Introduction to Photosynthesis:

Photosynthesis is the light-driven reaction which converts energy - poor compounds like carbon dioxide and water to energy rich carbohydrates. Our knowledge of photosynthesis however started from the inquisitiveness and experimentation of early scientists. In this unit, we are going to begin a series of units that will treat the subject matter of photosynthesis. This unit will be mostly historical in nature. We will look at the early experiments carried out by pioneers, and how one thing led to the other until we got to the level we are today in the knowledge of photosynthesis.

A good understanding of photosynthesis will hinge on an understanding of the nature of light. We will also look at the pioneering efforts of Isaac Newton and Einstein that led to an understanding of the nature of light. We will reserve other issues on photosynthesis to subsequent units.

History of the Elucidation of Photosynthesis:

Early scientists had no idea that the sun supplies the earth with virtually all its energy or that green plants trap energy and produce the invisible gases that we breathe. Ancient Greeks rather, regarded plants as "soil eater" because they noted that fertilizing the soil increases plants growth. Plants utilize CO₂ made by combustion or exhaled by animals and that animals inhale and use O₂ released by plants. Priestley's experiments presented first logical description of how air remained "pure" and able to support mouse despite burning of candles and breathing of animals. Though, he didn't realize it. Priestley's experiments were first revelation that plants generate oxygen.

Ingenhousz reported that plants in dark pollute air and make it injurious to animals. He also said that sun by itself has no power to mend air without concurrence of plants. Senebier also stated that air (CO₂) generated by animals and plants in darkness-stimulated production of purified air (O₂) through plants in light. Therefore by late 1700s, biologists know that at least 2 gases partake in photosynthesis. Work completed by Lavoisier and other illustrated that the gases were CO₂ and O₂. Ingenhousz adapted thoughts of Lavoisier and recommended that plants don't simply exchange good air for bad air rather; plants absorb carbon from carbon dioxide. Throwing out at time oxygen alone and keeping carbon to itself as nourishment. Ingenhousz's work was extended in 1804 by Swiss botanist and physician Nicholas de Saussure, who stated that about equal volumes of CO₂ and O₂ are exchanged in photosynthesis. He added final component of overall photosynthesis reaction when he illustrated that photosynthesis needs water.

Carbon dioxide + water + light → Organic material + oxygen

T.W. Engelmann, who is 1883 planned the experiment that of water containing oxygen requiring bacterium and utilized the prism to illuminate various parts of chloroplast with different colors of light. When Engelmann observed spirogyra through the microscope, he saw that bacteria clustered around parts of chloroplast illuminated by red and blue light. He stated that red and blue light are most efficient for generating oxygen in photosynthesis. Other biologists began to follow up Ingelhousz's result that light was needed for release of O₂. Julius Sachs reported that chlorophyll, photosynthesis pigment happens in chloroplast and that photosynthesis forms carbohydrates only in light. This Sachs stated overall reaction of photosynthesis as

nCO₂ + nH₂O + light → (Chlorophyll) (CH₂O)n + nO₂

In this reaction CH₂O is abbreviation for starch or other carbohydrates. Further research illustrated that Sachs conclusion was right. There is no recognized exception to the linking of chlorophyll with oxygen production.

By turn of twentieth century, many biologists accepted Ingenhousz's suggestion that oxygen released in photosynthesis came from carbon dioxide. Others though questioned the assumption. In 1920's a C.B. Van Niel started the study which would resolve the question and that became the milestone in biological research. Van Niel studied the photosynthesis bacterium which uses H2S as electron source and deposits sulphur as by-product. Photosynthesis in the bacteria happened as follows:

CO₂ + 2H₂S → light → CH₂O + H₂O + 2S

Van Niel's asserted that oxygen released in photosynthesis came from water not carbon dioxide. His analysis was based on analogies between roles of H₂S and H₂O and of 0₂ and sulphur.

