Photosynthesis

Photosynthesis is the process by which plants use the energy of light to pro­duce carbohydrates and molecular oxygen (O₂) from carbon dioxide (CO₂) and water:

6CO₂ + 6H₂O⇒sunlight ⇒ C₆H₁₂O₆ + 6O₂

Virtually all ecosystems on Earth depend on photosynthesis as their source of energy, and all free oxygen on the planet, including that in the at­mosphere, originates from photosynthesis. The overall reaction is the re­verse of respiration, which releases energy by oxidizing carbohydrates to produce CO₂ and water. Photosynthesis and respiration are linked ecolog­ically, being the cellular metabolic processes that drive the carbon and oxy­gen cycles.

Photosynthesis occurs in plants, photosynthetic protist (algae), and some bacteria. In plants and algae, it takes place within chloroplasts, whereas in bacteria it occurs on the plasma membrane and in the cytosol. The remain­der of this discussion will refer to photosynthesis in chloroplasts of plants.

An overview of the photosynthetic process.

Overview

Photosynthesis is divided into two sets of reactions: the light-dependent (light) reactions and the light-independent (dark) reactions. As their names imply, the first set depends directly on light, whereas the second set does not. Nevertheless, even the dark reactions will cease if the plants are de­prived of light for too long because they rely on the products of the light reactions.

The light reactions, which convert the energy in light into chemical en­ergy, take place within the thylakoid membranes of the chloroplasts, whereas the dark reactions, which use that chemical energy to fix CO2 into organic molecules, take place in the stroma of the chloroplast. In the light reactions, the energy of light is used to “split water,” stripping a pair of electrons from it (and causing the two hydrogens to be lost), thus generating molecular oxygen. The energy in light is transferred to these electrons, and is then used to generate adenosine triphosphate (ATP) and the electron carrier NADPH. These two products carry the energy and electrons generated in the light reactions to the stroma, where they are used by the dark reactions to synthesize sugars from CO₂.

Photosynthesis in a chloroplast.

The Light Reaction

The light reactions rely on colored molecules called pigments to capture the energy of light. The most important pigments are the green chloro­phylls, but accessory pigments called carotenoids are also present, which are yellow or orange. The accessory pigments capture wavelengths of light that chlorophylls cannot, and then transfer the energy to chlorophyll, which uses this energy to carry out the light reactions. These pigments are arranged in the thylakoid membranes in clusters, along with proteins and electron car­riers, to form light-harvesting complexes referred to as photosystems. Each photosystem has about two hundred chlorophyll molecules and a variable number of accessory pigments.

In most plants there are two photosystems, which differ slightly in how they absorb light. At the center of each photosystem is a special chlorophyll molecule called the reaction center, to which all the other pigments molecules pass the energy they harvest from sunlight. When the reaction-center chlorophyll absorbs light or receives energy from its accessory molecules, a pair of electrons on it becomes excited. These elec­trons now carry the energy from light, and are passed to an electron ac­ceptor molecule.

The fate of these electrons depends on which photosystem they arose from. Electrons from photosystem I are passed down a short electron trans­port chain to reduce NADP⁺ to NADPH (which also gains an H⁺ ion). Electrons from photosystem II are passed down a longer electron transport chain, eventually arriving at photosystem I, where they replace the electrons given up by photosystem I’s reaction center. Along the way, the energy re­leased by the electrons is used to make ATP in a process called pho­tophosphorylation. Many of the molecular details of this ATP-generating system are similar to those used by the mitochondrion in oxidative phos­phorylation. (Phosphorylation refers to the addition of a phosphate group to adenosine diphosphate [ADP] to form ATP.) Like the mitochondrion, the chloroplast uses an electron transport chain, and ATP synthetase to cre­ate ATP.

The end result of excitation of both photosystems is that electrons have been transferred from chlorophyll to NADP+, forming NADPH, and some of their energy has been used to generate ATP. While photosystem I gains electrons from photosystem II, the electrons lost by photosystem II have not been replaced yet. Its reaction center acquires these electrons by split­ting water. During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O₂), a waste prod­uct of photosynthesis. The reaction is:

2H₂O ⇒ O₂ + 4H⁺ + 4e^-

The carbon fixation cycle transforms simple, inorganic compounds of carbon into more complex forms of organic matter.

The Dark Reactions

The NADPH and ATP generated in the light reactions enter the stroma, where they participate in the dark reactions. Energy and electrons provided by ATP and NADPH, respectively, are used to incorporate CO₂ into car­bohydrate via a cyclic pathway called the Calvin-Benson cycle. In this com­plex pathway, the CO₂ is added to the five-carbon sugar ribulose bisphosphate to form a six-carbon unstable intermediate, which immediately breaks down to two three-carbon molecules. These then go through the rest of the cycle, regenerating ribulose bisphosphate as well as the three-carbon sugar glyceraldehyde phosphate. It takes three turns of the cycle to produce one glyceraldehyde phosphate, which leaves the cycle to form glucose or other sugars.

Some plants bind CO₂ into a four-carbon compound before perform­ing the Calvin-Benson cycle. Such plants are known as C4 plants or CAM plants, depending on the details of the CO₂ capture process.

References

Bishop, M. B., and C. B. Bishop. “Photosynthesis and Carbon Dioxide Fixation.” Journal of Chemical Education 64 (1987): 302-305.

Govindjee, and W. J. Coleman. “How Plants Make O₂.” Scientific American 262 (Feb­ruary 1990): 50-58.

Youvan, D. C., and B. L. Marrs. “Molecular Mechanisms of Photosynthesis.” Scien­tific American 256 (June 1987): 42-48.