Photosynthesis

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Photosynthesis is a very complex process carried out by green plants, blue-green algae, and certain bacteria. These organisms are able to harness the energy contained in sunlight, and via a series of oxidation-reduction reactions, produce oxygen and sugar, as well as other compounds which may be utilized for energy as well as the synthesis of other compounds.

The basic reaction of photosynthesis is:

CO₂ + H₂O + light energy ---> (CH₂O)n + O₂

The equation is the net result of two processes. One process involves the splitting of water. This process is really an oxidative process that requires light, and is often referred to as the "light reaction". This reaction may be written as:

12 H₂O + light energy-----------------------> 6 O₂ + 24 H+ + 24e-

The oxidation of water is accompanied by a reduction reaction resulting in the formation of a compound, called nicotinamide adenine dinucleotide phosphate (NADPH). This reaction is illustrated below:

NADP+ + H₂0 -------> NADPH + H+ + O

(oxidized form) (reduced form) (oxygen)

This reaction is linked or coupled to yet another reaction resulting in the formation of a highly energetic compound, called adenosine triphosphate, (ATP ). As this reaction involves the addition of a phosphate group (labeled, as Pi) to a compound called, adenosine diphosphate (ADP) during the light reaction, it is called photophosphorylation.

ADP + Pi ------------> ATP

Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule. Photophosphorylation consists of two pigments to excite, called PS1 (photosystem 1) and PS2 (photosystem 2). PS1 is better excited by light at about 700 nm, and is thus sometimes called P-700. PS2 cannot use photons of wavelength longer than 680 nm, and is sometimes called P-680. Photophosphorylation occurs when the electrons from water are excited by the light in the presence of P680. The energy transfer is similar to the chemiosmotic electron transport occurring in the mitochondria. Light energy causes the removal of an electron from a molecule of P680 that is part of Photosystem II. The P680 requires an electron, which is taken from a water molecule, breaking the water into H+ ions and O-2 ions. These O-2 ions combine to form the diatomic O2 that is released. The electron is "boosted" to a higher energy state and attached to a primary electron acceptor, which begins a series of redox reactions, passing the electron through a series of electron carriers, eventually attaching it to a molecule in Photosystem I. Light acts on a molecule of P700 in Photosystem I, causing an electron to be "boosted" to a still higher potential. The electron is attached to a different primary electron acceptor (that is a different molecule from the one associated with Photosystem II). The electron is passed again through a series of redox reactions, eventually being attached to NADP+ and H+ to form NADPH, an energy carrier needed in the Light Independent Reaction. The electron from Photosystem II replaces the excited electron in the P700 molecule. There is thus a continuous flow of electrons from water to NADPH. This energy is used in Carbon Fixation. Cyclic Electron Flow occurs in some eukaryotes and primitive photosynthetic bacteria. No NADPH is produced, only ATP. This occurs when cells may require additional ATP, or when there is no NADP+ to reduce to NADPH. In Photosystem II, the pumping to H ions into the thylakoid and the conversion of ADP + P into ATP is driven by electron gradients established in the thylakoid membrane.

 Photosynthesis is a two stage process. The first process is the Light Dependent Process (Light Reactions), requires the direct energy of light to make energy carrier molecules that are used in the second process. The Light Independent Process (or Dark Reactions) occurs when the products of the Light Reaction are used to form C-C covalent bonds of carbohydrates.

.The light reaction is a process by which organisms "capture and store" radiant energy as they produce oxygen gas. This energy is stored in the form of chemical bonds of compounds such as NADPH and ATP. The energy contained in both NADPH and ATP is then used to reduce carbon dioxide to glucose. This reaction, shown below, does not require light, and it is often referred to as the "dark reaction".

6 CO₂ + 24 H+ + 24 e- ------> C₆H₁₂O₆ + 6 H₂O

Carbon dioxide enters single-celled and aquatic autotrophs through no specialized structures. Land plants must guard against drying out (desiccation) and so have evolved specialized cells known as stomata to allow gas to enter and leave the leaf. The Calvin Cycle occurs in the stroma of chloroplasts. Carbon dioxide is captured by the chemical ribulose biphosphate (RuBP). RuBP is a 5-C chemical. Six molecules of carbon dioxide enter the Calvin Cycle, eventually producing one molecule of glucose.

The first stable product of the Calvin Cycle is phosphoglycerate (PGA), a 3-C chemical. The energy from ATP and NADPH energy carriers generated by the photosystems is used to attach phosphates to (phosphorylate) the PGA. Eventually there are 12 molecules of glyceraldehyde phosphate (also known as phosphoglyceraldehyde or PGAL, a 3-C), two of which are removed from the cycle to make glucose. The remaining PGAL molecules are converted by ATP energy to reform 6 RuBP molecules, and thus start the cycle again.

Currently, there is research being done on light-driven biological and chemical processes and their applications in sciences and engineering. Applied and basic interdisciplinary research on light-driven reactions such as photosynthesis and biomimetic reactions holds great promise in at least two areas. One is the development of novel light-driven technologies, such as light-responsive molecular-scale electronics (including miniaturized light-driven computer chips and optical switches), single-molecule optical probes and light-controlled enzymes. The other is to utilize organisms that can live and grow on solar energy for bioremediation purposes (cleaning up water supplies) and for biotechnological production of enzymes and pigments.

Click here for the Photosynthesis Quiz on Light Reactions.

Click here for the Photosynthesis Quiz on Dark Reactions.

Photosynthesis

More Photosynthesis!

Light Reactions