Sulphur bacteria

CO₂ + 2H₂S → light → CH₂O + H₂O + 2S

General equation

CO₂ + 2H₂x → light → CH₂O + H₂O + 2X

Green Plants

CO₂ + 2H₂G → light → CH₂O + H₂O + O₂

Van Niel's conclusion that O2 released by plants comes from water rather than CO₂ was tested in 1941 by Samuel Ruben and Martin Kamen, who uncovered cultures of chlorella (a green algae) to H₂O labeled with ¹⁸O₂, non-radioactive isotope of oxygen that could notice with mass spectrometer. Ruben and Kamen articulate that if oxygen released in photosynthesis came from water, then oxygen would be tagged with ¹⁸O₂. On the other hand, if oxygen were derived from carbon dioxide, it wouldn't be labeled with ¹⁸O₂. The results were striking, oxygen, not carbohydrates, were labeled with ¹⁸O₂.

CO₂ + 2H₂¹⁸O₂ → light → CH₂O + H₂O + ¹⁸O₂

These results established Van Niel's claim that oxygen released in photosynthesis comes from water not carbon dioxide. Further evidence for this conclusion was given by Robin hill and his coworkers, who found that isolated chloroplast, would release O₂ in absence of CO₂ if given the appropriate election acceptor for electrons removed from water. This light driven splitting of water in absence of CO₂ became known as Hill reaction and illustrated that:

1. Whole cells are needless for some of reactions of photosynthesis

2. Light-driven release of O₂ in photosynthesis is not associated directly to fixation of CO₂.

In 1951, a botanist found that electron acceptor in chloroplasts is NADP, the co-enzyme which can accept electrons. Afterward studies illustrated that NADP reduced in photosynthesis was reduced to CO₂ in photosynthesis. Likewise, reduction of NADP is driven by light.

The Nature of Light:

Light is a part of electromagnetic spectrum containing wavelengths visible to human eye. Almost all life relies on light. Understanding of light started about 300 years ago when Isaac Newton illustrated that White light which passed through the droplet of water, prism, or soap bubble would divide into the band of colors that if passed by another prism, could be recombined to create white light. Based on that experiment, Newton suggested that white light is actually the spectrum of colors ranging from violet to red. 200 years later, in 1860s, a Scottish Mathematician named James Maxwell illustrated that visible light that Newton separated into spectrum of colors is only small part of much larger spectrum of radiation.

In 1905, Einstein linked ideas of Newton and Maxwell by suggesting that light comprises of packets of energy known as photons. Intensity (i.e. brightness) of light depends on number of photons (i.e. amount of energy) absorbed per unit of time. Every photon carries the fixed amount of energy which is determined by how photon vibrates: slower the vibration, less energy carried by photon. Distance moved by photon during complete vibration is referred to as photon's wavelength. Stated another way, wavelength is distance between vibrational crests of photon. Wavelengths of visible light are estimated in nanometers (nm) or billionth (10-9) of a metre.

We distinguish different wavelength, as different colors. For instance, violet light has wavelength of approx 400nm that is about one fortieth the thickness of page. Energy of photon is quantum and is inversely proportional to wavelength of light. Longer the wavelength (that is longer distance traveled during vibration) the less energy per photon. Sunlight comprises of approx 4% ultraviolet radiation, 52% infrared radiation and 44% visible light. Each of these types of light has different energy and affects organisms differently. Ultraviolet radiation (UV) has too much energy for many biological systems. Certainly, its light energy photons frequently drive electrons from molecules that is why UV is also known as ionizing radiation. UV breaks weak bonds and causes sunburn. It is absorbed by O₂, ozone (O₃), and glass. Infrared radiation (R) doesn't have sufficient energy per photon to be helpful to living systems. Cells absorb IR radiation but this energy is inadequate to stimulate electrons. Thus, most of the energy of IR is converted instantly to heat. IR is absorbed by water and carbon dioxide, but goes through glass thus. Visible light has simply right amount of energy for biological reactions like photosynthesis. To have the effect though, light should first be absorbed. Light is absorbed by pigments.

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