Photosynthesis

People have long been interested in how plants obtain the nutrients they use for growth. The early Greek philosophers believed that plants obtained all of their nutrients from the soil. This was a common belief for many centuries.

In the first half of the seventeenth century, Jan Baptista van Helmont (1579-1644), a Dutch physician, chemist, and alchemist, performed important experiments which disproved this early view of photosynthesis. He grew a willow tree weighing 5 lb (2.5 kg) in a clay pot which had 200 lb (91 kg) of soil. Five years later, after watering his willow tree as needed, it weighed about 169 lb (76.5 kg) even though the soil in the pot lost only 2 oz (56 g) in weight. Van Helmont concluded that the tree gained weight from the water he added to the soil, and not from the soil itself. Although van Helmont did not understand the role of sunlight and atmospheric gases in plant growth, his early experiment advanced our understanding of photosynthesis.

In 1771, the noted English chemist Joseph Priestley performed a series of important experiments which implicated atmospheric gases in plant growth. Priestley and his contemporaries believed a noxious substance, which they called phlogiston, was released into the air when a flame burned. When Priestley burned a candle within an enclosed container until the flame went out, he found that a mouse could not survive in the "phlogistated" air of the container. However, when he placed a sprig of mint in the container after the flame had gone out, he found that a mouse could survive. Priestley concluded that the sprig of mint chemically altered the air by removing the "phlogiston." Shortly after Priestly's experiments, Dutch physician Jan Ingenhousz (1730-1799) demonstrated that plants "dephlogistate" the air only in sunlight, and not in darkness. Further, Ingenhousz demonstrated that the green parts of plants are necessary for" dephlogistation" and that sunlight by itself is ineffective.

As Ingenhousz was performing his experiments, the celebrated French chemist Antoine Lavoisier (1743-1794) disproved the phlogiston theory. He conclusively demonstrated that candles and animals both consume a gas in the air which he named oxygen. This implied that the plants in Priestley's and Ingenhousz's experiments produced oxygen when illuminated by sunlight. Considered by many as the founder of modern chemistry, Lavoisier was condemned to death and beheaded during the French revolution.

Lavoisier's experiments stimulated Ingenhousz to reinterpret his earlier studies of "dephlogistation." Following Lavoisier, Ingenhousz hypothesized that plants use sunlight to split carbon dioxide (CO₂) and use its carbon (C) for growth while expelling its oxygen (O₂) as waste. This model of photosynthesis was an improvement over Priestley's, but was not entirely accurate.

Ingenhousz's hypothesis that photosynthesis produces oxygen by splitting carbon dioxide was refuted about 150 years later by the Dutch-born microbiologist Cornelius van Niel (1897-1985) in America. Van Niel studied photosynthesis in anaerobic bacteria, rather than in higher plants. Like higher plants, these bacteria make carbohydrates during photosynthesis. Unlike plants, they do not produce oxygen during photosynthesis and they use bacteriochlorophyll rather than chlorophyll as a photosynthetic pigment. Van Niel found that all species of photosynthetic bacteria which he studied required an oxidizable substrate. For example, the purple sulfur bacteria use hydrogen sulfide as an oxidizable substrate and the overall equation for photosynthesis in these bacteria is:

On the basis of his studies with photosynthetic bacteria, van Niel proposed that the oxygen which plants produce during photosynthesis is derived from water, not from carbon dioxide. In the following years, this hypothesis has proven true. Van Niel's brilliant insight was a major contribution to our modern understanding of photosynthesis.

The study of photosynthesis is currently a very active area of research in biology. Hartmut Michel and Johann Deisenhofer recently made a very important contribution to our understanding of photosynthesis. They made crystals of the photosynthetic reaction center from Rhodopseudomonas viridis, an anaerobic photosynthetic bacterium, and then used x-ray crystallography to determine its three-dimensional structure. In 1988, they shared the Nobel Prize in Chemistry with Robert Huber for this ground-breaking research.

Modern plant physiologists commonly think of photosynthesis as consisting of two separate series of interconnected biochemical reactions, the light reactions and the dark reactions. The light reactions use the light energy absorbed by chlorophyll to synthesize labile high energy molecules. The dark reactions use these labile high energy molecules to synthesize carbohydrates, a stable form of chemical energy which can be stored by plants. Although the dark reactions do not require light, they often occur in the light because they are dependent upon the light reactions. In higher plants and algae, the light and dark reactions of photosynthesis occur in chloroplasts, specialized chlorophyll-containing intracellular structures which are enclosed by double membranes.