbon dioxide and water. Carbon dioxide is taken in through the leaves, and water is taken in mainly through the roots. Sunlight acts as the energy needed to run the reaction that yields glucose as the product the plant needs and oxygen as a waste product that is released into the environment.

In green plants and algae, the pigment molecules that initially absorb the light energy are chlorophyll and various carotenoids[?]. Bacteria contain various other pigments. It may be noted that the typical colors of photosynthetic organisms (green, brown, golden, or red) result from the light that is not absorbed by the pigment molecules.

The typical overall chemical reaction of photosynthesis is:

6H₂O + 6CO₂ + light → C₆H₁₂O₆ (glucose) + 6O₂

In simple English terms, this is carbon dioxide plus water plus light (energy) yields oxygen plus sugar. In animal bodies, this is exactly reversed: oxygen plus sugar yields carbon dioxide plus water plus energy. However, it is important to note that this chemical equation is highly simplified; in reality photosynthesis employs a very complex mechanism for the adsorbption and conversion of light into chemical energy, using chemical pathways with many important intermediates. Photosynthesis has two distinct stages, called the light reaction and carbon fixation[?], which typically occurs via the Calvin cycle.

The production of oxygen

It is interesting to note that the oxygen released during photosynthesis is not in fact derived from the carbon dioxide, but rather from the water molecules which are consumed in the reaction. This was first proposed in the 1930s by C. B. van Neil of Stanford University, while investigating photosynthetic bacteria, many of which do not release oxygen. One significant group of such organism are bacteria which use hydrogen sulfide instead of water in their photosynthetic pathway:

CO₂ + 2 H₂S → CH₂O + H₂O + 2S

Some of these produce globules of sulfur as a waste product instead of oygen, while others further oxidize it, producing sulfates. In general, photosynthesis requires a source of hydrogen with which to reduce carbon dioxide into carbohydrates. Van Neil's proposal was confirmed 20 years later by using the O¹⁸ isotope of oxygen as a tracer label to follow the fate of oxygen atoms during photosynthesis.

Light-dependant reaction

The first stage of the photosynthetic system is the light-dependant reaction, which converts solar energy into chemical energy. Light absorbed by chlorophyll or other photosynthetic pigments is used to drive a transfer of electrons and hydrogen from water (or some other donor molecule) to an acceptor called NADP[?]⁺, reducing it to the form of NADPH by adding a pair of electrons and a single proton (hydrogen nucleus). The water or other donor molecule is split in the process; it is the light reaction which produces waste oxygen.

The light reaction also generates ATP by powering the addition of a phosphate group to ADP, a process called photophosphorylation[?]. ATP is a versatile source of chemical energy used in most biological processes. Note, however, that the light reaction produces no carbohydrates such as sugars.

Both of these processes are accomplished via the mechanism of an electron transport chain. This is a series of proteins embedded in a biological membrane that transfers high-energy electrons from one to another, accomplishing various activities along the way as the electron drops in energy level. The electrons originate when a photon of sunlight strikes a chlorophyll molecule contained within a photosystem (cluster of associated pigment molecules) and excites one of its electrons, which is then captured by a primary acceptor protein.

The chlorophyll's electron can follow either of two different pathways, cyclic or non-cyclic.

Cyclic photophosphorylation

In cyclic electron flow, the electron originates in a pigment complex called photosystem I, passes from the primary acceptor to ferredoxin[?], then to plastoquinone[?] (a complex of two cytochromes similar to those found in mitochondria), and then plastocyanin[?] before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H⁺ ions across the membrane; this produces a concentration gradient which can be used to power ATP synthase. This pathway is known as cyclic photophosphorylation, and it produces neither O₂ nor NADPH.

Noncyclic photophosphorylation

The other pathway, noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. First, a photon is absorbed by the chlorophyll core of photosystem II, exciting two electrons which are transferred to the primary acceptor protein. The deficit of electrons is made up for by taking electrons from a molecule of water, splitting it into O₂ and H⁺. The electrons transfer from the primary acceptor to plastoquinone, then to plastocyanin, producing proton-motive force as with cyclic electron flow and driving ATP synthase.

Since the photosystem II complex replaced its lost electrons from an external source, however, these electrons are not returned to photosystem II as they would in the analogous cyclic pathway. Instead, the still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the primary acceptor protein, but this time are passed on to ferredoxin, and then to an enzyme called NADP⁺ reductase[?] which uses them to drive the reaction

NADP⁺ + H⁺ + 2e^- → NADPH

This consumes the H⁺ ions produced by the splitting of water, leading to a net production of O², ATP, and NADPH with the consumption of solar photons and water.

The Calvin cycle

The Calvin cycle is similar to the Krebs cycle in some regards. Carbon enters the Calvin cycle in the form of CO₂ and leaves in the form of a carbohydrate such as sugar, with the reaction being driven by ATP and NADPH. This ATP and NADPH is usually produced by the light reaction described above, but there is nothing inherent in the process which requires this to be the case; other sources of ATP and NADPH can be used, and in some cases are.