Photosynthesis

I

Introduction

Photosynthesis, process by which chlorophyll-containing organisms—green plants, algae, and some bacteria—capture energy in the form of light and convert it to chemical energy. Virtually all the energy available for life in the Earth's biosphere—the zone in which life can exist—is made available through photosynthesis.

A quite generalized, unbalanced chemical equation for photosynthesis is

CO2 + 2H2A + light energy → (CH2) + H2O + A2

The formula H₂A represents a compound that can be oxidized, that is, from which electrons can be removed; CO₂ is carbon dioxide; and CH₂ is a general formula for the hydrocarbons incorporated by the growing organism. In the vast majority of photosynthetic organisms—that is, algae and green plants—H₂A is water (H₂O) and A₂ is oxygen (O₂); in some photosynthetic bacteria, however, H₂A is hydrogen sulphide (H₂S). Photosynthesis involving water is the most important and best understood and, therefore, will be discussed here in detail.

Photosynthesis consists of two stages: a series of light-dependent reactions that are temperature-independent and a series of temperature-dependent reactions that are light-independent. The rate of the first series, called the light reaction, can be increased by increasing light intensity (within certain limits) but not by increasing temperature. In the second series, called the dark reaction, the rate can be increased by increasing temperature (within certain limits) but not by increasing light intensity.

II

Light Reaction

The first step in photosynthesis is the absorption of light by pigments. Chlorophyll is the most important of these because it is essential for the process. It captures light energy in the violet and red portions of the spectrum and transforms it into chemical energy through a series of reactions. Different forms of chlorophyll and other pigments known as carotenoids and phycobilins absorb slightly different wavelengths of light and pass the energy to a form of chlorophyll called chlorophyll A for the completion of the transformation process. These accessory pigments thus broaden the spectrum of light energy that can be fixed through photosynthesis.

Photosynthesis takes place within cells, in organelles called chloroplasts (in the leaves of plants) that contain the chlorophylls and other chemicals, especially enzymes, necessary for the various reactions. The chemicals involved are organized into units of the chloroplasts called thylakoids, and the pigments are embedded in the thylakoids in subunits called photosystems. Light is absorbed by the pigments, raising their electrons to higher energy levels. The energy is then transferred to a special form of chlorophyll A, called a reaction centre.

Two photosystems, numbered I and II, are recognized. Light energy is first trapped by photosystem II, and the energized electrons are boosted to an electron receptor. They are replaced in photosystem II by electrons from water molecules, and oxygen is released. The energized electrons are passed along an electron transport chain to photosystem I, and energy-rich adenosine triphosphate, or ATP, is generated in the process. Light energy absorbed by photosystem I is then passed to its reaction centre, and energized electrons are boosted to its electron acceptor. They are passed by means of another transport chain to energize the coenzyme (see Enzyme) nicotinamide adenine dinucleotide phosphate, or NADP, resulting in its reduction to NADPH₂. The electrons lost by photosystem I are replaced by those passed along the electron transport chain from photosystem II. The light reaction ends with the energy yield stored in the ATP and NADPH₂.

III

Dark Reaction

The dark reaction takes place in the stroma (matrix) of the chloroplast, where the energy stored in the ATP and NADPH₂ is used to reduce carbon dioxide to organic carbon. This is accomplished through a series of reactions known as the Calvin cycle, driven by the energy in the ATP and NADPH₂. At each turn of the cycle one molecule of carbon dioxide enters and is initially combined with a five-carbon sugar called RuBP (ribulose 1,5-biphosphate) to form two molecules of a three-carbon compound called PGA (3-phosphoglycerate). Three turns of the cycle—each of which consumes one molecule of carbon dioxide, two of NADPH₂, and three of ATP—produce a three-carbon molecule, glycer-aldehyde 3-phosphate, two molecules of which combine to form a six-carbon sugar, glucose. The RuBP is regenerated with each turn of the cycle.

Thus, the net effect of photosynthesis is the temporary capture of light energy in the chemical bonds of ATP and NADPH₂ through the light reaction, and the permanent capture of the energy in glucose through the dark reaction. Water is split during the light reaction to provide electrons which transfer the light energy to form ATP and NADPH₂. The oxygen given off as a by-product is the main source of atmospheric oxygen. Carbon dioxide is reduced in the dark reaction to provide the basis for the sugar molecule. The complete, balanced equation for photosynthesis in which water serves as the electron donor is

There are two other systems of photosynthesis known in plants using CO₂ and water. Most grasses and some dicotyledon plants, including many food plants such as potatoes and beans, use 4-carbon photosynthesis, synthesizing a 4-carbon molecule instead of PGA during the dark reaction. This system improves the efficiency of water use. Many succulent plants use CAM (crassulacean acid metabolism), a process where the CO₂ is taken in during the night and assimilated during the day. This allows such plants to live in very dry environments as they can reduce water loss by keeping their stomata (leaf pores) closed during the heat of the day (see Transpiration).

IV

Artificial Photosynthesis

Were chemists able to duplicate photosynthesis by artificial means, resulting systems would have enormous potential for tapping solar energy on a large scale. Much research is now being devoted to this effort. An artificial molecule that remains polarized sufficiently long to react usefully with other molecules has not yet been perfected, but the prospects are promising.